Increase In Atmospheric Oxygen Research Articles

Sustained increases in atmospheric oxygen and marine productivity in the Neoproterozoic and Palaeozoic eras

A geologically rapid Neoproterozoic oxygenation event is commonly linked to the appearance of marine animal groups in the fossil record. However, there is still debate about what evidence from the sedimentary geochemical record—if any—provides strong support for a persistent shift in surface oxygen immediately preceding the rise of animals. We present statistical learning analyses of a large dataset of geochemical data and associated geological context from the Neoproterozoic and Palaeozoic sedimentary record and then use Earth system modelling to link trends in redox-sensitive trace metal and organic carbon concentrations to the oxygenation of Earth’s oceans and atmosphere. We do not find evidence for the wholesale oxygenation of Earth’s oceans in the late Neoproterozoic era. We do, however, reconstruct a moderate long-term increase in atmospheric oxygen and marine productivity. These changes to the Earth system would have increased dissolved oxygen and food supply in shallow-water habitats during the broad interval of geologic time in which the major animal groups first radiated. This approach provides some of the most direct evidence for potential physiological drivers of the Cambrian radiation, while highlighting the importance of later Palaeozoic oxygenation in the evolution of the modern Earth system.

Stochastic Character Mapping, Bayesian Model Selection, and Biosynthetic Pathways Shed New Light on the Evolution of Habitat Preference in Cyanobacteria.

Cyanobacteria are the only prokaryotes to have evolved oxygenic photosynthesis paving the way for complex life. Studying the evolution and ecological niche of cyanobacteria and their ancestors is crucial for understanding the intricate dynamics of biosphere evolution. These organisms frequently deal with environmental stressors such as salinity and drought, and they employ compatible solutes as a mechanism to cope with these challenges. Compatible solutes are small molecules that help maintain cellular osmotic balance in high salinity environments, such as marine waters. Their production plays a crucial role in salt tolerance, which, in turn, influences habitat preference. Among the five known compatible solutes produced by cyanobacteria (sucrose, trehalose, glucosylglycerol, glucosylglycerate, and glycine betaine), their synthesis varies between individual strains. In this study, we work in a Bayesian stochastic mapping framework, integrating multiple sources of information about compatible solute biosynthesis in order to predict the ancestral habitat preference of Cyanobacteria. Through extensive model selection analyses and statistical tests for correlation, we identify glucosylglycerol and glucosylglycerate as the most significantly correlated with habitat preference, while trehalose exhibits the weakest correlation. Additionally, glucosylglycerol, glucosylglycerate, and glycine betaine show high loss/gain rate ratios, indicating their potential role in adaptability, while sucrose and trehalose are less likely to be lost due to their additional cellular functions. Contrary to previous findings, our analyses predict that the last common ancestor of Cyanobacteria (living at around 3180 Ma) had a 97% probability of a high salinity habitat preference and was likely able to synthesise glucosylglycerol and glucosylglycerate. Nevertheless, cyanobacteria likely colonized low-salinity environments shortly after their origin, with an 89% probability of the first cyanobacterium with low-salinity habitat preference arising prior to the Great Oxygenation Event (2460 Ma). Stochastic mapping analyses provide evidence of cyanobacteria inhabiting early marine habitats, aiding in the interpretation of the geological record. Our age estimate of ~2590 Ma for the divergence of two major cyanobacterial clades (Macro- and Microcyanobacteria) suggests that these were likely significant contributors to primary productivity in marine habitats in the lead-up to the Great Oxygenation Event, and thus played a pivotal role in triggering the sudden increase in atmospheric oxygen.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Mesoproterozoic oxygenation event: From shallow marine to atmosphere

AbstractOxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1%–1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%–1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ~20‰ to ~50‰ and a subsequent decline to ~10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca + Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.

