Temperature Estimates of Lower Miocene (Burdigalian) Coastal Water of Southern India Using a Revised Otolith Clumped Isotope Paleothermometer

Carbonate clumped isotope thermometry is based on the ordering of C-13 and O-18 in the carbonate lattice and is based on the relative abundance of (COO)-C-13-O-18-O-16 in CO2 produced through acid digestion of carbonate minerals. The major advantage of this technique is its non-dependency on the delta O-18 value of water from which the carbonate precipitated. Ghosh et al. (2007, ) previously published calibration data for fish otoliths referenced to heated gases and used the Gonfiantini O-17 parameter set in their data evaluation. Herein, we present a new clumped isotope (Delta(47)) calibration for aragonitic fish otoliths in the absolute reference frame using the International Union of Pure and Applied Chemistry O-17 correction. Our revised calibration equation for otolith is: Delta 47CDES=0.0364 +/- 0.005x(106)T2+0.2619 +/- 0.0657(R2=0.9,p-value<0.001). To test the accuracy of this calibration, we apply it to otoliths of modern Lutjanus lutjanus from the Bay of Bengal. The estimated average temperature (22.3 degrees C +/- 4.2 degrees C) for the Bay of Bengal and delta O-18(V-SMOW) composition of waters of -1.7 parts per thousand (+/- 0.5) are consistent with the onsite observations. We also apply the new calibration to well-preserved otoliths of genus Ambassidarum sp. and genus Gobiidarum sp. from lower Miocene (Burdigalian) sediments of the Quilon Formation, India to quantify coastal water conditions. Estimated average environmental water temperatures in their habitats were 12.9 degrees C +/- 1.7 degrees C, and the average delta O-18(V-SMOW) of ambient waters calculated yielded a value between -3.5 parts per thousand and -2.6 parts per thousand (V-SMOW) (mean: -2.9 parts per thousand +/- 0.4) and -4.4 parts per thousand, respectively. These results indicate delta O-18 values reflect the kinetic effects impacting the delta O-18 of fish otoliths independently of Delta(47), although we cannot fully preclude diagenesis.

Automated summary

Carbonate clumped isotope thermometry provides a method to estimate water temperature independently of the δO-18 value, and the new calibration technique presented in this study shows promising results in accurately determining environmental conditions in modern and ancient settings. The study suggests that the δO-18 values in fish otoliths reflect kinetic effects and environmental conditions, with potential diagenesis effects also taken into consideration.