Earth'S Oldest Reported Glaciation: Physical and Chemical Evidence From the Archean Mozaan Group (∼2.9 Ga) of South Africa

Distributions of sulfur isotopes in geological samples would provide a record of atmospheric composition if the mechanism producing the isotope effects could be described quantitatively. We determined the UV absorption spectra of 32 SO2, 33 SO2, and 34 SO2 and use them to interpret the geological record. The calculated isotopic fractionation factors for SO2 photolysis give mass independent distributions that are highly sensitive to the atmospheric concentrations of O 2, O3, CO2, H2O, CS2, NH3, N2O, H2S, OCS, and SO 2 itself. Various UV-shielding scenarios are considered and we conclude that the negative ⌬ 33 S observed in the Archean sulfate deposits can only be explained by OCS shielding. Of relevant Archean gases, OCS has the unique ability to prevent SO 2 photolysis by sunlight at >202 nm. Scenarios run using a photochemical box model show that ppm levels of OCS will accumulate in a CO-rich, reducing Archean atmosphere. The radiative forcing, due to this level of OCS, is able to resolve the faint young sun paradox. Further, the decline of atmospheric OCS may have caused the late Archean glaciation. mass independent fractionation ͉ carbonyl sulfide (OCS) ͉ sulfur dioxide (SO2) ͉ photochemistry ͉ greenhouse gases M ass independent fractionation (MIF) of sulfur isotopes has been found in geological samples older than 2.3 billion years (Ga) and is believed to be produced by the photochemical reactions of gaseous sulfur compounds in a low-oxygen reducing atmosphere (1-4). The rise of atmospheric oxygen (O 2 and O 3 ) would have oxidized atmospheric sulfur compounds to sulfate, preventing the accumulation of isotopic fractionation in two reservoirs. Moreover, it would have blocked UV sunlight inhibiting the supposed photochemical process(es) producing the isotope anomaly. In contrast, given reducing conditions, two isotopically distinct sulfur species such as elemental sulfur (polysulfur) and sulfate could be formed photochemically and precipitate at the surface, where the MIF signal could be preserved in sediment (3-5). Sulfur MIF in geological samples is regarded as indicating an anoxic atmosphere.This assertion relies on assumptions as to the origin of the MIF, although the mechanisms are still poorly known. Broad agreement in the ⌬ 36 S/⌬ 33 S ratio (ϷϪ1) in pre-2.3 Ga sedimentary rocks (1, 6, 7) and those produced in the laboratory by SO 2 photolysis suggest that photodissociation of gas-phase SO 2 most likely accounts for MIF in the Archean (2). Indeed, SO 2 photolysis is a key rate limiting step, particularly in a low-O 2 reducing atmosphere (4, 8). In addition, two mechanisms have been proposed whereby sulfur MIF could be formed under oxidizing conditions in the modern atmosphere: (i) Photolysis of SO 3 at altitudes greater than Ϸ50 km, where reaction between SO 3 and H 2 O is slow, is claimed to be responsible for the MIF found in modern stratospheric aerosol (8). In contrast, at lower altitudes, SO from SO 2 photolysis reforms SO 2 , so fractionation in SO 2 photolysis would not affect the isotopic compos...

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“…30). It has been known that the magnitude of the MIF decreased at approximately this time, followed by an increase (Fig.…”

Section: Effect Of Atmospheric Uv Shielding On Isotopic Fractionationmentioning

confidence: 75%

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

Ueno

Johnson

Danielache

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

138

124

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“…2.9 Ga Pongola Supergroup in southern Africa, with diamictites containing faceted and striated clasts and stratified siltstones with dropstones (Young et al, 1998;Crowell, 1999).…”

