Increase In Atmospheric Oxygen Research Articles

At the dawn of the aerobic biosphere: The effect of oxygen on the development of biota in the Proterozoic and Early Paleozoic

The distinctive features of biota at successive stages of its development in the Proterozoic and Early Paleozoic are analyzed. Based on this analysis the minimum possible and most probable oxygen contents of the atmosphere at each of these stages are determined. It is shown that Proterozoic organisms with the higher level of organization than the background ones first appeared in oxygen oases and that later, when the same mean oxygen content was achieved across the entire water area because of an increase in atmospheric oxygen, these organisms spread widely with an explosive increase in morphological and taxonomic diversity.

Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales

The atmosphere–ocean system experienced a progressive change from anoxic to more oxidizing conditions through time. This oxidation is traditionally envisaged to have occurred as two stepwise increases in atmospheric oxygen at the beginning and end of the Proterozoic Eon. Here, we present a study of the redox-sensitive element, uranium, in organic-rich shales to track the history of Earth's surface oxidation at an unprecedented temporal resolution. Fluctuations in the degree of uranium enrichment in organic-rich shales suggest that the initial rise of atmospheric oxygen ~2.4billionyr ago was followed by a decline to less oxidizing conditions during the mid-Proterozoic. This redox state persisted for almost 1billionyr, ending with a second oxygenation event in the latest Neoproterozoic. The U record tracks major fluctuations in surface oxygen level and challenges conventional models that suggest the Earth underwent a unidirectional rise in atmospheric oxygen during the Precambrian.

Mo isotopic composition of the mid-Neoproterozoic ocean: An iron formation perspective

The Neoproterozoic was a major turning point in Earth's surficial history, recording several widespread glaciations, the first appearance of complex metazoan life, and a major increase in atmospheric oxygen. Marine redox proxies have resulted in many different estimates of both the timing and magnitude of the increase in free oxygen, although the consensus has been that it occurred following the Marinoan glaciation, the second globally recorded “snowball Earth” event. A critically understudied rock type of the Neoproterozoic is iron formation associated with the Sturtian (first) glaciation. Samples from the <716Ma Rapitan iron formation were analysed for their Re concentrations and Mo isotopic composition to refine the redox history of its depositional basin. Rhenium concentrations and Re/Mo ratios are consistently low throughout the bottom and middle of the iron formation, reflecting ferruginous to oxic basinal conditions, but samples from the uppermost jasper layers of the iron formation show significantly higher Re concentrations and Re/Mo ratios, indicating that iron formation deposition was terminated by a shift towards a sulfidic water column. Similarly, the δ98Mo values are close to 0.0‰ throughout most of the iron formation, but rise to ~+0.7‰ near the top of the section. The δ98Mo from samples of ferruginous to oxic basinal conditions are the product of adsorption to hematite, indicating that the Neoproterozoic open ocean may have had a δ98Mo of ~1.8‰. Together with the now well-established lack of a positive Eu anomaly in Neoproterozoic iron formations, these results suggest that the ocean was predominantly oxygenated at 700Ma.

An indirect evidence of piezonuclear fission reactions: Geomechanical and geochemical evolution in the Earth’s crust

Piezonuclear reactions, which occur in inert and nonradioactive elements, are induced by high pressure and, in particular, by brittle fracture phenomena in solids under compression. These low energy reactions generally take place in nuclei with an atomic weight that is lower or equal to that of iron (Fe). The experimental evidence, obtained from repeatable measurements of neutron emissions, can be also recognized considering the anomalous chemical balances of the major events that have affected the Earth’s crust, oceans and atmosphere, over the last four billion years. These anomalies include: (i) the abrupt variations in the most abundant elements in correspondence to the formation of tectonic plates; (ii) the Great Oxidation Event (2.7 to 2.4 billion years ago), with a sharp increase in atmospheric oxygen and the subsequent origin of life; (iii) the increase of carbon and nytrogen concentrations in the primordial atmosphere.

