A stable isotope record documenting the Middle to Late Miocene climate transition from the Kutch Tertiary group, Gujarat (India)

Given its stance as the harbinger of modern-day climatic conditions ¹, the Miocene epoch (23.03 – 5.33 Ma) is perhaps, climatologically, the most important epoch in the Cenozoic era. Within this epoch, the Middle to Late Miocene transition offers a time window to study the effects of plummeting atmospheric CO₂ levels and consequent global cooling on ecosystems. Currently, we are witnessing a reversal of this scenario, namely global warming, which makes studying this transition crucial for verifying the model predictions regarding the future of our ecosystems. Herein, such a study documenting the paleoclimatic record preserved in the fossiliferous section of the Kutch region (India), a hitherto unexplored area, is presented. It spans two localities: Palasava (~14 Ma) & Tappar (~10 Ma), one each from the Middle and Late Miocene sub-epoch and utilises the carbonate phase from enamel remains of megafaunal herbivore mammals (Proboscideans & Rhinocerotids) as the investigatory tool. Enamel δ¹³C signature is indicative of ambient vegetation type due to the differential assimilation of C isotopes in diet plant tissue as a function of different carbon fixing mechanisms in C₃ and C₄ plants ². δ¹⁸O signal, on the other hand, is reflective of the environmental rainfall intensity and seasonality because the rainfall isotopic composition is a function of temperature and amount effect during precipitation. Large-bodied obligate drinker mammals are most efficient for these reconstructions ³, which justifies the choice of mammals in this study.

The Palasava and Tappar samples have ranges of +0.20‰ to +4.24‰ and -3.97‰ to +5.47‰, respectively, for δ¹⁸O values. The larger scatter within the latter indicates higher seasonality in the Late Miocene relative to the Middle Miocene, which aligns well with the idea of intensification of the Indian summer monsoon regime during the younger sub-epoch⁴. Parallelly, the δ¹³C signature for Palasava samples ranges from -11.23‰ to -9.42‰ while the Tappar ones are between -12.95‰ and -10.64‰. The former represents woodland browsing, whereas the latter indicates forest-woodland browsing. Both localities imply C₃-dominated environments. Since Tappar straddles the beginning of the Late Miocene, it is acceptable to think that C₃-dominated habitats must have persisted up till this time, and it was only much later and perhaps fuelled by enhanced rainfall seasonality that C₄ grasses became abundant and eventually, grassland expansion took place.

Conclusively, the observed trends agree with the ones seen for contemporaneous Siwalik samples and comply with the hypothesis of increasing rainfall seasonality towards the Late Miocene sub-epoch followed by eventual, and perhaps consequential, expansion of C₄ grasses during the later part of Late Miocene.

References:

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