Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion

GR-01024; No of Pages 35 Gondwana Research xxx (2013) xxx–xxx

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Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion S. Maruyama b,⁎, Y. Sawaki a, T. Ebisuzaki c, M. Ikoma d, S. Omori e, 1, T. Komabayashi a a

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan c RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan d Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan e Open University of Japan, 2-11, Wakaba, Mihama-ku, Chiba, 261-8586, Japan b

a r t i c l e

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Article history: Received 13 September 2012 Received in revised form 8 March 2013 Accepted 15 March 2013 Available online xxxx Keywords: Snowball Earth Leaking Earth The Cambrian explosion pO2 increase Nutrients

a b s t r a c t For life to have dramatically evolved and diversified during the so-called Cambrian explosion, there must have been significant changes in the environmental conditions of Earth. A rapid increase in atmospheric oxygen, which has been discussed as the key factor in the evolution of life, cannot by itself explain such an explosion, since life requires more than oxygen to flourish let alone survive. The supply of nutrients must have played a more critical role in the explosion, including an increase in phosphorus (P) and potassium (K) which are key elements for metabolisms to function. So, what happened at the onset of the Cambrian to bring about changes in environmental conditions and nutrient supply and ultimately evolution of life? An ultimate trigger for the Cambrian explosion is proposed here. The geotherm along subduction zones of a cooling Earth finally became cool enough around 600 Ma to allow slabs to be hydrated. The subduction of these hydrated slabs transferred voluminous water from the ocean to the mantle, resulting in a lowering of the sea level and an associated exceptional exposure of nutrient-enriched continental crust, along with an increase in atmospheric oxygen. This loss of water at the surface of the Earth and an associated increase in exposed landmass is referred to here as leaking Earth. Vast amounts of nutrients began to be carried through weathering, erosion, and transport of the landmass; rock fragments of the landmass would break down into ions during transport to the ocean through river, providing life forms (prokaryote) sufficient nutrients to live and evolve. Also, plume-driven dome-up beneath the continental crusts broadened the surface area providing a supply of nutrients an order magnitude greater than that produced through uplift of mountains by continental collision. Simultaneously, atmospheric oxygen began to increase rapidly due to the burial of dead organic matter by enhanced sedimentation from the emergence of a greater landmass, which ultimately inhibited oxidation of organic matter. Hence, oxygen began to accumulate in the atmosphere, which when coupled with a continuous supply of nutrients, resulted in an explosion of life, including an increase in the size. An enhanced oxygen supply in the atmosphere resulted in the formation of an ozone layer, providing life a shield from the UV radiation of the Sun; this enabled life to invade the land. In addition to a change in the supply of nutrients related to a leaking Earth, the evolution of life was accelerated through mass extinction events such as observed during Snowball Earth, possibly related to a starburst in our galaxy, as well as mutation in the genome due to radiogenic elements sourced from carbonatite magma (atomic bomb magma) in rift valley. There are two requirements to find a habitable planet: (1) the initial mass of an ocean and (2) the size of a planet. These two conditions determine the history of a planet, including planetary tectonics and the birth of life. This newfound perspective, which includes the importance of a leaking planet, provides a dawn of new planetary science and astrobiology. © 2013 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . Modern Earth . . . . . . . . . . . . . . . . 2.1. Climate system and nutrient transportation 2.2. Only two life-sustaining sites on the Earth

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⁎ Corresponding author at: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail address: [email protected] (S. Maruyama). 1 Present address: Open University of Japan, 2-11, Wakaba, Mihama-ku, Chiba, 261-8586, Japan. 1342-937X/$ – see front matter © 2013 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. http://dx.doi.org/10.1016/j.gr.2013.03.012

Please cite this article as: Maruyama, S., et al., Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion, Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.03.012

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2.3. Global material circulation from the surface to the core by plate and plume tectonics . . . . . . . . 2.4. Water circulation through plate tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Water budget during the global water circulation . . . . . . . . . . . . . . . . . . . . . . . . . 3. What happened at the end of the Precambrian? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Appearance of blueschist at the subduction zone: rapidly cooled mantle along the subduction zone . 3.2. Secular change of subduction zone geotherm since 4.0 Ga . . . . . . . . . . . . . . . . . . . . . 3.3. Initiation of return-flow of seawater into mantle . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Sea-level change around the onset of the Phanerozoic . . . . . . . . . . . . . . . . . . . . . . . 3.5. Surface environmental change during the Neoproterozoic toward the Cambrian . . . . . . . . . . 3.6. pO2 increase in atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Birth of metazoans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Origin of Snowball Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. Records of surface environmental change from 635 Ma to 488 Ma in South China . . . . . . . . . . 3.9.1. Geological constraints in South China: results from Japan–China program (2004–2012) . . . 3.9.2. Chemostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10. Relationship to the environmental change . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11. Relationship to mass extinctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12. Bio-mineralization and arms race during the Cambrian explosion . . . . . . . . . . . . . . . . . 3.13. Development of ozone layer and life invasion on-land . . . . . . . . . . . . . . . . . . . . . . . 3.14. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Supporting evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Sea-level change by Hallam (1989, 1990) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Sedimentary rocks through time (Ronov et al., 1991; Maruyama et al., 2013) . . . . . . . . . . . . 4.3. Secular change of oxygen isotope (Wallmann, 2001) . . . . . . . . . . . . . . . . . . . . . . . 4.4. Sr isotopic change of seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. The Cambrian explosion: the Big Bang of evolving life . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Extrinsic: what was the ultimate trigger? . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Intrinsic: both from Universe and from solid Earth, by speed up mutation through radiation 5.2. The relationship to the Snowball Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. The fate of the naked planet Earth; why has the Earth become a mega-life sustaining planet? . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The history of life on Earth, beginning some time in the Hadean, witnessed two distinct evolutionary phases: one at 2.1 Ga from prokaryotes to eukaryotes, and the other at 0.6 Ga from eukaryotes to metazoans (e.g., Maruyama et al., 2001, 2007; Payne et al., 2009). Though such an evolutional history of life appears relatively shortlived, the duration in each case is actually millions of years. Such change could be interpreted to be the result of a sudden increase in oxygen content in both the atmosphere and hydrosphere through increased activity of cyanobacteria, as observed at 2.1 Ga (from b1/1000 PAL to 1/100 PAL) and at 0.6 Ga (from 1/100 PAL to the present level 1 PAL) (Holland and Beukes, 1990; Holland, 1994, 2006). The topic of Cambrian explosion has long been discussed, including attributes to both intrinsic causes such as hybridization, increased grade of complexity, and evolution of regulatory genes (e.g., summary by Shu, 2008), and external influences such as a supercontinent cycle and ocean chemistry, in addition to a rapid increase of pO2. However, the ultimate trigger has not yet been confirmed. One of the most critical influences on the evolution of life is nutrient supply, long assumed to be forever adequately present in the oceans of Earth by some unknown processes. This assumption, though, becomes invalid when considering the active geological processes of Earth which bring about constant environmental change, especially those driven by endogenic activity such as plate tectonics and superplume development, and ultimate variations in the amounts of continental and oceanic crustal materials and subaerial exposure of land masses, as well as the ever changing ocean volumes (Maruyama et al., 2013). Though there was a significant enhancement in oxygen content since 0.6 Ga, with atmospheric and oceanic values both nearing 1 PAL, metazoans could not form during ancient times because of the lack of continuous supply of the major (e.g., K, P, Ca, Fe, and Mg) and minor

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(Mo, Zn, Mn, and others) nutrients (Maruyama et al., 2013). Considering much greater nutrient supplies (>1 million times as compared to previous supply), such as during times of enhanced exposure of continental crustal materials, and maintenance of a high-pO2 since 600 Ma up to now, one of the essential geologically-derived changes in the Earth system, consisting of (1) solid Earth, (2) hydrosphere, (3) atmosphere and (4) magnetosphere, would have accordingly occurred around the onset of Phanerozoic. The magnetosphere, which penetrates the other three systems, is caused by the convection of a liquid metallic iron core (Fig. 2). The biosphere is centered mainly in the hydrosphere, but also on-land and in the atmosphere with minor sub-ground microbial communities. But in the past, life was born in the hydrothermal system either in Hadean or earliest Archean, independently from Panspermia, and evolved in the hydrosphere throughout Precambrian time at least from 4.0 Ga until 0.6 Ga (Maruyama et al., 2001, 2007; Fig. 1). At the onset of Phanerozoic, metazoans were born, with life beginning to invade the land. Moreover during this time, life became extremely diversified. For example, all 35 phyla of metazoans alone appeared during the early Cambrian, referred to as the Cambrian explosion (Cloud, 1968; Gould, 1995; Shu, 2008). Significantly, the size of the biomass (total weight of whole life at a given geological time) would expand 1 million times during the Proterozoic–Phanerozoic transition, if considering the change in the Earth system, including the increase in land mass and the resulting supply of nutrients that are sufficient to explode the life system (Maruyama et al., 2013). In this work, we propose that the change in the Earth system was triggered by initiation of return-flow of seawater into the mantle. Some of the fundamental arguments and observations were given in our earlier paper (Maruyama and Liou, 2005). Here we enlarge arguments specifically with emphasis on the role of nutrients, size increase of biomass, and significance of emergence of metazoans that

Please cite this article as: Maruyama, S., et al., Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion, Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.03.012

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Fig. 1. History of life and Earth. 10 major events are shown on the bottom of the figure; (1) birth of the Earth–Moon system, (2) birth of the ocean and atmosphere, (3) birth of life, (4) birth of cyanobacteria (2.8 Ga), (5) 1st Snowball Earth (2.3 Ga), (6) birth of eukaryote (2.1 Ga), (7) 2nd Snowball Earth, (8) Cambrian explosion, (9) P–T boundary, and (10) birth of human being. From bottom to top, surface of solid Earth, ocean, atmosphere (XO2), ozone layer after 450 Ma, and geomagnetic intensity. Cloud like figure shows magnetic intensity through time. Blue bar on the bottom shows the glacial period. Future in 1.5 billion years is also shown. Modified after Maruyama et al. (2001), Maruyama and Santosh (2008a, 2008b).

enabled the Earth to be a mega-life habitable planet through the course of cooling of this rocky planet driven effectively by plate tectonics. 2. Modern Earth The key to understanding the drastic change of the Earth system at the end of the Precambrian is the global circulation of material on Earth, summarized in the following and schematically portrayed in Fig. 3. 2.1. Climate system and nutrient transportation mechanism into ocean Fig. 3 shows the global circulation of materials of the Earth. Only 11 km thick convection layer of dense atmosphere rests on the solid Earth. The top of troposphere, which is at an elevation slightly higher than Mt. Everest of the Himalayan mountain range, has an atmospheric pressure only about 10% of that at sea level. Convection in the troposphere circulates heat and water vapor from equatorial to polar regions. Due to the extraordinary high-speed, self-rotation of the Earth (40,000 km/day at equator), the convective cell is separated into five zones on the Earth, i.e., equatorial and polar and middle latitude in each hemisphere. Selective heating on the equatorial ocean causes high-temperature upwelling enriched in water vapor which turns its direction at the top of the troposphere horizontally northward or southward to transport heat and vapor to the polar regions. At middle-latitude, the circulation of heat and vapor makes a downward turn towards the surface of the ground or oceans where atmospheric pressure is greatest. Hence, the land surface at middle latitudes, 30–40° on both hemispheres, turns to desert regions. The wind blows dusts from the desert regions to oceans at global scale.

This is one of the most important processes to deliver nutrients to the oceans, such as Ca, K, Fe, P, Mo, Mg among others because desert-forming rocks are mainly granitic in composition. Ocean currents, driven by the wind system, transport the nutrients which source from the continents through river systems. If continents are absent, the nutrient supply is non-existent with ocean currents just following the wind system. In addition, the presence of continents modifies the ocean currents, forming complex patterns such as upwellings of deep ocean along the western coast of continents (e.g., along Peru and Namibia). Nutrient-enriched currents are restricted along the continents, called coastal currents. Deep ocean upwellings are enriched in dead organic matter, derived from the breakdown of surface materials, providing food to the ecosystem. The driving force of an ocean current is salinity, initiating east of Greenland with deep-sea downwelling, southwards through the Atlantic Ocean, and then making an east–northeast turn and eventually entering into Indian Ocean. Antarctica is enveloped by a circulating Antarctic current, mostly isolated from the equatorial region. The situation was different in the past until the Miocene, because South America was connected to Antarctica. Since separated from South America, Antarctica has been isolated and covered by ice. Due to accumulated snow, Antarctica became frozen after ca. 2.6 Ma (Ogg et al., 2008). The presence of a cold ice cap on the Antarctic continent, which provides a source of cold water continuously to the oceans, coupled with sun-warmed equatorial waters, effectively drives ocean currents. Water vapor-laden winds, which in general are driven by the selective heating of the Earth by the Sun, encounter mountains resulting in rainfall and/or snowfall. This wind and water activity erodes continental rock materials, which are eventually transported to the oceans through river systems. During the transportation

Please cite this article as: Maruyama, S., et al., Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion, Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.03.012

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Sun cosmic ray solar wind plasma Interplanetary space solar radiation solar radiation (X-ray, ultraviolet, & visible)

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process, clastic grains are cycled from rock fragments through gravel, sand, clay to submicron particles, and thus nutrients can be extracted into water as negative or positive ions through water–rock interaction that can be utilized by microorganisms. Another process occurs at the mid-oceanic ridge, which encircles the solid Earth over 56,000 km (Reymer and Schubert, 1986), reacting continuously with overlying seawater to deliver nutrients from mid-oceanic ridge basalt (MORB) to the surroundings. The amount of nutrient supply from MORB, however, is generally poor (ca. 10−6 times less than that of the surface, and its composition is incomplete for living creatures, because

P and Mo, and specifically K, are remarkably depleted, compared to that of granite (Maruyama et al., 2013). 2.2. Only two life-sustaining sites on the Earth The phenomenon of life is possible only where there is a steadystate supply of nutrients, and there is thermal energy in addition to water circulation. This is analogous to the case of living animals and human beings which survive from a steady supply of food and water. If these conditions are not satisfied, life will terminate. This scenario

Please cite this article as: Maruyama, S., et al., Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion, Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.03.012

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Fig. 3. (I) Global material circulation by climate and plate tectonics on the Earth. Surface material circulation driven by radiation of Sun, and global material circulation by plate tectonics from surface through deep mantle, and some volatiles output from central core to the surface through plumes are also shown. The speed of material circulation is extremely slower in a solid Earth, ca. 10−8 times than the surface. Nutrient delivery system on the modern Earth is dominated on the surface by climate with minor hydrothermal system at mid-oceanic ridge and hot springs by hot-spot magma or subduction zone magma. (II) There are two sites of sustaining life, one at deep-sea ridge hydrothermal system and the other on the surface of the Earth driven by Sun.

