Resistant degradation of petrogenic organic carbon in the weathering of calcareous rocks

Abstract

Weathering of rock-derived organic carbon (OC) is an important source of atmospheric CO₂ in the global carbon cycle, contributing to the long-term regulation of climates. Despite numerous investigations on clastic rocks, weathering behaviors of OC in calcareous rocks remain poorly constrained due to their conventional recognition of organic-lean features. Here, we analyzed bulk OC and biomarkers along weathering profiles of organic-rich calcareous rocks from the Green River Formation. Total OC and ramped-temperature pyrolysis/oxidation results reveal a minimal degradation of OC. Given a substantial fraction of bulk OC and a rich pool of biomolecules preserved in carbonate matrix, our results suggest that the limited bulk-level degradation is attributed to the shielding of OC in carbonate matrix. Furthermore, contrasting biomarker proxies are observed between free and associated forms, implying that carbonate-associated lipids are resistant to degradation during weathering; whereas clearly decreasing patterns of free lipids at the molecular level correspond to weathering-induced microbial degradation of excluded OC. Overall, our findings reveal that calcareous rocks might be an overlooked, yet understudied, carbon pool involved in the global organic carbon cycle.