Precambrian Atmospheric Oxygen and Banded Iron Formations: A Delayed Ocean Model

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PRECAMBRIAN ATMOSPHERIC OXYGEN AND BANDED IRON FORMATIONS: A DELAYED OCEAN MODEL KENNETH M. TOWE Department o f Paleobiology, Smithsonion Institution, Washington, DC 20560 (U.S.A.)

ABSTRACT Towe, K.M., 1983. Precambrian atmospheric oxygen and banded iron formations: a delayed ocean model. Precambrian Res., 20: 161-170. Evidence and arguments increasingly in favor of free oxygen in the Earth’s early atmosphere renew the constraints on the environmental significance of Precambrian banded iron formations. An early moist greenhouse atmosphere with a delay in, and gradual growth of, the world oceans offers a mechanism to provide a geochemically and mechanically segregated source of iron and silica for banded iron formation, while simultaneously ‘cannibalizing’ evidence for early Archean red beds. The model supports the high rates of weathering necessary to remove initially outgassed CO, quickly, favors continuity in early biogenic evolution, provides a mechanism for hydrogen and strontium isotope partitioning, and is consistent with iron oxide facies that are devoid of organic carbon or stromatolites that are not encrusted by iron oxide.

INTRODUCTION

As new evidence continues t o accumulate, it is increasingly clear that arguments in support of free oxygenin the Earth’s early atmosphere must be given serious consideration. The presence or absence of free oxygen is becoming less the subject of debate. Rather, the levels of such oxygen and its sources of supply are becoming the focus of attention. The presence of photolytically derived oxygen very early in Earth history, and prior t o the advent of oxygenic photosynthesis, requires a reconsideration of the conditions surrounding the origin and early evolution of life. As traditional atmospheric sources of primordial organic matter assume less importance in such scenarios, renewed roles for carbonaceous chondrites (Engel and Nagy, 1982) or comets (Lazcano-Araujo and O h , 1981), and hydrothermal vents (Corliss et al., 1981) come t o the fore. The many experimental successes in prebiotic chemistry that emphasize fluctuating wetting and drying conditions serve to re-emphasize the importance of a biologically effective, UV-protective ozone screen to the origin and early evolution of life (Towe, 1981). The view of the primitive Earth as covered with an

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oceanic ‘primordial soup’ beneath a more-or-less reducing atmosphere free of oxygen for billions of years is less certain than ever before. INFERENCES AND EVIDENCE F O R F R E E OXYGEN

A rock record is lacking for the first 700-800 M a of Earth history. Thus, with the evidence missing, or yet to be found, the nature of the environment at this time has been conjecture, and conjecture supporting the presence of free oxygen in the atmosphere can be assembled. One such argument related to the origin of life concerns the so-called UV-liquid water dilemma (Towe, 1981), in which peptide bonds formed through prebiotic dehydration reactions faced UV-destruction during evaporative exposure, but under continued subaqueous protection faced spontaneous hydrolysis and curtailment of sustained polypeptide chain elongation. Thus, the environments most favorable for the concentration and continued growth of complex prebiotic molecules (proteins and nucleic acids) would be subjected to UV-destruction in the absence of an effective oxygen/ozone screen. Utilization of visible light to promote later experiments with early biological photoenergy conversions leading to photosynthesis would also have been severely inhibited by a simultaneous UV-flux t o DNA-based life forms. In short, the continuity of both prebiotic and early biogenetic evolution would be difficult to maintain. Astronomical data gathered on the high UV-flux emitted by young stars (Gaustad and Vogel, 1982; Canuto et al., 1982) further emphasizes the importance of the UV problem to the early Earth at this time. Model support for the presence of photolytic free oxygen and ozone in this early atmosphere as a result of an enhanced UV-flux has already been produced (Canuto e t al., 1982). Greenhouse atmospheres, dictated by the need to prevent a frozen Earth (e.g., Owen et al., 1979) because of a lower stellar luminosity (Newman and Rood, 1977), are thus capable of generating free oxygen through photodissociation with ultimate loss of hydrogen to space. The levels of such oxygen are still open questions. They should be lower than modern levels yet high enough t o oxidize ferrous iron and provide a reasonable ozone screen. The Berkner-Marshall level of <1%present atmospheric level (PAL) seems reasonable. Even with the beginning of the rock record, the 3.8 Ga Isua deposits in Greenland, the first actual evidence for free oxygen is found. In addition to clastic (or volcanoclastic) sediments, chemical sediments implying free oxygen in a subaqueous medium are found - the first banded iron formations. Later, bedded gypsum deposits in Australian evaporite sequences that are 3.5 Ga-old indicate the presence of oxidized sulfur in the waters from which they precipitated (Groves et al., 1981). Coeval rocks interpreted as stromatolitic (Lowe, 1980; Walter et al., 1980) imply the presence of life, but do not provide unequivocal evidence of oxygenic photosynthesis. The sedimentological evidence for their intermittent dessication does, however, require some sort of UV-protection and suggests the existence of an ozone screen

