Strong evidence for a weakly oxygenated ocean-atmosphere system during the Proterozoic.

Earth's surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debated-foremost oxygen (O2) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstones-Fe-rich sedimentary rocks deposited in nearshore marine settings-as a proxy for O2 levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O2 in the shallow oceans, settings crucial to eukaryotic evolution. Low O2 in surface waters can be linked to markedly low atmospheric O2-likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O2 levels between ca 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O2 levels during this interval may have selected for life modes adapted to more oxygenated habitats.

Neoproterozoic syn-glacial carbonate precipitation and implications for a snowball Earth.

The Neoproterozoic 'snowball Earth' hypothesis suggests that a runaway ice-albedo feedback led to two intense glaciations around 717-635 million years ago, and this global ice cover would have drastically impacted biogeochemical cycles. Testing the predictions of this hypothesis against the rock record is key to understanding Earth's surface evolution in the Neoproterozoic. A central tenet of the snowball Earth hypothesis is that extremely high atmospheric CO2  levels-supplied by volcanic degassing over millions of years-would be required to overcome a strong ice-albedo feedback and trigger deglaciation. This requires severely diminished continental weathering (and associated CO2 drawdown) during glaciation, and implies that carbonate minerals would not precipitate from syn-glacial seawater due to a lack of alkalinity influxes into ice-covered oceans. In this scenario, syn-glacial seawater chemistry should instead be dominated by chemical exchange with the oceanic crust and volcanic systems, developing low pH and low Mg/Ca ratios. However, sedimentary rocks deposited during the Sturtian glaciation from the Adelaide Fold Belt-and contemporaneous successions globally-show evidence for syn-sedimentary dolomite precipitation in glaciomarine environments. The dolomitic composition of these syn-glacial sediments and post-glacial 'cap carbonates' implies that carbonate precipitation and Mg cycling must have remained active during the ~50 million-year Sturtian glaciation. These syn-glacial carbonates highlight a gap in our understanding of continental weathering-and therefore, the carbon cycle-during snowball Earth. In light of these observations, a Precambrian global biogeochemical model (PreCOSCIOUS) was modified to explore scenarios of syn-glacial chemical weathering, ocean chemistry and Sturtian carbonate mineralogy. Modelling results suggest that a small degree of chemical weathering during glaciation would have been capable of maintaining high seawater Mg/Ca ratios and carbonate precipitation throughout the Sturtian glaciation. This is consistent with a severe ice age during the Sturtian, but challenges predictions of biogeochemical cycling during the endmember 'hard snowball' models. A small degree of continental weathering might also help explain the extreme duration of the Sturtian glaciation, which appears to have been the longest ice age in Earth history.

Subglacial meltwater supported aerobic marine habitats during Snowball Earth.

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.