Putative Cryogenian ciliates from Mongolia

Research Article| December 01, 2011 Putative Cryogenian ciliates from Mongolia T. Bosak; T. Bosak 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar F. Macdonald; F. Macdonald 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar D. Lahr; D. Lahr 3Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar E. Matys E. Matys 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Geology (2011) 39 (12): 1123–1126. https://doi.org/10.1130/G32384.1 Article history received: 25 Apr 2011 rev-recd: 05 Jul 2011 accepted: 07 Jul 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. Bosak, F. Macdonald, D. Lahr, E. Matys; Putative Cryogenian ciliates from Mongolia. Geology 2011;; 39 (12): 1123–1126. doi: https://doi.org/10.1130/G32384.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Major lineages of modern eukaryotes, represented primarily by microscopic taxa, are thought to have originated during the Neoproterozoic, but microfossils older than 635 Ma rarely have unambiguous relationships to modern microscopic eukaryotes. Here we report exceptionally preserved 715–635 m.y. old eukaryotic tests in limestone strata of Mongolia. The ∼100-μm-long organic-rich three-dimensional tests have flask-like shapes, constricted necks, distinct and often thickened collars, and flexible walls composed of densely packed alveolar structures. The combined morphological and ultrastructural characters of these Cryogenian tests are remarkably similar to the tests of tintinnids, modern planktonic ciliates. Eukaryotes forming recalcitrant organic or mineral-rich tests before 635 Ma may have increased export and burial fraction of organic carbon, driving an increase in atmospheric oxygen and the subsequent radiation of metazoans. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Nature of and controls on volcanism in the ca. 2.32–2.06 Ga Pretoria Group, Transvaal Supergroup, Kaapvaal Craton, South Africa

The time of deposition of the Pretoria Group between 2.32 and 2.06Ga on South Africa's Kaapvaal Craton was characterized by the first major increase in atmospheric oxygen. It was accompanied by the extrusion of significant thicknesses of volcanic deposits, namely the Bushy Bend lavas of the Timeball Hill Formation, the Hekpoort Formation and the Machadodorp Volcanic Member of the Silverton Formation, marking the three major volcanic events within the Transvaal Supergroup which are thought to be precursors to the succeeding Bushveld Complex magmatism. The Bushy Bend Lava Member of the Timeball Hill Formation is characterized by subaqueous basaltic–andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the Transvaal Supergroup basin. The subaerial volcanism of the Hekpoort Formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material. Intermittent hiatuses in volcanism were marked by local lacustrine shale deposition. Finally, the Machadodorp Lava Member can be interpreted as the deposits of several seamounts aligned along a fissure, probably within an extensional environment.The geodynamic control inferred for the evolution of the Pretoria Group basin encompasses two cycles of prerift uplift, subsequent mechanical rifting and long lived thermal subsidence. The limited extent of the Bushy Bend lavas in the south of the Pretoria Group depository attests to the likelihood that volcanism accompanying the first rifting event was short-lived. In contrast, the second rifting cycle postulated for the Pretoria Group basin has a strong association with widespread and large scale volcanism of the Hekpoort–Ongeluk flood basalts, which may have been plume-related. The scale of the second cycle volcanism and its importance in influencing the upper part of the Pretoria Group basin-fill is underlined by the Machadodorp volcanism, probably related to hot spot volcanism as the Ongeluk–Hekpoort plume waned.

Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics

The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe 2+ oxidation kinetics to the Fe 2+ and Fe 3+ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen ( P O 2 ) between 2.5 and 2.0 Ga. We first estimated the compaction factor ( CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions ( M R ), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe R increased gradually within the time window of ∼2.5–2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn R also increased gradually similar to Fe R but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe 2+ and Mn 2+ in paleosols ranging in age between ∼2.5 and 1.9 Ga. We have modeled P O 2 variations during the Paleoproterozoic by applying Fe 2+ oxidation kinetics to the Fe 2+ and Fe 3+ concentrations in paleosols, which enabled us to derive an Fe 2+ oxidation term referred to as ψ. Possible changes in temperature and P CO 2 during this time window and their effects on resulting models of P O 2 evolution have been also considered. We assumed four cases for the calculations of P O 2 variations between 2.5 and 2.0 Ga: no change in either temperature or P CO 2 , long-term change in only P CO 2 , long-term changes in both temperature and P CO 2 , and short-term fluctuations of both temperature and P CO 2 during the possible, multiple global-scale glaciations. The calculations indicate that P O 2 increased gradually, linearly on the logarithmic scale, from <∼10 −6 to >∼10 −3 atm between 2.5 and 2.0 Ga. Our calculations show that the P O 2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.

