TRACING MICROBIAL SOURCES OF AMORPHOUS ORGANIC MATTER HELPS RECONSTRUCT PALEOENVIRONMENTAL EVOLUTION OF HYPERSALINE SHALES FROM THE EOCENE QIANJIANG FORMATION

Efforts to understand the role of microbial communities in controlling the composition and preservation of sedimentary organic matter (OM) in hypersaline environments face the challenge of assessing stratigraphic variability in such OM-lean sequences. Thus, development of an effective approach that enables determination of the amorphous OM (AOM) sources for a sequence of rhythmites from the Eocene Qianjiang Formation in the Jianghan Basin affords promise for advancing this aim, especially in combination with stratigraphic variations of microbial biomarkers and carbon isotopes. Collectively, these data enable characterization of microbial sources of OM and elucidation of geochemical proxies that record temporal changes in environmental conditions during sediment deposition, including redox status and water column stratification. Petrographic evidence for variations in AOM composition quantified as proportions of light versus dark material in fluorescent light correlates strongly with the sterane/hopane ratios that record the balance of OM contributions from algae versus bacteria and reflect its extent of degradation. Hence, the characteristics of the AOM reflect temporal differences in microbial communities in response to fluctuating environmental conditions, including alternating episodes of water column stratification that create anoxic and mixing events promoting bottom water oxygenation, likely corresponding to drier and wetter climate cycles caused by changes in precipitation and lake levels. The sequence of saline rhythmites in the Jianghan Basin records these hydrologic cycles, which are also interrupted by clastic dilution events that lower OM contents in the sediments and may serve to reset the stratification cycle. Thus, the ability to better characterize and quantify AOM through an innovative petrographic method advances the understanding of the role of microorganisms as sources of AOM and in its preservation, which is enhanced by integration with biomarker and isotopic data. Adopting this research approach in investigating other hypersaline lacustrine sequences may further benefit the recognition of controls on AOM in these depositional settings.