Analysis of Molecular Biomarkers Covalently Bound Within Neoproterozoic Sedimentary Kerogen

Catalytic hydropyrolysis (HyPy) is a powerful analytical technique for fragmenting macromolecular organic matter, such as kerogen (insoluble sedimentary organic matter), and releasing covalently-bound molecular constituents including branched and cyclic biomarker hydrocarbons. Here we illustrate our molecular approach to paleobiology with lipid biomarker data collected from rock bitumens and kerogens hosted within sedimentary units of the Neoproterozoic Huqf Supergroup, South Oman Salt Basin, Sultanate of Oman. We emphasize that parallel analyses of free and bound biomarker pools affords more confidence that we have correctly identified syngenetic compounds. One enigmatic class of compounds that is prominent in many late Proterozoic and Cambrian sedimentary rocks and oils, including from the Huqf Supergroup, is a series of C14-C30 mid-chain methylalkanes which were originally denoted X-peaks. Despite their abundance in the Precambrian rock record, little is known about the organisms responsible for their biosynthesis. Here we propose a possible origin of X-peak methylalkanes from colorless sulfur bacteria (a very heterogeneous group of chemolithotrophic γ-proteobacteria). In modern marine settings, these bacteria are abundant mat formers wherever a sedimentary sulfide-rich horizon intersects the seafloor producing a steep geochemical redox gradient. This condition may have been met more commonly on shallow marine shelves in late Neoproterozoic basins and these benthic mats may have acted as environmental buffers consuming hydrogen sulfide. If our hypothesis is correct, proliferation of sulfide-oxidizing benthic microbial mats, commencing in the late Cryogenian in South Oman Salt Basin, implies unique and specific benthic conditions during the evolution of the earliest metazoans.