The temporal distribution of Earth's supermountains and their potential link to the rise of atmospheric oxygen and biological evolution

We use the distribution of low-Lu and low-Lu/Dy zircons, derived from the eroded roots of mountains, where trace amounts of zircon compete with abundant garnets for heavy rare earth elements, to identify periods of extensive high mountain (supermountain) formation. The data reveal that Earth has two, and they correlate with periods when the average metamorphic pressure of orogenic belts exceeded 1.2 GPa, the pressure at which metamorphic garnet becomes abundant. The first supermountains formed at 2.0-1.8 Ga, during the assembly of Nuna, and the second at 650-500 Ma, during the amalgamation of Gondwana. The 650-500 Ma event has been previously recognized and named the Transgondwanan Supermountain, but the 2.0-1.8 Ga Nuna Supermountains have not. Our data also show that mountain building was limited during the Boring Billion, between 1.8 and 0.8 Ga.Mountain building results in high rates of erosion and sedimentation, and the two periods of supermountain formation are associated with voluminous sedimentation, whereas sediment production during the Boring Billion was more limited. Enhanced erosion would have increased the supply of bio-limiting nutrients such as phosphorous to the oceans, potentially increasing primary productivity and the flow of energy through ecosystems. Enhanced carbon production and sedimentation, both driven by supermountain erosion, are also expected to lead to increases in atmospheric oxygen. During Transgondwanan Supermountain erosion, increasing oxygen and nutrient levels may be connected to the proliferation of chlorophyte algae after 650 Ma and the emergence of large, animal-like organisms 75 Myr later. Targeted research of Nuna-aged sediments is needed to evaluate whether rapid erosion of supermountains is linked to geochemical and biotic events, such as the disappearance of banded iron formations at 1.85 Ga, the emergence of the first macroscopic organisms (Grypania) at 1.9 Ga, and the radiation of early eukaryotes, which become visible in the fossil record at 1.65 Ga.

(Not) an elementary question: How to mobilize the iron in the wall?

Photosynthesis, respiration, and countless other processes are dependent on proteins that carry iron (Fe) as an essential cofactor. When Fe was recruited for biological functions early in evolution, it was an available element. The Earth's atmosphere was reducing with oxygen limited to very low pressure. Thus, Fe was present as comparatively soluble Fe sulfides. With the increase in atmospheric oxygen, the bioavailability of Fe dramatically decreased because the solubility of Fe hydroxides is very low ( Williams, 2002 Williams R.J.P. The fundamental nature of life as a chemical system: the part played by inorganic elements. J. Inorg. Biochem. 2002; 88: 241-250 Crossref PubMed Scopus (32) Google Scholar ). This gradual change proceeded extremely slowly for more than a billion years before a more rapid increase finally led to today's oxygen levels about 500 million years ago. Life's dependency on Fe drove the evolution of intricate mechanisms to increase the availability of Fe outside of cells, to take up mobilized Fe and to deliver it to the many intracellular target sites. Fe transport, the synthesis of chelating molecules, and transcriptional networks regulating the activity of these processes play key roles in practically all organisms from bacteria to humans.

Possible link between Earth\u2019s rotation rate and oxygenation

The biotic and abiotic controls on major shifts in atmospheric oxygen and the persistence of low-oxygen periods over a majority of Earth’s history remain under debate. Explanations of Earth’s stepwise pattern of oxygenation have mostly neglected the effect of changing diel illumination dynamics linked to daylength, which has increased through geological time due to Earth’s rotational deceleration caused by tidal friction. Here we used microsensor measurements and dynamic modelling of interfacial solute fluxes in cyanobacterial mats to investigate the effect of changing daylength on Precambrian benthic ecosystems. Simulated increases in daylength across Earth’s historical range boosted the diel benthic oxygen export, even when the gross photosynthetic production remained constant. This fundamental relationship between net productivity and daylength emerges from the interaction of diffusive mass transfer and diel illumination dynamics, and is amplified by metabolic regulation and microbial behaviour. We found that the resultant daylength-driven surplus organic carbon burial could have shaped the increase in atmospheric oxygen that occurred during the Great and Neoproterozoic Oxidation Events. Our suggested mechanism, which links the coinciding increases in daylength and atmospheric oxygen via enhanced net productivity, reveals a possible contribution of planetary mechanics to the evolution of Earth’s biology and geochemistry.