holds true even for micro-organisms. Considering those conditions, the Earth has only two life-sustaining places: one is the surface of the Earth fed by a climate driven by Sun. The other is in endogenic vent systems, bet exemplified by the deep-sea hydrothermal system driven by MORB magma, though the biomass at mid-oceanic ridge is 10−6 times smaller than that of the surface of the Earth, i.e., negligibly small (see Maruyama et al., 2013). Though hydrothermal systems are manifested at and near the surface of the Earth generally associated with the development of island arcs and hotspots, they are minor compared to the MOR system. 2.3. Global material circulation from the surface to the core by plate and plume tectonics The speed of nutrient supply by wind or river on the surface of Earth or by hydrothermal fluid circulation at mid-oceanic ridge is fast compared to the speed of material circulation within the solid Earth. Plate velocity is an order of cm/year, whereas wind blows a few m/s to several tens of m/s. Therefore, the speed of material circulation is ca. 10 8 times faster on the surface of the Earth when compared to that in the solid mantle, assuming surface material circulation approximates 1 m/s. This difference means that the solid mantle moves independently from the surface, with interaction among the two in a time scale of days to millions of years only through magmatism (including volcanism and volatile release from magma bodies along conduits such as faults, fractures, and joints), earthquakes, and deep-seated basement structural control of magma and volatiles such as water. However, in a time scale much longer than 1 million years, material circulation drastically affects the surface environments like the Cambrian explosion. We will discuss it later in this paper. Related to plate production, MORB production ranges from 25 km3/year to 5 km3/year for subduction zone-related, andesiteenriched magmas on the modern Earth (Reymer and Schubert, 1986). OIB magma production is around 0.1 km3/year (Reymer and Schubert, 1986). Slab subduction equals plate production at MOR (Reymer and Schubert, 1986). Oceanic plates, formed at MOR, subduct at consuming plate boundaries which are predominantly located in the circum-Pacific

and in the northern margin of Indian Ocean. Calculated length of slabs over the last 150 Ma ranges from over 20,000 km in length under East Asia to several thousands of km in other regions (Engebretson et al., 1985; Lithgow-Bertelloni and Richards, 1998). Some of these may have already sunk down onto the D″ layer directly above the core–mantle boundary (CMB). Subducted slabs capped by deep-sea sediments with or without trench-turbidites are dehydrated, releasing fluids at subduction zones down to a depth of 660 km, which metasomatically alter the hanging wall mantle wedge. Subducted water-enriched fluids decrease mantle viscosity and melting temperature, and ultimately yield arc magmas of andesite or basaltic andesite (Tatsumi, 1986, 1989; Tatsumi and Maruyama, 1989; Tatsumi and Kogiso, 1997). In some cases of youthful plate subduction, the subducted slab itself may partially melt to supply TTG magma directly to the surface (e.g., Defant and Drummond, 1990). At a depth of 660 km (Fig. 4), subducted slabs may become stagnant, not readily being able to penetrate further because of: (1) a Clapeyron negative reaction of γ phase olivine to result in perovskite + wustite, (2) a viscosity jump between the upper and lower parts of the mantle, and (3) a decrease in the viscosity of the descending slab through the reaction of (1) (see a summary by Maruyama et al., 2007). Moreover, at the base of the MTZ, specifically directly below continents, ca. 10 times the volume of TTG materials could be present compared to that of total surface continents (Kawai et al., 2010). In addition, metallic iron can be produced through a perovskite-forming reaction at 660 km (Komiya et al., 2004; Komiya and Maruyama, 2007). Once stagnant at 660 km depth, slabs would finally collapse and move down to the CMB, referred to as slab avalanche (Honda et al., 1993; Honda and Yuen, 1994; Maruyama, 1994). An increase in the amount of slab-graveyards is now clearly observed at the CMB through tomographic images (Fukao et al., 1994; Zhao, 2007, 2009, see references in Fukao et al., 2001; Maruyama et al., 2007), including even beneath major oceans. The presence of past supercontinents and slab-graveyards below the Pacific Ocean and the Indian Ocean back to 1.0 Ga has been correlated in previous studies (e.g. Maruyama et al., 2007). MORB slab aggregates circulate through time, driven by horizontal tectonics along the CMB, referred to as anti-plate tectonics (Maruyama

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Recycling of slab Arc (continent)

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tectonics

solid inner core Fig. 4. Global material circulation from top to the bottom of mantle. Modified after Maruyama et al. (2007).

et al., 2007), at speeds 2–4 times faster than the plate velocity at the surface to ultimately reach the volatile-enriched superplume region where recycled MORB would preferentially partly melt because of a 100 K lower melting temperature than that of the surrounding peridotites. Partial melting of recycled MORB yields dense melt, which aggregates along CMB; this is imaged as an ultra-slow velocity zone (ULVZ), whereas buoyant Ca-perovskite-enriched residue (andesitic) would rise upwards as small-scale clustered plumes ultimately yielding a superplume at macro-scale (Maruyama et al., 2007). Rising plumes enriched in MORB restites cause heterogeneity of the mantle, episodically manifesting OIB at the surface of the Earth, as typically shown by island chains in Hawaii–Emperor seamounts (Hanan and Graham, 1996; Hirose, 2007; Kogiso, 2007). The origin of these chain-forming isolated islands can be easily explained by the heterogeneous distribution of recycled MORB in the underlying mantle. We now evaluate the global circulation of volatile components, such as water and CO2. MORB, erupted at the mid-oceanic ridge, includes 0.1 wt.% water with 0.01 wt.% CO2, whereas arc-related andesite degasses 4–5% water from primary magmas with 0.4–0.5 wt.% CO2. In the case of OIB, 0.5 wt.% water and 0.05% CO2 are transported from the mantle to the surface on average (Maruyama et al., 2007). We can thus estimate the inputs of water and CO2 to the surface environment through the transferal of around 5 × 1013 tons/year and 1.5 × 1013 tons/year, respectively. On the other hand, there is greater difficulty in estimating the amount of water and CO2 removed from surface to the mantle through descending slabs. 2.4. Water circulation through plate tectonics Phase diagrams of MORB + Water and Peridotite + Water are the key to discussing plate-tectonic-driven circulation of water through the formation of hydrated slabs at the MOR and subduction into deep mantle at trenches, in addition to the subduction zone geotherm. Fig. 5 shows both diagrams together with subduction zone geotherms, A, B, C and D (Maruyama and Okamoto, 2007). Path A depicts the dehydration of an old plate such as the Cretaceous Pacific slab descending under NE Japan, whereas D depicts dehydration

of a subducting young plate (0.1 Ma) like the Juan de Fuca, northwestern USA. Because the geothermal gradients are not straight lines in a P– T space, the expression of geothermal gradients is not yet completely understood. Presumably, the calculated geotherms concave to the temperature axis (Figs. 5 and 6, Maruyama and Okamoto, 2007) could be bent to some extent, at 60–100 km depth range because of convectional heating of the mantle wedge above the slab. Nevertheless, the essential part of the discussion of the fate of hydrated slabs is still valid based on these geotherms, as delineated in Fig. 5. Path A can direct hydrous silicates in MORB crust and underlying slab peridotite down to depths reaching 300 km where lawsonite and other hydrous silicates break down, releasing water-rich fluids through a series of continuous reactions (Maruyama and Okamoto, 2007). Fluids from a descending slab react to form phases A and E in the mantle wedge. A dragged thin and cold part of the mantle wedge would thus be hydrated, though at a subsequent stage it would be dehydrated if the temperature would rise above the maximum stability limit of phase E as shown in Fig. 5B. In the case of geotherm B, hydrated MORB crust can transport water down to depths reaching 175 km, though it is impossible for the underlying slab peridotite to do so since the stability limit of antigorite serpentine equals a depth of 150 km (Fig. 5B). In the case of D (young slab), hydrated MORB crust dehydrates through partial melting of MORB crust at depths ranging from 25 to 70 km. Minor fluids included within magma react with the hanging wall to become amphibole peridotite, occurring in minor amounts, being restricted only along the triangular wedge corner, and being less stable at depths less than 100 km. Hence, there is no way to transport surface water into a mantle wedge >100 km, as well as slab peridotite (Fig. 5). Thus, schematic illustration of water transportation mechanism is drawn along the cross section of an arc–trench system in which oceanic slabs with different ages subduct (Fig. 6), showing that young slabs cannot transport surface seawater into the deep mantle (Cases C and D), whereas slabs older than 50 Ma can transport seawater into the deep mantle and even down to the MTZ (410–660 km) if the slabs are older than 100 Ma (Fig. 6, Omori and Komabayashi, 2007). The dehydration embrittlement hypothesis of mantle earthquakes along subduction zone

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MORB + Water

(GPa)

stability field (PH O=Ptotal) of hydrous phases stability field of anhydrous phases melting region of oceanic crust

13

15

A B 3.5 C/km C O

old plate

8

250km

O

7 C/km

7

5

D

6 5 4

200km

150km coesite quartz

te

100km

nd mo te dia aphi gr ori

e

3

250km

7 200km

150km coesite quarz

8

chl

4

9

entin

young plate

D

350km

e ovit stish site e co

serp

nd mo te dia aphi gr

β

300km

10

P (GPa)

P (GPa)

e ovit stishesite co

srou hyd melt +

C

11 300km

9

A B

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350km

10

6

400km

13

2 C/km

11

stability field of anhydrous phases

14

400km

O

12

2

E

14

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stability field (PH O=Ptotal) of hydrous phases

e

15

16

450km

2

ph as

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ph ph +ase A as e E

(GPa)

7

3

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amphibole 2

solid

dry solidus

2

50km

50km

us

1

1

0

0 0

100 200 300 400 500 600 700 800 900 10001100 1200 1300 1400 1500

T (OC)

0

100 200 300 400 500 600 700 800 900 10001100 1200 1300 1400 1500

T (OC)

Fig. 5. (a): Phase diagrams of MORB + Water and (b): Peridotite + Water. A, B, C and D show subduction zone geotherm right above the descending slab. Modified after Maruyama and Okamoto (2007).

3. What happened at the end of the Precambrian?

has been established, though cyclic (Fig. 7). If this is correct, seismic epicenters correspond to the site of the dehydration reaction of the descending slab. Fig. 8 shows the stability field of hydrous silicates of the Peridotite + Water system in the whole upper mantle and the topmost lower mantle (Omori and Komabayashi, 2007). The stability field of dense hydrous silicates has a wide region at the high-temperature part, with depths ranging from 410 to 660 km (MTZ) in a P–T space. The normal subduction zone geotherm, however, never reaches this region, except in the case of the very cold geotherm. The simple and clear relationship between seismic epicenters and slab ages around the circum-Pacific subduction zone indicates that subduction of an old slab (>100 Ma) transports surface ocean water into the MTZ.

The Precambrian Earth system was distinct from the modern Earth, as exemplified by low-pO2, dearth of sedimentary rocks, smaller life forms, and a significantly smaller volume of biomass (e.g., Maruyama et al., 2001, 2013). A deficient nutrient supply to the ocean can be aptly explained by a smaller or nearly absent landmass in the Archean and relatively minor during the Proterozoic, as detailed below. Drastic change of the Earth system occurred at the boundary of the Precambrian and the Cambrian in a broad sense. An ultimate cause of the drastic change was the secular cooling of the subduction zone geotherm. In the following sections, we will show the observations, mechanism, and final processes that led to the evolution of metazoans.

2.5. Water budget during the global water circulation

3.1. Appearance of blueschist at the subduction zone: rapidly cooled mantle along the subduction zone

We can estimate the water budget for the global water circulation by balancing output of water from magmas at the MOR, OIB and subduction zone, as well as input of water into mantle by hydrated slabs from the trench as discussed in the previous section. Transportation of surface water into the mantle is difficult to quantitatively estimate, because of the difficulty in computing the volume of hydration at the MOR for both the MORB crust and the underlying mantle. Moreover, the total slab is cut at the outer wall of the trench before subduction by bending the lithosphere, where hydration of the deeper part of the plate propagates down to 50 km depth along fractures (Fig. 9, Maruyama and Liou, 2005). In spite of such uncertainty, the global budget of water circulation is clearly negative, pointing to a leaking Earth, as seen through time through changing sea-level, mass of sedimentary rocks, and Sr isotopic signature of seawater as detailed below.

Regional metamorphic belts have been forming since the Archean and display strongly penetrative fabrics along shear zones. Such shear zones occur only along decollement zones (Benioff plane) directly above descending slabs through plate tectonics. Therefore, the P–T conditions recorded in these regional metamorphic belts reflect past subduction zone geotherms that correspond to temperature on the top of descending slab at different depths. The P–T conditions for regional metamorphic belts from Archean to the youngest of Miocene age have been compiled extensively by Maruyama et al. (1996), Hayashi et al. (2000), Komiya et al. (2002), in addition to a classic work by Grambling (1981), and more recently by Brown (2007). A consensus in these papers includes: (1) rapid change of the P/T ratio, (2) first appearance of blueschist (BS hereafter) at ca. 700 Ma, (3) lawsonite-bearing schist restricted in the Phanerozoic, first pointed out by de Roever (1957), and (4) general pressure increase

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A

volcano volcano

0 110

300

hydrous hydouce plate plate

B 110

dry water transportation

300

water reservoir

km

upper mantle lower mantle

660

km

C

volcano volcano

hydrous hydouce plate plate

D

volcano volcano

0

0

110

stability field of hydrous phases

dry

hydrous plate

dry

300 440

440

660

water transportation

water reservoir upper mantle lower mantle

660

300

dry

440

440

110

hydrous hydouce plate plate

volcano volcano

0

upper mantle lower mantle

km

660

upper mantle lower mantle

km

Fig. 6. Water circulation from surface to deep mantle. Paths A, B, C and D mean the P–T conditions right above descending slab (Maruyama and Okamoto, 2007). Note the water transportation from surface to mantle transition zone (410–660 km) is possible only along the cold subduction zones (A and B).

from less than 15 kb to 50–70 kb at the Precambrian–Cambrian boundary. The BS, or glaucophane schist, is a rock type characteristic of high-P/T physical conditions (Miyashiro, 1961, 1973, 1994), originally pointed out by Eskola (1921). However, glaucophane generally has a wide range of stability from low-pressure to extremely high-pressure, hence Miyashiro (1957) differentiated it into two types: Al over Fe 3 + and Fe 2 + over Mg. The conventional field name of BS has become widely accepted, while keeping in mind that the end-member of Na-amphibole, riebeckite, is stable at less than 3– 4 kb (Maruyama et al., 1986). For this reason, riebeckite-bearing metamorphic rocks are present even in banded iron formations such as the Hamersley Group, Australia, formed at 2.5 Ga. On the other hand, crossite, which is intermediate between end-members glaucophane and riebeckite, is restricted to a high-P/T metamorphic belt that is stable at pressure and temperature ranges, 5–15 kb and 300–700 °C, respectively (Maruyama et al., 1986; Evans, 1990). Considering the solid-solution effect, the oldest apparent crossitebearing schist is ca. 700 Ma Aksu BS, western China, on the northern

margin of Tarim craton (Nakajima et al., 1990), with no reports of earlier occurrences in the Archean and Early–Middle Proterozoic. Instead, all the Precambrian regional metamorphic belts and rocks belong to intermediate to low-pressure facies series (Maruyama et al., 1996). Metamorphic pressure is yet another significant indicator of paleoenvironmental conditions, because it reflects the maximum depth to which subducted materials reached, and immediately thereafter (20–30 m.y.) returned back to the surface through wedge-driven tectonic forcing during shallowing of the angle of subduction, and through the process called “wedge extrusion” (Maruyama, 1990; Maruyama et al., 1994, 1996). The metamorphic pressure seems to have suddenly increased at around 650 Ma from less than 15–20 kb to 60 kb (Fig. 10). More recent estimates show much higher pressures presumably up to 70 kb. The 580 Ma Kokchetav massif, Kazakhstan (Maruyama et al., 2002; Katayama and Maruyama, 2009), and 560 Ma B-type lawsonite eclogite from eastern Australia (Och et al., 2003) are the oldest examples of diamond-bearing A-type collisional belts. A similar Neoproterozoic (620 Ma) collisional UHP–HP unit has been identified along the eastern margin of the West African craton (Maruyama and Liou, 1998; Jahn et al.,

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Alaska

Kamchatka Kurile

1

NE Japan

14

18

12 1

5 1

3 1

Sumatra

Mexico

40Ma

1 Mariana 3 1

9

New Hebrides

4 Bolivia

1 1

4

1 4

Tonga

40Ma

Chile

40Ma

15

A.

