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Ediacaran seawater temperature: Evidence from inclusions of Sinian halite
Precambrian Research 184 (2011) 63–69
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Ediacaran seawater temperature: Evidence from inclusions of Sinian halite Fanwei Meng a,b , Pei Ni a,∗ , James D. Schiffbauer c , Xunlai Yuan b , Chuanming Zhou b , Yigang Wang d , Maolong Xia d a
State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluid Research, Department of Earth Science, Nanjing University, Hankou Road 22#, Nanjing, China Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, China c Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA d Southwest Oil and Gas Field Company Exploration and Development Research Institute, PetroChina, Chengdu, China b
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 31 August 2010 Accepted 26 October 2010
Keywords: Ediacaran Seawater temperature Inclusion Halite
a b s t r a c t Seawater temperatures throughout Earth’s history have been suggested to illustrate a long-term cooling trend from nearly 70 ◦ C at ∼3500 Ma to around 20 ◦ C at ∼800 Ma. The terminal Neoproterozoic prior to the “Cambrian Explosion” is a key interval in evolutionary history, as complex multicellularity appeared with the advent of the Ediacara fauna. These organisms were likely the first that required higher levels of atmospheric and dissolved marine oxygen for their sustainability. It is known that most modern macroinvertebrates are intolerant of temperatures in excess of 45 ◦ C. Perhaps more importantly, these high seawater temperatures limit the potential of dissolved oxygen, and therefore become an integral part of this evolutionary story. Previously, our understanding of seawater temperature during the terminal Neoproterozoic comes only from 18 O/16 O and 30 Si/28 Si ratios ascertained from a limited number of cherts. Isotopic ratio methods for assessing seawater temperatures are inherently indirect and have a wide range of oscillation. However, maximum homogenization temperatures (Thmax ) of primary fluid inclusions in halite provide a direct means of assessing brine temperature, and have been shown to correlate well with average maximum air temperatures. The oldest halites date to the Neoproterozoic–lower Paleozoic (∼700–500 Ma), and Ediacaran representatives can be found in Sichuan Province, China, which do preserve primary fluid inclusions for analysis via cooling nucleation methods. We utilized halite samples from the Changning-2 well, correlative to the Dengying Formation (551–542 Ma), to provide a direct assessment of terminal Neoproterozoic seawater temperature. Our measurements indicate that seawater temperatures where these halites formed are highly similar to tropical Phanerozoic seawater temperature estimates. From compiled paleotemperature data, the decline in seawater temperatures over the course of the Proterozoic, accompanied by the reduction of seawater salinity with the sequestration of salt in massive halite deposits in the Neoproterozoic, allowed the ocean system to accumulate more dissolved oxygen, and potentially paved the way for the evolutionary innovation of complex multicellularity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Precambrian hosted several astonishing advancements in biological evolution. Indeed, this timeframe encompasses four of the six evolutionary megatrajectories in the history of life, including the evolution of the last common ancestor of extant life, the metabolic diversification of bacteria and archaea, the evolution of the eukaryotic cell, and the origin of multicellularity (Knoll and Bambach, 2000). This should not come as a surprise, however, as the Precambrian occupies nearly 90% of Earth’s history, a vast time interval for evolutionary development. As the pace of evolution can be rapid, the hindrance in the development of complex multi-
∗ Corresponding author. Tel.: +86 25 83597124; fax: +86 25 83592393. E-mail address: [email protected] (P. Ni). 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.10.004
cellularity until over 3000 million years after the first hints of life presents numerous queries in our understanding of the history and nature of evolution on the Precambrian Earth. It is likely that a series of switches needed to be turned-on prior to the so-called “explosion” of metazoan life. Some of the previously proposed switches include, but are not limited to, internal genetic switches, such as the Hox gene complex, behavioral or ecological switches, including the evolution of macrophagy and increasing predation pressures, as well as numerous external switches, such as the increase of atmospheric oxygenation and changes in seawater chemistry (e.g., see Cloud, 1976; Peterson et al., 2005). One switch – or perhaps more accurately a compilation of interconnected switches – that was undoubtedly a prerequisite to metazoan evolution, is the reduction of seawater salinity and temperature along with the concurrent change in the potential for increased hydrospheric oxygen solubility (Knauth, 2005).
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If evolutionary change were dictated only by internal switches, one may suggest – with the presence of metaHox genes in primitive organisms such as sponges (Coutinho et al., 2003) – that complex multicellularity may have had the potential to arise much more swiftly than what we have observed in the fossil history of life. It seems applicable, therefore, to invoke external pressures, such as environmental restrictions, as levels of containment for life’s impending evolutionary developments. Indeed, many traditional explanations for major Precambrian evolutionary events invoke specific thresholds in atmospheric oxygen levels; for instance, for both metabolic function as well as the biosynthesis of membranesupporting sterols, the origin of Eukarya have been suggested to require at least 1% of the present atmospheric level (PAL) of oxygen (e.g., Jahnke and Klein, 1983; Han and Runnegar, 1992). Further, one of the more accepted explanations for the appearance of complex multicellularity in the Ediacaran is the increase in atmospheric oxygen content to a required threshold level (suggested to be between 1 and 10% PAL, depending on potential adaptation of Ediacara organisms to dysaerobic conditions) for metazoan respiration (e.g., Runnegar, 1982, 1991; Catling et al., 2005). While atmospheric oxygenation is undoubtedly important, as Knauth (2005) stated, “the level of oxygen dissolved in water is the real issue.” Seawater oxygen solubility is highly constrained by both ocean temperature and salinity, which likely fluctuated significantly over the course of the geologically- and environmentally-evolving Precambrian Earth. The most continuous records of seawater temperature come from the complementary stories told by ␦18 O and ␦30 Si data, which have been collected and compiled from marine cherts (Knauth, 2005; Robert and Chaussidon, 2006). These isotopic data (along with other published data, for instance, see Perry, 1967; Knauth and Epstein, 1976; Knauth and Lowe, 1978) have been interpreted as evidence for a long-term cooling trend of the oceans through the Precambrian. From Knauth’s (2005) estimates using a hypothetical 10–20 ◦ C seawater temperature for Cenozoic oceans (see also Knauth and Lowe, 2003) and compiled oxygen isotope data, the Archean ocean surface temperature ranged from approximately 55–85 ◦ C, which agrees with paleotemperature estimates of ∼70 ◦ C ascertained from silicon cycle models constructed from silicon isotopic data (Robert and Chaussidon, 2006). Furthermore, isotopic evidence of a hot Archean ocean has been linked to microbial evolution by studies of thermostability in early microbial proteins (Gaucher et al., 2008). However, it is important to note that recent isotopic data (Hren et