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(Walter et al., 1980). Either oxygenic photosynthesis evolved very early on, and a t least by Isua time, or photodissociation was the source of the early oxygen. Although the source of any prebiotic oxygen must have been atmospheric photolysis, later sources may have been augmented by photosynthesis. Some biochemical arguments against an initial role for photosynthesis have been presented (Towe, 1978; Schwartz and Dayhoff, 1978). Mineralogic and geochemical data from younger Archean and early Proterozoic rocks add to the evidence. There are Archean pillow basalts with oxidized crusts implying the presence of free oxygen during a period of submarine weathering (Dimroth and Lichtblau, 1978). Archean calcites contain levels of Fez+and Mn2+in their structures that are lower, by orders of magnitude, than expected had these carbonates been formed in anoxic waters (Veizer, 1978). Archean (Shegelski, 1980; Schau and Henderson, 1983) and early Proterozoic (Button, 1979; Gay and Grandstaff, 1980) weathering profiles in Canadian and South African rocks indicate oxidative conditions during a subaerial weathering process. Geological evidence for reducing conditions cannot prove the general absence of oxygen in past atmospheres, especially if the O2-levels were lower, because reducing environments are widespread locally today under a clearly oxygenic atmosphere. Thus, the presence of pyrite and uraninite in modern detrital sediments (Simpson and Bowles, 1977), coupled with the kinetic studies on the behavior of such minerals (Grandstaff, 1976) have markedly lowered the significance of these minerals for the interpretation of Precambrian atmospheres (cf., Clemmey and Badham, 1982). It may be concluded that inferences for the interval before the earliest rock record (pre-3.8 Ga) combined with evidence from the subsequent rock record indicate that free oxygen was not only present in the early environment (in lower than PAL), but occurred generally in the atmosphere and locally in the hydrosphere. As support for free oxygen grows, the origin and significance of the banded iron fomiations requires re-examination. THE BANDED IRON FORMATIONS REVISITED

Central to most arguments regarding early Precambrian atmospheric oxygen are the iron-rich bedded chert deposits known collectively as the banded iron formations. The banded iron formations (BIF) are first and foremost, subaqueous oxidized chemical precipitates. They, therefore, differ substantially from their subaerial oxidized analogues, the clastic red beds. It is this distinction, perhaps more than any other, that has placed the BIF into a position of such importance in the interpretation of Precambrian atmospheres. By themselves, the BIF lose much of this importance simply because they occur in rocks of all ages from the earliest Precambrian to the clearly oxygenic atmospheres of the Phanerozoic (Schultz, 1966; Cloud, 1973). Although their abundance in the Precambrian is significant, it is in conjunction with the temporal distribution of red beds and other terrestrial deposits