>100Ma

B.

50Ma< <100Ma

C.

<50Ma

Fig. 7. Frequency of seismicity on the Earth. Three different subduction zones are defined, according to the age of slab, older than 100 Ma (A), between 50 Ma and 100 Ma (B), and younger than 50 Ma(C). Systematic difference of depth of earthquakes are seen, (C) is shallower than 100 km depth, intermediate for (B) from shallow to deep down to 660 km, and (A) dominated by deep earthquake. Modified after Omori and Komabayashi (2007), Zhao et al. (2007).

2001). UHP–HP belts are common in the Phanerozoic collisional orogenic belts (see recent summary by Dobrzhinetskaya et al., 2011), specifically in Asia, and subordinately in other parts of Gondwana (see maps by Maruyama et al., 1996). 3.2. Secular change of subduction zone geotherm since 4.0 Ga The temperature at a given depth, e.g., the Moho depth, 30 km (10 kb), was higher than 700 °C before 1.0 Ga, decreasing below 600 °C at 750 Ma, and then gradually lowering to 200 °C, particularly since 600 Ma (Fig. 10). Secular decrease of temperature at a given

depth is clearly seen in both A-type and B-type belts (Maruyama et al., 1996). In addition, the radiometric ages of on-land BS belts roughly correspond to the age of ridge-subduction (Maruyama and Seno, 1986; Maruyama et al., 1996; Maruyama, 1997). This means that the subduction zone geotherm must be cold enough to produce BS, even in the case of hot plate subduction which is a modern analog of Archean Earth. This is due to effective cooling by the hanging wall mantle convection in the Phanerozoic. Rapid decrease of the subduction zone geotherm would result through the liberation of water-rich free fluids derived from the dehydration of hydrous silicates. With a deepening of the stability fields of

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

Depth, km transition zone 200

300

400

500

600

700

e ng plum

lli

Up-we

800

900

Average mantle adiabat

2

1500

Temperature,oC

(i) Stagnation

Pv+Pc

hy-Wad/Rin

Fo+En

3 (ii) Penetration

4

1000

DHMS

Hot slab Cold slab

Ser 500

5

1

10

15

20

25

30

35

Pressure, GPa Fig. 8. Stability filed of hydrous silicates in whole upper mantle and topmost lower mantle (Komabayashi et al., 2004, 2005). Hot and standard mantle geotherm cannot deliver slab-water into the wet MTZ (410–660 km depth). Only the cold slab can transport surface water into MTZ. Blue region is the stability field of hydrous silicates in mantle.

hydrous silicates through time (Fig. 10), the region of liberated free fluids in the mantle wedge both widened and deepened resulting in significant cooling of the subduction zone wedge in a relatively short time, from 700 °C to 200 °C at 30 km depth within 200 m.y. This process of “runaway growth” cool down of the mantle wedge through time resulted in a conduit for the transferal of water down to mantle transition zone (410–660 km depth range: MTZ as a water storage room) (Fig. 11). This may have caused the petrogenesis of subduction zone magma from slab-melting which sourced TTG magma (Defant and Drummond, 1990; Martin, 1993) to hydrous mantle wedge melting which yielded basaltic andesite with high H2O and its fractionated dacite, which are all common in Phanerozoic orogens (e.g., Tatsumi et al., 1986, 2008).

whereas the 4000 Ma–1000 Ma data indicate path D during a time when surface ocean water had not yet been transferred into the mantle below 70 km. The 750–1000 Ma data suggest presumably path C or between paths C and D, indicating that wedge mantle >70 km started to be hydrated, but not yet propagated into the deeper MTZ. Fig. 12 suggests the contrasting behavior of the water circulation system before and after the Precambrian–Cambrian boundary. 3.4. Sea-level change around the onset of the Phanerozoic If the leakage of sea water into the mantle began in the late Neoproterozoic, at ca. 750–700 Ma, the volume of ocean water would have decreased through time, because hydrated slabs, mainly the upper portion of the oceanic slab (MORB crust) with subordinate amounts of slab peridotite, started to dehydrate at greater depths than Moho, releasing water-rich fluids which in turn resulted in the hydration of the hanging wall of mantle wedge, a small corner of arc-trench gap (Maruyama and Liou, 2005). Hydration and resulting expansion of the underlying mantle through serpentinization would result in the uplift of the continental margin (Fig. 13). The stability field of antigorite in

3.3. Initiation of return-flow of seawater into mantle P–T conditions collected from the Archean to the Early–Middle Proterozoic regional metamorphic belts are plotted on P–T phase diagrams of MORB + Water and Peridotite + Water (Fig. 12). The 630 Ma–present data clearly indicate a dominance of A and B types in the Phanerozoic suggesting water transportation into the MTZ,

(2) seawater input down to 50~60km depth bending

(1) seawater input at MOR

100km

200km

300km

410km

410km

hydrous wadsleyite 520km

hydrous ringwoodite 660km

660km

Fig. 9. Two-step hydration of oceanic slab; first at MOR and second right before the trench by bending (Maruyama and Liou, 2005). Bending at trench-outer wall may insert water along the fracture down to 50–60 km depth. Also shown are double seismic zone converged at 200 km depth and counter flow convection within a wedge mantle.

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

11

Fig. 10. Secular variation of P–T conditions of regional metamorphic belts over the world (Maruyama et al., 1996; Maruyama and Liou, 1998). Left: P–T condition of regional metamorphic belt from Archean to the present. See more details and references in Maruyama et al. (1996) and Maruyama and Liou (1998). Right: (1) Archean to Proterozoic subduction zone geotherm and (2) Neoproterozoic to Phanerozoic subduction zone geotherm. Also shown are calculated subduction zone geotherm from 0 Ma to 50 Ma.

the Peridotite + Water system is shown in Fig. 12, which also highlights the subduction zone geotherm around 630 Ma. The wedge mantle must have absorbed 6.5 times more water after 630 Ma than before, because antigorite peridotite can contain 6.5 wt.% water compared to amphibole peridotite (1.0 wt.% water), which is stable before 630 Ma (Fig. 13). The leaking Earth began at a trench such as the modern Pacific plate along the western Pacific region where older slabs are subducting. Through time, serpentine becomes stable in the wedge corner of all subduction zones (Maruyama and Liou, 2005).

3.5. Surface environmental change during the Neoproterozoic toward the Cambrian If a rapid drop of sea-level began some time in the Neoproterozoic by the initiation of return-flow of seawater into mantle, the surface environment must have changed drastically immediately after the emergence of a huge landmass as discussed below. First, if hydrated slabs transport surface water efficiently into the mantle wedge as a runaway growth process, the total mass of ocean

Arc

(1)

hydrated MORB crust

(2) wedge mantle convection

410km stability field (3) of DHS

660km 2nd continents (TTG) Fig. 11. A mechanism to cool down the mantle wedge by the runaway process. The dehydrated water-rich fluids played a critical role by the enlargement of fluid flows by decreasing temperature and increasing pressure of hydrous minerals in the descending slabs. Rapid enlargement of stability field of dense hydrous silicates in the mantle wedge occurred since Neoproterozoic from (1) through (2) to (3). At 660 km, second continents are present due to the granite subduction (Kawai et al., 2010).

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MORB + Water stability field (PH O=Ptotal) of hydrous phases stability field of anhydrous phases melting region of oceanic crust 2

13

A B 3.5 C/km C O

11

old plate

P (GPa)

8 O

7 C/km

7

nd mo te dia aphi gr

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rite

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e

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chlo

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8

entin

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D

e ovit stish site e co

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350km

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9 250km

s- β rou hyd melt +

C

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e ovit stishesite co

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400km

11 300km

9

A B

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350km

10

6

stability field of anhydrous phases

13

O

2 C/km

12

2

14

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450km

stability field (PH O=Ptotal) of hydrous phases

15

E

14

16

450km

e

15

ph as

16

Peridotite + Water

(GPa)

ph ph +ase A as e E

(GPa)

100km

amphibole 2

solid

dry solidus

2

50km

50km

us

1

1

0

0 0

100 200

300

400

500 600

700

800 900 1000 1100 1200 1300 1400 1500 O

T ( C)

0

100 200

300

400

500 600

700

800 900 1000 1100 1200 1300 1400 1500

T (OC)

Fig. 12. P–T conditions of regional metamorphic belts over the world are plotted on phase diagrams of MORB + Water and Peridotite + Water (Fig. 5). Phanerozoic subduction zone geotherms show path A or B, whereas Archean to Early–Middle Proterozoic subduction zone geotherms show path D.

would have decreased rapidly presumably during the Snowball Earth event, some time between 750 and 635 Ma; 635 Ma is the end of the Marinoan Snowball Earth (see Okada et al., 2013, this volume). The surface exposure of continents, i.e., landmass, is very sensitive to sea-level change (Fig. 14). For example, the average thickness of the ocean on modern Earth is 3800 m (Reymer and Schubert, 1984). Topography of the Earth exhibits a bimodal distribution with two flat planes, one stemming from continents with granite as the major rock type and the other from ocean floor composed of MORB (Fig. 14). The present sea-level yields a landmass that is 32% of the total surface of the Earth. If the sea-level rises 600 m, landmass is reduced only 10%, which was the case before 600 Ma, and even less during the Neoproterozoic (> 750 Ma), as discussed later. Assuming a 600 m drop in sea-level from 750 Ma to 600 Ma, the landmass must have expanded from 10% to 32%. During this period, the supercontinent Rodinia started to rift, suggesting that a number of rising mantle plumes uplifted continents as domes (dome-up) to heights 2000–3000 m above sea-level as seen in modern African rift regions. These domes would have subsequently subsided to make rift valleys, and resulting formation of oceans such as Paleo-Pacific and Iapetus (Fig. 16, Maruyama, 1994; Maruyama et al., 1997, 2007; Li et al., 2008; Rino et al., 2008). In addition to the drop of sea-level, an activated mantle selectively uplifted continents to supply huge amounts of granitic sediments into the oceans through far-reaching river systems such as the present Nile, Mississippi, and Amazon. Moreover, the emergence of the huge landmass, Rodinia, diversified the climate, including generating deserts such as the modern Saharan Desert in North Africa. While the river systems deliver nutrients widespread, the atmosphere carries eolian dusts on a global scale. It is important that well-balanced nutrients are delivered to oceans globally, because the supply of nutrients by a river system is restricted to the continental margin. Hence, the open sea is barren like desert without carriage by air, especially when considering the embryonic development of life. The global distribution of eolian dust contributes to improved conditions for fostering life in the open seas, including enhancing the

activation of photosynthesis. To increase pO2 in atmosphere, marine algae and cyanobacteria are important, as well as on-land plants. If we kill all of the on-land plants, as well as stop the wind-driven supply of nutrients over the oceans, pO2 in the atmosphere diminishes to b1/100 PAL (Falkowski and Isozaki, 2008; Payne et al., 2009). Diversified surface environments appeared as a result of the emergence of a huge landmass, specifically the supercontinent Rodinia and its subsequent separation into a number of continents and intervening oceans. Separated continents were later amalgamated to make Gondwana at 540 Ma, marking the Ediacaran/Cambrian boundary age (Fig. 16). At this time, wet regions, swamps, lakes, river systems, and deserts appeared on the continents, In addition, mountain-building by continent–continent collisions within Rodinia and Gondwana would have promoted efficient delivery of nutrients. Above all, the Mozambique collisional orogeny extending over 7000 km, about twice as long as the Himalayan mountain range, would have contributed efficiently as nutrient-delivery agent, because vertical elevation plays a role in erosion, weathering, and transportation of nutrients (Fig. 15). During the breakup and dispersion of Rodinia from 650 Ma to the amalgamation of Gondwana at 540 Ma, the isolated continental masses traveled a distance covering the Paleo-Pacific Ocean and the Iapetus–Rheic Sea (Figs. 16 and 17). If some of them were located in polar regions, ice caps and cold sea regions would accelerate ocean currents, as seen today (Fig. 3). Paleogeographic reconstruction of continents and a supercontinent in the Neoproterozoic shows their south-pole-centered spatial distribution (e.g., Torsvik et al., 2001; Evans, 2003, 2006; Li et al., 2008). Global ocean currents and deep upwellings along the western side of the continents as observed today must have been drastically different when there was a supercontinent or even semi-supercontinents such as the Gondwana and Laurasia segmented from the once supercontinent Pangaea. Furthermore, the geologic times of the most active mantle convection were Grenvillian and Pan-African, peaking at 1.0 Ga and at 0.8 Ga, respectively, because they have the peak of zircon age population through time (Fig. 18) (modified after Rino, 2007). This is due to the water infiltration into the mantle which perturbs mantle dynamics

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

A

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B 3OC/km

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er

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750

Ma

2 Stage III

Ch 1.0

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MOHO (30km)

0 400

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de

sc

hydrated

en

din

gs

lab

Fig. 13. A: Propagation of hydration front in mantle wedge through cooling. Stage I: hydration at shallow level b70 km from the Archean to Early–Middle Proterozoic. Stage II: Neoproterozoic hydration of wedge corner by enlargement of antigorite peridotite by serpentinization down to 120 km depth. Hydrated mantle wedge pushed up continental margin, and Stage III: Ediacaran–Cambrian serpentinization propagates downwards to link mantle transition zone below 410 km where five times more water than the total volume of surface ocean can be stored (Maruyama and Liou, 2005). Vertical arrows show the uplift of continental margin. B: Water content of Peridotite + Water system together with subduction zone geotherms in the Phanerozoic at 630 Ma, and before 1000 Ma. Note the number enclosed by square means water content for each rock type depending on stability fields of hydrous minerals. A drastic change of water content must have occurred some time in the Neoproterozoic. 6.5 times more water could have been swallowed in mantle wedge by the enlargement of antigorite stability field. C: Hydration uplift of active continental margin due to the hydration of mantle wedge, propagating hydration front into deep mantle since 750 Ma, uplifted active continental margin ca. 300 km wide, vertically >1000 m along trench margin >3000 km, same as today. Panel B is modified after Maruyama and Liou (2005).

by a drop in both viscosity and the melting T (Maruyama and Liou, 2005).

3.6. pO2 increase in atmosphere pO2 has long been considered to be the most important factor for the birth of metazoans (e.g., Holland and Beukes, 1990; Han and Runnegar, 1992; Kasting, 1993; Holland, 1994; Knoll, 1992; Lovelock and Kump, 1995; Farquhar et al., 2000). In general, atmospheric pO2 has been estimated to increase from 10−2 PAL to 1 PAL at the Precambrian/ Phanerozoic boundary (Fig. 1), although no direct quantitative evidence is available. However, indirect evidence comes from biological investigations such as the production of multi-cellular animals through collagen-producing reactions (Towe, 1970). The redox state of the atmosphere could semi-quantitatively constrain the degree of oxidation. Hematite- and goethite-bearing sedimentary rocks are common during the Cambrian. For example, the Old Red sandstone appeared in the Late Paleozoic, and continues to present-day, except for short periods of super-anoxia associated with mass extinction (e.g., Isozaki, 1997).