A paleosol record of the evolution of Cr redox cycling and evidence for an increase in atmospheric oxygen during the Neoproterozoic.

Atmospheric oxygen levels control the oxidative side of key biogeochemical cycles and place limits on the development of high-energy metabolisms. Understanding Earth's oxygenation is thus critical to developing a clearer picture of Earth's long-term evolution. However, there is currently vigorous debate about even basic aspects of the timing and pattern of the rise of oxygen. Chemical weathering in the terrestrial environment occurs in contact with the atmosphere, making paleosols potentially ideal archives to track the history of atmospheric O2 levels. Here we present stable chromium isotope data from multiple paleosols that offer snapshots of Earth surface conditions over the last three billion years. The results indicate a secular shift in the oxidative capacity of Earth's surface in the Neoproterozoic and suggest low atmospheric oxygen levels (<1% PAL pO2 ) through the majority of Earth's history. The paleosol record also shows that localized Cr oxidation may have begun as early as the Archean, but efficient, modern-like transport of hexavalent Cr under an O2 -rich atmosphere did not become common until the Neoproterozoic.

Structure and evolutionary implications of the earliest (Sinemurian, Early Jurassic) dinosaur eggs and eggshells

One of the fossil record’s most puzzling features is the absence of preserved eggs or eggshell for the first third of the known 315 million year history of amniote evolution. Our meagre understanding of the origin and evolution of calcareous eggshell and amniotic eggs in general, is largely based on Middle Jurassic to Late Cretaceous fossils. For dinosaurs, the most parsimonious inference yields a thick, hard shelled egg, so richly represented in the Late Cretaceous fossil record. Here, we show that a thin calcareous layer (≤100 µm) with interlocking units of radiating crystals (mammillae) and a thick shell membrane already characterize the oldest known amniote eggs, belonging to three coeval, but widely distributed Early Jurassic basal sauropodomorph dinosaurs. This thin shell layer strongly contrasts with the considerably thicker calcareous shells of Late Jurassic dinosaurs. Phylogenetic analyses and their Sinemurian age indicate that the thin eggshell of basal sauropodomorphs represents a major evolutionary innovation at the base of Dinosauria and that the much thicker eggshell of sauropods, theropods, and ornithischian dinosaurs evolved independently. Advanced mineralization of amniote eggshell (≥150 µm in thickness) in general occurred not earlier than Middle Jurassic and may correspond with a global trend of increase in atmospheric oxygen.

Step-like growth of the continental crust in South China: evidence from detrital zircons in Yangtze River sediments

Detrital zircons from nine river sand samples from the upper, middle and lower streams of the Yangtze River (the world's third largest river) and its two largest tributaries, the Han and Jialing rivers have been analyzed for U–Pb–Lu–Hf–O isotope compositions. Zircons from the upper Yangtze River cluster in age groups at 0–100, 200–300, 400–500, 700–1000, 1800–1900 and 2300–2500 Ma, with a marked peak at 41 Ma diagnostic of magmatism on the Tibetan Plateau. Zircons from the middle and lower Yangtze River and its tributaries exhibit broadly similar age groups at 100–300, 400–500, 700–900, 1800–2000 and 2300–2700 Ma, except for the lack of Cenozoic ages.The Yangtze River catchment is covered by thick Phanerozoic sedimentary rocks and so Archean-aged zircons are likely to be under-represented in modern river sands. We therefore applied the approach of Dhuime et al. (2012) to calculate a crustal growth curve for South China, and the resultant curve has two inflections. The older inflection at ~2.8 Ga is slightly younger than that of Dhuime et al. (2012), suggesting the onset of plate tectonics in the Yangtze Craton. The younger inflection at ~1.8 Ga appears to mark the onset of another period of relatively high crustal growth rate in South China. This period of increased crustal growth rate in the Mesoproterozoic is coeval with the breakup of the supercontinent Columbia, suggesting that the Mesoproterozoic crustal growth in South China may be related to supercontinent dispersal.Comparison with global crustal growth curves based on detrital zircons highlights the distinct step-like pattern of crustal growth in South China. This emphasizes the potential of comparing regional with global crustal growth curves, and the changes in the rates of crustal growth in South China appear to have been controlled by changes in regional geodynamics. The maximum δ18O values in the zircons analyzed increase markedly at the end of the Archaean, and these increases therefore accompany the estimated increases in atmospheric oxygen at that time. It is therefore suggested the atmospheric O2 levels were associated with increased crustal weathering, the development of more elevated δ18O sediments and their increasing incorporation in the generation of crustally derived magmas from ca. 2.5 Ga.