al., 2009; Blake et al., 2010) challenge the commonly held view of a hot Archean world, pointing instead to a temperate and perhaps biologically active primitive ocean. Using a combined approach of oxygen (␦18 O) and hydrogen (␦D) isotopes from cherts of the ∼3420 Ma Buck Reef Chert in South Africa, Hren et al. (2009) provide evidence suggesting that these cherts formed in equilibrium with waters below ∼40 ◦ C. Supporting these conclusions and adding yet another facet, Blake et al. (2010) ascertained ␦18 OP from phosphates of the ∼3500–3200 Ma Barberton Greenstone Belt, South Africa. Due to their similarity with modern phosphates, these data again suggested a marine temperature of less than ∼40 ◦ C in the Archean and a developed biological phosphorus cycle (Blake et al., 2010). Thus, while seawater temperatures in the Archean have become debated in light of recent evidence, multiple estimates for Archean ocean salinity place the likely range between 1.2 and 2× that of modern ocean surface waters (Holland, 1978; Knauth, 1998). This elevated salinity may have persisted through the Archean due to the absence of continental cratons, as salt was not sequestered in halite deposits forming in evaporitic environments (Knauth, 2005). Furthermore, oxygen solubility in these early oceans was limited by elevated water temperatures and salinity, presumably resulting in a predominantly anoxic ocean full of anaerobic, thermophillic, and at least halotolerant if not
halophillic prokaryotes. One can easily imagine, then, that the physical and chemical properties, certainly including oxygen solubility, of this early ocean are far different than the ocean we know today. In the Proterozoic Earth, on the other hand, seawater temperatures declined as continents became larger, more stable, and longer-lived. From Knauth’s (2005) oxygen isotope dataset and Robert and Chaussidon’s (2006) silicon isotopes, it is suggested that seawater temperatures reached typical Phanerozoic levels, an estimated 20 ◦ C, by approximately 1200–800 Ma, during the assembly and near the break-up of the supercontinent Rodinia. During the break-up of Rodinia, the ocean temperatures of the Neoproterozoic Earth likely fluctuated dramatically, with global glaciation snowball events and warming periods during interglaciation; unfortunately, there seems to be a comparative lack of cherts during the Neoproterozoic and lower Paleozoic, which results in a scarcity of data for paleotemperature assessments during this biologically significant time interval (Knauth, 2005). Thus, seawater temperatures during the key geologic interval of metazoan origination and evolution is yet unresolved. Further, once in the Neoproterozoic, salt began precipitating out of seawater in large evaporite basins – such as the one described here. The resulting lowered oceanic salinity, in conjunction with the cooling of ocean temperatures associated with global glaciations, would allow dissolved oxygen to more readily accumulate in the oceans. This compilation of interconnected events not only limited the ecological ranges of anaerobic, thermophillic, halotolerant/-phillic prokaryotes that had once dominated the oceans, but also may have resulted in enough dissolved oxygen in the ocean system to support metazoan respiration – and most importantly corresponded in time with the rise of complex metazoans, as evidenced by the earliest fossil representatives of metazoan life at ∼632 Ma (Yin et al., 2007). We provide additional data herein for seawater paleotemperature reconstruction of the terminal Neoproterozoic not based on isotopic analysis, but rather from halite fluid inclusion homogenization temperature assessments.
2. Halite fluid inclusions and geological setting of collected samples Paleotemperature reconstructions using 18 O/16 O and 30 Si/28 Si methods vary largely with diagenetic influences and provide only relative estimates; whereas homogenization of primary halite fluid inclusions, by utilizing the cooling nucleation method, provides a direct means to evaluate seawater paleotemperature (Roberts and Spencer, 1995; Benison and Goldstein, 1999; Lowenstein et al., 1998; Satterfield et al., 2005; Liu et al., 2007). The cooling nucleation method of measuring homogenization temperatures in original, all-liquid, primary fluid inclusions has been successfully used in Permian, Pleistocene, and modern halites (Benison and Goldstein, 1999; Lowenstein et al., 1998; Satterfield et al., 2005). Maximum homogenization temperatures of fluid inclusions (Thmax ) in halite (from both laboratory-produced crystals and modern samples from Death Valley, CA) match the assessed brine temperatures during halite precipitation, and additionally correlate well with the average maximum air temperatures (Lowenstein et al., 1998). Therefore, maximum homogenization temperatures, calculated from a series of measured homogenization temperatures (Th) in halite cores, reflect the maximum brine temperature and provide a close approximation for local air temperatures. Primary halite has two crystal forms: cumulate crystals and chevron crystals in sedimentary surrounding (Roberts and Spencer, 1995; Benison and Goldstein, 1999; Lowenstein et al., 1998). Cumulate halite forms at the air–brine interface and can record the surface water temperature, which is directly comparable to air temperature (Lowenstein et al., 1998). Chevron halite, on the
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Fig. 1. Location (denoted by star) of the Changning-2 well core used in the study.
other hand, forms at the evaporite basin bottom, and therefore records the benthic brine temperature (Benison and Goldstein, 1999). Cumulate salt formed at surface–brine can be found in water of any depth, however chevron salt is formed in shallow water because dense brine circulates to the bottom to precipitate minerals. In deep, density-stratified water environments, bottom growth is suppressed (Hovorka et al., 2007). In shallow water settings, Thmax of the inclusions in cumulate halite and chevron halite illustrate a similar homogenization temperature range (Benison and Goldstein, 1999). Two time intervals in geologic history host the majority of the Earth’s known halite deposits. Approximately 40% of Phanero-
zoic salts can be found from the Permo-Triassic boundary to the late Early Jurassic (∼250–180 Ma). The second major interval of salt deposition ranges from the Neoproterozoic–lower Paleozoic (∼700–500 Ma), which constitute the oldest halites on Earth (Knauth, 2005). The halites examined in this study are from the Neoproterozoic time interval, and were sampled from the Changning-2 well of the Sichuan Province, southwestern China (Fig. 1, Sichuan Petroleum Bureau, 1973; Lin and Cao, 1998). The Sinian halites are found in the local structure of Changning Anticline, located centrally in the Changning Depression, Southwestern Sichuan-Guizhou Province Subsiding Belt. The Sinian-age (635–542 Ma) beds of the sampled Changning-2 well range from
Fig. 2. Stratigraphy of the Changnig-2 well assessed from drill core and field observation.
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Table 1 Stratigraphic position of Changning-2 well. Geological time
Formation
Thickness (m)
Lithology
Cambian
Jiuladong Formation
1019.50–1870 m or so
Lower part is black shale, and a parallel unconformity is between the Formation and lower Maidiping Formation Carbonaceous chert interbedded with dolomite and shale The formation is divided 2 sections according to lithology. Upper section (1975–2492.5 m) mainly are dolomite, and lower section dolomite bear grapestone; Lower Section (2492.5–2992.50 m) is dolomite and halite, dolomite contain anhydrite without grapestone, halite‘s top is glauberite. 2 Section‘s base is white chert. The Formation is not overly drilled through. Upper part is mainly sandstone. Lower part is mudstone.