The important question is why high-pO2 oxygenic atmosphere began to develop during the Phanerozoic and was maintained until now. The production of an oxygenic atmosphere is derived from simple photosynthetic reaction: CO2 + H2O = CH2O + O2 driven by the Sun. Though, this reaction itself cannot increase atmospheric pO2 without the back-reaction of oxygen consumption. An analogy are the leaves of a tree which turn yellow during the Fall and defoliate to be oxidized back to CO2 and H2O in atmosphere, ultimately consuming the free oxygen in atmosphere. To stop this back-reaction, the organic matter such as leaves must be separated from atmospheric O2. In nature, this is performed through sedimentation. Once incorporated within sediments, organic matter cannot be oxidized and are converted to coal, oil, and natural gases depending on the temperature and pressure at depth; the predominant occurrence is kerogen, or simply organic matter in mudstone and sandstone. To increase the atmospheric oxygen, the amount of sedimentary rocks must have increased, and to maintain high atmospheric pO2, the continuous formation of sedimentary rocks is crucial. The increased free oxygen is spent through oxidation of on-land rocks or consumed by animals and by plants during nighttime, hence

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12

No land-mass

10

Mt. Everest (Himalaya)

8

Elevation (km)

6 4 2 600Ma 250Ma

0

0Ma

2

snowball periods (-1km)

4 6 8 10 10

20

30

40

50

60

70

80

90

Mariana trench 100

Surface (%) Fig. 14. Average elevation of the Earth's surface and the position of present-day sea-level (Moores, 1993; Maruyama and Liou, 1998, 2005). Note the very sensitive elevation profile of continental crust to sea-level change. If sea-level was 600 m higher than today, the landmass reduces from 32% (today) to only 10% (Neoproterozoic, see text). If the Earth has had 10 km thick more water, no landmass on the Earth presumably no life appeared hence no metazoan and no civilization.

continuous sedimentary burial of organic matter is very important to keep high pO2 in the atmosphere. The life-forming reaction is a one-way reaction spending CO2 + H2O + N2 + Nutrients = Organic matter (life) + O2 (plant) under the radiation of the Sun. Atmospheric CO2 was extremely abundant in the Precambrian time (Shibuya et al., 2010), but it has been reduced to only 400 ppm in the atmosphere, whereas nutrients such as P, K, Mo, Ca, Fe, Mg and others were extremely poor in the Precambrian ocean. The major turning point came at the Precambrian–Cambrian boundary, because a huge landmass appeared, supplying nutrients from the granitic continents to the ocean, and continuous formation of sedimentary rocks enabled maintenance of high pO2 in the atmosphere. More

importantly, because an increase in the supply of oxygen and nutrients occurred simultaneously, metazoans could appear, thrive, and evolve. Since there was no continuous supply of nutrients prior to the Precambrian–Cambrian boundary, it was impossible for metazoans to exist. These factors must have strongly affected the biological evolution of life forms and expansion in size (Payne et al., 2009). 3.7. Birth of metazoans The birth of the metazoans was the result of an increase in the amount of nutrients at global scale, particularly along the continental margin, i.e., the continental shelf transitioned from b− 800 m to a

Extensive erosion (nutrient supply) Sun

Before 600Ma

After 600Ma

troposphere 11km

folded mountains sea-level landmass ocean

troposphere 600m drop

rift

11km ocean

continental crust plume

Fig. 15. Topographic change before (left) and after (right) the lowering of sea-level, and the effects by mountain-building and dome-up bulges by rising plume heads for rifting. Combination of these two effects caused the delivery of enormous amounts of sediments to oceans which supplied huge amounts of nutrient into ocean.

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Fig. 16. Paleogeographic maps from Rodinia break-up at ca. 750 Ma to the formation of Gondwana at 540 Ma, roughly spanning the Ediacaran to the onset of the Cambrian, covering the Cambrian explosion period. The birth place of metazoan could be at rifts in South China in the central part of Rodinia during the birth of Pacific superplume and breakup of Rodinia. South China was isolated in a super-ocean and migrated to attach the peripheral part of Gondwana at 540 Ma. The faunal provinces of Ediacaran fauna is after 580 Ma. For more details, see text and Santosh et al. (this issue).

region b− 200 m with increased visible light to grow a new ecosystem with a niche of animals and plants. The emergence of a landmass is the cause of a continuous supply of nutrients, and an uplift of a landmass enhances the supply of nutrients. The most effective type of uplift is plume-driven dome-up which affects a broader area, reaching 7000 km long and 2000–3000 km high, as exemplified by the present-day African rift valley. Also, this process has continued for more than 20 m.y. Mountain-building, which exposes UHP–HP belts from deep crustal and mantle depths, also contributes to the supply of nutrients, as seen in the Alps and the Himalaya during the Cenozoic. However, it is an order of magnitude less effective in supplying nutrients than the dome-up process (Santosh et al., this issue). The most active period of subduction-zone magmatism is recorded during the Neoproterozoic, demonstrated by the zircon populations of river-mouth sands over the world (Fig. 17). Moreover, the UHP–HP metamorphic belt first appeared at ca. 800–600 Ma in West Africa, becoming widespread in the Phanerozoic (Maruyama et al., 1996; Maruyama and Liou, 1998; Brown, 2007; Dobrzhinetskaya et al., 2011). Dome-up and mountain-building strongly affects the climate system, which includes a change to the wind-circulation system, as seen during the Miocene in Africa, although the process depends on the longitudes and latitudes, as well as the distribution pattern of the continents. In addition, unique magmatism occurs in the rift system with extremely nutrient-enriched volcanic rocks, an aspect that will be discussed later. Similar to the case of the birth of humans in the African rift valley, the first metazoan must have appeared at one place somewhere on the Earth. We consider South China as the probable bearer of it

because of the abundant and diverse fossils, particularly for the period of time ranging from the Ediacaran to the Early Cambrian. Fig. 19 shows the paleogeographic reconstruction of Rodinia and its breakup at ca.700 Ma, and subsequent dispersion of continents including S. China. S. China was relatively small in size, hence it was significantly affected by the average age of oceanic lithosphere riding the S. China Block (Fig. 19. bottom panel). S. China gradually subsided below sea-level, following a decrease in the global sea-level, caused by an initiation of return-flow of seawater into the mantle. On major continents supported gravitationally by the presence of a buoyant mantle keel like Laurentia, continental-shelf regions exposed to atmospheric conditions formed a global-scale unconformity. Thus, S. China was a reasonable site to bear and evolve life along the continental rifts and shelf during the migration in Paleo-Pacific Ocean and the final docking with the Gondwana margin by 560 Ma. Fig. 20 shows the site of a rift (structurally-controlled sedimentary basin) and associated ore of phosphorites during the rifting stage of Rodinia. It also shows ca. 3000 km-long cross-section showing modification related to the manifestation of the Pacific superplume which caused the breakup of Rodinia (Maruyama et al., 2007). The origin of phosphorous and other nutrients could be strongly related to alkaline magmas derived from the deep mantle, specifically carbonatite may have brought well-balanced nutrients to shallow-marine, rift-system environments supplying food to the evolved metazoans and enabling diversification in this specific environment. On the other hand, carbonatite is enriched in U, K, and Th which may have caused internal radiation through the decay of radiogenic elements. Around the volcanoes immediately after the eruption of the carbonatite magma, most living creatures were killed by direct or indirect internal radiation

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A)

B

Pangea

Gondwana

Supercontinent

Rodinia

Columbia (Nuna)

Fig. 17. A: Rodinia breakup for the preferential elevation by dome-up by underlying plumes and mountain-buildings by preceding continent–continent collision orogeny. B: Collision– amalgamation of continents to form Gondwana to form mountain-buildings. These events, in addition to sudden decrease of sea-level caused the global supply of nutrients into oceans.

100 80 60 40 20 0 4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Fig. 18. Zircon age population to show the most active period of magmatism through time, presumably reflecting most active plate tectonics driven by mantle convection at 2.7–2.6 Ga, 2.2–2.1 Ga, 1.1–1.0 Ga, 0.6–0.7 Ga and 0.1–present. The most active mantle activity was 1.1–1.0 Ga and 0.7–0.6 Ga. This could be the result of water transportation into mantle (Maruyama and Liou, 1998, 2005). Source of river sand zircon is shown on the left. Red curve shows the apparent growth curve of continental crust of the Earth normalized to 100% today. First half of the earth history only 50% was formed suggesting huge amount of continental crust gone into deep mantle (Kawai et al., 2010). Presence of supercontinent Columbia (Nuna), Rodinia, Gondwana and Pangea are shown. (covered area of the continents as drainaged region reach to ca. 40%). Modified after Rino (2007).

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

770Ma

Cryogenian

Kaigas 750 735 0

Sturtian 700

-1

715

drop

up

17

Ediacaran

680

up

Gaskier

Marinoan 650 635 660 drop

Cambrian

Ordovician

Baikonur 550542

600

500 488

up >600m drop by leaking Earth

-2 sea level drop

-3 -4

Sclat

er cu

-5 Cap carbonate

-6 km Erosion

Erosion

~700 Ma

60 o N Tarim

tillite

n immeria

C 30 o N

Greater India

Erosion

Deposition

600-550 Ma delta SCB

Aus.

Tarim

tillite NCB

60 o N

30 oN

SCB

Siberia

East Antarctica 0

o

Aus.

PALEO-PACIFIC 0

Laurentia

A

fr

Baltica

ic

an

bl

oc

ks

oS

30

o

60

rve

South American blocks

S

E. Ant.

o

Western Tasmania? NCB

o 30 S Af r i c Am a er n an ica d n b So loc uth ks

Siberia Laurentia

Baltica

Fig. 19. Figure illustrating why S. China was under shallow marine environment during Ediacaran–Cambrian periods, different from most continents over the world. Upper figure shows schematic illustration of sea-level change in Neoproterozoic Snowball and Cambro-Ordovician time. 2 km fluctuation of up and down is caused by Snowball events. Since the onset of Ediacaran age, relatively small continent, S. China has been subsided through time, following Sclater curve, in spite of lowering global sea-level ca. 600 m at least. On major continents like Laurentia, being supported gravitationally by continental keel or tectosphere, global unconformity was formed to expose continental shelf regions. Below: Paleogeographic reconstruction of Rodinia breakup at 700 Ma and migration of S. China block in a Paleo-Pacific Ocean at 600–550 Ma (Ediacaran age).

through the food chains. Extensive damage of the genome (DNA) by radiation would have promoted gene mutation, adjustment by gene shuffling, and duplication (Maruyama et al., 2013). This is the most probable scenario to explain why all small shelly fossils (SSFs) occur in phosphorous ores formed during 540–520 Ma in S. China (Shu, 2008), in addition to the initial stage of rifting of the Rodinia supercontinent at 700–800 Ma (Fig. 19). The Snowball Earth event, which occurred in the Neoproterozoic, has been proposed to have triggered the Cambrian explosion. For example, Hoffman and Schrag (2002) proposed the concept of “Necking” to explain the termination of biological radiation triggered by a lowering of temperature due to snowball event, since temperature is sensitive to metabolism. They proposed that the life, survived in hot springs, suddenly evolved to bear all 35 phyla of metazoans after the extensive melting of glaciers. But this idea was not accepted by most scientists, because Cambrian radiation occurred much later than the end of snowball event, i.e., 635 Ma, whereas the Cambrian explosion occurred during 540–520 Ma. It is obvious that there is no genetic relationship between the Cambrian explosion and the Snowball Earth event. However, the birth of the first metazoan (sponge) may be related to the Snowball Earth event, because the known oldest record of sponge coincides with the timing of the Snowball Earth event, presumably between the Marinoan and the Sturtian (Peterson et al., 2004). In the following section, we propose a direct relationship between the birth of the first metazoan and the Snowball Earth event, which includes a landmass sufficient in size to provide nutrients for the initiation and proliferation of life. Other

contributions to the birth could include galactic cosmic radiation (GCR) by a starburst and a 50% decrease of geomagnetic intensity during the Neoproterozoic time.

3.8. Origin of Snowball Earth Historical review of the concept of Snowball Earth has been summarized by Hoffman (1999) and Hoffman and Schrag (2000, 2002). In the following section, we summarize the current consensus on the degree of snowball state, with not more than ca. 1 km thick of ice sheet because of the steady-state heat-flow from the solid Earth (Tajika, 2003). The first aspect is the duration of glaciation and snowball state based on geologic records of tillite beds combined with paleo-latitudes. Two snowball states of the Sturtian (710–685 Ma) and the Marinoan (660– 635 Ma) are recognized together with local glaciations of Gaskiers (582 Ma), Kaigas (770–730 Ma) and Baikonur (542 Ma, Chumakov, 2010). Thus, the Snowball Earth event in the Neoproterozoic can be recognized from 770 Ma until 542 Ma in a broad sense, so the total time range covers 228 m.y. However, almost 60% of this period records warm environmental conditions. Moreover, two engines, hot and cold global ocean currents, played a critical role in effectively driving nutrient circulation. Also, the transition time to deglaciation was characterized by the presence of a huge landmass and glacial erosion, yielding glacier ‘milk’ (fine-grained soils are best suited for agriculture soils). Therefore, the period of Snowball Earth was, generally speaking, the best surface environment to grow and evolve metazoans by warm environment with nutrient

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

of ca. 840-740 Ma + Location igneous activities

A

Extension axes, including failed continental rift and ocean ridges sedimentary basin

+

Madag

ascar

+

?

Siberia + + + India Australia + + + ? + ++ ++ + + East + + + + + Antarctica ++ + + + ? Laurentia South African ? blocks China + ++ ? + South American blocks +

Baltica

Pacific superplume

B Australia

A (North)

S. China

B (South)

rift

0 4km

oceanic crust

+

+

+ +

+

+

+

continental crust

Upwelling?

+ + +

oceanic crust

major phosphorite ore Fig. 20. Birth of first metazoan sponge in the rifts during the breakup of Rodinia supercontinent ca. 840–740 Ma. Red dots mean locality of phosphorous ore, blue color in Australia means sedimentary basins, and pale red color means the dome-up region by the mantle upwelling triggered by the birth of Pacific superplume. Cross section A–B shows the origin of phosphorous ores by the continental rift, brought by strongly alkaline rocks. Modified after Maruyama (1994) and Maruyama et al. (1997).

supply, intermittently associated with glacial periods with minor but concentrated snowball state. The second aspect is the cause of the Snowball Earth. First, regarding the debate on the periodic cycle of global warming and cooling, we propose duration of ca. 228 m.y. Traditionally, fluctuation of greenhouse gas of XCO2 was thought to be the cause, as input from the mantle by volcanism remained unbalanced compared to output from the surface into mantle/crust including organic matter (kerogen and carbonate) buried in sediments. However, the estimated XCO2 values through time do not support the glacial–non-glacial periodicity in the Phanerozoic (Veizer et al., 2000). Instead, Shaviv and Veizer (2003) proposed that a fluctuation in the amount of clouds is controlled by galactic cosmic rays (GCR) through time. This, roughly speaking, corresponds to periodic collision of the galactic arms of our solar system up to six times through the last 600 m.y. On the other hand, workers who insist on greenhouse gas have proposed other kinds of greenhouse gas for the cause of the Snowball Earth, such as methane hydrate seen at the Paleocene/Miocene boundary (Kennett and Stott, 1991; Weissert, 2000). But even in this case, methane derived from gas hydrates on the ocean floor cannot simply explain the movement of methane gas into hydrosphere–atmosphere, because it is immediately consumed by bacteria and does not remain as a greenhouse gas in the atmosphere for long time. Numerical simulation groups attempted to show that the stability field of the state of Snowball Earth fluctuated through time, suggesting an extremely strong negative greenhouse effect (e.g., Tajika, 2003). The

deglaciation process can be easily interpreted based on the accumulation of greenhouse gases through magmatism under the condition of no input of atmospheric CO2 into the ocean, because an ice cap is covering the entire ocean. Hence, CO2 in the atmosphere accumulates, finally leading to the melting of ice sheets along the surface of the ocean. The difficulty is how to explain the depletion of greenhouse gas in the atmosphere, such as CO2, when the snowball state initiated. The most critical evidence opposing the depletion of greenhouse CO2 comes from zircon population data from river-mouth sands (Fig. 18). The Neoproterozoic has been identified as the most active period of magmatism over geologic time pointing to large amounts of mantle CO2 being released into the atmosphere. CH4 didn't engage being unrelated to magmatism and to balance it between mantle and atmosphere. Svensmark (2007) proposed a new idea that extraterrestrial events controlled the snowball state of the Earth, i.e., starburst originating from the Milky Way Galaxy during the Neoproterozoic time, and a consequent increase in GCR which caused more clouds that diminished the Sun's radiation on the surface of the Earth. About 1% increase of albedo causes 1 K decrease of the average temperature of the Earth's surface. The total modern Earth is covered by clouds generally ca. 50% ± 2% during the last 30 years on average. Even a 1% fluctuation in cloud coverage would greatly influence the climate, being more effective than CO2 greenhouse gas for the past 50 years. Corroborative with the idea by Svensmark (2007), a positive correlation among the amount of GCR and number of sunspots is clearly seen during the last 50 years. He proposed that the Earth is protected by