Earth's Second Wind

Studies of molybdenum isotopes in sediments support the idea that increases in atmospheric oxygen in the Devonian period drove increases in animal size.

The evolution of the marine phosphate reservoir

Phosphorus is a biolimiting nutrient that has an important role in regulating the burial of organic matter and the redox state of the ocean-atmosphere system. The ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks can be used to track dissolved phosphate concentrations if the dissolved silica concentration of sea water is estimated. Here we present iron and phosphorus concentration ratios from distal hydrothermal sediments and iron formations through time to study the evolution of the marine phosphate reservoir. The data suggest that phosphate concentrations have been relatively constant over the Phanerozoic eon, the past 542 million years (Myr) of Earth's history. In contrast, phosphate concentrations seem to have been elevated in Precambrian oceans. Specifically, there is a peak in phosphorus-to-iron ratios in Neoproterozoic iron formations dating from ∼750 to ∼635 Myr ago, indicating unusually high dissolved phosphate concentrations in the aftermath of widespread, low-latitude 'snowball Earth' glaciations. An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere. The snowball Earth glaciations and Neoproterozoic oxidation are both suggested as triggers for the evolution and radiation of metazoans. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.

Geomechanical and Geochemical Evidence of Piezonuclear Fission Reactions in the Earth’s Crust

Abstract: Piezonuclear reactions, which occur in inert and non‐radioactive elements, are induced by high pressure and, in particular, by brittle fracture phenomena in solids under compression. These low‐energy reactions generally take place in nuclei with an atomic weight that is lower or equal to that of iron (Fe). The experimental evidence, obtained from repeatable measurements of neutron emissions [Strain 45, 2009, 332; Strain (in press); Phys. Lett. A. 373, 2009, 4158], can be also recognised considering the anomalous chemical balances of the major events that have affected the Earth’s crust, oceans and atmosphere, over the last 4 billion years. These anomalies include (i) abrupt variations in the most abundant elements in correspondence with the formation of tectonic plates; (ii) the ‘Great Oxidation Event’ (2.7–2.4 billion years ago), with a sharp increase in atmospheric oxygen and the subsequent origin of life; (iii) the current climate acceleration partially because of ‘carbon pollution’. Natural piezonuclear reactions are induced by fault sliding and plate subduction phenomena.

Anomalous negative excursion of carbon isotope in organic carbon after the last Paleoproterozoic glaciation in North America

Early Paleoproterozoic time (2.5–2.0 Ga) spanned a critical phase in Earth's history, characterized by repeated glaciations and an increase in atmospheric oxygen (the Great Oxidation Event (GOE)). Following the last and most intense glaciation of this period, marine carbonates record a large positive excursion of δ13C value (termed the “Lomagundi event”) between about 2.2 and 2.1 Ga coinciding with the global appearances of red beds and sulfates, which suggest an accumulation of high levels of atmospheric oxygen. Here we report the discovery of large negative excursions of δ13C in organic matter (down to −55‰) from quartzose sandstones (of the Marquette Range and the Huronian Supergroups, North America) intermediate in age between the last Paleoproterozoic glaciation and the possible onset of the Lomagundi event. The negative excursion is concomitant with the appearance of intensely weathered quartzose sandstones, which may represent hot and humid conditions. There are some interpretations that potentially explain the negative excursions: (1) redeposition of older 13C‐depleted kerogen, (2) later post‐depositional infiltration of oil, (3) active methane productions by methanogens in shallow‐marine environments, or (4) dissociation of methane hydrate. If the latter two were the case, they would provide clues for understanding the environmental change connecting the intense glaciation and an increase in oxygen.

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1 M and 1 M d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2 M 1 illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian–Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes

Geochemical data suggest that oxygenation of the Earth's atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between approximately 2.45 and 2.2 Gyr ago, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era ( approximately 800-542 Myr ago), ultimately leading to oxygenation of the deep ocean approximately 580 Myr ago, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth's atmosphere-hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters approximately 2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45-2.2 Gyr ago (the Great Oxidation Event). In approximately 1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (delta(53)Cr up to +4.9 per thousand), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life.

