EVOLUTION OF MARINE REDOX LANDSCAPE CONTROLLED BY THE PHYTOPLANKTON COMMUNITY STRUCTURE

It is proposed that relatively small extent of sulfidic (H₂S-ernchment) seafloor (1-10% of modern seafloor area) would be sufficient to scavenge seawater Mo, retarding the evolution of eukaryotes during the ‘boring billion’ (1.8-0.8 Ga), while the radiation of eukaryotes immediately after the Marinoan snowball Earth event (635 Ma) occurred in the context of ocean oxygenation. Persistent sulfidic condition, in addition to low atmospheric O₂ level, requires sufficient supply of organic matter, which consumes seawater oxidants (O₂, nitrate, MnO₄, and ferric iron), allowing bacterial sulfate reduction in water column. Organic matter was mainly produced by phytoplanktons in the surface ocean, including the prokaryotic cyanobacteria and eukaryotic phytoplanktons, the latter of which might be represented by acanthomorphic acritarchs. As compared with cyanobacteria, eukaryotic phytoplanktons have larger size and thicker walls with more complex structures, and thus organic matter produced by eukaryotes might have higher chance of escaping from oxidation in seawater. For above reasons, change of phytoplankton community structure might modify the redox profile of water column below the euphotic zone. In order to evaluate how marine redox landscape could be affected by the phytoplankton community structure, here we developed a 1-D biochemical model to simulate the vertical redox profile in water column at various phytoplankton community structures. Our modeling results indicate that, similar to an increase of O₂ level in atmosphere, with invariant organic matter input, an increase in the fraction of eukaryotic phytoplanktons tends to attenuate the thickness of sulfidic zone in water column. Thus, in addition to low O₂ level in atmosphere, cyanobacteria as the predominant primary producer might be another important reason for the widespread sulfidic continental margins in the ‘boring billion’. Similarly, significant reduction of sulfidic seafloor after the Marinoan snowball Earth might be the direct consequence of diversification of eukaryotes, though atmospheric O₂ level may not necessarily increase concurrently. We conclude that ocean oxidation caused by phytoplankton community structure evolution in surface ocean could predate the oxygenation in atmosphere.