Maidiping Formation Sinian
100 mor so
Hongchunping Formation
1975–2992.5 m
Labagang Formation
2992.5–3308.08 m
1975 to 3308.8 m in section, from top to base, occupying the Hongchunping and Labagang Formations. The Cambrian Maidiping Formation unconformably contacts the overlying black shale of Cambrian Jiulaodong Formation; and the lowermost portion of the well does not extend beyond the Labagang Formation (Fig. 2, Table 1). The Labagang Formation can roughly be correlated to the well-known Doushantuo Formation, which, from U-Pb zircon dating of volcanic ash beds, ranges in age from 635 to 551 Ma (Condon et al., 2005). The halite sampled for examination is located in the middle Hongchunping Formation and specifically ranges in the Changning-2 well section from 2645 to 2855 m (Sichuan Petroleum Bureau, 1973), which is correlative to the 551–542 Ma Dengying Formation. In addition to halite, the salt-bearing strata predominantly consist of grapestone-bearing dolomites, anhydrites, and glauberites (Sichuan Petroleum Bureau, 1973), which were likely formed in a low-energy subtidal zone, similar to modern lowenergy marine environments below 9 m in depth (Cao, 2002). According to sedimentary facies mapping and paleogeography, the salt-bearing unit of the Sichuan Province covers over 20,000 km2 in area; the paleogeographically proximal Changning Depression likely also houses evaporitic rock units (Lin and Cao, 1998). These salt-bearing units were formed during a marine regression after the Chengjiang orogeny (Lin and Cao, 1998). In bedded salts like those collected, halite cores can show parallel bands of minute fluid inclusions similar to parallel bands of cumulate halite and chevron halite, inherited from original sedimentary textures, with rims of recrystallized salts (Roedder, 1984). In shallow brines, fluid inclusions in both cumulate and chevron crystals can be used to estimate paleotemperatures due to their similar temperature of formation and resultant Th range (Benison and Goldstein, 1999). The Sinian halite examined here has numerous preserved primary inclusions and sedimentary textures, identical to bands of cumulate halite and chevron halite (Fig. 3).
3. Methods The Changning-2 well of the Sichuan Province, southwestern China, was drilled in the 1970s using drilling mud prepared with saturated brines to prevent dissolution of bedded halite. Since the initial drilling, the core has been stored in sealed plastic (at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences) to prevent alteration by atmospheric moisture. The analytical methods utilized in this study follow those of Benison and Goldstein (1999). Unpolished halite cleavage chips were prepared by cleaving 1–2 mm-thick fragments with a razor blade. The halite samples were placed in a HAIER freezer at −15 ◦ C for two weeks. This temperature is not cold enough to freeze the fluid inclusions, and also conserves the original size and shape of the inclusion. After the two-week cooling period, ∼5% of the fluid inclusions produce vapor bubbles. Once removed from the freezer, the samples were placed on a Linkam THMS600 heating/freezing stage at −15 ◦ C, and were held at constant temperature until the nucleated vapor bubbles within the inclusions were located. Subsequently, the Linkam stage was slowly warmed at a rate of approximately 0.5 ◦ C per minute at lower temperatures (up to ∼15 ◦ C), and 0.1 ◦ C per minute upon reaching the arbitrary 15 ◦ C threshold. The Linkam THMS600 stage used in this study was calibrated using synthetic fluid inclusions, precise to approximately ±1.0 ◦ C between a temperature range of 0–50 ◦ C. As the vapor bubbles in the fluid inclusions become gradually smaller and homogenized to liquid during the heating process, the Th value was observed and recorded (Benison and Goldstein, 1999). In total, 24 Sinian halite cleavage chips of the same stratigraphic level were investigated for fluid inclusions, 15 of which contained microthermometrically measureable inclusions (58 different inclusions were analysed).
Fig. 3. Primary fluid inclusion textures in Ediacaran halite.
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Table 2 Homogenization temperature data for primary fluid inclusions in Sinian halite. Sample #
Homogenization temperatures (◦ C)
Pr-1 Pr-4 Pr-5 Pr-6 Pr-7 Pr-13 Pr-14 Pr-15 Pr-16 Pr-17 Pr-19 Pr-21 Pr-22 Pr-23 Pr-24
15.0 20.2 30.5 22.5 20.1 19.7 20.2 19.2 17.8 13.3 21.1 20.2 21.0 17.3 18.4
Range (◦ C) 18.7 21.9 39.4 26.5 25.3 – 20.5 20.5 18.2 16.4 22.1 21.7 24.5 18.0 –
21.4 25.4 – – – – 20.8 26.7 – 18.1 22.2 22.5 24.7 19.7 –
23.2 26.1 – – – – 22.9 27.0 – 19.5 23.0 23.1 – 20.9 –
4. Results and discussion The Th values measured from 58 Sinian halite primary fluid inclusions ranged from 13.3 ± 1.0 ◦ C to 39.4 ± 1.0 ◦ C, with the majority of measurements falling between 20–25 ± 1.0 ◦ C (mean = 23.1 ◦ C, median = 22.15 ◦ C, mode = 20.2 ◦ C, 1 = 5.0 ◦ C, n = 58; Table 2; Fig. 4). Within individual chips, Th measurements exhibited a mean range of 8.2 ◦ C (with a maximum of 17.7 ◦ C and a minimum of 0.4 ◦ C; Table 2). The primary fluid inclusions measured from individual chips came from the same or adjacent fluid inclusion bands, which illustrate relatively consistent homogenization temperatures. For example, sample Pr-16 (Table 2) contains two measurements from inclusions from within the same fluid inclusion band and illustrates a range of 0.4 ◦ C (see also Fig. 14 of Benison and Goldstein, 1999). Because only ∼5% of the inclusions produced vapor bubbles, not all of the halite cleavage chips containing primary fluid inclusions were measurable; however, previous investigation suggests a minimum of 30–40 Th measurements to assess the maximum brine temperature during crystal growth (Lowenstein et al., 1998). Because the maximum homogenization temperatures of the fluid inclusions (Thmax ) in the sampled halites have been shown to be equivalent to the maximum brine temperatures (Lowenstein et al., 1998), we can suggest that the local maximum seawater temperatures during
– 28.0 – – – – 22.9 28.1 – 22.6 30.3 37.9 – 21.9 –
– – – – – – 25.7 29.4 – – – – – 32.8 –
– – – – – – 26.1 30.3 – – – – – – –
– – – – – – 26.3 – – – – – – – –
8.2 7.8 8.9 4.0 5.2 – 6.1 11.1 0.4 9.3 9.2 17.7 3.7 15.5 –
Mean (◦ C) 19.6 24.3 35.0 24.5 22.7 – 23.2 25.9 18.0 18.0 23.7 25.1 23.4 21.8 –
the precipitation of the Sinian halites of the Sichuan Province corresponds to approximately 39.4 ± 1.0 ◦ C. Paleomagnetic data indicate that the Sichuan Province of South China was equatorially located during the Ediacaran Period (Yang et al., 2004). The average air temperature in equatorial regions today ranges from 25 to 35 ◦ C, with maxima rarely greater than 38 ◦ C, and average surface water temperatures in these modern evaporative settings should be quite similar to average local air temperatures (Lowenstein et al., 1998). Further, from data collected on the modern South Equatorial Current, a broad westward flowing current running between the west coast of Africa (Gabon, Republic of Congo region) and east coast of Brazil, near surface water temperatures annually range between 26–28 ◦ C, and 22–24 ◦ C at 100 m depth. This current is constrained between ∼4◦ N and ∼15–25◦ S latitudes (depending on seasonality), and illustrates a stable annual cycle of temperature fluctuation, showing only about 4 ◦ C variability (Mayer et al., 1998). The Thmax data collected here, with the peak of the distribution centered on the range of 20–25 ◦ C, are therefore consistent with measured modern temperatures of equatorial currents. Additionally, according to the measured Thmax from the Sinian halites, we can suggest that the estimated highest tropical seawater temperature during the Ediacaran Period (39.4 ± 1.0 ◦ C) was similar to Phanerozoic maximum seawater temperatures at the same latitude. Indeed, Permian halite fluid inclusions record
Fig. 4. Temperature-frequency distribution of measured homogenization temperatures.
