Pondering the Precambrian #42

Proterozoic:

NeoProterozoic:

Changes to the carbon cycle and environment  were detected from the Ediacaran/Cambrian boundary in China.

Using boron to detect the salinity of the paleooceans gives some interesting twists.

Evolution of multiple basins from the Neoproterozoic are described.

Is there a biochemical signal dividing metazoan clades into two?

Did animal-like embryos predate actual metazoans (animals).

How NOT to become a metazoan.

A Cryogenian interglacial deposit has been found in China.

When did the Sturtian glaciation begin during the Cryogenian in South China?

Mesoproterozoic:

Where was South China in the Nuna/Columbia and Rodinia supercontinents during the Mesoproterozoic?

The Stenian Copper Harbor and Nonesuch Formations have a marine origin.

PaleoProterozoic:

Evidence of explosive basalt volcanism found from the paleoproterozoic.

In French Guiana, a gold rich Rhyacian Period formation has been found.  It contains sedimentary layers within the volcanic ones.  Are they fossiliferous?

After the Lomagundi-Jatuli Event, the carbon-sulfur cycle seemed to return to strong activity in the aftermath in the Rhyacian/Orosirian Periods.

Archean:

It appears during the GOE, the ocean basins remained anoxic.

Earth's magnetic field may have been in place during the Archean.

Evidence of subduction from the Late Archean comes from Australia.

The Kaapvaal Craton may be a remnant of the Pilbara Craton that split off in the MesoArchean.

Extraterrestrial impacts might have triggered bursts in plate tectonics.

Ancient microbes played an important role in warming the early Earth.

Hadean:

The impact history of Earth should NOT be correlated to other planets in the solar system.

Origin of Life:

There is a hyperacidic, hypersaline geothermal system in Dallol, Ethiopia that is lifeless and that has implications for the origin of life.

DNA is only one of several possible genetic information molecules.

Lifelike chemistry has been created in the lab.

Ribose and other sugars have been found in meteorites.

Some of the earliest animal fossils are possibly chemical 'gardens' that produce pseudofossils.

An Alternate Explanation for Jasper Formation Prior to the Great Oxidation Event

Dust to dust: Evidence for the formation of “primary” hematite dust in banded iron formations via oxidation of iron silicate nanoparticles

Authors:

Rasmussen et al

Abstract:

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63-2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from grey-green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between grey-green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment-water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica.

Tracking Copper Through the Great Oxidation Event

Cu isotopes in marine black shales record the Great Oxidation Event

Authors:

Fru et al

Abstract:

The oxygenation of the atmosphere ∼2.45–2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth’s redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ65CuERM-AE633) in organic carbon-rich shales spanning the period 2.66–2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in 65Cu, along with the preferential removal of 65Cu by iron oxides, left seawater and marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ65Cu values coincides with a shift to negative sedimentary δ56Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ65Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Great Oxygenation Event had a Precedessor 100 Million Years Earlier

Earth scientists from the University of Alberta, University of Waterloo, Arizona State University, University of California Riverside, and Georgia Institute of Technology have found evidence that Earth's transition to a permanently oxygenated atmosphere was anything but smooth.

Their paper, published this month in Science Advances, uses geochemical data from sedimentary rocks in Western Australia to show that a burst of O2 production by photosynthetic cyanobacteria temporarily increased O2 concentrations in Earth's atmosphere and shallow oceans roughly 2.5 billion years ago.

"We are tracking atmospheric changes through time to understand how oxygen increased to the level needed to support complex life," says Rob Creaser, professor of earth and atmospheric sciences at the University of Alberta. "When the Earth first formed, there was no oxygen in the atmosphere. Our analytical facilities here at the U of A allowed us to conduct precise analyses of this rock sample to understand the tempo at which that oxygen built up through photosynthesis.

Creaser's lab at the University of Alberta in the Canadian Centre for Isotopic Microanalysis is one of only a few in the world with the ability to take the precise measurements of osmium needed to conduct this type of analysis. "Without this type of facility, we wouldn't be able to write this paper or investigate this process."

The new data suggest that O2 levels in the Earth's atmosphere fluctuated until enough O2 finally accumulated to create a permanently oxygenated atmosphere around 2.4 billion years ago, a transition widely known as the "Great Oxidation Event." "The onset of Earth's surface oxygenation may have been a complex process characterized by multiple 'whiffs' of O2 until a tipping point was crossed," says Creaser's former PhD student and UAlberta alumnus Brian Kendall, a professor of Earth and Environmental Sciences at the University of Waterloo and lead author on the paper.

