Carbon Isotope Excursions of Archean Organic Matter (~3.0-2.5Ga) from Four Drilling Cores at the Hamersley Basin, Western Australia

Archean biological activities have played an important role for the evolution of the earth's surface environment. In particular, the carbon isotopic composition of Archean organic matter has been used to infer types and predominance of the biological activities. As a part of Archean Biosphere Drilling Project (ABDP), we determined carbon isotopic compositions of bulk organic matter in Archean shales (~3.0-2.5Ga) from four drilling cores at the Hamersley Basin, Western Australia. Organic carbon from Mosquito Formation (~2.9-3.2Ga) has ~-31% (relative to PDB) with much less variation through a ~130m core section, suggesting normal photosynthesis by algae and/or cyanobacteria using rubisco. At the ~2.8Ga black shale interbedded in Mount Roe basalt, organic matter is extremely 13C-depleted (~-50%) through ~20m, implying abundant methane assimilation. Through a ~140m core section in the Jeerinah Formation (~2.7Ga), carbon isotopic composition is highly variable between -44 and -35% with high organic carbon contents (~8 wt%). Then at the Mount McRae Shale (~2.5Ga), organic carbon is gradually enriched in 13C, varying from -37 and -32% through a ~25m core section. The carbon isotope excursion of this study indicates that methane assimilation suddenly occurred in the Hamersley Basin at ~2.8Ga, followed by gradually recovering to normal photosynthesis around ~2.5Ga.