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

19

Fig. 21. The relationship among Sun's magnetic field (heliosphere), geomagnetic field and GCR. If Sun's magnetic field turns weak, or GCR increases by starburst, the Earth would be strongly affected by GCR showers to cause more clouds to turn glacial period or even to Snowball Earth state.

the Sun's heliosphere against GCR, so that more GCR could attack the Earth when solar activity was inactive, resulting in an increase in cloud cover, thereby cooling the Earth (Fig. 23). The story, however, is not as simple as that proposed by Svensmark, because the Earth is also protected by a strong magnetosphere that shuts down the solar winds which would otherwise blow out GCR. Thus, the amount of GCR (clouds) fluctuates due to the variable magnetic intensity of the Earth. Maruyama and Santosh (2008a, 2008b) proposed the combination of two factors, starburst and the Earth's magnetic intensity. The ratio of magnetic intensity over GCR through time is shown in Fig. 22C. The concordance between cool/warm climate, documented by the presence or absence of glacial deposits of tillites, is clear. This suggests that the intensity of the GCR bombardment could be controlled not only by starburst but also by geomagnetic intensity. Also, Kirschvink et al. (1997) proposed a heretical idea of true polar wander (TPW), based on the polar wander path from the Early Ediacaran to the Early Carboniferous. The model shows an instantaneous 90° rotation of the solid Earth, maintaining the same rotation axis and core, with the rotated solid Earth returning back to the same position as before in a relatively short time, explaining the extensive change in the surface environment on the Cambrian Earth (Fig. 22). However, there is a radius difference ca. 21 km between polar and equatorial regions. Whereas the polar regions transitioned into a warm climate, the equatorial regions transitioned to land with 15 km above sea-level under cold climate. If the apparent polar wander path (APWP) is correct, then it is difficult to explain the anomalously fast plate velocity (over 40 cm/year) from Early to Middle Cambrian time. To avoid this, the above model proposes an interval of TPW in Ediacaran–Cambrian Earth. However, the observed changes in the surface environment from the Neoproterozoic to the Cambrian don't fit the expected scenario. i.e., (1) Snowball Earth, (2) birth of metazoans and (3) rapid evolution of metazoans in the Early Cambrian, cannot be explained by TPW model, even though the model of TPW might be correct. The alternative interpretation of APWP is the change of geomagnetic field from dipole to quadrupolar with lowering geomagnetic intensity (Fig. 23). If this is correct, the APWP from Early (younger than 540 Ma) to Middle Cambrian (older than 520 Ma) does not necessarily mean abnormally fast plate velocity. Moreover, the above mentioned

surface environmental changes, and birth and evolution of metazoans can be interpreted well as follows. Fig. 23. shows the possible scenario of Snowball Earth and subsequent Cambrian explosion combined with lowering of sea-level. Neoproterozoic starburst was not so remarkable as that occurred at 2.3 Ga, whereas geomagnetic intensity was 50% weaker than that of today, and even lower in Early Cambrian, presumably accompanied by the change from dipole to quadru-polar magnetic field, which in turn caused increased GCR bombardment on the Earth. This generated the Snowball Earth environment through an increase in clouds (Fig. 23). The surface environment of the Earth during the Snowball Earth (Fig. 23) suffered from extensive GCR bombardment, making it difficult for life to survive under extremely low-temperature conditions, except in hot springs. Ice sheet thickness on oceans never exceeded 1 km because of the steady-state heat-flow from the solid Earth. Continuous plate subduction transferred surface water into the deep mantle, lowering the sea level, and by the end of Marinoan Snowball Earth (635 Ma), the strong di-pole geomagnetic field was restored, shielding Earth from GCR bombardment. Sea level dropped over 600 m, which enabled the continental shelf to emerge as a heaven for the evolution of metazoans, because of the availability of visible light and an enriched supply of nutrients from the continents (Fig. 23). 3.9. Records of surface environmental change from 635 Ma to 488 Ma in South China Since 2004, Japan (Tokyo Institute of Technology)–China (Northwest University) cooperative research program investigated the Cambrian Explosion using over 23 drill cores from the South China craton, spanning from the Marinoan glacial period (680 Ma) to the end of the Cambrian (488 Ma). A special issue of Gondwana Research on the Cambrian Explosion was first published in 2008, with subsequent findings highlighted in the following section with particular focus on the chemostratigraphic studies combined with new paleontological discoveries (Fig. 24). The main observations include: (1) paleogeographic constraints on the birth of metazoans, which occurred in a rift system dating back to 750 Ma, after the Sturtian Snowball Earth and before the

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx snowball(2)

15

snowball(1)

glaciation

glaciation

10 5

A

0 2.0

cosmic-ray flux

1.0

B 20

Second bombardment of meteorite First bombardment

0.0

warming

15 10

C

5

cooling

0

0.5 SN

1.0

SN : snowball Earth

2.0 SN

2.5

3.0

3.5

4.0

4.4 4.6

time (Ga) TPW

(D)

A

A

mantle

Middle Cambrian

6357km

outer core

MidVendian

(R)

Early Carboniferous

S

S

N

N

Early vendian Earliest Cambrian SilurianDevonian

Im

700(?)-350

Only mantle rotation remaining core same as before! Fig. 22. A (top): Intensity variation of geomagnetic field through time compiled after Kohno and Tanaka (1995), Yoshihara et al. (2002), and Evans (2000). Panel B shows cosmic ray flux (GCR) through time, modified after starburst rate through time (Svensmark, 2007). C: Ratio of magnetic intensity over GCR through time (in this paper). Note the co-relationship between cool/warm climates documented by the presence of glacial deposits of tillites. D: Left shows the pole position of polar wander path from Early Ediacaran to early Carboniferous by Evans (1998). Note the abnormal and unrealistic high-speed from Earliest Cambrian (b540 Ma) to Middle Cambrian (>520 Ma) over 7000 km long distance indicates >40 cm/year plate velocity under the assumption of dipole geomagnetic field. Evans (2003) proposed the idea of true polar wander schematically shown in middle and right figures. True polar wander (TPW) means that solid mantle with crust rotates 90° in a short time and return back to the same position afterwards, ca. 20 m.y. under the fixed rotation axis.

Marinoan Snowball Earth, (2) Metazoans evolved in the South China block, isolated in Paleo-Pacific Ocean, and (3) rapidly diversified animals suddenly evolved in a rift system on the Gondwanan margin. These three points can be related to: (1) stem evolution in a rift system within supercontinent Rodinia (ca. 750 Ma), (2) migration in a Paleo-Pacific Ocean during Ediacaran radiation (635–560 Ma), and (3) crown evolution leading to the diversification of life forms into 35 phyla of metazoans after amalgamation of Gondwana at 540 Ma. Evidence for the initiation of subduction along the southern margin of the S. China block is the 530 Ma jadeitite metasomatic rocks that formed along the subduction zone, revealed in SW Japan (Kunugiza and Maruyama, 2011; Matsumoto et al., 2011). Moreover, the oldest lawsonite eclogite and BS have been reported from eastern Australia beneath the ca.600 Ma ophiolite (Watanabe et al., 1988; Maruyama et al., 1996). If Australia and S. China were both formed along the marginal part of Gondwana, subduction must have already started by 560 Ma along Australia and by 530 Ma along the South China (Fig. 24).

The crown evolution occurred in a short time range of 540–520 Ma within a phosphorite ore body, but the next stage to make larger animals would have been related to bio-mineralization in a tidal and/or nearly closed rift system, separated from major open oceans. An ‘arms race’ started after the Middle Cambrian in shallow marine rifts in Gondwana. 3.9.1. Geological constraints in South China: results from Japan–China program (2004–2012) Drill sites in the S. China craton cover more than 500 km in a north–south direction and over 1200 km in the east–west direction sampling environments ranging from shallow marine to slope basin. A total of 25 drill cores were obtained during an 8-year period. A summary of the geology and drill sites is given in Fig. 25. The fundamental stratigraphic framework has been established by Chinese geologists (see compilation by Zhu et al., 2007a, 2007b). A simplified major stratigraphic column is shown in ascending order as a representative

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

ON tic c

solar wind

Ga la c

os mic

21

Ga

lac

tic

co

sm

ic

ray

Ontgoing longwave radiation 235 W m-2

ra

y

OFF

+

magnetospheric ring current

solar wind

atmosphere

Cosmic Ray clouds

photic zone

barrier reef patch reef

O2 O2

lagoon

87

Sr

S. China continental crust

O2 O2 O2

upw ellin g

tillite

CO2

S. China continental crust

Fig. 23. Right side panel: Quadrupolar geomagnetic field weakened the intensity to turn Snowball Earth state by the increased GCR showers. Thick cloud covered the solid Earth and resultant formation of frozen Earth with b1 km thick ice sheets on oceans. Left side panel: When the Earth's magnetic field turned back to dipole state with strong intensity, GCR decreased to reduce the amount of cloud to return back to warm Earth, and already lowered sea-level enabled the continental shelf regions to be a paradise where visible light became available to evolve metazoans in addition to continuous supply of nutrients.

standard stratigraphy of the Doushantuo (black shale dominated; 635 Ma–551 Ma), Deng-ying (carbonate-dominated; 551 Ma–542 Ma), Yanjahe (alternating shale and carbonate; 542 Ma–526 Ma) and Sujintuo (black-shale dominated with carbonate; 526 Ma–521 Ma) Formations (Fig. 24). After the completion of the regional mapping, reference sections were established based on 62 sites, and δ13C carbonate analyses were used as an index of environmental change. Empirically, the compiled geologic information seems to correlate well with other observations such as paleontological changes. A positive δ13C shift corresponds to an improved environment for evolving life, whereas a negative shift to adverse conditions such as mass extinction (Zhu et al., 2007a, 2007b). However, whereas the application of this empirical relationship does not work perfectly for deeper paleoocean environments (Jiang et al., 2007), it appears to be valid for shallow marine platform regions (Zhu et al., 2007a, 2007b).

3.9.2. Chemostratigraphy In addition to δ13C carbonate, we have measured δ 13C of kerogen in mudstone, δ18O in carbonate and mudstone (Ishikawa et al., 2008, 2011), and 87Sr/86Sr in carbonate (Sawaki et al., 2008). We have also evaluated 44Ca/ 42Ca (Sawaki et al., this issue), 88Sr/86Sr (Ohno et al., 2008), trace elements, P and Mn (Sawaki et al., 2008), N isotope, carbonate Ce*, Eu* for redox state index of ocean (Komiya et al., 2008), and molecular fossils with S isotope study. A brief summary of the results is discussed below. The original definition of the Precambrian/Cambrian boundary highlights traces of bioturbation, suggesting appearance of animals. When compared to megafossils which seldom remain in sedimentary rocks, bioturbation trace fossils are more commonly preserved (Stanley and Yang, 1994). However, the new discovery of megafossils from South China improves our understanding of the Cambrian explosion, proposed by Gould (1995) and Conway-Morris (2003). The onset of the Cambrian explosion is now defined by the appearance

of SSFs at the beginning of the Early Cambrian (Fig. 24); SSFs is a conventional name that covers a wide range of fossils classified into five stages in the Early Cambrian by Steiner et al. (2007). Rapid differentiation of SSFs in the Early Cambrian is now obvious. 3.9.2.1. Mega-fossil radiation at 520 Ma. At least by 520 Ma, the first fish appeared in the Chenjiang Formation as an ancestor of the vertebrate, in addition to trilobites among other life. After the Tomotian (520 Ma), the burst of megafossils began, presumably from a group of SSFs; Echinodermata, Hemichordata, Annelida, Mollusca, Nemertea, Arthropoda, Priaspulida, Anthozoa Hydrozoa, Caliucspongia, and Demospongia (Fig. 24). 3.9.2.2. Ediacaran radiation after Gaskiers at 580 Ma. Two periods of radiation are identified: the earlier period at 580–560 Ma, well recorded in Weng'an Biota, and the latter period ranging from 560 Ma to 542 Ma, by a series of abundant biota including Miahohe, Gaojiashan, Wulingshan, and Xillingxia Biota. This is the so-called Ediacaran radiation. The earlier radiation is characterized by embryos and larvae of animals (Xiao and Knoll, 2000), but the idea of animal embryos has been recently criticized, even with the presence of bilaterians (Chen et al., 2001; Butterfield, 2011). Frondose Ediacaran fossils, weakly calcified metazoans, and mineralized metazoans, all seem to be multi-cellular algae. In contrast, some fossils from the White Sea such as Kimberella and Yorgia are most likely bilaterians, suggesting movement in the shallow seas (Fedonkin, 2003). 3.9.2.3. Ediacaran before Gaskiers: (635–580 Ma). Only Lantian Biota are known from the S. China craton, and all megafossils seem to be multi-cellular algae. Protozoa, such as an ameba, could be present though not yet identified. Acritarchs and metazoan, such as a sponge, have been confirmed, though not yet in China.

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Age (Ma)

cold

0

5

)

87

0.708

Sr/86Sr 0.709

0.710

pO2 low

high

7 6

Gondwana Aus. SE. Asia SCB

E. Ant.

rifting

Cont. arc 5

Baikonur

Gaskiers

Molcular fossil analysis

Ediacaran

560-490 Ma

8

542

582

Fossil Record MEP-ore

SSFs Archaeocyathids Trace and bioturbation

513

-5

Acanthomorphic acritarchs Macroscopic multi-cellular algae Embryo & larvae Ediacaran fauna Calcified metazoans

501

warm

Cambrian Early Mid. Late N-D T A B/T

488

δ13Ccarb (

Temperature

Echinodermata Hemichordata Annelida Mollusca Nemertea Arthropoda Priapulida Anthozoa Hydrozoa Calcispongia Demospongia

22

4

5

3

4

Subduction initiation : 560-530 Ma ACS Paleo-Pacific Ocean

Japan

600 -560 Ma 60 o N

Tarim

tillite 3

30 oN

SCB

2 Aus.

2

Paleo-Pacific Ocean

0o

Western Tasmania?

E. Ant.

NCB

1 o

30 S Af Am rica er n an ica d n b So loc uth ks

Siberia Baltica

Laurentia

1 750 -600 Ma

60 o N Tarim

tillite

635

30 o N

Marinoan

delta

ian

Cimmer Greater India

643

Aus.

NCB SCB

Siberia

East Antarctica

Cryogenian

0o

Laurentia

A

fr

Baltica

ic

an

bl

oc

ks

oS

30

Sturtian

o

60

South American blocks

S

Fig. 24. Post-Marinoan stratigraphy, physico-chemical environmental index, biological evolution, and paleogeography during the birth and rapid evolution of metazoans. Right side panel: (1): Rodinia rifting (750–600 Ma) and birth place of metazoan, sponge in a rift with phosphorous ore, (2) migration of S. China Block (600–560 Ma) in Paleo-Pacific Ocean, and (3) Post-collisional rifting and Cambrian radiation (540–520 Ma). The final Cambrian diversification occurred after collision against Gondwana near Australia. ACS means accretionary complex formed at trench. Left side panel: A summary of post-Marinoan stratigraphy in South China from cap carbonate (635 Ma) to the end of the Cambrian 488 Ma. δ13C carbonate, as an index of environmental change, 87Sr/86Sr ratio of carbonate as an index of nutrients supply, and small unconformity or glacial record, and pO2 in seawater are shown together. Rightmost column shows the metazoan evolution through time. Tree of metazoans, fossil records, and mass extinctions are shown together. References are from Sawaki et al. (2008).