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Fig. 5. Dissolved oxygen in seawater, as a function of salinity and temperature (for present, 1 PAL, and 10% PAL O2 ), concluded from homogenization temperature range assessed in this study. Example metazoan dissolved O2 requirement of ∼0.5 ml/l for dissolved O2 illustrated with hypothetical trajectories for the evolution of dissolved O2 in seawater over time; curves A and B represent minimum salinity in the Archean with a steady decline through the Precambrian and high initial salinity with steep drop-off associated with deposition of Neoproterozoic salt deposits, respectively, after schematic diagram presented in Knauth (2005) with experimentally determined algorithms by Weiss (1970). Knauth (2005) points out that, with 10% PAL O2 and seawater temperature estimates of 15–25 ◦ C, the reduction of marine salinity resulting from the Neoproterozoic salt deposits can feasibly shift dissolved O2 above the required threshold for metazoan respiration, as indicated by the Neoproterozoic arrow. As shown by the red bar, our data corroborate the calculated seawater temperature range at this time interval necessary for the threshold dissolved O2 to be achieved. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
homogenization temperatures ranging from 21 ◦ C to 50 ◦ C (Benison and Goldstein, 1999), which is similar to but slightly higher than the range recorded from the measured Sinian halites. The highest estimated tropical Ediacaran seawater temperatures according to halite fluid inclusion Th are additionally consistent with the highest temperatures calculated by silicon and oxygen isotope analyses (nearing ∼40 ◦ C; Knauth, 2005; Robert and Chaussidon, 2006). While geochemical models of the early Earth ocean system typically use temperatures of 25 ◦ C for estimating oxygen solubility, the early ocean should have much higher temperatures and salinities than modern ocean approximations, and therefore the amount of dissolved marine O2 should be limited (Knauth, 2005). Once in the Ediacaran, however, the salinity and temperature of the ocean system had likely declined significantly, and therefore allowed for more dissolved marine oxygen (Fig. 5). In addition to the amount of dissolved oxygen, the decreased seawater temperature may have played an important role in metazoan evolution, as modern macroinvertebrates (with the obvious exception of deep sea vent communities) rarely tolerate temperatures ≥45 ◦ C (Lamberti and Resh, 1985). 5. Conclusions The soft-bodied organisms of the Ediacara biota, known from a worldwide distribution of fossil impressions and restricted in age from 575 Ma to 541 Ma (Xiao and Laflamme, 2008), were the first complex multicellular organisms that likely required higher dissolved marine oxygen levels for metabolic functions than what is known or inferred, either environmentally or from the perspective of biological requirements, from the earlier Proterozoic and Archean biosphere. With fluid inclusion homogenization data from Ediacaran halites, we supply seawater paleotemperature data to a timeframe that notably lacked evidence from isotopic analyses. Given the presence of the large salt deposits from which our data was ascertained, we can conclude that the Ediacaran seawater salinity and temperature had finally reached the threshold level at which the increased capacities for dissolved marine oxy-
gen could support the evolution of more complex multicellular forms. The question of the external, whether environmental or ecological, switch that may have led to the evolutionary innovations observed during the Cambrian explosion has multiple potential answers, such as biological responses to increased atmospheric and oceanic oxygenation in addition to major changes in seawater chemistry (e.g., Cloud, 1976; Kempe and Kazmierczak, 1994; Brennan et al., 2004), increasing predation pressures (Stanley, 1976), ecological recovery from a snowball-induced mass extinction (e.g., Narbonne and Gehling, 2003), or evolutionary ramifications of the origin of mesozooplankton (e.g., Signor and Vermeij, 1994; Butterfield, 1997). Certainly one of the longeststanding ideas revolves around the increase of oxygen, which could support metabolic functions of larger metazoans (e.g., see Knoll and Carroll, 1999), and was likely necessitated for collagen biosynthesis vital for metazoan biomineralization processes (suggested at >10% PAL; Towe, 1970). While the rise of oxygen, however, cannot directly explain the emergence of metazoans, it should almost certainly be considered a prerequisite switch in the origin of animal life. If atmospheric oxygen had already reached levels capable of supporting larger complex eukaryotes by the Ediacaran (Canfield and Teske, 1996), oceanic salinity decreased following the enormous accumulation of salt in halites between ∼700 and 500 Ma (Land et al., 1988; Knauth, 2005), and seawater temperatures were trending toward increased solubility of dissolved gases through the Proterozoic (Knauth, 2005), then all of the interconnected switches were in place for the increase of oceanic oxygenation leading up to the widespread appearance of complex metazoans by the Cambrian explosion. The homogenization temperature data provided here suggest that tropical ocean waters in the latest Ediacaran were very similar to Phanerozoic seawater temperatures at the same latitude. Acknowledgments This investigation was supported by China Postdoctoral Science Foundation (2007), Jiangsu Province Postdoctoral Science Founda-
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Ediacaran seawater temperature: Evidence from inclusions of Sinian halite Fanwei Meng a,b , Pei Ni a,∗ , James D. Schiffbauer c , Xunlai Yuan b , Chuanming Zhou b , Yigang Wang d , Maolong Xia d a
State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluid Research, Department of Earth Science, Nanjing University, Hankou Road 22#, Nanjing, China Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, China c Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA d Southwest Oil and Gas Field Company Exploration and Development Research Institute, PetroChina, Chengdu, China b
a r t i c l e
i n f o
Article history: Received 21 February 2010 Received in revised form 31 August 2010 Accepted 26 October 2010
Keywords: Ediacaran Seawater temperature Inclusion Halite
