Geochemistry of carbon and sulfur in the 2.7 Ga stromatolitic carbonate and shale (ABDP#10 core) from Meentheena, Western Australia

Abstract:

Earth’s surface environments about 2.7 Ga ago likely experienced drastic changes such as rapid continental growth, negative excursion of organic carbon isotopes, and positive excursion of mass-independently fractionated sulfur isotopes. Discovery of biomarkers indicating cyanobacteria, although questionable, was also claimed. The coeval shallow ocean could have contained appreciable amount of molecular O₂ produced by cyanobacteria, but it has been unknown whether deep sea was oxygenated by ocean circulation. In order to unravel the shallow water environment, we obtained modern-weathering-free 2.7 Ga drilcore samples of stromatolitic carbonate and shale by Archean Biosphere Drilling Project (ABDP#10 core) in Meentheena, NE Western Australia. The purpose of this study is to extract information regarding the surface ocean chemistry by abundance (of various species) and corresponding stable isotope compositions of carbon and sulfur.

Based on mass balance calculation, we obtained a much reduced fraction of organic carbon to total carbon (f_org) to be 0.11 (c.f., Phanerozoic average is 0.2), suggesting that aerobic and/or anaerobic decomposition of organic matter was vigorous. The very low δ¹³C_org values suggest carbon recycling involving methanogenesis by methanogen (e.g., 2CH₂O → CH₄ + CO₂) accompanied by large isotope fractionation. Generated CH₄ was oxidized by methylotroph to CO₂, which was reused to form organic matter by biochemical processes such as photosynthesis. Either O₂ or SO₄^2- was required to facilitate CH₄ oxidation, suggesting availability of oxidized species in the 2.7 Ga shallow sea environment. Abundance of S_py with near zero δ³⁴S_py values suggest syndepositional and/or diagenetic formation of bacteriogenic pyrite utilizing sulfate formed by oxidation of mantle-derived sulfur (δ³⁴S ≈ 0‰). A plot of S_py vs. C_org abundance shows a trend falling between a Phanerozoic normal marine trend (S_py/C_org = 0.36) and a freshwater trend (S_py << C_org), suggesting formation of S_py in environments fluctuating between near normal marine and fresh water conditions with low SO₄^2- concentration.

We suggest that the 2.7 Ga shallow water near Meentheena was slightly oxic in lacustrine setting.