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2nd link.

Did Multicellularity in Cyanobacteria Help Drive the Great Oxidation Event?

Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils

Authors:

Schirrmeister et al

Abstract:

Cyanobacteria are among the most ancient of evolutionary lineages, oxygenic photosynthesizers that may have originated before 3.0 Ga, as evidenced by free oxygen levels. Throughout the Precambrian, cyanobacteria were one of the most important drivers of biological innovations, strongly impacting early Earth's environments. At the end of the Archean Eon, they were responsible for the rapid oxygenation of Earth's atmosphere during an episode referred to as the Great Oxidation Event (GOE). However, little is known about the origin and diversity of early cyanobacterial taxa, due to: (1) the scarceness of Precambrian fossil deposits; (2) limited characteristics for the identification of taxa; and (3) the poor preservation of ancient microfossils. Previous studies based on 16S rRNA have suggested that the origin of multicellularity within cyanobacteria might have been associated with the GOE. However, single-gene analyses have limitations, particularly for deep branches. We reconstructed the evolutionary history of cyanobacteria using genome scale data and re-evaluated the Precambrian fossil record to get more precise calibrations for a relaxed clock analysis. For the phylogenomic reconstructions, we identified 756 conserved gene sequences in 65 cyanobacterial taxa, of which eight genomes have been sequenced in this study. Character state reconstructions based on maximum likelihood and Bayesian phylogenetic inference confirm previous findings, of an ancient multicellular cyanobacterial lineage ancestral to the majority of modern cyanobacteria. Relaxed clock analyses provide firm support for an origin of cyanobacteria in the Archean and a transition to multicellularity before the GOE. It is likely that multicellularity had a greater impact on cyanobacterial fitness and thus abundance, than previously assumed. Multicellularity, as a major evolutionary innovation, forming a novel unit for selection to act upon, may have served to overcome evolutionary constraints and enabled diversification of the variety of morphotypes seen in cyanobacteria today.

Cyanobacteria: The Great Hub of Anaerobe and Obligate Aerobe Genomes

Deciphering Primordial Cyanobacterial Genome Functions from Protein Network Analysis

Authors:

Harel et al

Abstract:

The Great Oxidation Event (GOE) ∼2.4 billion years ago resulted from the accumulation of oxygen by the ancestors of cyanobacteria. Cyanobacteria continue to play a significant role in primary production and in regulating the global marine and limnic nitrogen cycles. Relatively little is known, however, about the evolutionary history and gene content of primordial cyanobacteria. To address these issues, we used protein similarity networks, containing proteomes from 48 cyanobacteria as the test group, and reference proteomes from 84 microbes representing four distinct metabolic groups from most reducing to most oxidizing: methanogens, obligate anaerobes (nonmethanogenic), facultative aerobes, and obligate aerobes. These four metabolic groups represent extant bioinformatic proxies for ancient redox chemistries, extending from an anoxic origin through the GOE and ultimately to obligate aerobes. Analysis of the network metric degree showed a strong relationship between cyanobacteria and obligate anaerobes, from which cyanobacteria presumably arose, for core functions that include translation, photosynthesis, energy conservation, and environmental interactions. These data were used to reconstruct primordial functions in cyanobacteria that included nine gene families involved in photosynthesis, hydrogenases, and proteins involved in defense from environmental stress. The presence of 60% of these genes in both reaction center I (RC-I) and RC-II-type bacteria may be explained by selective loss of either RC in the evolutionary history of some photosynthetic lineages. Finally, the network reveals that cyanobacteria occupy a unique position among prokaryotes as a hub between anaerobes and obligate aerobes.