3.9.2.4. Early radiation before Ediacaran, back to Cryogenian radiation for the birth of first metazoan. The first appearance of Acritarchs dates back to the middle Proterozoic (e.g., a summary by Knoll, 2000), appearing to be multi-cellular algae. A most important question is when the oldest metazoan appeared on Earth. Based on a tree of animals and genome biology, Peterson et al. (2004) predicted that it was sometime during the Sturtian snowball at 664 Ma; molecular fossils have been reported from Oman with ages between Sturtian and Marinoan Snowball Earth event. In summary, (1) first appearance of metazoans could have occurred during Snowball Earth, followed by (2) Ediacaran radiation by two-steps before and after the Gaskiers glaciation (580 Ma). After the mass extinction of Ediacaran fauna and flora at 542 Ma, (3) SSFs appeared at the onset of Cambrian, and rapidly evolved and diversified into mega-metazoans by the end of Early Cambrian. 3.10. Relationship to the environmental change Fossil records summarized above with tree of animals, which is estimated from developmental biology and genome biology, are compared with chemostratigraphy, as indirect indices of: (1) nutrient supply (Sr

isotope), (2) pO2 (redox reactions), and (3) atmospheric temperature (presence or absence of tillite, and δ 18O carbonate) (Fig. 24). Clearly, the paleontological change coincides with chemostratigraphic curves, with at least a 5-step evolution, including abrupt changes in environmental factors during the post-Marinoan evolution until 520 Ma, by the end of Atdabanian, with the appearance of first vertebrates, fish, in the Chenjiang Formation. Significantly, the nutrients supply could be the major cause to evolve post-Marinoan metazoan through time, but sudden depletion of PO43− in the seawater was due to the appearance of skeleton-bearing animals, as well as sufficient supply of Ca, NO3, Fe and Mn in the platform environments. Nutrient supply has continuously increased through time with a strong pulse at 542 Ma. The 87Sr/86Sr ratio nearly reached the river water value at 542 Ma (Fig. 24), indicating that the shallow marine sea was isolated from open oceans, and the presence of evaporite minerals suggests an unusual tidal geochemical environment in the rifted region with abundant phosphorite ores that provide the birth place of SSFs, i.e., the Cambrian explosion in the strict sense (Fig. 24). Phosphorite ores occur globally at the Precambrian/ Cambrian boundary (Fig. 25, modified after Stanley and Yang, 1994). To bear vertebrates, phosphorus was critically important with regional distribution predominant in S. China, Australia, and central to SE Asia.

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

5 3 4

23

6 7 12 11 8

10 9 13

2

1 14

Australia

Hainan

Tarim

Ordos

SW China

Vietnam

Pakistan

Mongolia

Altai-Sayan

S. Kazakhstan

Maurutania

Morocco

Volta

Brazil

Deposits estimeted to contain more than 100 million tonnes of phosphate rock Deposits estimeted to contain between 1 million and 100 million tonnes of phosphate rock Minor occurrences, including deposits for which resources estimates are not known

Mid

Early

521

Cambrian

Late

Age (m.y.)

Ediacaran Cryogenian

635

Proterozoic

542

Major phosphorites

Minor phosphorites

Top of glaciogen sediments

Fig. 25. Phosphorous ore deposits between the Neoproterozoic to the Cambrian time over the world. Heterogeneous distribution reflects the original rift positions during Rodinia breakup. Bottom figure shows the stratigraphy of major 14 phosphorous ores. Note the concentration of ore formation at Ediacaran–Cambrian boundary. Modified after Stanley (2006).

Specifically, the occurrence of life dates back to the Neoproterozoic, with the Early Cambrian in S. China the birth place of metazoan. The Cambrian explosion was initiated in S. China and migrated throughout Gondwana (Fig. 24 right side upper panel).

pO2 has not systematically increased through time. Instead, it decreased periodically, appearing to be related to frequent mass extinctions from 635 Ma to 520 Ma, and likely at the end of the Cambrian at 488 Ma (Fig. 24).

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

3.11. Relationship to mass extinctions Although Gould (1995) did not consider the role of mass extinction during the Cambrian explosion, its common occurrence has been documented by Chinese paleontologists (Zhu et al., 2007a, 2007b). There are 2 listed, including: (1) 580 Ma Gaskiers glaciation, suggested by a strong negative shift of δ13C carbonate, though mass extinction has not yet been demonstrated due to poor fossils records; (2) 560 Ma mass extinction of large Acanthomorph Acritarchs, (3) 542 Ma mass extinction of Ediacaran fossils, (4) 520 Ma mass extinction of SSFs, (5) 515 Ma mass extinction of Archaeocyathids, (6) 499 Ma mass extinction of Marjumild Biomere, (7) 496 Ma mass extinction of Pterocephalid Biomere, and (8) 488 Ma mass extinction of Ptychaspid Biomere. Thus a total of 7 + 1 times of mass extinctions have been proposed (Zhu et al., 2007a, 2007b). The significance of these mass extinctions will be discussed in a later section.

3.12. Bio-mineralization and arms race during the Cambrian explosion Biomineralization in a broad sense includes biologically-induced formation of more than 77 distinct mineral species; e.g., carbonates, sulfates, sulfides, silica, iron oxides, manganese oxides, phosphates, hydroxides and halogenides. One of the most remarkable features of the Late Neoproterozoic biotic evolution was the invention of biomineralization; i.e. the in vivo formation of inorganic mineral crystals, such as phosphate (CaPO4), calcite/aragonite (CaCO3), and silica (SiO2) (Fig. 26). This invention allowed some metazoan groups to have survival advantages; possibly expansion in size, morphological variation, protection from predators, and more physical functions. The most successful group utilizing biomineralization was the vertebrates that consequently developed concentrated nerve systems along with calcified bones equipped with a nerve center, i.e. a brain. The great complexity and diversity of extant life was finally accomplished by the appearance of dominant biomineralizing organisms around the Precambrian–Cambrian boundary. From studies of the oldest fossils with hard skeletons, the appearance of phosphate-forming animals seemed to be the most critical for modern life.

In the beginning, biomineralization of animals likely started as a coating on the bodies of animals, probably in the form of absorption of microscopic inorganic minerals over organic material. This type of mineralization is passive because the crystallization is controlled directly by physicochemical conditions of surrounding seawater or by neighboring substrates, in particular the over-saturation of dissolved elements in seawater to precipitate minerals. Under such unusual conditions, some animals became capable of encoding this mineral-coating mechanism in the genome, and biomineralizing ancestors may have adapted more advanced mechanisms to perform mineralization within their bodies; i.e. incorporating nutrients from outside through membranes and controlling the physico-chemical conditions for crystallization in microcosm. Once established, such endo-cellular mineralization can be performed following the information kept in the genome regardless of external conditions. On the other hand, under appropriate conditions, biomineralized crystals are protected from resolving, as can be seen in the internal skeletons of vertebrates. Except for the 2.1 Ga magnetobacteria that formed magnetite (magnetococci), biomineralization of most animals started in the Late Neoproterozoic (Fig. 26) when the large phosphorite deposits were formed for the first time during the history of the Earth. The most successful animal for biomineralization, i.e. vertebrates, appeared in the Cambrian; however, the preparation at a genome level was probably already finished in the Late Neoproterozoic. Elements such as Na, K, and Ca were essential for animals not only for the construction of bones — but also for forming of effective nerve systems, muscles, and enzymes. Ancestral vertebrates with these attributes may have developed in certain geologic settings. The first fish, the oldest lineage of vertebrates that branched off from chordates, are found in the Early Cambrian Chengjian fauna in South China (D. Shu et al., 1996a; D.G. Shu et al., 1996b). During the late Neoproterozoic, South China was located in a low-latitude and was undergoing rifting within the semi-supercontinent Gondwana (Fig. 24). The birthplace of fish was likely a lake within a rift that appeared in the early stage of continental breakup. The water chemistry of such intra-continental lakes was probably unique because they were isolated from oceans, as well as saturated in elements such as Ca, HCO3, Fe, V, P, Mn and Zn, derived from weathered continental crust. Modern Lake

Snowball earth

Snowball earth

100% Continental crust 0 abundance

Supercontinent Amount of phosphorites

Sedimentary carbonate BIF

stromatolite magnetotactic bacteria eucaryote demospongia multi-cellular algae metazoa SSFs phosphatic shells calcareous shells

Biologic- & biomineralogic events

3.0

2.0

1.0

0

Age (Ga) Fig. 26. Bio-mineralization through the last 3.0 Ga. Also shown are growth rate of continental crust, supercontinent, amount of phosphorites and formation of sedimentary carbonate and BIF. Note the two period of bio-mineralization right after the Snowball earth period Paleoproterozoic and Neoproterozoic. Calcite bio-mineralization is older than 2.9 Ga.

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx

Victoria is a close analog where the unusually diversified fish group Cichlids flourishes, diversifying into more than 400 species during the last ten thousand years (Kobayashi et al., 2009). At the end of the Proterozoic, an explosion had begun in the number and types of shells and microskeletons made of minerals such as Ca-carbonate, Ca-phosphate or silica. Such minerals are the products of biomineralization which involves the selective extraction and uptake of elements from a local environment and their incorporation into functional structures under strict biological control. 3.13. Development of ozone layer and life invasion on-land Living life on the surface of the Earth is protected from risks of death by the Sun and the Universe by two or even three barriers: (1) Sun's magnetosphere called heliosphere, (2) the Earth's magnetosphere, and (3) ozone layers (Fig. 21). The heliosphere is not familiar to geologists, though important when considering the Snowball Earth events, because a starburst event may have occurred during the Neoproterozoic (Maruyama and Santosh, 2008a, 2008b). If so, the Earth could have been extensively damaged by the Universe, through the collapse of heliosphere. The next barrier is a strong geomagnetosphere protecting life on the surface of the Earth from solar winds of the Sun that often accompanies super-flares which are also dangerous for the fragile civilization of present-day human beings. Though the present-day strong geomagnetosphere is formed by a dipole geomagnetic field, its history is generally vague. Specifically, the Neoproterozoic from 1.0 Ga to 0.54 Ga may have been characterized by weak geomagnetic field with quadrupole geomagnetic fields (Maruyama and Santosh, 2008a, 2008b). This speculation is based on lower magnetic intensity (Fig. 22) and frequent change of apparent polar positions (Evans, 2003; Li et al., 2004). Combined with a starburst, a weak geomagnetic field through quadrupole geomagnetism could help explain an enhanced bombardment of GCR during the Neoproterozoic time (Maruyama and Santosh, 2008a, 2008b). Fig. 21 shows the possible paleogeography of the Universe centered by our solar system. The third barrier of life is the ozone layer that shields the Earth from ultraviolet rays (UV) which destroy DNA. Without an ozone layer, life can't survive on land. The first record of plants on the Earth is found in the Ordovician, predominantly in the Silurian, which archived the invasion of wet regions by moss and various fern types (see Kawahata, 2011). The Devonian–Carboniferous was a time of thickly vegetated forests with resultant high pO2 up to 30%. By this time, relatively large dragonfly and animals appeared under high pO2 and sufficient nutrients. Such high content of free oxygen in the atmosphere was derived by steady-state formation of huge amounts of sedimentary rocks in which organic matter was buried. With the volume of sedimentary rocks suddenly increased on the Earth, as well as pure organic matter such as limestone and chert, as well as oil, coal, and natural gases, it must have been a golden era for life. Increased oxygen content in the atmosphere started to leak above the troposphere (11 km thick) into the stratosphere, which is 40–50 km above the ground. Free oxygen reacted with solar protons and electrons to form O3, i.e., ozone. The steady-state formation of free oxygen by photosynthesis is the source of the ozone layer, which forms a barrier between life and UV exposure. After the formation of the ozone layer, all living creatures from microorganisms to animals and plants, expanded their living domains to land, making the Phanerozoic the golden era for life (Fig. 24). 3.14. Summary The expected scenario at the end of Neoproterozoic, a time of significant change in surface environmental conditions, is summarized as follows. Appearance of BS at subduction zones represents the secular cooling of the subduction zone, indicating the initiation of return-flow of seawater into mantle. This diminished the volume of the oceans on

25

Earth, causing the emergence of a huge continental landmass above sea-level composed of TTG (tonalite–trondhjemite–granodiorite). The drop of sea-level promoted global supply of nutrients to enable life to evolve into large multi-cellular animals with resultant increase of high atmospheric pO2. Diversified surface environments also diversified life. Increase in photosynthesis activity generated more free oxygen in atmosphere to raise the content to present-day level, and free oxygen began leaking into the stratosphere, 40–50 km above the ground to form an ozone layer. After the appearance of the barrier to protect life on-land, metazoans expanded the living domain from ocean to land, diversifying their forms up to 35 phyla during the Early Cambrian time (540– 520 Ma). This is the result of diversified surface environment not only in physical conditions (climate) but also chemical conditions. The birth of metazoans and plants in the Phanerozoic was the result of a drastic change in the Earth system, triggered by the initiation of return-flow of seawater into the mantle (Fig. 27). 4. Supporting evidence The above-mentioned scenario can be examined by independent observations below. These are (1) sea-level change through time, (2) sedimentary rocks through time, (3) secular change of oxygen isotope of seawater, and (4) Sr isotope change through time. 4.1. Sea-level change by Hallam (1989, 1990) Sea-level change is the most fundamental check point to test the model proposed herein. The coastline is defined by the relation to sea level. In other words, it is an oceanward limit of sedimentation and a landward limit of unconformity. A global unconformity geologic contact is well preserved on-land. The Ediacaran–Cambrian boundary, which is a good example of a global-scale unconformity in general, clearly indicates that almost all continents were exposed above sea-level at the onset of the Cambrian. On the other hand, in the Archean, there is no global unconformity, except a short period of 2.7–2.6 Ga. This means that the Archean Earth was mostly covered by oceans and the exposed landmass was extremely small. Another global unconformity was at 2.1–1.8 Ga, remaining mainly on Laurentia. This period in the Precambrian was punctuated, and not continuous like in the Phanerozoic. Hallam (1989, 1990) presented a sea-level change curve, based on the on-land geology, as mentioned above, but details remain under several assumptions. The most important aspect is that there is no way to reconstruct the Precambrian curve, because of the absence of a continuous global unconformity. This suggests that the Precambrian sea level was higher than any level in the Phanerozoic. In the Phanerozoic, there are two culminated peaks, one at the early Cretaceous, and the other was 600 m higher in Late Paleozoic (Fig. 28). On the other hand, sea level was 200 m lower than today at the P/T boundary. These fluctuations of sea-level are explained by glaciation or non-glaciation, because ice sheets on land would decrease sea-level, and ±200 m can be well explained by calculation and preservation during the last 2 Ma glacial period (see Geologic Time Table, Ogg et al., 2008). However, in the view of sea level fluctuation during the non-glacial periods such as the Cretaceous and the Late Paleozoic, the sea level was significantly higher, reaching 300 m in the Cretaceous and 600 m in Late Paleozoic (Fig. 28). This requires alternate interpretations such as the widely accepted dominance of young buoyant plates below the ocean, e.g., Cretaceous pulse period (120–80 Ma). If we follow the Sclater curve that depends on the age of a slab, the Cretaceous pulse can be interpreted by the predominant occurrence of young slabs, hence buoyant plates on the Earth pushed up sea levels landward, i.e., transgression time. Similarly, the other peak of sea level can be explained by another pulse of high sea level, because of the pulsing activity of plate tectonics. Such a pulse must be present from the Middle to Late Paleozoic, because of accelerated TTG activity on-land.

