Lipid biomarkers and stable carbon isotopes can track the evolution of microbial communities and carbon cycling, respectively, in preserved organic matter. Traditionally, carbon isotopes are measured in bulk materials, though technological advances allow us to measure the carbon isotope composition of individual hydrocarbon molecules, providing direct insights into the carbon source and microbial metabolism used for biosynthesis. During sedimentary diagenesis, lipids are incorporated into a complex macromolecular matrix—(proto)-kerogen. Lipids associated with or tightly bound within the kerogen matrix are not routinely analysed, thus a large repository of geochemical information relating to mode of binding and preservation remains unexplored.
Presented here are two studies of marine organic matter preservation and cycling, from the enigmatic late Ediacaran Shuram excursion of South Oman and from the modern hypersaline microbial mats of Guerrero Negro, Baja California, Mexico.
From South Oman, we present the first detailed bulk isotope, lipid biomarker, and compound-specific carbon isotope compilation from an organic-rich, deeper water facies that captures the unique ‘Shuram excursion’—a carbon isotope excursion in carbonate (down to -12‰) that is not paralleled in magnitude by the coeval organic carbon phases. Our results definitively demonstrate that preserved organic matter is syngenetic and marine-derived and has two distinct organic carbon isotopic sources. The extractable alkane hydrocarbons preserve a smaller overall magnitude carbon isotope excursion in phase with a primary surface seawater origin.
From Guerrero Negro, we investigated kerogen-bound lipid biomarker profiles from two cores of different salinity and compare them to lipids extracted using conventional techniques. Additionally, to investigate the mode of binding and incorporation of sterols and hopanoids into kerogen during diagenesis, we extracted kerogen-bound lipids of pre-extracted microbial mat residues that were subjected to three selective chemical degradations. Our findings indicate a suite of lipid biomarkers, including sterols and hopanoids, are rapidly incorporated into kerogen via strong covalent linkages and preserved during sedimentary diagenesis. The absence of sterols or other polycyclic biomarker compounds classes is unlikely the result of taphonomic bias in periods of Earth history where benthic organic matter source inputs are pronounced.