MULTIPLE INCREASES IN ATMOSPHERIC OXYGEN AND MARINE PRODUCTIVITY THROUGH THE NEOPROTEROZOIC AND PALEOZOIC

It is widely believed that Earth’s oxygenation proceeded in two steps, with the second occurring in the late Neoproterozoic. Records of redox-sensitive trace metals (RSMs) in anoxic shales are one of the strongest lines of evidence for a Neoproterozoic oxygenation event, where increased RSM concentrations are interpreted to represent a decrease in the proportion of seafloor area covered by reducing sinks. Here, we present statistical learning analyses of a large dataset of Tonian-Carboniferous geochemical data and associated geological context assembled by the Sedimentary Geochemistry and Paleoenvironments Project. Contrary to previous analyses of raw data, we show that (when statistically deconvolved from other geochemical and geological context data) there was no major stepwise increase in Mo or U concentrations until the Devonian. Our analyses do, however, indicate there was a stepwise increase in shale total organic carbon (TOC) around the Ediacaran-Cambrian boundary, followed by further increases in the Paleozoic. Higher organic carbon burial is expected to correlate with higher atmospheric oxygen and marine productivity on geologic timescales – suggesting that other biogeochemical processes may have buffered marine Mo-U concentrations despite changing surface oxygenation. We combine the cGENIE Earth system model with a three-sink Mo-U mass balance model and the CANOPS biogeochemical model to investigate the range of atmospheric oxygen and marine productivity scenarios consistent with the deconvolved trends we observe in Mo, U and TOC. We show that organic carbon and trace metal burial rates have different sensitivities to increasing atmospheric oxygen and productivity. TOC is sensitive to surface oxygen changes at low atmospheric oxygen levels that lead to widespread reducing bottom water conditions, while RSMs are more sensitive at higher levels. Thus, deconvolved geochemical signals can be explained by multiple increases in atmospheric oxygen and marine productivity through the late Neoproterozoic and Paleozoic that differently affected TOC and RSM records. We therefore suggest the transition to broadly modern levels may have proceeded across this entire time interval, with implications for hypotheses linking these environmental changes to the evolution of early animal ecosystems.