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6CO2+6H2O

Fomation of ozone layer

C6H12O6+6O2 buried!

life on land

increased oxygen in the atmosphere new coast line birth of large rivers deposition of thick sediments lowering of sea-level

Oceanic Plate hydration of mantle wedge

Leaking Earth

410km

low-T part 660km Fig. 27. Return-flow of water into mantle caused a series of surface environmental changes. (1) Return-flow of seawater into mantle (2) dropped sea-level, (3) emerged huge landmass, (4) birth of huge rivers, (5) formation of huge amounts of granitic sediments, (6) increased oxygen content in atmosphere, (7) formation of ozone layer (8) which finally enabled metazoans and plants to invade on land. Modified after Maruyama and Liou (2005).

The sea level curve before 600 Ma can be roughly estimated by the Sr isotopic change in carbonate, reflecting seawater composition which is, in turn, controlled by a flux of weathered TTG crusts as seen later. The extended sea level curve before 600 Ma indicates a

This classic interpretation of the increase of young plates can be re-examined through mantle overturn between lower and upper mantle, while evaluating the effects of an expanding and contracting Earth (Tsuchiya et al., 2013).

Glacial period SN

Kai St M snowball

Gas Bai

Emergence of large landmass Return flow of seawater

0.709 87

86

Sr/ Sr carbonates 0.708

87

86

Sr/ Sr Carbonates

Veizer et al.,1986

sea

0.707

-lev

600m lost

0.706

el

600m 0.707

Cretaceous pulse

chan

ge

Global unconformity

Global unconformity

300m 0.706 0m

1300 Ma

E/C

P/T T/J

E/K

Mass Ext. Ice age Superplume

African cold downwelling

?

Earth's rotation (days/yr) Plate activity

spin-down

440 low 1

No. of continents

2

3

12

10

Rodinia (supercontinent)

950

900

Cryogenian

850

low 2

5

spin-down

410 4

2

high 8

6

10

800

750

700

Ediacaran

650

600

550

spin-up

370 low 11

8

5

3

375

spin-down

365

high 2

1

3

4

5

6

7

Pan gea

Gondwana

Tonian

1000

Asian African hot surperplume cold downwelling

Pacific hot surperplume

spin-up

400 extremely high

Cam

Ordovician

500

Silurian Devonian Carboniferous Permian Triassic

450

400

350

300

250

Jurassic

200

150

Cretaceous

100

Cenozoic

50

0

Fig. 28. Sea-level change curve in the Phanerozoic and Neoproterozoic back to 1000 Ma. Sea-level was higher than 600 m in Neoproterozoic. Assuming the empirical co-relation between Sr isotopic ratio with sea-level in the Phanerozoic (Shields and Veizer, 2002), the sea-level must be fluctuated during the Neoproterozoic Snowball Earth event, and at least 600 m higher than today. Also shown are mass extinction, ice age, activities of superplumes, Earth's rotation speed, averaged plate speed, number of continents and supercontinents. Modified after Maruyama and Liou (2005).

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much higher sea level than in the Phanerozoic, and fluctuated to some extent, presumably related to the Marinoan and Sturtian glaciations (Fig. 28).

27

rocks over 4.0 Ga, based on the geologic map of the Earth, combined with TTG growth curve through time (Fig. 18). The results approximate Ronov et al. (1991), except the presence of additional two minor peaks at 1.8–2.1 Ga and 2.7–2.6 Ga (Fig. 18). 4.3. Secular change of oxygen isotope (Wallmann, 2001)

Direct evidence of lost seawater into the mantle in the Phanerozoic is the amount of sedimentary rocks formed through geologic time. Sediment volume at a given geologic age reflects the space of at an exposed landmass to a first order of approximation. Ronov et al. (1991) pointed out that the Phanerozoic is a time of sedimentary rocks, and these rocks have increased with time since the Phanerozoic (Fig. 29). This is a mirror image of increasing pO2, consistent with the expected scenario mentioned in this paper, supporting the idea that an ultimate cause of decreasing seawater by return-flow of seawater into the mantle began after the Neoproterozoic time. Independently from Ronov et al. (1991), Maruyama et al. (2013) showed the formation of sedimentary

To interpret the secular change of oxygen isotope δ 18O of carbonate (seawater), Wallmann (2001) reached the conclusion that a slope change in the secular variation of δ 18O of carbonate can be explained by the loss of ca. 400 m thick seawater into the mantle by hydrothermal interaction at mid-oceanic ridge and subduction of altered MORB into deep mantle after 600 Ma. His conclusion is the same as that of Maruyama and Liou (2005), although the amount of lost seawater is different, over 600 m thick as estimated by Maruyama and Liou (2005), and 400 m by Wallmann (2001). Oxygen isotopes are controlled by not only hydrothermal interaction at mid-oceanic ridge, but also dehydration reaction at different

Sedimentary mass (1020 g/m.y.)

4.2. Sedimentary rocks through time (Ronov et al., 1991; Maruyama et al., 2013)

20

Age distribution of sedimentary rocks (Ronov et al., 1991)

10

4000

3000

2000

1000

0

0

Age (Ma) Gondwana Rodinia Pangea

Nuna

embryonic continent

600m 0

?

1 magma ocean

2 3

subduction

volume of continental crust

100

4km

Supercontinent

Huge landmass

80

No continents

intra-oceanic arcs

90

sedimentary rocks

40

20

Granitic rocks (andesitic) 4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Age (GPa) Fig. 29. Sedimentary rocks through time, A: Ronov et al. (1991) and B: Maruyama et al. (2013). Note the predominant formation in the Phanerozoic.

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P–T conditions along the subduction zone, near surface biological fixation by carbonate, as well as buried oil, coal, and other organic compounds in sedimentary rocks. The fate of these sediments is not quantitatively well known particularly along subduction zones. In spite of these uncertainties, the conclusions of the various studies are consistent. 4.4. Sr isotopic change of seawater 87

Sr/ 86Sr ratio of carbonate is considered to reflect the past seawater composition (Veizer et al., 1999; Shields and Veizer, 2002). Although this ratio is easily altered by secondary processes such as surface weathering, diagenesis and low-grade metamorphism, the minimum value could reflect secular change. Fig. 30 shows the compiled result through time (Shields and Veizer, 2002). In this figure, there are a few remarkable changes of Sr isotopic ratio, one at 600 Ma, another at 2.1 Ga, and yet another probably at 2.7 Ga, though less clear. This is consistent with compiled data on the amount of sedimentary rocks through time by Maruyama et al. (2013). 87 Sr/ 86Sr ratio of carbonate is basically controlled by two endmembers, one from riverline flux of eroded TTG materials indicative of the amount of exposed landmass, and the other from the hydrothermal flux of MORB at the mid-oceanic ridge. In the ocean, the extremely high continental value is mixed with the extremely low MOR value. The 87Sr/86Sr ratio of seawater is homogeneous, because of the relatively short residence time of Sr (i.e., 4.1 million years) in the oceans (Veizer et al., 1989), hence the observed value of carbonate Sr isotope is determined by a different mixing ratio in the binary system. If we assume that MORB production rate was more or less constant through time, the ratio reflects the amount of exposed landmass at a given geologic age (Fig. 30). The secular trend of Sr isotopic ratio records a spectacular curve. A remarkable increase in 87Sr/86Sr is present at ca. 600 Ma, temporally consistent with the emergence of huge landmass on the Earth, and also notably with the increased volume of sedimentary rocks, as proposed by Ronov (1994) and Maruyama et al. (2013), all of which corroborate the model in this paper. 5. Discussion The Cambrian explosion, called the Big Bang of evolving life, has received considerable attention not only from geologists and paleontologists, but also geophysicists and astronomers, particularly after the recognition of the Snowball Earth events before the Cambrian explosion. In the following section, we will discuss an ultimate trigger of the Cambrian explosion.

Average value of continents 0.710 0.708 0.706 87Sr/86Sr

seawater composition

0.704 0.702

Mantle Contribution

4000

3000

2000

1000

0

0.700

Age Ma Fig. 30. Secular variation of 87Sr/86Sr carbonate is shown here modified after Shields and Veizer (2002). Also shown is the evolution curve of Sr isotope through time. Extremely rapid increase in 87Sr/86Sr is present during the Neoproterozoic ca. 800–600 Ma which can be explained by the increase in nutrients derived from granitic continents.

5.1. The Cambrian explosion: the Big Bang of evolving life The sudden appearance of metazoans was a peculiar event, because the preceding life form was of a microscopic scale ca. 1/1,000,000, when compared to modern animals. Preston Cloud (1948) realized the significance of this event as a milestone of the history of evolving life, naming it the Cambrian explosion. The triggers of the Cambrian explosion can be classified into (1) extrinsic and (2) intrinsic based on the summary by Signor and Lipps (1982), and more recently by McCall (2006). 5.1.1. Extrinsic: what was the ultimate trigger? The surface environment from the Neoproterozoic to Cambrian was one of the most drastic in terms of mantle dynamics, pointed out long ago by investigators such as Valentin and Moores (1970) who recognized a close relation among the supercontinent cycle and evolving life (mass extinction and faunal and floristic diversifications). The Cambrian explosion occurred when the semi-supercontinent Gondwana was formed at 540 Ma and subsequently, Pangaea rifted at the P/T boundary (largest mass extinction in the Phanerozoic), and another major mass extinction of dinosaurs occurred at K/T boundary. Applying his idea to the Cambrian explosion, the breakup of the supercontinent Rodinia could be related to the first step of the Cambrian explosion, as the Ediacaran period (635–542 Ma) was followed immediately after the Marinoan Snowball Earth event. The second step could be the Ediacaran–Cambrian boundary, 542 Ma, when SSFs appeared immediately after the boundary. By the end of the Early Cambrian at 520 Ma, all 35 Phyla of metazoans appeared (Shu, 2008). Collision–amalgamation– mountain building of Gondwana at 540 Ma would bring nutrients to the oceans, diversification of environment, and a change in ocean chemistry, all of which affected surface environments (Shu, 2008). As an extrinsic trigger, partial pressure of atmospheric oxygen was pointed out 50 years ago to be the most critical factor to the origin and evolution of life (Cloud, 1968; Schopf, 1993; Knoll and Carroll, 1999; Canfield et al., 2006; Fike et al., 2006; Yin and Yuan, 2007). Ocean chemistry can be controlled by sea-level fluctuation, specifically precipitation of carbonate and phosphorite (Knoll, 1991; Knoll and Walter, 1992). But these processes cannot address the genetic relations among pO2, tectonic environment, supercontinent cycle, ocean chemistry, and rapid biological diversification. Here, we attempt to solve this problem and propose a genetic scenario. A drastic change of the Earth system was triggered by (1) the initiation of return-flow of seawater into the mantle, (2) lowering of sealevel to expose a huge landmass, at least 3 times larger than before with the help of mountain-building through collision orogeny and dome-up of crustal materials by rising plumes and resulting rifting of the continents, (3) global supply of nutrients, (4) growth of microorganisms and metazoans in the diversified surface environment, (5) increase of atmospheric pO2 which resulted from an increase in sedimentary rocks stemming from a rapid drop of sea-level, and (6) increased biological activity resulting in elevated atmospheric O2 and finally an ozone layer, as a shield for life. The ozone barrier enabled the invasion of life onto continents, beginning a golden era of life in the Phanerozoic (Fig. 24). 5.1.2. Intrinsic: both from Universe and from solid Earth, by speed up mutation through radiation The above scenario explains the Cambrian explosion; a biological response to an external environmental change. The physical and chemical environmental changes were the external forces to genome change. Gould (1995) noted that the Cambrian explosion was not caused by mass extinctions, but rather by the birth of metazoans in a short time. During the relatively short time, extraordinary biological evolution occurred; through the Phanerozoic evolution of life, there are no corresponding examples of the Cambrian explosion. New phylum did not appear even after the extensive mass extinctions such as observed

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during the P/T event (Gould, 1995). Following the excellent summary of the Cambrian explosion by Gould (1995), a perturbation in the evolution of life of which paleontologists refer to as “Trial-and-Error Experiment of Animal Evolution”, including the discovery of strange animal fossils such as 5 eye-bearing animals, Opabinia, animals with strange mouth structure, Anomalocaris, animals full of spines on upper and lower body, Hallucigenia, possible ancestor of vertebrates, Pikaia and so on (see a review by Conway-Morris, 2003). However, these fossils are all from Middle–Late Cambrian Burges Shale, Canada, and not from Early Cambrian when the actual explosion occurred. The fossiliferous Early Cambrian platform sediments are missing in most areas over the world, hence the critical information was absent until 1990. However, the discovery of new fossils from the Early Cambrian drastically changed the image of the Cambrian explosion. A number of excellent fossil discoveries came from South China (see a review by Shu, 2008), such as the first fish, Haikouichthys and presumably Myllokunmiongia (Shu et al., 1999; Shu, 2003; Shu et al., 2003). Paired eyes and unusual antero-dorsal lobe were also described (Shu, 2003). Early lower chrodates and other deuterostomes such as Cheungkongella, Cathaymyrus and Haikouella are other representative examples (D. Shu et al., 1996a; D.G. Shu et al., 1996b; Shu et al., 2001; Shu, 2003; Shu et al., 2003; Shu, 2005). From these discoveries, it became evident that first vertebrate appeared in the Early Cambrian. Moreover, extinct Phylum Vetulicolia among others was also recognized (Shu et al., 2001; Conway-Morris, 2003; Shu et al., 2003). The next target for the origin of 35 phyla is small shelly fossils (SSFs). Steiner et al. (2007) classified SSFs from S. China into 5 zones with time in the Early Cambrian, as well as summarized the evolution of the SSFs. The most radical evolution occurred after Stage I, with Stage II being the most diversified stage, and finally disappearing at the end of the Early Cambrian ca. 520 Ma. This is the entity of the Cambrian explosion.

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How can the rapid diversification of metazoans in the Early Cambrian be best explained? Some have long discussed hybridization, for example, a critical point pass the complexity grade, and evolution of regulatory genes (Shu, 2008). The birth of the arms race could be related to birth of skeleton-bearing organisms to overcome fighting. Birth of large animals has drastically changed the biological system flow, called biological pump (Rothman et al., 2003). Among them, biomineralization would have played the critical role for evolution of metazoan. Triggers may be oversaturation of key elements in a closed environment, and thereafter coded by genome, because all SSFs are present in phosphorous ores (Shu, 2008). The isolated geochemical environments have been well-recognized through Sr isotopic ratios. The next question is the origin of phosphorous ore. Where does it come from? Although the research is on-going, the most probable answer is rift-related carbonatite magma, i.e., mantle-derived magma. In other words, atomic bomb magma enriched in U caused local mass extinction, and nutrients, which were elevated in P, K, Ca, Fe, Mg, Mo among others, evolved SSFs by the internal exposures to the radiation of radiogenic elements through nutrient-enriched food chains (Maruyama et al., 2013). Gene shuffling, insertion, and duplications (Fig. 31) occurred for life to survive, probably by random processes. Adapted to the environments, resultant new species remained to overcome the arms race. The final intrinsic reason is mutation in genome triggered by radiogenic elements, provided by solid Earth. 5.2. The relationship to the Snowball Earth After the concept of the Neoproterozoic Snowball Earth was established (715–680 Ma and 660–635 Ma), Hoffman and Schrag (2000, 2002) proposed the relationship to the Cambrian explosion. They proposed a termination of the radical evolution of life, by lowering

From Starburst to Cambrian Explosion II Snowball Earth: 770-635Ma

I Starburst: 900-600Ma Gaskier

Great Magellanic Clouds

Sun-Earth

(Present)

Cosmic rays

Perseus Orion spur

weakened geomagnetism

Marinoan

Au EA

Sgr-Carina Black hole

Sturtian

Cloud

Kaigas

S NA SA

South China

Norma

Scutum-Crux

III Genome level diversification: 770-635Ma IV Cambrian explosion: Ca.540-520Ma Cosmic ray

Mutation

O2+Nutrients enlarged shelf

Genome duplication & shuffling

O2

dropped sea-level

Photic Zone

coating (biomineralization) Fig. 31. From galaxy to genome. A scenario of the Cambrian explosion controlled by Universe. I: Starburst occurred during 900–600 Ma. Structure of our Milky Way Galaxy is shown together with the location of our Solar system. Assuming rotation of our solar system is cyclic ca. 600 m.y., the paleogeographic position of our Solar system is estimated. II: Snowball Earth by weakening geomagnetic intensity combined with starburst, see the details in text. III: Genome level diversification by increased mutation 770–635 Ma (birth of sponge as a first metazoa), and IV: The Cambrian explosion after ending of starburst and recovery of strong magnetic intensity. In addition, drop of sea level was critical to make continental shelf as photic zone.