a b s t r a c t Seawater temperatures throughout Earth’s history have been suggested to illustrate a long-term cooling trend from nearly 70 ◦ C at ∼3500 Ma to around 20 ◦ C at ∼800 Ma. The terminal Neoproterozoic prior to the “Cambrian Explosion” is a key interval in evolutionary history, as complex multicellularity appeared with the advent of the Ediacara fauna. These organisms were likely the first that required higher levels of atmospheric and dissolved marine oxygen for their sustainability. It is known that most modern macroinvertebrates are intolerant of temperatures in excess of 45 ◦ C. Perhaps more importantly, these high seawater temperatures limit the potential of dissolved oxygen, and therefore become an integral part of this evolutionary story. Previously, our understanding of seawater temperature during the terminal Neoproterozoic comes only from 18 O/16 O and 30 Si/28 Si ratios ascertained from a limited number of cherts. Isotopic ratio methods for assessing seawater temperatures are inherently indirect and have a wide range of oscillation. However, maximum homogenization temperatures (Thmax ) of primary fluid inclusions in halite provide a direct means of assessing brine temperature, and have been shown to correlate well with average maximum air temperatures. The oldest halites date to the Neoproterozoic–lower Paleozoic (∼700–500 Ma), and Ediacaran representatives can be found in Sichuan Province, China, which do preserve primary fluid inclusions for analysis via cooling nucleation methods. We utilized halite samples from the Changning-2 well, correlative to the Dengying Formation (551–542 Ma), to provide a direct assessment of terminal Neoproterozoic seawater temperature. Our measurements indicate that seawater temperatures where these halites formed are highly similar to tropical Phanerozoic seawater temperature estimates. From compiled paleotemperature data, the decline in seawater temperatures over the course of the Proterozoic, accompanied by the reduction of seawater salinity with the sequestration of salt in massive halite deposits in the Neoproterozoic, allowed the ocean system to accumulate more dissolved oxygen, and potentially paved the way for the evolutionary innovation of complex multicellularity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Precambrian hosted several astonishing advancements in biological evolution. Indeed, this timeframe encompasses four of the six evolutionary megatrajectories in the history of life, including the evolution of the last common ancestor of extant life, the metabolic diversification of bacteria and archaea, the evolution of the eukaryotic cell, and the origin of multicellularity (Knoll and Bambach, 2000). This should not come as a surprise, however, as the Precambrian occupies nearly 90% of Earth’s history, a vast time interval for evolutionary development. As the pace of evolution can be rapid, the hindrance in the development of complex multi-
∗ Corresponding author. Tel.: +86 25 83597124; fax: +86 25 83592393. E-mail address: [email protected] (P. Ni). 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.10.004
cellularity until over 3000 million years after the first hints of life presents numerous queries in our understanding of the history and nature of evolution on the Precambrian Earth. It is likely that a series of switches needed to be turned-on prior to the so-called “explosion” of metazoan life. Some of the previously proposed switches include, but are not limited to, internal genetic switches, such as the Hox gene complex, behavioral or ecological switches, including the evolution of macrophagy and increasing predation pressures, as well as numerous external switches, such as the increase of atmospheric oxygenation and changes in seawater chemistry (e.g., see Cloud, 1976; Peterson et al., 2005). One switch – or perhaps more accurately a compilation of interconnected switches – that was undoubtedly a prerequisite to metazoan evolution, is the reduction of seawater salinity and temperature along with the concurrent change in the potential for increased hydrospheric oxygen solubility (Knauth, 2005).
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If evolutionary change were dictated only by internal switches, one may suggest – with the presence of metaHox genes in primitive organisms such as sponges (Coutinho et al., 2003) – that complex multicellularity may have had the potential to arise much more swiftly than what we have observed in the fossil history of life. It seems applicable, therefore, to invoke external pressures, such as environmental restrictions, as levels of containment for life’s impending evolutionary developments. Indeed, many traditional explanations for major Precambrian evolutionary events invoke specific thresholds in atmospheric oxygen levels; for instance, for both metabolic function as well as the biosynthesis of membranesupporting sterols, the origin of Eukarya have been suggested to require at least 1% of the present atmospheric level (PAL) of oxygen (e.g., Jahnke and Klein, 1983; Han and Runnegar, 1992). Further, one of the more accepted explanations for the appearance of complex multicellularity in the Ediacaran is the increase in atmospheric oxygen content to a required threshold level (suggested to be between 1 and 10% PAL, depending on potential adaptation of Ediacara organisms to dysaerobic conditions) for metazoan respiration (e.g., Runnegar, 1982, 1991; Catling et al., 2005). While atmospheric oxygenation is undoubtedly important, as Knauth (2005) stated, “the level of oxygen dissolved in water is the real issue.” Seawater oxygen solubility is highly constrained by both ocean temperature and salinity, which likely fluctuated significantly over the course of the geologically- and environmentally-evolving Precambrian Earth. The most continuous records of seawater temperature come from the complementary stories told by ␦18 O and ␦30 Si data, which have been collected and compiled from marine cherts (Knauth, 2005; Robert and Chaussidon, 2006). These isotopic data (along with other published data, for instance, see Perry, 1967; Knauth and Epstein, 1976; Knauth and Lowe, 1978) have been interpreted as evidence for a long-term cooling trend of the oceans through the Precambrian. From Knauth’s (2005) estimates using a hypothetical 10–20 ◦ C seawater temperature for Cenozoic oceans (see also Knauth and Lowe, 2003) and compiled oxygen isotope data, the Archean ocean surface temperature ranged from approximately 55–85 ◦ C, which agrees with paleotemperature estimates of ∼70 ◦ C ascertained from silicon cycle models constructed from silicon isotopic data (Robert and Chaussidon, 2006). Furthermore, isotopic evidence of a hot Archean ocean has been linked to microbial evolution by studies of thermostability in early microbial proteins (Gaucher et al., 2008). However, it is important to note that recent isotopic data (Hren et al., 2009; Blake et al., 2010) challenge the commonly held view of a hot Archean world, pointing instead to a temperate and perhaps biologically active primitive ocean. Using a combined approach of oxygen (␦18 O) and hydrogen (␦D) isotopes from cherts of the ∼3420 Ma Buck Reef Chert in South Africa, Hren et al. (2009) provide evidence suggesting that these cherts formed in equilibrium with waters below ∼40 ◦ C. Supporting these conclusions and adding yet another facet, Blake et al. (2010) ascertained ␦18 OP from phosphates of the ∼3500–3200 Ma Barberton Greenstone Belt, South Africa. Due to their similarity with modern phosphates, these data again suggested a marine temperature of less than ∼40 ◦ C in the Archean and a developed biological phosphorus cycle (Blake et al., 2010). Thus, while seawater temperatures in the Archean have become debated in light of recent evidence, multiple estimates for Archean ocean salinity place the likely range between 1.2 and 2× that of modern ocean surface waters (Holland, 1978; Knauth, 1998). This elevated salinity may have persisted through the Archean due to the absence of continental cratons, as salt was not sequestered in halite deposits forming in evaporitic environments (Knauth, 2005). Furthermore, oxygen solubility in these early oceans was limited by elevated water temperatures and salinity, presumably resulting in a predominantly anoxic ocean full of anaerobic, thermophillic, and at least halotolerant if not