Evidence of Pre Great Oxidation Event Aerobic Photosynthesis

Selenium isotopes support free O2 in the latest Archean

Authors:

Stüeken et al

Abstract:

Selenium (Se) undergoes redox transformations and isotopic fractionations at relatively high redox potentials and could therefore provide insight into changes in oceanic and atmospheric O2 levels over Earth's history. We test this idea with Se data from the 2.5 Ga Mount McRae Shale (Hamersley Basin, Australia), which records temporary enrichments in abundances and isotopes of other redox-sensitive elements indicating a "whiff of oxygen" in Earth's atmosphere before the Great Oxidation Event. Se isotopic ratios expressed as δ82/78Se and abundances relative to crustal background show significant positive excursions of up to 1.1‰ and an enrichment 13 times above background, respectively, overlapping with excursions in Mo and N isotopes and abundances. Because Se has a relatively high redox potential and photosynthetic oxidation pathways are unknown, our data thus suggest that Se was mobilized by free O2 during this interval. The isotopic fractionation likely occurred during transport of Se oxyanions from the site of weathering to the outer shelf. Although O2 could have been produced locally on land and may not necessarily have increased in the global atmosphere, our results strengthen the inference of an early origin of oxygenic photosynthesis long before the Paleoproterozoic Great Oxidation Event. This is the first report of a Se isotope excursion in the Precambrian rock record, and it confirms that Se isotopes can serve as a useful redox proxy in deep time.

The Emergence of Continents was Crucial for Great Oxidation Event

Benthic perspective on Earth’s oldest evidence for oxygenic photosynthesis

Authors:

Lalonde et al

Abstract:

The Great Oxidation Event (GOE) is currently viewed as a protracted process during which atmospheric oxygen increased above ∼10−5 times the present atmospheric level (PAL). This threshold represents an estimated upper limit for sulfur isotope mass-independent fractionation (S-MIF), an Archean signature of atmospheric anoxia that begins to disappear from the rock record at 2.45 Ga. However, an increasing number of papers have suggested that the timing for oxidative continental weathering, and by conventional thinking the onset of atmospheric oxygenation, was hundreds of million years earlier than previously thought despite the presence of S-MIF. We suggest that this apparent discrepancy can be resolved by the earliest oxidative-weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere, such as biological soil crusts and freshwater microbial mats covering riverbed, lacustrine, and estuarine sediments. We calculate that oxygenic photosynthesis in these millimeter-thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox-sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant S-MIF signature. As continental freeboard increased significantly between 3.0 and 2.5 Ga, the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass-balance perspective. These observations help reconcile evidence for pre-GOE oxidative weathering with the history of atmospheric chemistry, and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the GOE.

A Continguous (!?!!!) Section Across the Great Oxidation Event From Siderian/Rhyacian PaleoProterozoic Australia

Sedimentology of the Paleoproterozoic Kungarra Formation, Turee Creek Group, Western Australia: A conformable record of the transition from early to modern Earth

Authors:

Van Kranendonk et al

Abstract:

This paper presents the first, detailed sedimentological analysis of the Paleoproterozoic Kungarra Formation, the lowermost of three formations comprising the Turee Creek Group in Western Australia, which was deposited across the rise in atmospheric oxygen (the Great Oxidation Event, or GOE) and the transition from early to modern Earth.

The data show that the Kungarra Formation has a gradational, conformable lower contact with underlying banded iron-formation of the Hamersley Group and predominantly comprises an upward-shallowing succession from deepwater shales and siltstones, through rippled fine-grained sandstones and stromatolitic carbonates, to tidal flat deposits that immediately underlie coastal–fluvial deposits of the overlying Koolbye Formation.

At the base of the Kungarra Formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron-formation that pass upsection to units of non-magnetic shale and ferruginous chert and grey chert, reflecting a gradual loss of iron from the world's oceans accompanying the rise of atmospheric oxygen. A falling stage systems tract is recognised above this transition in the Hardey Syncline area, capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock. Two glacio-eustatic cycles are recognised within the middle to upper parts of the Kungarra Formation, each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition.

Stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest, in contrast to previous models of a north-northeastward deepening foreland basin. The lack of seismites or internal unconformities within the formation precludes a foredeep setting. Rather, deposition is interpreted as having occurred within an intracratonic basin, with detritus sourced from erosion of uplifted bedrock to the southeast.

Fresh Water Lakes Played an Important Role in the Proterozoic Great Oxygenation Events

Enhanced organic carbon burial in large Proterozoic lakes: Implications for atmospheric oxygenation

Authors:

Spinks et al

Abstract:

The burial of organic carbon in sedimentary systems has been a fundamental part of the carbon cycle throughout the geological record, and was instrumental in major oxygenations of the atmosphere in the early Palaeoproterozoic and Neoproterozoic. While much focus has been placed on the burial of carbon in Precambrian marine carbonate and organic carbon-rich rocks deposited around the time of these major oxygenations, such deposits yield little information on the evolution of the atmosphere in the significant time between. There is, however, growing evidence from terrestrially deposited sediments to suggest the surface environment may have been at least intermittently well-oxygenated from the late Mesoproterozoic. Hence Proterozoic sediments deposited in terrestrial near-surface environments are useful targets for the study of atmospheric evolution during a time which is hitherto poorly understood.

