Nascent Eocene Indian Monsoons recorded in oyster shells from the western Indian margin

Abstract. The origin, intensification and paleogeographic drivers of the South Asian Monsoon (SAM) since the Eocene, remain poorly understood mainly due to the paucity of suitable seasonal proxies coupled with climate simulations. This study evaluates the seasonal climate variability and potential presence of SAM-like conditions based on geochemical proxy-based sub-annual climatic reconstruction recovered from late-middle Eocene fossil oyster shell material from the Kutch basin (India), interpreted in the framework of Community Earth System Model (CESM) climate simulations. A thick-shelled honeycomb oyster species Pycnodonte kachchhensis is used as a natural palaeoclimate archive. Bulk sediment organic carbon isotope stratigraphy places the oyster horizon within the late Bartonian, after the Middle Eocene Climatic Optimum (MECO) from which a previous oyster fossil study provides comparison. Clumped isotope thermometry indicates low seasonal variability with sea surface temperatures of 31^+2.2_−2.1 to 36^+2.8_−2.7 °C and evaporative conditions (δ¹⁸O_w: 0.6^+0.4_−0.4 to 1.6^+0.6_−0.5 ‰), within a restricted late Bartonian lagoonal setting. Trace element profiles in oyster shells indicate a low-nutrient environment and capture around two years of the oysters’ lifespan, reflected in seasonal variations along the shell growth axis. Stable isotope data from the multiyear shell archive indicates annual temperature drops associated with periods of higher rainfall to evaporation. The observed pattern is consistent with CESM simulation showing a nascent SAM as well as precipitation outside the main monsoon months due to moisture transport from the open seaway between India and Eurasia. However, a modern-like monsoon with strong seasonal contrasts and pronounced winter temperature minima was not yet established in the study area, possibly owing to the tropical palaeolatitude of the region (~8° N) and palaeogeographic factors such as the open northern seaway of Greater India. Observed climatic and seasonal variability are comparable to the underlying MECO oyster record, despite the early Bartonian hyperthermal conditions and the gradual decline in pCO₂ toward the late Bartonian, suggesting a limited sensitivity of equatorial climate to atmospheric CO₂.