halophillic prokaryotes. One can easily imagine, then, that the physical and chemical properties, certainly including oxygen solubility, of this early ocean are far different than the ocean we know today. In the Proterozoic Earth, on the other hand, seawater temperatures declined as continents became larger, more stable, and longer-lived. From Knauth’s (2005) oxygen isotope dataset and Robert and Chaussidon’s (2006) silicon isotopes, it is suggested that seawater temperatures reached typical Phanerozoic levels, an estimated 20 ◦ C, by approximately 1200–800 Ma, during the assembly and near the break-up of the supercontinent Rodinia. During the break-up of Rodinia, the ocean temperatures of the Neoproterozoic Earth likely fluctuated dramatically, with global glaciation snowball events and warming periods during interglaciation; unfortunately, there seems to be a comparative lack of cherts during the Neoproterozoic and lower Paleozoic, which results in a scarcity of data for paleotemperature assessments during this biologically significant time interval (Knauth, 2005). Thus, seawater temperatures during the key geologic interval of metazoan origination and evolution is yet unresolved. Further, once in the Neoproterozoic, salt began precipitating out of seawater in large evaporite basins – such as the one described here. The resulting lowered oceanic salinity, in conjunction with the cooling of ocean temperatures associated with global glaciations, would allow dissolved oxygen to more readily accumulate in the oceans. This compilation of interconnected events not only limited the ecological ranges of anaerobic, thermophillic, halotolerant/-phillic prokaryotes that had once dominated the oceans, but also may have resulted in enough dissolved oxygen in the ocean system to support metazoan respiration – and most importantly corresponded in time with the rise of complex metazoans, as evidenced by the earliest fossil representatives of metazoan life at ∼632 Ma (Yin et al., 2007). We provide additional data herein for seawater paleotemperature reconstruction of the terminal Neoproterozoic not based on isotopic analysis, but rather from halite fluid inclusion homogenization temperature assessments.
2. Halite fluid inclusions and geological setting of collected samples Paleotemperature reconstructions using 18 O/16 O and 30 Si/28 Si methods vary largely with diagenetic influences and provide only relative estimates; whereas homogenization of primary halite fluid inclusions, by utilizing the cooling nucleation method, provides a direct means to evaluate seawater paleotemperature (Roberts and Spencer, 1995; Benison and Goldstein, 1999; Lowenstein et al., 1998; Satterfield et al., 2005; Liu et al., 2007). The cooling nucleation method of measuring homogenization temperatures in original, all-liquid, primary fluid inclusions has been successfully used in Permian, Pleistocene, and modern halites (Benison and Goldstein, 1999; Lowenstein et al., 1998; Satterfield et al., 2005). Maximum homogenization temperatures of fluid inclusions (Thmax ) in halite (from both laboratory-produced crystals and modern samples from Death Valley, CA) match the assessed brine temperatures during halite precipitation, and additionally correlate well with the average maximum air temperatures (Lowenstein et al., 1998). Therefore, maximum homogenization temperatures, calculated from a series of measured homogenization temperatures (Th) in halite cores, reflect the maximum brine temperature and provide a close approximation for local air temperatures. Primary halite has two crystal forms: cumulate crystals and chevron crystals in sedimentary surrounding (Roberts and Spencer, 1995; Benison and Goldstein, 1999; Lowenstein et al., 1998). Cumulate halite forms at the air–brine interface and can record the surface water temperature, which is directly comparable to air temperature (Lowenstein et al., 1998). Chevron halite, on the
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Fig. 1. Location (denoted by star) of the Changning-2 well core used in the study.
other hand, forms at the evaporite basin bottom, and therefore records the benthic brine temperature (Benison and Goldstein, 1999). Cumulate salt formed at surface–brine can be found in water of any depth, however chevron salt is formed in shallow water because dense brine circulates to the bottom to precipitate minerals. In deep, density-stratified water environments, bottom growth is suppressed (Hovorka et al., 2007). In shallow water settings, Thmax of the inclusions in cumulate halite and chevron halite illustrate a similar homogenization temperature range (Benison and Goldstein, 1999). Two time intervals in geologic history host the majority of the Earth’s known halite deposits. Approximately 40% of Phanero-
zoic salts can be found from the Permo-Triassic boundary to the late Early Jurassic (∼250–180 Ma). The second major interval of salt deposition ranges from the Neoproterozoic–lower Paleozoic (∼700–500 Ma), which constitute the oldest halites on Earth (Knauth, 2005). The halites examined in this study are from the Neoproterozoic time interval, and were sampled from the Changning-2 well of the Sichuan Province, southwestern China (Fig. 1, Sichuan Petroleum Bureau, 1973; Lin and Cao, 1998). The Sinian halites are found in the local structure of Changning Anticline, located centrally in the Changning Depression, Southwestern Sichuan-Guizhou Province Subsiding Belt. The Sinian-age (635–542 Ma) beds of the sampled Changning-2 well range from
Fig. 2. Stratigraphy of the Changnig-2 well assessed from drill core and field observation.
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Table 1 Stratigraphic position of Changning-2 well. Geological time
Formation
Thickness (m)
Lithology
Cambian
Jiuladong Formation
1019.50–1870 m or so
Lower part is black shale, and a parallel unconformity is between the Formation and lower Maidiping Formation Carbonaceous chert interbedded with dolomite and shale The formation is divided 2 sections according to lithology. Upper section (1975–2492.5 m) mainly are dolomite, and lower section dolomite bear grapestone; Lower Section (2492.5–2992.50 m) is dolomite and halite, dolomite contain anhydrite without grapestone, halite‘s top is glauberite. 2 Section‘s base is white chert. The Formation is not overly drilled through. Upper part is mainly sandstone. Lower part is mudstone.
