4:45 PM - 5:00 PM
[BPT26-03] Dynamics of Proterozoic oceanic euxinia and its impact on the biosphere
Keywords:Proterozoic, oceanic redox states, ocean biogeochemical cycle model, euxinia
To investigate the dynamics of oceanic biogeochemical response during the Paleoproterozoic eon, we improved upon the CANOPS model, in which coupled C-N-O-P-S marine biogeochemical cycles and a series of redox reactions were adequately taken into account. We newly developed an open system modeling approach for marine sulfur cycling in which the oceanic sulfur balance is explicitly evaluated.
In this study we show that a plausible cause and effect chain of events that is consistent with the geological records of the LJE aftermath can be obtained when a dramatic decrease in atmospheric oxygen level (from > 50%PAL to < 1%PAL; PAL=present atmospheric level) is assumed: the deoxygenation of an atmosphere-ocean system stimulates the microbial sulfate reduction, resulting in a transitional (~20 Myr) expansion of euxinia until the ocean reaches a new steady state. A mass balance calculation of Mo also demonstrates that LJE aftermath is marked by an expansion of euxinia to ca. 9-40% of the whole seafloor. Under such conditions Mo levels would decrease to as low as ca. 2.0-6.5 nM where nitrogen fixation by Fe-Mo nitrogenase is very sensitive to Mo concentration. We therefore suppose that waxing and waning of the atmospheric oxygenation state in the Paleoproterozoic could have affected evolution and diversification of the aerobic bacteria and/or eukaryotes through dynamic oceanic euxinia.