Geostable molecules and the Late Archean 'Whiff of Oxygen'

Geoscientists continue to debate the course of oxygenation of Earth's surface environment. The prevailing scenarios include the one articulated by Cloud, Holland and Walker which proposes an initially anoxic or very low O2 atmosphere but one where oxygen-releasing Photosystem II appeared early. A strong imbalance between sources & sinks kept the atmospheric pO2 low until the 'Great Oxidation Event' or GOE at c. 2.45 Ga. An alternative scenario is that the GOE marks the first appearance of oxygen-releasing photosynthesis (Kopp et al., 2005). However, there is a wealth of geochemical and paleontological data that is consistent with the presence of both cyanobacteria (e.g. Bosak et al., 2009) and traces of environmental oxygen (e.g. Anbar et al., 2007) several hundred million years prior to the GOE. Here we report new studies of 2.7-2.5 Ga sedimentary rocks from the Transvaal Supergroup of the Kaapvaal Craton and from the Mt. Bruce Supergroup of the Pilbara Craton. We used improved analytical techniques which identified and excluded minor sources of contaminating hydrocarbons. We also focused on mineral-occluded hydrocarbons that show distribution patterns that co-vary with lithology and replicated earlier identifications of molecular fossils that require oxygen for their biosynthesis (Waldbauer et al., 2008, 2011). Some sediment horizons in the studied sections contain diagenetic products of pigments characteristic of phototrophic green sulfur bacteria and their co-variance with inorganic proxies, each leading to similar paleoenvironmental reconstructions and confirming the authenticity of this signal. The carotenoid-derived biomarkers, although diagnostic for anoxygenic phototrophs, indicate that the seas of the Hamersley and Transvaal Basins provided an intermittent supply of hydrogen sulfide to the photic zone. Indirectly, this suggests replenishment of the marine sulfate pool from the oxidative weathering of metal sulfides. Organic sulfur in these same sediments exhibits a 'MIF' signal that is significantly amplified compared to co-occurring pyrite sulfur. Limited isotopic exchange between the organic and inorganic sulfur pools suggests Archean origin of these organic sulfur compounds. We also report new results from the 2012 Agouron Pilbara drilling project. Anbar A.D. et al. A whiff of oxygen before the great oxidation event. Science 317, 1903-1906. (2007). Bosak T. et al., Morphological record of oxygenic photosynthesis in conical stromatolites. Proc. Natl. Acad. Sci. USA 106:10939-10943 (2009). Kopp, R.E. et al.,The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis. Proc. Natl. Acad. Sci. USA 102: 11131-11136 (2005). Waldbauer J.R. et al., Late Archean molecular fossils from the Transvaal Supergroup record the antiquity of microbial diversity and aerobiosis. Precambrian Research 169, 28-47 (2008). Waldbauer J.R. et al., 2011. Microaerobic steroid biosynthesis and the molecular fossil record of Archean life. Proceedings of the National Academy of Sciences (USA) 108, 13409-13414