Petrology and geochemistry of the Boolgeeda Iron Formation, Hamersley Basin, Western Australia

The Boolgeeda Iron Formation and overlying Turee Creek Group, Hamersley Basin, Western Australia, represent a conformable succession of sediment deposited between 2.45 and 2.22 Ga. This interval of geologic history is of significant interest because it spans the Great Oxidation Event (GOE), when oxygen first accumulated in Earth’s atmosphere. Here we present geochemical and petrographic data from the uppermost 45 m of the Boolgeeda Iron Formation and an additional 30 m of the overlying Kungarra Formation mudstones, as sampled from the Turee Creek Drilling Project 1 drill core (TCDP1). This core captures the termination of BIF deposition in the Hamersley Basin and coincides with a global decline in BIF deposition in the Paleoproterozoic. We provide a continuous, high resolution chemostratigraphic dataset of major and trace element concentrations, as well as Fe speciation data, to assess the relationship between the rise of atmospheric oxygen and the subsequent decline in BIF deposition. We also highlight the interplay between local and global controls on the preservation of redox signatures, including the rise and fall of local base-level, input of weathered continental material, influx of reduced sulfur species associated with the continental weathering, and global increases in atmospheric oxygen. We interpret Boolgeeda deposition to have taken place under shallow water, oxic conditions overlying anoxic ferruginous deep water. Intermittent periods of oxidative weathering led to influxes of redox sensitive trace elements from land as continental sulfide minerals were weathered. This also led to the temporary disappearance of MIF-S, but O2 remained below the threshold capable of completely eliminating S isotope signatures associated with the MIF-S recycling.

An Archaean oxygen oasis

The first of two stepwise increases in atmospheric oxygen occurred at the end of the Archaean eon. Analyses of sulfur and iron isotopes in pyrite reveal a near-shore environment that hosted locally oxygenated conditions in the Mesoarchaean era.

A theory of atmospheric oxygen.

Geological records of atmospheric oxygen suggest that pO2 was less than 0.001% of present atmospheric levels (PAL) during the Archean, increasing abruptly to a Proterozoic value between 0.1% and 10% PAL, and rising quickly to modern levels in the Phanerozoic. Using a simple model of the biogeochemical cycles of carbon, oxygen, sulfur, hydrogen, iron, and phosphorous, we demonstrate that there are three stable states for atmospheric oxygen, roughly corresponding to levels observed in the geological record. These stable states arise from a series of specific positive and negative feedbacks, requiring a large geochemical perturbation to the redox state to transition from one to another. In particular, we show that a very low oxygen level in the Archean (i.e., 10-7 PAL) is consistent with the presence of oxygenic photosynthesis and a robust organic carbon cycle. We show that the Snowball Earth glaciations, which immediately precede both transitions, provide an appropriate transient increase in atmospheric oxygen to drive the atmosphere either from its Archean state to its Proterozoic state, or from its Proterozoic state to its Phanerozoic state. This hypothesis provides a mechanistic explanation for the apparent synchronicity of the Proterozoic Snowball Earth events with both the Great Oxidation Event, and the Neoproterozoic oxidation.

