Warm Middle Miocene Indian Ocean Bottom Water Temperatures: Comparison of Clumped Isotope and Mg/Ca‐Based Estimates

Abstract

Abstract The middle Miocene is an important analogue for potential future warm climates. However, few independent deep ocean temperature records exist, though these are important for climate model validation and estimates of changes in ice volume. Existing records, all based on the foraminiferal Mg/Ca proxy, suggest that bottom water temperatures were 5–8°C warmer than present. In order to improve confidence in these bottom water temperature reconstructions, we generated a new record using carbonate clumped isotopes (Δ 47 ) and compared our results with Mg/Ca‐based estimates for the Indian Ocean at ODP Site 761. Our results indicate temperatures of 11.0 ± 1.7°C during the middle Miocene Climatic Optimum (MCO, 14.7–17 Ma) and 8.1 ± 1.9°C after the middle Miocene Climate Transition (MCT, 13.0–14.7 Ma), values 6 to 9°C warmer than present. Our record also indicates cooling across the MCT of 2.9 ± 2.5°C (uncertainties 95% confidence level). The Mg/Ca record derived from the same samples indicates temperatures well within uncertainty of Δ 47 . As the two proxies are affected by different non‐thermal biases, the good agreement provides confidence in these reconstructed temperatures. Our Δ 47 temperature record implies a ~0.6‰ seawater δ 18 O change over the MCT, in good agreement with previously published values from other sites. Our data furthermore confirm overall high seawater δ 18 O values across the middle Miocene, at face value suggesting ice volumes exceeding present‐day despite the warm bottom water temperatures. This finding suggests previously underappreciated additional influences on seawater δ 18 O and/or a decoupling of ice volume and ocean temperature.