Archean biological activity evolved with environ mental changes of the earth's surface inferred from biomarkers and stable isotopic compositions

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Photoautotrophic organisms evolving O_2 were already active as early as 3.5 Ga ago from studies of microfossils, and chemical fossils such as carbon isotopic compositions of organic matter. Eukaryotic biomarkers were also reported to be present in the Archean ocean. The popular theory, however, has been that the evolved O_2 was consumed to oxidize the earth's terrestrial environment and the atmosphere should be essentially O_2-free until 2.2 Ga ago. The Jeerinah Formation ( 2.7 Ga) in the Hamersley Basin, Western Australia is mainly composed of organic carbon-and pyrite-rich black shales. In this study, using a drilling core section, chemical and isotopic compositions of sedimentary rock samples of the Jeerinah Formation were determined to characterize biological activites involving redox conditions of the ocean.The Jeerinah shales contain high amounts of organic carbon (up to 8 wt%) and pyrite sulfur (up to 7.5 wt%). The isotopic composition of organic carbon varies systematically thr … More ough the stratigraphic section from -45 to -35% in δ^<13>C. The carbon isotope excursion could be explained by two-endmember mixing of organic matters between isotopically heavy component produced by normal photoautotrophs and light component produced by methane-assimilated heterotrophs. The carbon isotope excursions are accompanied with sulfur isotope excursions, from -10 to +10% in δ^<34>S. Sulfur isotope fractionation should be more than 20% by sulfur-reducing bacteria (SRB). The organic carbon contents are positively correlated with the nitorogen contents (N/C=0.006 by atomic). Bulk nitrogen isotopic composition falls mainly in a range from 0 to +5% in δ^<15>N ; these values are similar to those of common organisms in modern oceans. The shale with highest organic carbon content has a δ^<15>N value -%, probably indicating nitrogen fixation by cyanobacteria as a primary producer. However, the shale has an intermediate δ^<13>C value (-40%), possibly due to a contribution from chemoautotrophs or heterotrophs such as methanotrophs and SRB. Molybdenum concentrations are positively correlated with organic carbon content except two samples. The Mo could be weathered under oxic condition, and fixed in organic materials. A negative Eu shift is observed during positive shifts in δ^<13>C of organic carbon and δ^<34>S of sulfide. Such element behaviors suggest that weathering, transportation and fixation mechanism of Mo and REE should be affected under oxic condition. In contrast to a currently popular view that Archean oceans were entirely anoxic the results of our study suggest that the redox structure and biological activity in the Hamersley Basin were much more variable. Less