Diagenetic overprint to a negative carbon isotope anomaly associated with the Gaskiers glaciation of the Ediacaran Doushantuo Formation in South China

Ediacaran carbon isotope records contain significant and long-lasting negative anomalies for which convincing explanations have not been given. We recognized a diagenetic overprint contributing to the 120-m-thick negative anomaly interval in the upper part of the Ediacaran Doushantuo Formation at Yangjiaping, Hunan province in South China. Coarse-grained carbonate facies deposited in an outer shelf setting of the Yangtze platform provided an opportunity to examine diagenetic components by measuring micro-drilled sub-samples. Scattered δ13C values of bulk samples (+5 to −14‰) in this anomaly interval are here attributed to the secondary precipitation of highly 13C-depleted (up to −33‰) calcite cement on dolomite sedimentary grains of less negative δ13C. This diagenetic overprint was not observed in the intervals above or below the anomaly. In these intervals the δ13C difference between the cements and the grains is insignificant. Strontium isotopic variation in the anomaly interval is identical to that of the interval with negative δ13C anomaly in the middle Doushantuo Formation reported from the Three Gorges area, which is presumably correlated with the Gaskiers glaciation. This correlation scheme indicates a long-term hiatus (∼35myrs) between the anomaly horizon and the overlying Dengying Formation at Yangjiaping. We suggest that the anaerobic oxidization of reduced carbon (organic carbon or methane) by sulfate reduction was the most likely process responsible for 13C-depleted secondary calcite, which progressively filled intergranular porosity. An increase in atmospheric oxygen during Ediacaran and the Gaskiers glaciation enhanced oxidative weathering and elevated seawater sulfate concentrations to a level sufficient for diagenetic alteration. This co-occurred with an increase in 87Sr/86Sr from 0.7080 to 0.7085 in middle Ediacaran. We also show that the δ13C profile of the Doushantuo Formation at Yangjiaping is similar to that at Weng’an in Guizhou province, another section recording an outer shelf depositional environment. Although further study is needed, correlation between the Yangjiaping Unit 3 with the δ13C-scattered interval in Weng’an suggests that the embryo-bearing phosphorites at Weng’an were formed likely before the Gaskiers glaciation.

Could land‐based early photosynthesizing ecosystems have bioengineered the planet in mid‐Palaeozoic times?

AbstractThe Ordovician and Silurian periods were times of major geological activity as regards palaeogeography, volcanism and climate change, the last of these evidenced by a series of cooling episodes and glaciations that climaxed in the Hirnantian (Late Ordovician). The presence of cryptospores in the Darriwilian (Middle Ordovician) marked the advent of higher plants on land. A critical survey of direct (mega‐ and microfossils) and some indirect evidence in succeeding rocks indicates the presence of algae, Bacteria, Cyanobacteria, Fungi, probable lichens, cryptophytes and basal tracheophytes. Similar associations of photosynthesizers and decomposers occur today in cryptogamic covers (CCs), for example biological crusts, except that bryophytes replace cryptophytes (basal embryophytes) and tracheophytes are absent. Thus, extant CCs, which make significant contributions today to global carbon and nitrogen fixation and prevention of erosion, provide an excellent analogue for the impacts of early land vegetation on both lithosphere and atmosphere. As a prerequisite to assessing impacts in Ordovician–Silurian times, with particular consideration of parameters used by climate modellers, the effects of a number of abiotic factors on the growth and survival of extant cryptogamic ground covers and their environmental impacts are reviewed. Factors include photosynthetically active radiation, ultraviolet radiation, temperature, water, oxygen, carbon dioxide, nitrogen, phosphorus, iron, surface roughness and albedo. A survey of the nature and extent of weathering facilitated by such vegetation concludes that it was limited based on depth of weathering when compared with that from rooted tracheophytes today, with minor effects on carbon dioxide drawdown. As global net productivity from Ordovician–Silurian CCs was very probably lower than today, and while the small fraction of intractable material in their organic carbon would have resulted in a more rapid turnover of terrestrial biomass, we conclude that there was decreased possibility of long‐term organic carbon burial. Hence, there would have been very limited increase in atmospheric oxygen and decrease in carbon dioxide resulting from carbon burial.