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that the BIF assume their pivotal role in the interpretation of Precambrian atmospheres. This is because of the source of supply of the oxygen used t o oxidize the iron. If there was free oxygen in the hydrosphere but, as is commonly supposed, in the atmosphere, then BIF are possible, but none (less than -10-13 red beds or other evidence of terrestrial oxidation are not. An algal-cyanobacterial source of free oxygen localized in the hydrosphere, in proximity t o and in balance with the supply of ferrous iron could explain both the BIF and the absence of evidence for terrestrial oxidation. In other words, oxygen produced in the water by photosynthesis must be combined with ferrous iron or sulfide so as not to evade into the atmosphere to permit the formation of red beds. This is the essence of the heuristic proposal outlined by Cloud (1973) to explain the major Precambrian BIFs. On the other hand, if there was free oxygen in the atmosphere (regardless of its source and with due allowance for kinetics) then both subaerial and subaqueous oxidations are possible. Therefore, as newer geological evidence continues to emerge showing that terrestrial oxidation has taken place, both prior t o and synchronous with the deposition of BIF, the importance of these banded chemical precipitates to the qualitative composition of the atmosphere is devalued. When this newer evidence is added t o the other arguments in favor of atmospheric oxygen, as above, the idea of a direct relationship between photosynthetic oxygen and precipitated iron requires reassessment. A major advance in our understanding of banded iron formations was the quantitative assessment of possible iron sources made by Holland (1973). Using the Hamersley Basin of Australia as a model, he was able t o demonstrate that neither subaerial weathering nor volcanic emanations were likely to have been the immediate sources of soluble iron. This left the oceans, and Holland (1973) proposed that the upwelling of bottom waters could have provided the necessary quantities of dissolved iron to explain this deposit, and similar Lake Superior-type accumulations where clastic detritus is notably sparse. If the direct supply of soluble iron for the major Proterozoic BIF was necessarily the deeper ocean bottom waters then there is an important corollary to the Holland assessment: free oxygen need not be absent from the atmosphere in order to maintain the anoxic surface waters required for the free movement of weathered ferrous iron to the basin. What is required is that the deeper basins remain stagnant and anoxygenic for reasonable lengths of time, a viewpoint expressed by many (Hough, 1958; Govett, 1966; Drever, 1974; Degens and Stoffers, 1976; Veizer, 1983; and others). Acceptance of the Holland assessment that ocean bottom waters are the only plausible immediate source €or the iron in the Superior-type BIF also raises the question of what the indirect source of the iron was. In other words, what were the massive and continuous oceanic sources of supply for iron? Can a choice be made between the two available alternatives: volcanic emanations and chemical weathering? Both were certainly involved, but vol-

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canic emanations appear to be insufficient unless more evidence for intensive submarine volcanism at the beginning of the Proterozoic is forthcoming. Holland’s (1973) objections t o weathering apply very well to local conditions that surround a basin such as the Hamersley, but they do not negate a worldwide weathering source. With the necessity for an anoxic atmosphere softened by the Holland assessment, the weathering process could have taken place under oxygenic conditions. A DELAYED OCEAN MODEL

The delayed ocean model begins with the environmental conditions hypothesized by Towe (1981) and it is an extension of this speculative proposal. Like other models, it is of course tentative, and offered in the spirit of focussing attention to the problem. The early Archean environment (pre-3.8 Ga) consisted of an Earth surrounded by a moist greenhouse atmosphere (principally H, 0 and COz). Derived from an initial, but incomplete outgassing, it was an atmosphere disturbed and augmented by late-stage bombardment impacts into limited liquid water reservoirs. Beneath this wet atmosphere lay a generally mafic lithospheric crust with primarily shallow bodies of water, many near hydrothermal vents. Differentiated ‘continental’ masses were small, few in number, and of generally low relief. No oceans as such existed at first and the area of land (‘oceanic’ crust) exposed to the early atmosphere exceeded that covered by liquid water. Terrestrial silicate weathering could therefore proceed rapidly to initiate the necessary removal of CO, . The moist atmosphere provided the necessary conditions for the photolytic production of oxygen and development of the important UV-protective ozone screen necessary for DNA-based life to begin at the Earth’s surface. The abundant land areas provided the important hydration-dehydration locales necessary for the formation and continued growth and experimentation with peptide bonds in protein evolution. The mafic (‘oceanic’) crust exposed to this early moist atmosphere was subjected t o extensive weathering - a mafic laterization. A clay and oxide-rich weathering profile (perhaps analogous to Martian soils?) began to establish itself. Surficial oxidation of iron (and sulfur) served t o protect early experimentation with life-forming processes and the in the limited hydrosphere was, thus, kept low. Indeed, although the atmosphere was on the oxidizing side of neutral, the limited hydrosphere was more likely to have been generally reducing, and especially so near hydrothermal vents. The majority of early sedimentary environments were likely shallow water. The general absence of continental relief would preclude major areas of coarse clastics. Such sediments would be local and related at first to impact-generated crater relief and later t o volcaniclastic activity, as in the Isua rocks and in Algoma-type BIF. With the subsequent reduction of atmospheric CO, , first through silicate dissolution during weathering and then through the deposition of carbonates, and with a general tendency for tec-