Formation of supercontinents linked to increases in atmospheric oxygen

Atmospheric oxygen concentrations in the Earth’s atmosphere rose from negligible levels in the Archaean Era to about 21% in the present day. This increase is thought to have occurred in six steps, 2.65, 2.45, 1.8, 0.6, 0.3 and 0.04 billion years ago, with a possible seventh event identified at 1.2 billion years ago. Here we show that the timing of these steps correlates with the amalgamation of Earth’s land masses into supercontinents. We suggest that the continent–continent collisions required to form supercontinents produced supermountains. In our scenario, these supermountains eroded quickly and released large amounts of nutrients such as iron and phosphorus into the oceans, leading to an explosion of algae and cyanobacteria, and thus a marked increase in photosynthesis, and the photosynthetic production of O2. Enhanced sedimentation during these periods promoted the burial of a high fraction of organic carbon and pyrite, thus preventing their reaction with free oxygen, and leading to sustained increases in atmospheric oxygen. Atmospheric oxygen levels on Earth rose in at least six distinct steps and an examination of the timing of the steps suggests that they coincided with the formation of supercontinents and supermountains. This leads to the hypothesis that increased erosion of these supermountains released large amounts of nutrients to the oceans, stimulating productivity and the release of oxygen to the atmosphere. The subsequent burial of organic carbon along with the mountain sediments would have sustained the increased oxygen levels.

Modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans

The increase in atmospheric oxygen ca. 2.4 Ga had a significant impact on the geochemical cycling of Fe. The history of environmental oxygenation may be recorded in the Fe isotope composition of Archean and Proterozoic sediments, but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations. Here we present Fe isotope data and iron/aluminum ratios from the Black Sea oxic shelf and euxinic basin. The isotope data demonstrate that shelf Fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin. We propose that there is net transport of isotopically light Fe from sediments of the shelf to those of the distal, anoxic basin, consistent with enrichments in reactive Fe seen in the deep basin. The low δ 56 Fe benthic Fe flux is generated during the coupling of microbial Fe(III) reduction or sulfidization with Fe 2+ aq oxidation. Low δ 56 Fe values reported previously from Late Archean sedimentary pyrites may be an isotopic fingerprint of analogous Fe redox cycling in the Late Archean oceans. This interpretation implies shallow-water Fe redox recycling in the Late Archean. We predict that the light isotopic compositions of the Late Archean will prove to be distinct from those of the Early Archean, before Fe redox cycling became an important process, and we infer that this difference may be related to the presence of oxygen in the surface ocean.

Oxidation of the Ediacaran Ocean

Oxygenation of the Earth's surface is increasingly thought to have occurred in two steps. The first step, which occurred approximately 2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period ( approximately 800-542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates, and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (approximately 635 to approximately 548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth's history.

Neoproterozoic–Early Cambrian isotopic variation and chemostratigraphy of the Lesser Himalaya, India, Eastern Gondwana

The Rodinia supercontinent had fragmented by 750 Ma and East Gondwana (India, Australia and Antarctica) separated from West Laurentia. Baltica, Africa and South America occupied other side of the Rodinia. Neoproterozoic rifting, breakup of Rodinia low latitude glaciation and global warming events have been recorded from the Lesser Himalaya of India. Chemostratigraphy of the Blaini–Krol–Tal succession indicates Precambrian–Cambrian transition lies in the Lower Tal Formation ( δ 13C = − 4‰ PDB). The Krol belt in the Lesser Himalaya is characterized by positive δ 13C values (+ 1 to 6‰ PDB). The appearance of multicelluler Ediacaran life in the Upper Krol is consistent with hypothesis that increase in atmospheric oxygen played a major role in metazoan evolution and Cambrian diversification of life on Earth. The base of the Terminal Proterozoic System in the Lesser Himalaya is established in the Blaini Formation. The pink cap carbonate of the Blaini Formation shows negative δ 13C value (− 3‰ PDB) and this Blainian glaciation is correlated with Marinoan glacial event other regions of the Eastern Gondwana and South China, parts of Siberia and North Africa. Neoproterozic–Early Cambrian chemostratigraphy in the Lesser Himalaya is consistent in the isotopic variation where there is carbon isotopic similarity between Neoproterozoic Bambui Group in Central Brazil, South America and the Krol Formation of the Lesser Himalaya, India. In the northeastern Lesser Himalaya, India the Neoproterozoic sedimentary succession shows well developed carbonate the Buxa Dolomite shows significantly positive C-isotope ratios ( δ 13C = + 3.7 to + 5.4‰ PDB). The O-isotopic data also shows remarkable consistency with the δ 18O values fluctuating within a narrow range between − 8.9 and − 7.2‰ VPDB. These isotopic results from the Eastern Lesser Himalaya correspond to the Terminal Proterozoic C-isotopic evolution, followed by oscillations during the Precambrian–Cambrian transition in the Lesser Himalaya in Eastern Gondwana.