Gold in the oceans through time

During sedimentation and diagenesis of carbonaceous shales in marine continental margin settings, Au is adsorbed from seawater and organic matter and becomes incorporated into sedimentary pyrite. LA-ICPMS analysis of over 4000 sedimentary pyrite grains in 308 samples from 33 locations around the world, grouped over 123 determined ages, has enabled us to track, in a first order sense, the Au content of the ocean over the last 3.5 billion years. Gold was enriched in the Meso- and Neoarchean oceans, several times above present values, then dropped by an order of magnitude from the first Great Oxidation Event (GOE1) through the Paleoproterozoic to reach a minimum value around 1600 Ma. Gold content of the oceans then rose, with perturbations, through the Meso- and Neoproterozoic, showing a steady rise at the end of the Proterozoic (800 to 520 Ma), which most likely represents the effects of the second Great Oxidation Event (GOE2). Gold in the oceans was at a maximum at 520 Ma, when oxygen in the oceans rose to match current maximum values. In the Archean and Proterozoic, the Au content of seawater correlates with the time distribution of high-Mg greenstone belts, black shales and banded iron formations, suggesting that increases in atmospheric oxygen and marine bio-productivity, combined with the higher background of Au in komatiitic and Mg-rich basalts were the first order causes of the pattern of Au enrichment in seawater. We suggest the lack of major Au deposits from 1800 to 800 Ma, is explained by the low levels of Au in the oceans during this period.

The role of symbiosis in the transition of some eukaryotes from aquatic to terrestrial environments

Symbiosis may have played a far greater role in biological evolution than was previously thought. The symbiosis that made the colonization of land by plants possible was as a consequence of the development of arbuscular mycorrhizae. However, the present review draws attention to the role of lichens in assisting in this transition and to the phenomenon of lichenization. The recent discovery of lichen fossils in marine phosphorites in China and molecular clock estimates indicate that lichenized fungi were already present in Precambrian seas and, like contemporary species, played a role as pioneers in occupying new habitats. There is evidence that the holistic properties of associations between fungi and cyanobacteria and/or green algae facilitated the transition onto land and the subsequent colonization of terrestrial habitats. A key role in this process was played by poikilohydry. The algal components of delichenized fungi, along with lichens and photosynthetic aquatic organisms all contributed to the increase in atmospheric oxygen. Lichens, fungi and cyanobacteria settling on land were undoubtedly important in the formation of soils and thereafter enhancing their fertility. It is suggested that vascular and other green plants were able grow on these primitive soils that were stabilized by the growth of lichens, algae and cyanobacteria in a similar way to those which play a role in desert crusts at the present time.

The chemical conditions of the late Archean Hamersley basin inferred from whole rock and pyrite geochemistry with Δ33S and δ34S isotope analyses