Maidiping Formation Sinian
100 mor so
Hongchunping Formation
1975–2992.5 m
Labagang Formation
2992.5–3308.08 m
1975 to 3308.8 m in section, from top to base, occupying the Hongchunping and Labagang Formations. The Cambrian Maidiping Formation unconformably contacts the overlying black shale of Cambrian Jiulaodong Formation; and the lowermost portion of the well does not extend beyond the Labagang Formation (Fig. 2, Table 1). The Labagang Formation can roughly be correlated to the well-known Doushantuo Formation, which, from U-Pb zircon dating of volcanic ash beds, ranges in age from 635 to 551 Ma (Condon et al., 2005). The halite sampled for examination is located in the middle Hongchunping Formation and specifically ranges in the Changning-2 well section from 2645 to 2855 m (Sichuan Petroleum Bureau, 1973), which is correlative to the 551–542 Ma Dengying Formation. In addition to halite, the salt-bearing strata predominantly consist of grapestone-bearing dolomites, anhydrites, and glauberites (Sichuan Petroleum Bureau, 1973), which were likely formed in a low-energy subtidal zone, similar to modern lowenergy marine environments below 9 m in depth (Cao, 2002). According to sedimentary facies mapping and paleogeography, the salt-bearing unit of the Sichuan Province covers over 20,000 km2 in area; the paleogeographically proximal Changning Depression likely also houses evaporitic rock units (Lin and Cao, 1998). These salt-bearing units were formed during a marine regression after the Chengjiang orogeny (Lin and Cao, 1998). In bedded salts like those collected, halite cores can show parallel bands of minute fluid inclusions similar to parallel bands of cumulate halite and chevron halite, inherited from original sedimentary textures, with rims of recrystallized salts (Roedder, 1984). In shallow brines, fluid inclusions in both cumulate and chevron crystals can be used to estimate paleotemperatures due to their similar temperature of formation and resultant Th range (Benison and Goldstein, 1999). The Sinian halite examined here has numerous preserved primary inclusions and sedimentary textures, identical to bands of cumulate halite and chevron halite (Fig. 3).
3. Methods The Changning-2 well of the Sichuan Province, southwestern China, was drilled in the 1970s using drilling mud prepared with saturated brines to prevent dissolution of bedded halite. Since the initial drilling, the core has been stored in sealed plastic (at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences) to prevent alteration by atmospheric moisture. The analytical methods utilized in this study follow those of Benison and Goldstein (1999). Unpolished halite cleavage chips were prepared by cleaving 1–2 mm-thick fragments with a razor blade. The halite samples were placed in a HAIER freezer at −15 ◦ C for two weeks. This temperature is not cold enough to freeze the fluid inclusions, and also conserves the original size and shape of the inclusion. After the two-week cooling period, ∼5% of the fluid inclusions produce vapor bubbles. Once removed from the freezer, the samples were placed on a Linkam THMS600 heating/freezing stage at −15 ◦ C, and were held at constant temperature until the nucleated vapor bubbles within the inclusions were located. Subsequently, the Linkam stage was slowly warmed at a rate of approximately 0.5 ◦ C per minute at lower temperatures (up to ∼15 ◦ C), and 0.1 ◦ C per minute upon reaching the arbitrary 15 ◦ C threshold. The Linkam THMS600 stage used in this study was calibrated using synthetic fluid inclusions, precise to approximately ±1.0 ◦ C between a temperature range of 0–50 ◦ C. As the vapor bubbles in the fluid inclusions become gradually smaller and homogenized to liquid during the heating process, the Th value was observed and recorded (Benison and Goldstein, 1999). In total, 24 Sinian halite cleavage chips of the same stratigraphic level were investigated for fluid inclusions, 15 of which contained microthermometrically measureable inclusions (58 different inclusions were analysed).
Fig. 3. Primary fluid inclusion textures in Ediacaran halite.
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Table 2 Homogenization temperature data for primary fluid inclusions in Sinian halite. Sample #
Homogenization temperatures (◦ C)
Pr-1 Pr-4 Pr-5 Pr-6 Pr-7 Pr-13 Pr-14 Pr-15 Pr-16 Pr-17 Pr-19 Pr-21 Pr-22 Pr-23 Pr-24
15.0 20.2 30.5 22.5 20.1 19.7 20.2 19.2 17.8 13.3 21.1 20.2 21.0 17.3 18.4
Range (◦ C) 18.7 21.9 39.4 26.5 25.3 – 20.5 20.5 18.2 16.4 22.1 21.7 24.5 18.0 –
21.4 25.4 – – – – 20.8 26.7 – 18.1 22.2 22.5 24.7 19.7 –
23.2 26.1 – – – – 22.9 27.0 – 19.5 23.0 23.1 – 20.9 –
4. Results and discussion The Th values measured from 58 Sinian halite primary fluid inclusions ranged from 13.3 ± 1.0 ◦ C to 39.4 ± 1.0 ◦ C, with the majority of measurements falling between 20–25 ± 1.0 ◦ C (mean = 23.1 ◦ C, median = 22.15 ◦ C, mode = 20.2 ◦ C, 1 = 5.0 ◦ C, n = 58; Table 2; Fig. 4). Within individual chips, Th measurements exhibited a mean range of 8.2 ◦ C (with a maximum of 17.7 ◦ C and a minimum of 0.4 ◦ C; Table 2). The primary fluid inclusions measured from individual chips came from the same or adjacent fluid inclusion bands, which illustrate relatively consistent homogenization temperatures. For example, sample Pr-16 (Table 2) contains two measurements from inclusions from within the same fluid inclusion band and illustrates a range of 0.4 ◦ C (see also Fig. 14 of Benison and Goldstein, 1999). Because only ∼5% of the inclusions produced vapor bubbles, not all of the halite cleavage chips containing primary fluid inclusions were measurable; however, previous investigation suggests a minimum of 30–40 Th measurements to assess the maximum brine temperature during crystal growth (Lowenstein et al., 1998). Because the maximum homogenization temperatures of the fluid inclusions (Thmax ) in the sampled halites have been shown to be equivalent to the maximum brine temperatures (Lowenstein et al., 1998), we can suggest that the local maximum seawater temperatures during
– 28.0 – – – – 22.9 28.1 – 22.6 30.3 37.9 – 21.9 –
– – – – – – 25.7 29.4 – – – – – 32.8 –
– – – – – – 26.1 30.3 – – – – – – –
– – – – – – 26.3 – – – – – – – –
8.2 7.8 8.9 4.0 5.2 – 6.1 11.1 0.4 9.3 9.2 17.7 3.7 15.5 –
Mean (◦ C) 19.6 24.3 35.0 24.5 22.7 – 23.2 25.9 18.0 18.0 23.7 25.1 23.4 21.8 –
the precipitation of the Sinian halites of the Sichuan Province corresponds to approximately 39.4 ± 1.0 ◦ C. Paleomagnetic data indicate that the Sichuan Province of South China was equatorially located during the Ediacaran Period (Yang et al., 2004). The average air temperature in equatorial regions today ranges from 25 to 35 ◦ C, with maxima rarely greater than 38 ◦ C, and average surface water temperatures in these modern evaporative settings should be quite similar to average local air temperatures (Lowenstein et al., 1998). Further, from data collected on the modern South Equatorial Current, a broad westward flowing current running between the west coast of Africa (Gabon, Republic of Congo region) and east coast of Brazil, near surface water temperatures annually range between 26–28 ◦ C, and 22–24 ◦ C at 100 m depth. This current is constrained between ∼4◦ N and ∼15–25◦ S latitudes (depending on seasonality), and illustrates a stable annual cycle of temperature fluctuation, showing only about 4 ◦ C variability (Mayer et al., 1998). The Thmax data collected here, with the peak of the distribution centered on the range of 20–25 ◦ C, are therefore consistent with measured modern temperatures of equatorial currents. Additionally, according to the measured Thmax from the Sinian halites, we can suggest that the estimated highest tropical seawater temperature during the Ediacaran Period (39.4 ± 1.0 ◦ C) was similar to Phanerozoic maximum seawater temperatures at the same latitude. Indeed, Permian halite fluid inclusions record
Fig. 4. Temperature-frequency distribution of measured homogenization temperatures.