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tonic cooling, more atmospheric water condensed and the oceans began to grow. Continued outgassing (post 3.8 Ga) coupled with increased continental growth and basin development through isostatic readjustment, resulted in a continuous transfer of the initially oxidized weathering products to increasingly deeper basins. Aluminum and trace elements are geochemically partitioned during weathering and erosion. Large amounts of iron and silica would move into the growing oceans where reduction of the iron through interaction with organic matter placed it into solution. As oceanic circulation patterns developed the Superior-type BIF would tend to replace the Algoma-type. Dissolved iron (and the silica) was brought into the shallow surface waters to interact with atmospheric oxygen causing its precipitation as hydrous ferric oxide. The overall sedimentary environment was one of worldwide transgression because the oceans were steadily growing in size - an overstep leading t o the process known as marine replacement (cf. Dunbar and Rodgers, 1957, p. 142). By this process the earlier Precambrian red beds were gradually ‘cannibalized’ and mostly eliminated as their iron was transferred by erosion to oceanic repositories. With this gradual rise in eustatic sea level, the exposed areas of increasingly larger continents remained generally low in relief and, thus, consistent with the worldwide paucity of clastic sediments so characteristic of many later banded iron formations. By 2.0 Ga-ago the cycle would have been completed and the major episode of banded iron formation come to an end; subsequent BIFs would not be precluded, but would never again be as abundant.

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COMPARISONS WITH OTHER MODELS

Most current models begin with an immediately outgassed and condensed ocean system equilibrated with COz . With few differentiated continents, the early condensed oceans would cover the land surface to a considerable depth (e.g., Hargraves, 1976). Initially, the outgassed carbon dioxide would have to be dissolved in these oceans at very high concentrations in order to provide the high atmospheric partial pressures (1000 PAL) considered necessary to offset the reduced solar constant and prevent a frozen Earth (Owen et al., 1979). In the delayed ocean model, a greenhouse is provided by water and COz with the potential for runaway mediated by the more rapid removal of both water and COz in surface weathering reactions and in photolysis. The bulk of the photolytic oxygen is used for early oxidation of weathered iron and sulfur rather than primarily to oxidize atmospheric components such as CH, and NH, (e.g., Hart, 1978). With little land area exposed above sea level, the dominant geochemical reactions in all instant ocean models would necessarily have been the submarine weathering of mafic silicates. The rates should have been low because submarine reactions are limited by the rate at which fresh rock is exposed at the ocean bottom. Higher rates of silicate weathering remove COz more rapidly where warm temperatures combine