Fungi, Weathering, and the Emergence of Animals

A billion years separated the advent of one-celled eukaryotes from that of multicellular animals. This rise of the animals may have been triggered by increases in atmospheric oxygen resulting from mineral weathering accelerated by ancient soil biota.

Active Microbial Sulfur Disproportionation in the Mesoproterozoic

The environmental expression of sulfur compound disproportionation has been placed between 640 and 1050 million years ago (Ma) and linked to increases in atmospheric oxygen. These arguments have their basis in temporal changes in the magnitude of 34S/32S fractionations between sulfate and sulfide. Here, we present a Proterozoic seawater sulfate isotope record that includes the less abundant sulfur isotope 33S. These measurements imply that sulfur compound disproportionation was an active part of the sulfur cycle by 1300 Ma and that progressive Earth surface oxygenation may have characterized the Mesoproterozoic.

Evidence for Paleoproterozoic cap carbonates in North America

The early Paleoproterozoic Snowy Pass Supergroup of the Medicine Bow Mountains and Sierra Madre, Wyoming, USA and Huronian Supergroup, Ontario, Canada were deposited along the present-day southern flank of the Wyoming and Superior cratons. Whereas three discrete levels of glacial diamictite are developed in both successions, carbonate strata are known only directly above the middle diamictite (Vagner and Bottle Creek formations in Medicine Bow Mountains and Sierra Madre, respectively, and Espanola Formation in southern Ontario) in these thick correlative siliciclastics-dominated strata. The carbonates from each succession record negative δ 13C values (−4.0 to −0.8‰, V-PDB) and attenuated carbon isotopic difference between organic and inorganic phases. Oxygen in carbonates is strongly depleted in 18O suggesting exchange with hot fluids, which is consistent with pervasive recrystallization of carbonates and remobilization of elements. However, the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between δ 13C and either δ 18O values or trace element concentrations supports a primary origin for 13C-depleted carbonates, which are interpreted here to reflect anomalous oceanic compositions. The intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post-glacial transgression. Although these early Paleoproterozoic carbonates are similar to Neoproterozoic ‘cap dolomites’ in stratigraphic position and carbon isotopic compositions, the older post-glacial accumulations begin with limestone and lack many of the sedimentary structures typical of Neoproterozoic deposits. Furthermore, it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above Neoproterozoic diamictites. The differences might reflect lower overall carbonate saturation in early Paleoproterozoic oceans which contrasts sharply with Archean and later Paleoproterozoic intervals and higher siliciclastic inputs in rift environments, which shut down carbonate deposition. Geological and geochemical indicators suggest a stepwise increase in atmospheric oxygen across the Paleoproterozoic glacial epoch. The tempo and mode of atmospheric oxygen rise has significant consequences for the abundance of the important greenhouse gases CH 4 and CO 2 and hence for oceanic acidity. If we accept that atmospheric oxidation of methane to carbon dioxide resulted in each of the three discrete glaciations, it implies that atmospheric CH 4 remained high throughout the interval and that pulsed oxidation events, plausibly linked to higher primary productivity and lower hydrothermal activity, led to surface refrigeration. If correct, the unique presence of cap carbonate above the middle Paleoproterozoic diamictite may reflect an appropriate balance of CO 2 and CH 4 sufficient to provide enough alkalinity to seawater through silicate weathering, but not so high that carbonate preservation would be inhibited by enhanced acidity.