Middle to Late Jurassic stable isotopes and element ratios of fossils from western India: Developing a reference temperature curve for northeastern Gondwana

Abstract

The stable isotope (δ13C, δ18O) and element (Mg/Ca, Sr/Ca) composition of 374 well-preserved belemnites, bivalves, and brachiopods of the Kachchh and Jaisalmer basins in western India were used to reconstruct climatic changes in the Middle and Late Jurassic time interval. Absolute water temperatures reconstructed from δ18Oshell values depend on the used δ18Osea value and equation, but indicate a long-term temperature decrease of around 8–9 °C in the study area from the Bajocian to the Tithonian. Measured Mg/Ca ratios also point to decreasing temperatures, but additionally reflect species-specific factors and a lower Mg/Ca ratio of Jurassic seawater compared to today. The recorded cooling in western India agrees with previous results from neighbouring Madagascar and the Indian Himalayas and can be connected to a drift of Eastern Gondwana into higher latitudes. This palaeogeographic shift is accompanied by changes in sedimentation indicating an increasingly humid climate and in the composition of ecosystems reflecting increasingly cooler water temperatures. High-resolution stable oxygen isotope analyses of three oyster shells point to seasonal temperature changes of 4–5 °C in the Kimmeridgian. The recorded δ13C values reflect sea-level changes with a positive excursion in the Oxfordian connected to a major transgressive event. Sr/Ca ratios are observed to follow changes in global ocean chemistry.