The Oxfordian stable isotope record (δ18O, δ13C) of belemnites, brachiopods, and oysters from the Kachchh Basin (western India) and its potential for palaeoecologic, palaeoclimatic, and palaeogeographic reconstructions

The study combines the results of stable isotope (δ13C, δ18O) analyses of 187 belemnites, brachiopods, and oysters from the Middle to Upper Jurassic (Upper Callovian to Kimmeridgian) of the Kachchh Basin in western India. Generally, belemnites show lower δ13C and higher δ18O ratios than the benthic fauna. It is proposed that this discrepancy is caused by different life habitats of both groups with belemnites spending major parts of their life in deeper water outside the Kachchh Basin and entering shallow areas only occasionally (e.g., during seasonal migrations to spawning grounds). A mode of life as fast-swimming, nektonic predators similar to many modern squid species seems plausible. The study emphasises that results of stable isotope analyses should be evaluated separately for each fossil group. In addition to the influence of diagenetic alteration, weathering, salinity changes, and sea-level fluctuations, the ecology of the analysed fossil taxa has to be considered. Temperatures calculated from δ18O ratios point to relatively warm conditions around the Middle to Late Jurassic transition with no evidence for a glacial phase which had been proposed for this boundary. A distinct fall in temperatures is observed during the Middle Oxfordian reaching a minimum in the early Late Oxfordian. It is tempting to connect this event with the break-up of Gondwana causing stronger upwelling in the northwestern Tethys or alternatively the influx of polar water by the opening of the Transgondwanan Seaway. However, such connections cannot be verified without further data, preferably from localities in the southern Malagasy Gulf (e.g., Madagascar) and from younger horizons (Kimmeridgian–Tithonian).