Thus far, little attention has been paid to the contribution of large lakes and intercontinental basins to the global burial of organic carbon, and thus the progressive oxygenation of the atmosphere, especially given that the highest rates of organic carbon burial in modern aquatic environments occur in lacustrine settings, in stark contrast to the low rates observed in the contemporary marine realm. Here, we report high burial rates of organic carbon in large lacustrine systems of late Mesoproterozoic to early Neoproterozoic age, which are comparable with modern lacustrine systems, and significantly higher than modern and ancient marine deposits. These data emphasise the significance of lakes as a global repository for organic carbon, and imply Proterozoic lakes were at least as efficient, and perhaps as important, as modern lakes in the global burial of organic carbon. Such findings suggest large Proterozoic lakes and epicontinental basins played a crucial role in the progressive oxygenation of the atmosphere before the major Neoproterozoic oxygenation.

Was There a MesoProterozoic Oxygen Crash After the Great Oxidation Event?

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

Authors:

Planavsky et al

Abstract:

The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

Fossil Evidence of Iron Oxidizing Bacteria From the Great Oxidation Event

Fossil evidence of iron-oxidizing chemolithotrophy linked to phosphogenesis in the wake of the Great Oxidation Event

Authors:

Crosby et al

Abstract:

The oxygenation of Earth's atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives. For example, chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors. A growing number of oxygen-utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive-gradient habitats. Polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits. Here we present fossil evidence of iron-oxidizing bacteria preserved as filamentous iron oxides within phosphatic Paleoproterozoic stromatolites. The filaments include twisted stalks similar to those produced by modern iron-oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients. Fossil iron-oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of O2-Fe(II) gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and/or an active iron redox pump.

Was There a Great Oxidation Event (or two) During the MesoArchean?

Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India

Authors:

Mukhopadhyay et al

Abstract:

It is widely believed that atmospheric oxygen saturation rose from less than 10–5 present atmospheric level (PAL) in the Archean to greater than 10–2 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga, but it is unclear if any earlier oxygenation events occurred. Here we report U-Pb zircon data indicating that a pyrophyllite-bearing paleosol, from Keonjhar in the Precambrian Singhbhum Craton of eastern India, formed between 3.29 and 3.02 Ga, making it one of very few known Archean paleosols globally. Field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow-marine siliciclastic sediments. A negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile. The presence of redox-sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects. We suggest that a transient atmospheric oxygenation event occurred at least 600 m.y. prior to the GOE and ∼60 m.y. prior to a previously documented Archean oxygenation event. We propose that several pulsed and short-lived oxygenation events are likely to have occurred prior to the GOE, and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis.

Evidence of Oxygenated Marine Bottom Waters at the NeoArchean/PaleoProterozoic Boundary?!?!

Petrology And Geochemistry Of The Wangjiazhuang Banded Iron Formation And Associated Supracrustal Rocks From The Wutai Greenstone Belt In The North China Craton: Implications For Their Origin And Tectonic Setting

Authors:

Wang et al

Abstract:

The Wutai greenstone belt (WGB) is one of the most extensively studied greenstone belts in China. Together with the Hengshan and Fuping complexes, these three associations compose the central segment of the Trans-North China Orogen (TNCO) in the North China Craton (NCC). The Wangjiazhuang banded iron formation (BIF) is located in the bottom of the Jingangku Formation of the WGB. The associated supracrustal rocks consist of meta-basalts (amphibolites), meta-felsic volcanic rocks (leptynite) and metapelites (mica schist), which have experienced amphibolite-facies metamorphism. Amphibolites are commonly intercalated with the BIF. SIMS zircon U-Pb analyses on amphibolites suggest that the Wangjiazhuang BIF was formed at ca. 2543 ± 4 Ma. Combined with most Neoarchean Algoma-type BIFs in the NCC, these features indicate that a significant tectothermal event of the NCC have occurred at ∼2.5 Ga. Mineral assemblages of the Wangjiazhuang BIF are composed of quartz, magnetite, amphibole, and minor garnet, pyrite and calcite. The precursor deposits of this BIF were likely ferric-oxyhydroxides, fine-grained carbonate oozes, silicate phases rich in Al-Ca-Mg-Fe and amorphous silica. The appearance of garnet and ferro-pargasite, high concentrations of Al2O3, HFSEs, Sc, and positive correlations among Al2O3, TiO2, HFSEs and REE indicate that there was a significant terrigenous input. Even so, the Wangjiazhuang BIF samples display distinctively seawater-like REE + Y profiles, characterized by positive La and Y anomalies and HREE enrichment relative to LREE in PAAS-normalized REE diagrams. Consistently positive Eu anomalies are also observed, which are typically from high-T hydrothermal fluids. In addition, the true negative Ce anomalies recorded in the Wangjiazhuang BIF might indicate the onset of bottom-water oxidation at the Archean-Proterozoic boundary, at least in restricted basins. Amphibolites have geochemical affinity with both MORB- and arc-like components, and the trace element characteristics of leptynites are also consistent with a subduction zone signature. These features suggest that the Wangjiazhuang BIF was deposited in a back-arc basin. Oceanic subduction, coupled with contemporary depleted mantle upwelling related to back-arc basin extension, can account for the typical interaction between these two components.

Increased Continental Weathering Made the Great Oxygenation Event Possible?

Proterozoic oxygen rise linked to shifting balance between seafloor and terrestrial weathering

Authors:

Mills et al

Abstract:

A shift toward higher atmospheric oxygen concentration during the late Proterozoic has been inferred from multiple indirect proxies and is seen by many as a prerequisite for the emergence of complex animal life. However, the mechanisms controlling the level of oxygen throughout the Proterozoic and its eventual rise remain uncertain. Here we use a simple biogeochemical model to show that the balance between long-term carbon removal fluxes via terrestrial silicate weathering and ocean crust alteration plays a key role in determining atmospheric oxygen concentration. This balance may be shifted by changes in terrestrial weatherability or in the generation rate of oceanic crust. As a result, the terrestrial chemical weathering flux may be permanently altered—contrasting with the conventional view that the global silicate weathering flux must adjust to equal the volcanic CO2 degassing flux. Changes in chemical weathering flux in turn alter the long-term supply of phosphorus to the ocean, and therefore the flux of organic carbon burial, which is the long-term source of atmospheric oxygen. Hence we propose that increasing solar luminosity and a decrease in seafloor spreading rate over 1,500–500 Ma drove a gradual shift from seafloor weathering to terrestrial weathering, and a corresponding steady rise in atmospheric oxygen. Furthermore, increased terrestrial weatherability during the late Neoproterozoic may explain low temperature, increases in ocean phosphate, ocean sulfate, and atmospheric oxygen concentration at this time.

Evidence of Widespread Aerobic Iodine Cycle in Rhyacian PaleoProterozoic

An iodine record of Paleoproterozoic surface ocean oxygenation

Authors:

Hardisty et al

Abstract:

Constraining oxygen levels in the early Precambrian surface ocean has been a longstanding goal, but efforts have been challenged by the availability of suitable proxies. Here we present a novel approach, iodine geochemistry, which broadens our perspective by providing constraints on shallow, carbonate-dominated marine settings. Iodate (IO3–) persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals, allowing iodine-to-calcium ratios (I/Ca) in shallow carbonates to be used as a paleoredox indicator. Our data from a series of Mesoarchean through Paleoproterozoic carbonates deposited under shallow-marine conditions reveal a progressive surface ocean oxygenation in the early Paleoproterozoic. These data seem to indicate that a largely anoxic surface ocean extended throughout the Archean until the Great Oxidation Event (GOE) at ca. 2.4 Ga, implying that previous inferences of pre-GOE oxygen production may reflect oxygen oases, transient oxidation events, or oxygen levels below those required for IO3– accumulation. The data suggest formation and persistence of IO3– and, consequently, surface ocean oxygen concentrations of at least 1 μM during the GOE. Following the initial rise of oxygen, carbonate-associated iodine in globally extensive carbonate units deposited during the Lomagundi positive carbon isotope excursion at ca. 2.22–2.1 Ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event, synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions.

