Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea

Abstract

The disappearance of iron formations from the geological record approximately 1.8 billion years (Gyr) ago was the consequence of rising oxygen levels in the atmosphere starting 2.45-2.32 Gyr ago. It marks the end of a 2.5-Gyr period dominated by anoxic and iron-rich deep oceans. However, despite rising oxygen levels and a concomitant increase in marine sulphate concentration, related to enhanced sulphide oxidation during continental weathering, the chemistry of the oceans in the following mid-Proterozoic interval (approximately 1.8-0.8 Gyr ago) probably did not yet resemble our oxygen-rich modern oceans. Recent data indicate that marine oxygen and sulphate concentrations may have remained well below current levels during this period, with one model indicating that anoxic and sulphidic marine basins were widespread, and perhaps even globally distributed. Here we present hydrocarbon biomarkers (molecular fossils) from a 1.64-Gyr-old basin in northern Australia, revealing the ecological structure of mid-Proterozoic marine communities. The biomarkers signify a marine basin with anoxic, sulphidic, sulphate-poor and permanently stratified deep waters, hostile to eukaryotic algae. Phototrophic purple sulphur bacteria (Chromatiaceae) were detected in the geological record based on the new carotenoid biomarker okenane, and they seem to have co-existed with communities of green sulphur bacteria (Chlorobiaceae). Collectively, the biomarkers support mounting evidence for a long-lasting Proterozoic world in which oxygen levels remained well below modern levels.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Although there are a few instances of locally developed mid-Proterozoic euxinia (e.g., Shen et al, 2003;Brocks et al, 2005;Lyons et al, 2009), the idea of global-scale deep euxinia has also recently fallen out of favour. All available data that constrain the marine landscape in the midProterozoic ocean are consistent with only locally developed euxinic conditions along productive continental margins (e.g., in oxygen-minimum zones) or within intracratonic basins (Scott et al, 2008;Poulton et al, 2010;Planavsky et al, 2011;Lyons et al, 2012;Reinhard et al, 2013).…”

Paleoproterozoic sterol biosynthesis and the rise of oxygen

“…The derived steroidal and hopanoid hydrocarbons have similar thermodynamic stabilities and their concentrations in sedimentary rocks and petroleum decrease similarly as thermal maturity increases (Farrimond et al, 1998;Peters et al, 2005;van Graas, 1990). Yet the majority of Paleoproterozoic and Mesoproterozoic sedimentary rocks described that are known to preserve biomarkers appear to be devoid of eukaryotic steranes, but do contain C 27 -C 35 hopane hydrocarbons of bacterial origin (Blumenberg et al, 2012;Brocks et al, 2005;Flannery and George, 2014;Luo et al, 2015). From a micro-and macrofossil perspective, ample evidence exists for the presence of unambiguously eukaryotic fossil assemblages in the Mesoproterozoic (Butterfield, 2000;Javaux et al, 2001;Javaux et al, 2004;Knoll et al, 2006;Zhu et al, 2016).…”

“…In doing so, they also identified criteria by which earlier reports of steranes, in particular, could be evaluated. Multiple studies of the 1.64 Ga Barney Creek Formation, which has been penetrated by wells drilled into terraines that are at or below the maturity threshold for oil generation (Crick et al, 1988) reveal the presence of a wide range of biomarker hydrocarbons including acyclic isoprenoids, tricyclic terpanes, pentacyclic hopanoid and gammacerane triterpanes, and C 40 carotenoid-derived saturated and aromatic hydrocarbons (Brocks et al, 2005;Brocks and Schaeffer, 2008;Summons et al, 1988). A rigorous examination of the steranes in Barney Creek Formation sediments shows their presence but with a unique distribution dominated by 4-methyl and desmethyl triaromatic steroids at levels of 60-130 p.p.m.…”

Distribution of microbial terpenoid lipid cyclases in the global ocean metagenome

“…Because hopanoid lipids display regional and temporal patterns in their distributions, there must be taxonomic and/or physiological signals preserved in the records of these compounds. For example, 3-methylhopanes were reported at 1.64 Ga from the Barney Creek formation of Australia (Brocks et al, 2005). These compounds were interpreted to record an abundance of type I aerobic, methanotrophic bacteria in the upper water column of the Mesoproterozoic ocean at this time.…”