SULFUR, TRACE METALS, MICROBIAL COMMUNITY STRUCTURE, AND THE EVOLUTION OF ATMOSPHERIC OXYGEN

Major changes in the structure and composition of prokaryotic and eukaryotic populations have induced profound effects on biogeochemical cycles over Earth history. Atmospheric oxygen was no exception, but why it rose when it did is a matter of debate. Molecular biomarkers in Archean shales were used to infer the existence of oxygenic photosynthesizers (cyanobacteria), and eukaryotes at ~2.7 Ga. But, this is now in doubt, as the biomarkers likely post-date peak metamorphism of the rocks at 2.2 Ga. Stromatolites extend back to the Paleoarchean (ca. 3.49 Ga), and have long been taken as fossil cyanobacterial mats, but even modern shallow-water stromatolites can form from anoxygenic photoautotrophs. Banded iron-formations (BIFs) did not require direct (Fe²⁺)_aq to Fe³⁺ via O₂ from cyanobacteria, but were modulated by Fe(II)-oxidizers. These results seem to do away with an uncomfortably long (~300 Myr) delay between the apparent advent of cyanobacteria and inexorable O₂ rise by ~2.4 Ga. Before cyanobacteria, we suggest the microbial biosphere was dominated by photoferrotrophs. Archean oceans had high [Fe²⁺]_aq, but low [PO₄^3-]_aq and very low [SO₄^2-]_aq. Evidence suggests O₂ began to affect geochemical cycles of redox-sensitive metals, S and N in the oceans by 2.5 Ga. After, the atmosphere underwent a step-wise irreversible O₂ accumulation at ~2.4 Ga. Population doubling-time and diffusion models show that cyanobacteria should colonize the planet on timescales of months to years after origin. We propose that competition with photoferrotrophs and nutrient limitations stymied cyanobacterial takeover before ~2.45 Ga. We report preliminary results from a database of coupled multiple S-isotopes and trace metals (e.g. Ni, Co) reported for sulfides in Precambrian banded iron-formations to explore these secular changes.