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with high runoff (Holland, 1978; Walker et al., 1981).Thus, the conditions expected with a moist greenhouse-delayed ocean model would provide greater potential for runoff at warmer temperatures. Exposed land areas (‘oceanic’ crust) would have been the sites of very active silicate weathering to provide the abundant clay minerals considered useful for hydration-dehydration experiments in prebiotic organic polymerization reactions. Although prebiotic reactions at hydrothermal vents (Corliss et al., 1981) are not precluded by either model, it should be noted that for maintaining continuity in prebiotic polymerization chemistry, the addition of numerous dehydration sites to submarine hot spring sources gives the delayed ocean model an advantage. Alone, hot springs can produce one cycle polymerizations, but it seems difficult t o maintain continuity for evolutionary development of initial polypeptides in a flowing gradient where the products formed at one temperature are moved to a lower temperature locale (impeding further peptide bond formation). In comparing models, the hydrogen isotope data are important because they indicate differences between the deuterium composition of mantle water and mean ocean water over geologic time. The argument is as follows: ocean water is isotopically enriched in deuterium (heavier) with respect to mantle water. This implies a partitioning of juvenile (original) water at some time in the distant past. The deuterium composition of juvenile water was fixed by its chondritic origin at planetary formation (Anders and Owen, 1977). Therefore, either ocean water became heavier or mantle water became lighter with respect to original water. As there is no plausible mechanism for the preferential loss of deuterium or enrichment of protium in mantle rocks, it is the oceans that seem t o have changed. The most likely mechanism for this fractionation is the preferential gravitational escape of protium following the photodissociation of water vapor in the upper atmosphere. Very limited geological evidence exists to support this (Fig. 10, Knauth and Epstein, 1976). In the moist atmosphere-delayed ocean model advocated here, the mantle waterocean water hydrogen isotope reservoirs would have diverged early on and much of the oxygen produced by the photodissociation was used in the weathering of the early mafic crustal materials exposed on land. In other models, the process of photodissociation is considered to be very limited, and what oxygen does form is rapidly recombined with hydrogen to keep the atmosphere anoxic. Little hydrogen escapes; isotopic partitioning between ocean and mantle waters is limited. Mantle buffering models (e.g., Veizer et al., 1982) require that massive amounts of ocean water be pumped back into the basaltic crust. This would rapidly mix the two reservoirs and thereby also inhibit hydrogen isotopic partitioning between them. On the other hand, the 87Sr/86Sranomaly discussed by Veizer et al. (1982) is equally consistent with a delayed ocean model that specifically allows for the ‘continental’ weathering of mafic ‘oceanic’ crust, thereby giving the strontium ratios in Archean carbonates their mantle-like appearance.

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Finally, with respect to the Superior-type banded iron formations, two comparisons are worth mentioning: (1)BIF oxide facies are well-known to be remarkably free of organic carbon. Anoxic atmosphere models require that hydrospheric oxygen produced locally by photosynthesis in the basin waters be used t o oxidize iron and, thus, unavailable for oxidation of the algal carbon (Van Valen, 1971, Towe, 1978), which must be accounted for somewhere. For example, in the Hamersley Basin BIF, 6000 billion tons (6 x 10l8 gm) of photosynthetic carbon should have been produced t o account for the 100,000 billion tons (1020 gm) of iron estimated t o exist there. The idea (Cloud, 1973) that the magnetite in oxide facies could be diagenetically derived from the reduction of six moles of hematite for every mole of organic carbon is acceptable. However, if the algal communities living in these quiet waters died back following episodes of photosynthetic precipitation of hydrous iron oxides, then most of their organic remains should appear in the iron-poor cherty bands alternating with the iron-rich bands. If so, the organic carbon was not in immediate proximity t o the bulk of the hematite it was supposed t o have reduced. The use of atmospheric oxygen (from any source) is more readily reconcilable with oxide facies that are so deficient in organic carbon and yet considered deposited in very quiet waters where most of the dead phytoplankton would be expected to have accumulated. (2) Superior-type BIF are generally clastic-free precipitates notably depleted in aluminum and titanium (Lepp, 1966). A submarine volcanic source of iron and silica would allow for mechanical segregation of clastics, but would make geochemical segregation difficult. An indirect weathering source for iron and silica t o deepening ocean basins allows for both. Thus, clasticfree upwelling water, reduced in its aluminum and trace element content, permits the precipitation of hydrous iron oxides on contact with surface atmospheric oxygen in shallow waters, while it also permits the formation of pure Fe-silicates rather than aluminous clay minerals (Govett, 1966; Klein and Bricker, 1977) in the oxygen-poor offshore silicate facies. Shoreward continental platform carbonates need not be iron-rich (Larue, 1981). Stromatolites, if they were formed by oxygenic photosynthesizers, need not be draped with iron oxides, and coeval red beds are permitted, not prohibited. Reduced facies, as they are today, can be expected. ACKNOWLEDGMENT

I thank R.F. Fudali and anonymous referees for reading early drafts of the manuscript and offering useful suggestions, and I thank numerous colleagues for providing references and valuable discussion. REFERENCES Anders, E. and Owen, T., 1977. Mars and Earth: origin and abundance of volatiles. Science, 198: 453-465.

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