Nitrogen isotope evidence for anoxic deep marine environments from the Mesoarchean Mosquito Creek Formation, Australia

Abstract Current evidence for oxygenated environments in the Mesoarchean is limited to the shallowest marine and fluvio-lacustrine settings. It is not until the Neoarchean that signs of oxygenated surface waters above outer shelf and basinal depositional environments become evident. In order to further explore the Mesoarchean redox landscape for signs of basinward surface water oxygenation, we present nitrogen and carbon isotope ratios from the turbiditic Mosquito Creek Formation of the Nullagine Group (∼2.9 Ga). The δ 15 N and δ 13 C org values are invariant around −1.8‰ and −32‰ respectively throughout a 70 m section of drill-core, suggesting an ecosystem dominated by nitrogen fixers (anaerobic nitrogen cycling) and CO 2 fixation by the Calvin Cycle. When compared with other Archean isotopic data, these results (i) provide further evidence that the Mosquito Creek Formation was deposited in a marine basin, and (ii) contain δ 15 N values that highlight the prevalence of nitrogen fixation by Mo-nitrogenase and the dearth of aerobic nitrogen metabolisms in the Mesoarchean.