The well-preserved late Archean sedimentary rocks of the Fortescue and Hamersley Basins in Western Australia offer fascinating insights into early earth ocean chemistry prior to the Great Oxidation Event (GOE). In this study, we use a combination of whole rock geochemistry, LA-ICPMS trace element analysis of sedimentary pyrite and pyrrhotite and SHRIMP-SI sulfur isotope analyses to elucidate the chemical changes in these sedimentary rocks. These proxies are used to examine chemical conditions of the ocean during the late Archean. Two to three periods of oxygen enrichment prior to the deposition of banded iron formations (BIF) can be identified. One minor stage of general increase in whole rock enrichment factors and trace element content of pyrite is observed up stratigraphy in the Jeerinah Formation, Fortescue Basin and a more substantial stage is present in the Paraburdoo and Bee Gorge Members of the Wittenoom Formation, Hamersley Basin. Some of the trace element enrichments indicate organic matter burial flux (Ni, Cr, Zn, Co and Cu) which suggests an increase in biological productivity. If the increased biological activity reflects an increase in cyanobacteria activity then an associated increase in oxygen is likely to have occurred during the deposition of the Bee Gorge Member. An increase in atmospheric oxygen would result in continental weathering of sulfide and other minerals, increasing the trace element content of the water column via erosion and avoiding excessive depletion of trace elements due to drawdown in seawater. Since some of these trace elements may also be limiting nutrients (such as Mo and Se) for the cyanobacteria, the degree of biological productivity may have further increased due to the increasing amount of trace elements introduced by oxygenation in a positive feedback loop. These periods of increased productivity and oxygen rise stopped prior to the onset of BIF deposition in the Hamersley Basin. This may be due to the ocean reaching an oxidation threshold, enabling the precipitation of hematite and magnetite BIF. The BIF deposition caused depletion of ocean nutrients such as phosphate, severely limiting the growth of cyanobacteria, and thus limiting further oxygen production.

Oxygenation of Ediacaran Ocean recorded by iron isotopes

The increase in atmospheric oxygen during the late Neoproterozoic Era (ca. 800–542Ma) may have stimulated the oxygenation of the deep oceans and the evolution of macroscopic multicellular organisms. However, the mechanism and magnitude of Neoproterozoic oxygenation remain uncertain. We present Fe isotopes, Fe species and other geochemical data for two sections of the Doushantuo Formation (ca. 635–551Ma) deposited after the Nantuo glacial episode in the Yangtze Gorge area, South China. It is highlighted that highly positive δ56Fe values reflect a lower oxidation rate of Fe(II)aq under ferruginous conditions, and in turn near zero δ56Fe values indicate oxidizing conditions. Our study suggests that during the deposition of the bottom of Member II of the Doushantuo Formation the shallow seawater was oxic, but the deep water was characterized by ferruginous conditions, which is consistent with a redox chemical stratification model. Subsequent anoxic conditions under shallow seawater, represented by positive δ56Fe and negative δ13Ccarb excursions, should be ascribed to the upwelling of Fe(II)aq and dissolved organic carbon (DOC)-rich anoxic deep seawater. The oxidation of Fe (II)aq and DOC-rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well-known negative δ13Ccarb excursions (ENC2) and positive δ56Fe excursions in the middle of Doushantuo Formation. Meanwhile, the upwelling Fe (II)aq- and DOC-rich anoxic deep seawater could increase photosynthetic activity. The balance between oxygen consumption and production was most important criteria for the oxygenation of Early Ediacaran Ocean and diversity of eukaryotic organisms.

Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era

The oxygenation of the Earth's deep oceans is often thought to have triggered the evolution of simple animals. A review article proposes that instead, the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea. The Neoproterozoic era (about 1,000 to 542 million years ago) was a time of turbulent environmental change. Large fluctuations in the carbon cycle were associated with at least two severe — possible Snowball Earth — glaciations. There were also massive changes in the redox state of the oceans, culminating in the oxygenation of much of the deep oceans. Amid this environmental change, increasingly complex life forms evolved. The traditional view is that a rise in atmospheric oxygen concentrations led to the oxygenation of the ocean, thus triggering the evolution of animals. We argue instead that the evolution of increasingly complex eukaryotes, including the first animals, could have oxygenated the ocean without requiring an increase in atmospheric oxygen. We propose that large eukaryotic particles sank quickly through the water column and reduced the consumption of oxygen in the surface waters. Combined with the advent of benthic filter feeding, this shifted oxygen demand away from the surface to greater depths and into sediments, allowing oxygen to reach deeper waters. The decline in bottom-water anoxia would hinder the release of phosphorus from sediments, potentially triggering a potent positive feedback: phosphorus removal from the ocean reduced global productivity and ocean-wide oxygen demand, resulting in oxygenation of the deep ocean. That, in turn, would have further reinforced eukaryote evolution, phosphorus removal and ocean oxygenation.