Paleoclimate and paleoenvironmental reconstructions of the Kutch Basin, India, during the Eocene: Insights from geochemical and micropaleontological signatures

Eocene is one of the most eventful epochs of Earth’s geologic history, ranging from a series of global warming events in the early Eocene (~56-47 Ma) to an icehouse transition at the end of Eocene to Oligocene. Kutch, a pericratonic rift basin at the western boundary of India, hosts thick sedimentary strata with intervening marine deposits of Eocene with hiatus at Lutetian (~47-42 Ma) and Priabonian (~37-33 Ma). Given Kutch's equatorial paleolatitudinal position (~10°S-10°N) during the Eocene, it is particularly interesting to investigate the basin's response to global climatic perturbations. Bulk sediment samples from Naredi (Ypresian), Harudi, and Fulra Limestone (Bartonian) formations are collected at 0.5-meter intervals or at lithological changes, whichever provided a finer resolution. Stable isotope analyses of organic carbon (δ¹³C_org) from the samples are performed. A pronounced negative δ¹³C_org excursion indicative of Eocene Thermal Maximum (ETM2) is recorded from the first fossil-bearing shale beds with the occurrence of larger benthic foraminifera (LBF). The Assilina Limestone Unit, the topmost fossiliferous bed of the Naredi Formation, records the Early Eocene Climatic Optimum (EECO). The top of Naredi and basal Harudi Formation has relatively lower δ¹³C_org values in the range of -24 to -26‰ (VPDB). δ¹³C_org values are relatively higher from the overlying coquina shell beds, and another negative excursion (~1.5‰ VPDB) has been observed concurrently to the Nummulites obtusus bed. This negative excursion is likely linked to the Middle Eocene Climatic Optimum (MECO) warming event. An abundance of other LBFs like Nummulites spectabilis and Nummulites vredenburgi have been noticed at this level. No major shift in the δ¹³C_org profile has been noticed in the overlying carbonate platform deposit of the Fulra Limestone Formation. The limestone is highly fossiliferous, dominated by diverse species of LBF such as Discocyclina, Assilina and Nummulites. This investigation indicates a correlation between sea-level rise and global warming events during the Eocene epoch in the paleo-equatorial Kutch Basin. Elevated temperatures and increased nutrient input during the EECO and MECO fostered the growth and diversification of LBF like Nummulites and Assilina. These warm, nutrient-rich conditions enabled LBF to achieve high diversity, abundance, larger sizes, and an expanded latitudinal range in the middle Eocene, demonstrating their adaptability to global warming. The observed negative δ¹³C excursion in the Naredi and Harudi formations is likely linked to sea-level fluctuations and changes in terrestrial carbon input to the marine environment. This study contributes to a better understanding of the complex interplay between climate change, sea-level fluctuations, and carbon cycling at the Kutch Basin during the Eocene.