Biomarker and carbon isotope signatures constrain changes in organic matter inputs during the Oligocene-Miocene interval of Kutch Basin, western India: climatic and tectonic implications

The Late Oligocene-Miocene time interval represents a critical phase in Earth’s history, due to significant mountain-building events, drastic global climatic shift and resultant intensified silicate weathering. Despite its significance, the influence of fluctuating climatic conditions, particularly monsoon-driven shifts on organic matter (OM) sources, sediment input, and carbon cycling during this interval remains poorly understood. To address these gaps, we present lipid biomarkers data (n-alkanes and hopanoids), along with δ13Corg and TOC measurements, from the Oligocene-Miocene sections of the Kutch Basin, western India. The n-alkane distribution reveals a marked shift in OM sources: short-chain n-alkanes (n-C15-20) dominate in the Oligocene, indicating significant marine OM input, whereas long-chain n-alkanes (n-C27-35) were prevalent in the Early–Middle Miocene, reflecting enhanced terrestrial contributions. Biomarker indices (ACL, CPI, TAR) and hopanoid abundances further support this transition. Correspondingly, δ13Corg values shift from marine signatures (–17.53‰ to –21.48‰) in the Late Oligocene to more terrestrial values (–18.45‰ to –26.23‰) in the Early–Middle Miocene. Relatively higher Oligocene TOC compared to the Miocene reflects enhanced marine OM contribution; however, uniformly low TOC values (<2%) throughout both intervals reflect dilution due to coarse-grained lithology and predominantly oxic depositional conditions. The increased terrestrial inputs during the Miocene correlate with accelerated Himalayan–Tibetan uplift and strengthening of the South Asian Monsoon. Collectively, these results highlight significant changes in rainfall patterns, seasonality, and OM sources across the Oligocene–Miocene transition.