Ecological and hydrological reconstruction of the western Indian coastal ocean during the Early Miocene (Burdigalian) based on the oxygen and carbon isotopes of multiple ostracod species.

Calcareous valves of various ostracod species from the Miocene (Burdigalian) Quilon Formation, Kerala Basin, southwest India, were separated and identified up to the species level. The 15 most abundant species were selected to determine the carbon and oxygen isotope composition, with 2 to 5 replicates to assess the variation among individual valves within each species. The δ¹³C ratios range from 0.56 to -4.65‰ VPDB with a standard deviation range between 0.08 to 0.53‰. The δ¹⁸O ratios varied between -2.57 to -4.25‰ VPDB with a standard deviation between 0.12‰ and 0.46‰. The seawater δ¹⁸O values were calculated using the empirical equation by Kim and Neil (1997), and they range between -3.08‰ to -0.01‰ (VSMOW), with an average of -1.85‰ (VSMOW). This study also tries to categorise the species into distinct habitat groups, namely the open ocean, mixed estuarine and shallow-marine environment with significant coastal upwelling influence, based on their isotopic composition. The results were compared with the habitats of their extant relatives at the family and genus levels, as well as information derived from valve ornamentations. Ostracods, namely Phlyctenophora meridionalis, Paranesidea cf. gajensis, Bairdoppilata sp., and Krithe autochthona inhabited a range of settings from shallow to deeper marine environments. The species Aurila singhi, Paractinocythereis gujaratensis, Stigmatocythere sp., Actinocythereis sp., Trachyleberis sp., Neocyprideis murudensis, Pokornyella chaasraensis, and Tenedocythere keralaensis are identified to inhabit an estuarine or shallow-marine environment influenced by freshwater influx. Whereas Paijenborchellina prona, Cytherelloidea sp., and Loxoconcha confinis show an indication of a shallow-marine environment with significant coastal upwelling influence.