Understanding Oxygen's Rise in the PaleoAtmosphere

The rise of oxygen in Earth’s early ocean and atmosphere

Authors:

Lyons et al

Abstract:

The rapid increase of carbon dioxide concentration in Earth’s modern atmosphere is a matter of major concern. But for the atmosphere of roughly two-and-half billion years ago, interest centres on a different gas: free oxygen (O2) spawned by early biological production. The initial increase of O2 in the atmosphere, its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth’s history.

Great Oxidation Event had Profound Impact on Phosphorus in the Orosirian PaleoProterozoic

Two billion years ago the Earth system was recovering from perhaps the single-most profound modification of its surface environments: the oxygenation of the atmosphere and oceans. This led to a series of major changes in global biogeochemical cycles, as a team around Aivo Lepland of the Norwegian Geological Survey NGU reports in the latest online edition of "Nature Geoscience".

This also resulted in the distribution of one of life's key elements, phosphorous. Studies on the unique organic-rich Zaonega rock formation preserved in Carelia, NW Russia, with an age of around two billion years has revealed an astonishing result: "The formation of Earth's earliest phosphorites was influenced strongly, if not controlled completely, by the activity of sulfur bacteria", says co-author Richard Wirth of the GFZ German Research Centre for Geosciences, who analyzed the rock samples with an electron microscope. "This activity occurred in an oil field setting that had been influenced by active volcanism and associated venting and seeping." In the modern world, sulfur bacteria inhabit upwelling vent and seep areas known as "Black Smokers" and mediate phosphorite formation. The authors therefore conclude that the formation of the earliest worldwide phosphorites 2 billion years ago can be linked to the establishment of sulfur bacteria habitats, triggered by the oxygenation of the Earth.

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The Orosirian PaleoProterozoic Zaonega Formation in Russia Suggests Caution With Interpreting Paleo Isotope Ratios

Petrography and geochemistry of carbonate rocks of the Zaonega Formation, Russia: Documentation of 13C-depleted non-primary calcite

Authors:

Črne et al

Abstract:

The ca. 2.0 Ga Zaonega Formation in the Onega Basin of NW Russia represents a deep-water, mixed siliciclastic-carbonate depositional system with voluminous mafic volcanism. It is typified by extremely organic-rich rocks (TOC > 40 wt%) and represents one of the earliest known episodes of oil/asphalt generation. These rocks have been inferred to archive one of the largest negative δ13C excursions in Earth history, one that followed and/or partially overlapped with the 2.2–2.06 Ga worldwide Lomagundi-Jatuli carbonate carbon isotopic excursion to high values and thought to be linked to the Paleoproteorozoic oxygenation of Earth's surface environments.

In order to assess the post-depositional integrity of the carbonate carbon isotopic signal (δ13Ccarb) of the Zaonega rocks, we examined in detail the petrography and geochemistry of eight carbonate beds (0.3 to 0.9 m thick) from different stratigraphic levels of the formation. The range of δ13C values for a single bed can be as much as 17 ‰, with calcite being significantly depleted in 13C relative to co-existing dolomite; the 13C-depleted calcite likely formed by involvement of carbon derived from diagenetic and catagenetic alteration of organic matter possibly abetted by volcanic CO2. The presence of calcite ± talc ± phlogopite ± actinolite indicates metamorphic reaction of dolomite with quartz, or possibly K-feldspar, in the presence of water; commonly accompanied by degassing of 13C-enriched CO2, this caused further 13C depletion of newly formed calcite. The least altered dolomite is documented in central parts of thick dolostone beds with variably calcitized margins. This dolomite is considered as the earliest and possibly primary carbonate phase, potentially recording the δ13C signal of the ambient seawater. Among the studied carbonates two stratigraphic intervals contain the least-altered dolomite exhibiting δ13C values of +8 ‰ for the middle part of the formation, and −2 and −4 ‰ for the upper part. All other beds, with δ13C ranging from −19 to +3 ‰, are considered to have been variably depleted in their 13C content by post-depositional processes and therefore cannot be reliably used for assessing the carbon isotope composition of Paleoproterozoic seawater. Our results emphasise the importance of distinguishing primary versus secondary (or later) isotopic compositions in studies of carbonate rocks used for reconstruction of global environmental change.

Oxygen Level Fluctuation During the Great Oxidation Event in the Rhyacian PaleoProterozoic

Oxygen dynamics in the aftermath of the Great Oxidation of Earth’s atmosphere

Authors:

Donald E. Canfield et al

Abstract:

The oxygen content of Earth’s atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth’s oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth’s oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest–lived positive δ13C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.