Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen ...comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O
currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O
sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O
that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.
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.
The rise of oxygen on the early Earth (about 2.4 billion years ago) caused a reorganization of marine nutrient cycles, including that of nitrogen, which is important for controlling global primary ...productivity. However, current geochemical records lack the temporal resolution to address the nature and timing of the biogeochemical response to oxygenation directly. Here we couple records of ocean redox chemistry with nitrogen isotope (
N/
N) values from approximately 2.31-billion-year-old shales of the Rooihoogte and Timeball Hill formations in South Africa, deposited during the early stages of the first rise in atmospheric oxygen on the Earth (the Great Oxidation Event). Our data fill a gap of about 400 million years in the temporal
N/
N record and provide evidence for the emergence of a pervasive aerobic marine nitrogen cycle. The interpretation of our nitrogen isotope data in the context of iron speciation and carbon isotope data suggests biogeochemical cycling across a dynamic redox boundary, with primary productivity fuelled by chemoautotrophic production and a nitrogen cycle dominated by nitrogen loss processes using newly available marine oxidants. This chemostratigraphic trend constrains the onset of widespread nitrate availability associated with ocean oxygenation. The rise of marine nitrate could have allowed for the rapid diversification and proliferation of nitrate-using cyanobacteria and, potentially, eukaryotic phytoplankton.
Molybdenum (Mo) isotopes have great potential as a paleoredox indicator, but this potential is currently restricted by an incomplete understanding of isotope fractionations occurring during key ...(bio)geochemical processes. To address one such uncertainty we have investigated the isotopic fractionation of Mo during adsorption to a range of Fe (oxyhydr)oxides, under variable Mo/Fe-mineral ratios and pH. Our data confirm that Fe (oxyhydr)oxides can readily adsorb Mo, highlighting the potential importance of this removal pathway for the global Mo cycle. Furthermore, adsorption of Mo to Fe (oxyhydr)oxides is associated with preferential uptake of the lighter Mo isotopes. Fractionations between the solid and dissolved phase (Δ
98Mo) increase at higher pH, and also vary with mineralogy, increasing in the order magnetite (Δ
98Mo
=
0.83
±
0.60‰)
<
ferrihydrite (Δ
98Mo
=
1.11
±
0.15‰)
<
goethite (Δ
98Mo
=
1.40
±
0.48‰)
<
hematite (Δ
98Mo
=
2.19
±
0.54‰). Small differences in isotopic fractionation are also seen at varying Mo/Fe-mineral ratios for individual minerals. The observed isotopic behaviour is consistent with both fractionation during adsorption to the mineral surface (a function of vibrational energy) and adsorption of different Mo species/structures from solution. The different fractionation factors determined for different Fe (oxyhydr)oxides suggests that these minerals likely exert a major control on observed natural Mo isotope compositions during sediment deposition beneath suboxic through to anoxic (but non-sulfidic) bottom waters. Our results confirm that Mo isotopes can provide important information on the spatial extent of different paleoredox conditions, providing they are used in combination with other techniques for evaluating the local redox environment and the mineralogy of the depositing sediments.
The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth's habitability
. Early atmospheric oxygenation occurred over a protracted period ...of extreme climatic instability marked by multiple global glaciations
, with the initial rise of oxygen concentration to above 10
of the present atmospheric level constrained to about 2.43 billion years ago
. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago
, which represents the estimated timing of irreversible oxygenation of the atmosphere
. Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron-sulfur-carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10
of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated.
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