Growth stress in oysters during early Bartonian: effects of high temperature and low seasonality in the Indian palaeotropics

Earth’s climate experienced an extreme transition going from the Eocene hothouse into Oligocene icehouse conditions. The Middle Eocene Climatic Optimum (MECO ~41Ma) briefly interrupted this cooling trend, initiated in the middle Lutetian (~44 Ma). The tropical position of the Indian subcontinent during the MECO provides an ideal setting to investigate low-latitude seasonality under hot climate conditions, considering the modern minimal seasonality in the tropics. To better understand the seasonality and its effect on shallow marine fauna, a high-resolution multiproxy study is carried out on oyster specimens of the type Flemingostrea pseudoflemingi from the early Bartonian palaeo-tropical Kutch Basin. Clumped isotope (Δ47) thermometry combined with δ18O analyses from excellently preserved shell specimens suggests low seasonal temperature fluctuations (Δ47-T: ~3°C) with a mean temperature of 34.4±2.0°C (95% CI) in the warmer season and 31.4±2.4°C in the months of highest precipitation. Oyster populations from distinct regions within the basin display varying sizes. Smaller variants, inhabiting a zone influenced by marine conditions (δ18Ow: -0.2±0.5‰ to 0.4±1.0‰) experienced slower growth rates (~18 µm/day) only during the warmer months. In contrast, larger variants, probably residing at shallower environment prone to more seasonal changes due to evaporation and freshwater input (δ18Ow: -1.0±0.8‰ to 1±0.7‰), maintained a steady growth rate (~58 µm/day) throughout the year. Although the observed seasonality is similar to modern tropical settings of the Indian and Pacific Ocean, the significantly warmer temperature range created a stressful environment for oysters, where seasonally diverse habitat and nutrient-rich freshwater input were critical to their growth.