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Fig. 5. Dissolved oxygen in seawater, as a function of salinity and temperature (for present, 1 PAL, and 10% PAL O2 ), concluded from homogenization temperature range assessed in this study. Example metazoan dissolved O2 requirement of ∼0.5 ml/l for dissolved O2 illustrated with hypothetical trajectories for the evolution of dissolved O2 in seawater over time; curves A and B represent minimum salinity in the Archean with a steady decline through the Precambrian and high initial salinity with steep drop-off associated with deposition of Neoproterozoic salt deposits, respectively, after schematic diagram presented in Knauth (2005) with experimentally determined algorithms by Weiss (1970). Knauth (2005) points out that, with 10% PAL O2 and seawater temperature estimates of 15–25 ◦ C, the reduction of marine salinity resulting from the Neoproterozoic salt deposits can feasibly shift dissolved O2 above the required threshold for metazoan respiration, as indicated by the Neoproterozoic arrow. As shown by the red bar, our data corroborate the calculated seawater temperature range at this time interval necessary for the threshold dissolved O2 to be achieved. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
homogenization temperatures ranging from 21 ◦ C to 50 ◦ C (Benison and Goldstein, 1999), which is similar to but slightly higher than the range recorded from the measured Sinian halites. The highest estimated tropical Ediacaran seawater temperatures according to halite fluid inclusion Th are additionally consistent with the highest temperatures calculated by silicon and oxygen isotope analyses (nearing ∼40 ◦ C; Knauth, 2005; Robert and Chaussidon, 2006). While geochemical models of the early Earth ocean system typically use temperatures of 25 ◦ C for estimating oxygen solubility, the early ocean should have much higher temperatures and salinities than modern ocean approximations, and therefore the amount of dissolved marine O2 should be limited (Knauth, 2005). Once in the Ediacaran, however, the salinity and temperature of the ocean system had likely declined significantly, and therefore allowed for more dissolved marine oxygen (Fig. 5). In addition to the amount of dissolved oxygen, the decreased seawater temperature may have played an important role in metazoan evolution, as modern macroinvertebrates (with the obvious exception of deep sea vent communities) rarely tolerate temperatures ≥45 ◦ C (Lamberti and Resh, 1985). 5. Conclusions The soft-bodied organisms of the Ediacara biota, known from a worldwide distribution of fossil impressions and restricted in age from 575 Ma to 541 Ma (Xiao and Laflamme, 2008), were the first complex multicellular organisms that likely required higher dissolved marine oxygen levels for metabolic functions than what is known or inferred, either environmentally or from the perspective of biological requirements, from the earlier Proterozoic and Archean biosphere. With fluid inclusion homogenization data from Ediacaran halites, we supply seawater paleotemperature data to a timeframe that notably lacked evidence from isotopic analyses. Given the presence of the large salt deposits from which our data was ascertained, we can conclude that the Ediacaran seawater salinity and temperature had finally reached the threshold level at which the increased capacities for dissolved marine oxy-
gen could support the evolution of more complex multicellular forms. The question of the external, whether environmental or ecological, switch that may have led to the evolutionary innovations observed during the Cambrian explosion has multiple potential answers, such as biological responses to increased atmospheric and oceanic oxygenation in addition to major changes in seawater chemistry (e.g., Cloud, 1976; Kempe and Kazmierczak, 1994; Brennan et al., 2004), increasing predation pressures (Stanley, 1976), ecological recovery from a snowball-induced mass extinction (e.g., Narbonne and Gehling, 2003), or evolutionary ramifications of the origin of mesozooplankton (e.g., Signor and Vermeij, 1994; Butterfield, 1997). Certainly one of the longeststanding ideas revolves around the increase of oxygen, which could support metabolic functions of larger metazoans (e.g., see Knoll and Carroll, 1999), and was likely necessitated for collagen biosynthesis vital for metazoan biomineralization processes (suggested at >10% PAL; Towe, 1970). While the rise of oxygen, however, cannot directly explain the emergence of metazoans, it should almost certainly be considered a prerequisite switch in the origin of animal life. If atmospheric oxygen had already reached levels capable of supporting larger complex eukaryotes by the Ediacaran (Canfield and Teske, 1996), oceanic salinity decreased following the enormous accumulation of salt in halites between ∼700 and 500 Ma (Land et al., 1988; Knauth, 2005), and seawater temperatures were trending toward increased solubility of dissolved gases through the Proterozoic (Knauth, 2005), then all of the interconnected switches were in place for the increase of oceanic oxygenation leading up to the widespread appearance of complex metazoans by the Cambrian explosion. The homogenization temperature data provided here suggest that tropical ocean waters in the latest Ediacaran were very similar to Phanerozoic seawater temperatures at the same latitude. Acknowledgments This investigation was supported by China Postdoctoral Science Foundation (2007), Jiangsu Province Postdoctoral Science Founda-
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