Section: Archaean and Palaeoproterozoic Glaciationsmentioning

confidence: 99%

Secular changes in sedimentation systems and sequence stratigraphy

et al. 2013

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________________________________________________________________________The ephemeral nature of most sedimentation processes and the fragmentary character of the sedimentary record are of first-order importance. Despite a basic uniformity of external controls on sedimentation resulting in markedly similar lithologies, facies, facies associations and depositional elements within the rock record across time, there are a number of secular changes, particularly in rates and intensities of processes that resulted in contrasts between preserved Precambrian and Phanerozoic successions. Secular change encompassed (1) variations in mantle heat, rates of plate drift and of continental crustal growth, the gravitational effects of the Moon, and in rates of weathering, erosion, transport, deposition and diagenesis;(2) a decreasing planetary rotation rate over time; (3) no vegetation in the Precambrian, but prolific microbial mats, with the opposite pertaining to the Phanerozoic; (4) the long-term evolution of the hydrosphere-atmosphere-biosphere system. A relatively abrupt and sharp turning point was reached in the Neoarchaean, with spikes in mantle plume flux and tectonothermal activity and possibly concomitant onset of the supercontinent cycle. Substantial and irreversible change occurred subsequently in the Palaeoproterozoic, whereby the dramatic change from reducing to oxidising volcanic gases ushered in change to an oxic environment, to be followed at ca. 2.4-2.3 Ga by the "Great Oxidation Event" (GOE); rise in atmospheric oxygen was accompanied by expansion of oxygenic photosynthesis in the cyanobacteria. A possible global tectono-thermal "slowdown" from ca. 2.45-2.2 Ga may have separated a preceding plate regime which interacted with a higher energy mantle from a ca. 2.2-2.0 Ga Phanerozoicstyle plate tectonic regime; the "slowdown" period also encompassed the first known global-scale glaciation and overlapped with the GOE. While large palaeodeserts emerged from ca. 2.0 -1.8 Ga, possibly associated with the evolution of the supercontinent cycle, widespread euxinia by ca. 1.85 Ga

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“…Of these, the best-known and best-preserved belong to the Huronian Supergroup of Canada and the Transvaal Supergroup of southern Africa. Although three discrete episodes of glaciation have been recorded in several widely spaced sedimentary successions in North America, there are very few carbonates in this interval (Young et al, 1998;Bekker et al, 2005).…”

Section: Carbonate Rocks Global Palaeoclimate Change and Glaciationmentioning

confidence: 99%

Sedimentation patterns during the Precambrian: A unique record?

Bose

Eriksson

Sarkar

et al. 2012

Marine and Petroleum Geology

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Although the similarities between depositional processes and products as well as the analogous controls on basinfilling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record, a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the Precambrian epoch. This paper searches for distinctiveness in the Precambrian sedimentary record, both siliciclastic and carbonate, through an extensive, though not exhaustive, review of the relevant literature augmented by new observations. While differences in Precambrian deltaic, aeolian, glacial and possibly also lacustrine deposits and settings appear to have been small, their large-scale development was controlled largely by a combination of temporal and geodynamic influences, essentially of global compass. In this regard the onset of the supercontinent cycle and major perturbations in palaeo-atmospheric composition appear to have been significant. Marine environments provide a 2 poor platform for Precambrian -Phanerozoic comparisons of sedimentation patterns, as those from the former period are preserved almost exclusively in epeiric settings, an environment essentially lacking on modern Earth. For the shallow marine carbonates, biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the Precambrian to a combination of metazons, protozoans and algae for the skeletal carbonates of the Phanerozoic. Despite it being widely recognized that Precambrian channel systems were braided in all environments (deltaic, tidal, alluvial, fluvial) as a consequence of the lack of vegetation and poor development of soils, the fluvial setting has some enigmatic aspects. Amongst these is evidence for ponding of muddy detritus in apparently sandstone bed-load dominated braided systems, with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for Precambrian fluvial systems. This is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the Phanerozoic.

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Earth'S Oldest Reported Glaciation: Physical and Chemical Evidence From the Archean Mozaan Group (∼2.9 Ga) of South Africa

Cited by 162 publications

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Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

Secular changes in sedimentation systems and sequence stratigraphy

Sedimentation patterns during the Precambrian: A unique record?

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