Biomarker proxies for redox-controlled biodegradation: Deciphering organic matter preservation and carbon cycling

Organic matter degradation is a vital component of the carbon cycle, which has been extensively studied in modern marine and lacustrine systems. However, the degradation process in their deep-time counterpart is poorly understood due to the lack of suitable proxies. We propose using biomarker concentrations for tracing biodegradation and carbon cycle that happened in the deep-time lacustrine systems, based on two hypotheses: 1) Hopanes will be enriched against total organic carbon (TOC) if the original organic matter has undergone significant reworking, as hopanoids are more recalcitrant than many organic compounds and are derived from heterotrophic bacteria during degradation; 2) The biomarker concentrations relative to sedimentary rock should positively correlate with TOC if the original organic matter undergoes limited biodegradation, while strong reworking will decouple biomarker concentrations and TOC. To test our hypotheses, we focus on the Songliao Basin in Northeast Asia, where the Cretaceous Qingshankou Formation records lacustrine expansion and contraction offering an exceptional natural laboratory to investigate the relationship among redox conditions, biodegradation, and biomarker concentrations. Our study shows that in more oxidized conditions (low TOC and Mo_EF), hopane concentrations relative to TOC (hopane/TOC) can be two orders of magnitude higher than those in more reducing conditions (high TOC and Mo_EF) indicating that hopane concentration reflects the intensity of redox-controlled biodegradation. We also analyze the relationships between biomarker concentrations (biomarker/rock) and TOC from the perspective of the microbial communities living in the ancient lacustrine system. The biomarkers, such as sterane, pristane, phytane, and gammacerane, generated by microbes living above the oxycline or in the transitional zone show no correlation with TOC, whereas the aryl isoprenoids generated beneath the oxycline correlate well with TOC. The distinct correlation patterns suggest that organic matter produced beneath the oxycline undergoes limited biodegradation and predominates the sedimentary organic matter, whereas organic matter generated from surface water is strongly reworked and could be the secondary contributor to sedimentary organic matter. We deduced that biodegradation is stronger in lacustrine than in marine environments after further compiling previously reported hopane concentrations of black shales and modern sediments. Our study highlights the application of biomarker concentrations for evaluating biodegradation and understanding carbon transfer from the photic zone to sediments in geological records.