Bacteriohopanepolyols with A-ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa. When preserved in ancient rocks, the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies. Considering both their occurrences in modern organisms, and their Proterozoic and Phanerozoic sedimentary distributions, hopanes carrying 2-methyl or 3-methyl substituents are proposed to be derived from cyanobacteria and oxygen-respiring methanotrophs, respectively. Here we report the distribution of methylhopanes in 2.72-2.56 billion-year-old, Neoarchean rocks from the Hamersley Province on the Pilbara Craton. The relative abundance of C31-3β-methylhopane, but not that of C31-2α-methylhopane, shows a strong correlation to the carbon isotopic composition of co-occurring kerogen (insoluble particulate organic matter). The unanticipated nature of this relationship provides evidence for a shallow-water locus of carbon cycling through aerobic oxidation of CH4 and, coincidentally, a means to demonstrate biomarker syngenicity. 2α-Methylhopanes are most abundant in both shale and carbonate from shallow-water sediments, and a positive correlation to carbonate abundance, contrasts to variable, but generally lower, 2α-methylhopane abundances of deeper water facies. These observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow-water ecosystems providing molecular oxygen, fixed carbon, and possibly fixed nitrogen.
- carbon isotope