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the temperature of the living environment, of which they referred to as necking. Freezing is fundamentally dangerous to the survival of life. Nevertheless, subsequent work did not follow such an idea in relation to the Cambrian explosion, because the timing of Snowball Earth, 770–635 Ma, particularly the snowball state ended at 635 Ma, whereas the Cambrian explosion occurred much later than the Marinoan snowball Earth, during a short time span (540–520 Ma). The obvious time difference (100–200 m.y.) indicates no direct relationship. However, the origin and subsequent biological radiation must be separated. The first metazoan was a sponge, appearing during the snowball event, between Sturtian (715–680 Ma) and Marinoan (660–635 Ma) glaciations. The so-called snowball period spans from local glaciation, Kaigas (770–735 Ma), through the snowball state of Sturtian and Marinoan, to Gaskiers (585–582 Ma), lasting for more than 190 m.y. Significantly, the total actual glacial period during this time range (770–580 Ma) was not so long, only lasting 40–50 m.y. Therefore, the so-called 190 m.y.-long snowball event was a rather warm and nutrient-enriched fertile environment for a significant time period. Episodic freezing and glaciation may have caused mass extinctions, presumably at 580 Ma, referred to as the Schuram excursion, and defined by a negative excursion of δ13C carbonate at global scale (Zhu et al., 2007a, 2007b). The Ediacaran radiation directly followed the Schuram excursion. Zhu et al. (2007a, 2007b) recognized eight mass extinctions during the time period ranging from the Ediacaran (635 Ma) to the end of the Cambrian (488 Ma): that is 8 times/147 m.y. The frequency is five times greater than the Big Five mass extinctions in post-Cambrian Phanerozoic (5/488 m.y). Therefore, the inference by Gould (1995) was incorrect.

The role of mass extinction and origin of metazoans, i.e., the first appearance of sponge are discussed in the following. The cause of the snowball state could be related to a starburst in our Milky Way Galaxy (Svensmark, 2007; Maruyama and Santosh, 2008a, 2008b). In the case of Neoproterozoic, the geomagnetic field of the Earth was decreased down to 50%, and quadrupolar geomagnetism may have appeared instead of strong dipole state (Fig. 32, Maruyama and Santosh, 2008a, 2008b). A weakened barrier of geomagnetic field coupled with an increased space activity by starburst would enhance GCR bombardment, and ultimately lead to an increase in cloud formation on the Earth. The combination of a weakened geomagnetism with a starburst could explain Snowball Earth (Svensmark, 2007) (Maruyama and Santosh, 2008a, 2008b). Here, we stress that elevated GCR over ca. 190 m.y. resulted in the mutation of life and ultimately the birth of first metazoan, the sponge, sometime between Sturtian and Marinoan Snowball Earth, during the peak stage of starburst. Origin of the first metazoan and subsequent mass extinctions could be caused by genome mutation, triggered by both GCR radiation over a long time and by atomic-bomb, carbonatite magma on the solid Earth in a nutrient rich environment (Fig. 33). In Fig. 33, the role of the Universe is schematically shown. After the birth of life some time in the Hadean, two distinct points of life evolution have been emphasized, one at 2.1 Ga for the evolution from Prokaryotes to Eukaryotes, and at ca. 0.6–0.5 Ga for the birth of metazoan (e.g., Maruyama et al., 2001; Maruyama and Santosh, 2008b). Ca. 200 m.y. ahead, Snowball Earth events are present, e.g., Early

Fig. 32. Neoproterozoic Snowball Earth. Five times more frequent mass extinctions than the Phanerozoic, and role of geomagnetism and Universe. Increased cosmic ray radiation accelerated the evolution of life through mutation, and its accumulated effects caused final Cambrian explosion of diversification of animals.

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Metazoa

Genome

Eucarya (Protozoa)

Birth of life

Plant

Bacteria Archaea

diversification diversification (body plan) Ediacaran fauna

Ed

mass extinction Bi-sex Multi-cellular

Acritarchs Bilaterian

2.9Ga Hadean 4.56

snowball

2.3Ga

K SMG

Archean Eo

4.0

snowball

Paleo 3.6

Proterozoic

Meso 3.2

Neo 2.8

Paleo 2.5

2.0

Phanerozoic

Meso 1.6

snowball Earth & glaciation

1.2

Neo

P

1.0

Age

M C subdivision

0.54 supercontinent

Nuna

3.0

2.7

2.4

Rodinia

Gondwana Pangea

2.2

Solid Earth geomagnetism

weak

Birth of the Earth

starbust, weak geomagnetism cosmic radiation

starbust, weak geomagnetism cosmic radiation Star burst

collision

collision

Great Magellanic cloud

Space

Galaxy

Fig. 33. History of life and the Earth. Two distinct periods of snowball state of the Earth and subsequent explosive evolution of life are shown. The 2.3 Ga snowball event was followed by the birth of Eukaryotes, and Late Neoproterozoic Snowball Earth was followed by the Cambrian explosion, i.e., birth of animals. Possible reason of starburst, collision of dwarf galaxy against our Galaxy at 2.3 Ga and 0.8–0.6 Ga is proposed. Tidal interference by Great Magellanic Cloud during Neoproterozoic remains one of the possibilities. Modified after Maruyama et al. (2001), Maruyama and Santosh (2008a, 2008b).

Proterozoic Snowball Earth is coincident with the biggest starburst in our Milky Way Galaxy, and Late Proterozoic Snowball Earth is coincident with starburst with simultaneous weakened geomagnetic intensity presumably by quadrupolar state. These coincidences are not accidental, but necessary outer forcing to accelerate mutation speed as an intrinsic factor.

5.3. The fate of the naked planet Earth; why has the Earth become a mega-life sustaining planet? There are two important factors to make a habitable planet: (1) the initial mass of ocean; the factor which allows plate tectonics to operate, and (2) size of a planet; the factor which controls the cooling speed of a planet. Here, a short summary of the formation of an initial ocean on Earth is given. It is difficult to produce a nearly naked planet in general, because H and O are the most common elements in a primordial solar nebula. Also, the presence of a “snowline” is another constraint; the inner zone in which there is difficulty to contain ocean water following planetesimal bombardments on all of the rocky planets from Mercury to Mars, especially because no hydrous silicates are available without a stability field of ice. To overcome this situation, ocean water on the Earth has been proposed to have sourced from the bombardment of chondritic materials from asteroid belts (e.g., Albarede, 2009), following the birth of a naked planet without an ocean or atmosphere (Maruyama et al., 2013). The presence of Jupiter and Saturn protected the Earth from icy meteorites from the Kuiper belt (Maruyama et al., 2013). Finally, the naked rocky planet Earth could have obtained an ocean at its surface with only a 3– 5 km-thick water column following the 4.4 Ga bombardments.

The initial mass of the ocean was key to making Earth habitable. If the ocean had been thinner than b 3 km, plate tectonics would not have operated, because the mid-oceanic ridge would have been higher than sea-level, resulting in a lack of hydration to the topmost plate. If the ocean had been thicker than > 5 km, an appropriate amount of landmass would not have appeared, and life could not have evolved due to an insufficient nutrient supply. If the water column of the ocean had been much thicker, plate tectonics would not have operated nor would a landmass have appeared, the result being no emergence of life. Plate tectonics played a critical role for the birth and sustainability of life by cleansing the poisonous ocean, which included producing ores at the MOR, and delivering them into the deep mantle through subduction, ultimately making nutrient-enriched TTG rocks along the subduction zone. Therefore, the initial mass of ocean is the determining factor to produce a habitable planet. In addition to the initial mass of the ocean, the size of a planet is another important constraint in making a habitable planet. The size of the planet controls the cooling speed of a planet, which is directly related to the timing of initiation of the return flow of sea water into the mantle. If the Earth was much smaller like Mars, it would have lost the surface ocean within 0.6 b.y. after its birth (Baker et al., 2007). On the other hand, if a Super-Earth was present in the habitable zone of our solar system, it could not have cooled down to initiate return-flow of seawater into the mantle within the lifetime (~ 10 Ga) of the Sun (Fig. 34). If seawater had not leaked into the mantle, there would not have been a sufficient supply of nutrients, and thus life would not have evolved on the Earth. These conditions, which control the evolution of a planet, are surely able to be applied to other planets both inside and outside of our

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continents upper mantle lower mantle

corresponding to D” layer magma oce sal an ba

ocean continent

Metallic core (liquid)

+ + +

su

pe

+

upper mantle lower mantle

rpl

um

e

+ +

Inner core

Anorthosite + KREEP

Outer core

+

+

superplume + lower mantle

+

core

mantle

+

mantle

+

upper mantle +

+

D” layer

+

ocean

Dry

No ocean

3.8km thick ocean

thick ocean

Moon

Mars

Earth

Super-Earth

R:1 8

R:1 2

R:1

R:2

Fig. 34. Size of planets to determine the timing of initiation of return-flow of seawater into mantle. Moon, Mars, Earth and Super-Earth are shown. Mars was too small to have lost surface ocean b9 km into mantle by 4.0 Ga (Baker et al., 2007). Moon was too small to keep atmosphere and ocean. Venus was too close to Sun, hence lost primordial ocean. Earth was appropriate in size. Primordial continents have been subducted into deep mantle in the Hadean or by the Early Archean, because of no successive formation of the composite crust of anorthosite + KREEP thereafter on the Earth. On the other hand, TTG has been accumulated on the surface by plate tectonics, and covered 1/3 on the Earth's surface now. Although about 10 times more TTG materials have subducted into mantle transition zone either by arc subduction or tectonic erosion of hanging wall TTG crust through time, continuous formation of TTG crust overcame against destruction of TTG crust. Primordial continents had annihilated from the surface of the Earth mainly in the Hadean, remaining first life as orphans on the surface (Maruyama et al., 2013). It took 4.0 b.y. to decrease surface water to expose huge landmass to drastically change Earth system in terms of material circulation driven by Sun. Super-Earth, even if it contains 3–5 km thick ocean initially, the return-flow of seawater into mantle would never happen within a lifetime of Sun. Structure of Super-Earth is after Valencia et al. (2010).

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S. Maruyama et al. / Gondwana Research xxx (2013) xxx–xxx Shigenori Maruyama is a Professor at the Department of Earth and Planetary Sciences, Tokyo Institute of Technology, graduated with a BSc (1972) from Tokushima University, Japan, and a PhD (1981) from Nagoya Univ., Japan. He became an assistant professor at Toyama University in 1978, a post-doc at Stanford University, USA, moved to the University of Tokyo in 1991 as an associate professor, and in 1994 he became a professor at the Tokyo Institute of Technology. He undertook extensive fieldwork in Japan 1971–1989, in California and the western coast of Canada 1981–1989, and over the world since1990, after he initiated the decoding Earth History program in over 25 countries. Since 1994 he has organized the multi-disciplinary program, “Superplume Project” supported by STA, Japan, combining geophysics, isotope geochemistry, UHP experiments, and world geology. Major results from this work were published in the edited book, “Superplumes; Beyond Plate Tectonics”, Springer, Holland, 569 p. (2007). Current interest is origin and evolution of life in the framework of Galaxy-Genome.

Yusuke Sawaki graduated with a PhD (2011) from Tokyo Institute of Technology (Tokyo Tech), Japan. He became a JSPS Postdoctoral Fellow at Japan Agency for MarineEarth Science and Technology in 2011 and moved to Tokyo Tech in 2012 as an Assistant Professor at the Department of Earth and Planetary Sciences, Tokyo Tech. He has joined the “decoding Earth History program” associated by Prof. Maruyama at Tokyo Tech. His main targets are sedimentary petrology and isotope geochemistry.

Toshikazu Ebisuzaki has been working in Astrophysics, Computational science, and Cosmic-ray physics. He got the degree of Ph.D in Astronomy from Department of Astronomy, University of Tokyo in 1986 and has been a chief scientist at RIKEN since 1995. In 2000, he won the First Prize of the Gordon Bell Prize in High Performance Computing. He is the principal investigator in Japan of the JEM-EUSO mission to explore the extreme energy cosmic rays, on-board the International Space Station. He has been interested in the study of the connection of the galactic environment to the Earth environment from the astrophysical point of view.

35 Masahiro Ikoma is an Associate Professor of Planetary Science at the University of Tokyo, Japan. He received his undergraduate training and his Ph.D. in Earth and Planetary Sciences at Tokyo Institute of Technology. After serving for seven and a half years as an assistant professor in the Department of Earth and Planetary Sciences at Tokyo Institute of Technology, in 2012 he moved to the University of Tokyo. In 2012, he received the Young Scientists' Prize for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology for his extensive theoretical studies of the formation and structure of giant planets in the solar and extrasolar systems.

Soichi Omori, an Associate professor of the Open University of Japan, graduated with a BSc (1989) from Waseda University, Japan, and a PhD (1998) from Waseda Univ., Japan. He became a research associate at Waseda University in 1994, moved to Tokyo Institute of Technology as a postdoc, in 2004 he became an assistant professor at the Tokyo Institute of Technology, in 2009 he became an associate professor at the Tokyo Institute of Technology, and in 2012 he became an associate professor at the Open University of Japan. He undertook a computer programming for the thermodynamic calculation in metamorphic petrology 1988–1998. Since 1994 he joined a collaboration with Titech-Stanford-Waseda research program for the UHP metamorphism. Now he is extending his research to multi-disciplinary subject in seismology, global material cycle, and early life on the basis of phase equilibrium petrology and mineralogy.

Tetsuya Komabayashi, an Assistant professor at the Department of Earth and Planetary Sciences, Tokyo Institute of Technology, received his Ph.D. in Earth and Planetary Sciences from the Tokyo Institute of Technology in 2005. After a one year post-doc, he was appointed as a faculty member at the current affiliation. His interests are focused on equilibrium properties of Earth's constituent materials such as the mantle silicate and oxide minerals and core iron phases in order to understand the structure and composition of the deep Earth's interior. While he is an experimental petrologist using a hightemperature diamond anvil cell, he is also trying to introduce the techniques of metamorphic petrology such as the petrogenetic grid to the deep Earth mineral physics.

Please cite this article as: Maruyama, S., et al., Initiation of leaking Earth: An ultimate trigger of the Cambrian explosion, Gondwana Research (2013), http://dx.doi.org/10.1016/j.gr.2013.03.012