A stable isotope record documenting the Middle to Late Miocene climate transition from the Kutch Tertiary group, Gujarat (India)
Authors: Priya Bhanu , Patnaik Rajeev , Ghosh Prosenjit 
Published: March 2024

Given its stance as the harbinger of modern-day climatic conditions 1, the Miocene epoch (23.03 &#8211; 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) &amp; 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 &amp; Rhinocerotids) as the investigatory tool. Enamel &#948;13C 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. &#948;18O 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,&#160;which justifies the choice of mammals in this study.The Palasava and Tappar samples have ranges of +0.20&#8240; to +4.24&#8240; and -3.97&#8240; to +5.47&#8240;, respectively, for &#948;18O 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 &#948;13C signature for Palasava samples ranges from -11.23&#8240; to -9.42&#8240; while the Tappar ones are between -12.95&#8240; and -10.64&#8240;. 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:Steinthorsdottir, M. et al. The Miocene: The Future of the Past. Paleoceanography. Paleoclimatology 36, (2021).
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