Oyster shells reveal low seasonality with winter precipitation in Bartonian India

A long-term cooling trend from middle Lutetian onwards (~44 Ma) was briefly interrupted by a short-lived warming event in the early Bartonian (~41 Ma), known as the Middle Eocene Climatic Optimum (MECO). At this stage, the Himalayan orogenesis was ongoing, and the mountain had not yet attained its current elevation, which now plays a crucial role in shaping Asia's seasonality and monsoon patterns. The Indian Subcontinent reached tropical latitudes in Eocene during its long voyage from the southern to the northern hemisphere, after its disjunction from Gondwanaland in the Early Jurassic. Given that seasonal temperature variation is relatively small at the tropics in the modern climate, exploring seasonality and monsoon patterns in the ice-free world of the early Bartonian can teach us about the response of seasonal variability to warming.

Kutch, a pericratonic rift basin located along the western margin of the Indian subcontinent, was at 6–7°N during the early Bartonian. The Harudi and Fulra Limestone formations, two successive Bartonian stratigraphic units (corresponding to SBZ 17), preserve sedimentary deposits with a moderate diversity of bivalves. Two oyster species—Flemingostrea sp. from upper Harudi Formation, and Pycnodonte sp. from basal Fulra Limestone—have been utilised to reconstruct the Bartonian climate and seasonality.

Dorso-ventral cross-sections of the shells underwent μXRF mapping, revealing the resilifers as the most pristine regions of the shells. High-resolution (25 µm) quantitative trace element profiles, conducted on the resilifers, show low concentrations of Mn and Fe indicative of good shell preservation. Incrementally sampled stable isotope (δ¹⁸O_carb and δ¹³C_carb) profiles display a sinusoidal pattern indicative of seasonal fluctuations throughout the oysters' growth. Subsamples corresponding to two of the lowest and highest δ¹⁸O_carb values from each shell were further analysed for clumped isotopes (Δ₄₇).

Clumped isotope thermometry, combined with δ¹⁸O records indicates that tropics experienced minimal seasonal temperature fluctuations (~3°C on average) in a range of 29±3°C to 36±2.8°C (95% Cl) in the early Bartonian. The oxygen isotope composition of the prevailing water body shows evidence for increased freshwater input during colder months.

This study suggests that early Bartonian seasonality range at tropical latitudes was similar than the modern one, but at a roughly 4 degree warmer level, with eventually higher precipitation during the colder season.