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

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 (Δ47) 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 Δ47. As the two proxies are affected by different non‐thermal biases, the good agreement provides confidence in these reconstructed temperatures. Our Δ47 temperature record implies a ~0.6‰ seawater δ18O change over the MCT, in good agreement with previously published values from other sites. Our data furthermore confirm overall high seawater δ18O 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 δ18O and/or a decoupling of ice volume and ocean temperature.

Deep Heat: Proxies, Miocene Ice, and an End in Sight for Paleoclimate Paradoxes?

“…For these reasons, the temperature reconstructions of Modestou et al. (2020) appear robust, and do not suffer from the nonthermal influences that complicate the interpretation of traditional stable isotope and trace element proxies in terms of absolute temperature (Figure 1). Indeed, these deep ocean temperature data are supported by mid/high‐latitude sea surface temperature (SST) estimates (Levy et al., 2016; Shevenell et al., 2004; Super et al., 2018), which adds support to the notion of a greatly reduced latitudinal SST gradient and therefore much warmer temperatures in the regions of deep water formation (Figure 2).…”

“…The clumped isotope data come from just one site, and modeling indicates substantial heterogeneity in the thermal response of the deep ocean as a result of changes in deep water formation driven by ice growth on Antarctica (Knorr & Lohmann, 2014). Of specific relevance to the results of Modestou et al (2020), deep water masses in the eastern Indian ocean are predominantly related to Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW)/Circumpolar Deep Water (CDW), with ODP Site 761 (∼2,200 m paleo water depth) currently sitting close to the depth of the boundary between AAIW and deeper water masses (Holbourn et al, 2004;Warren, 1981;Woo & Pattiaratchi, 2008). This raises the possibility that data from ODP Site 761 potentially relate to water masses that are a few degrees warmer than those bathing much of the deep ocean.…”

“…If correct, this would point to an unusual mode of Miocene deep water formation, rath-EVANS 10.1029/2020PA004174 Figure 2. Middle Miocene reconstructions of atmospheric CO 2 (Super et al, 2018;Sosdian et al, 2018), mid latitude sea surface temperature (SST) from Mg/Ca (Shevenell et al (2004) reinterpreted following Gray & Evans (2019)) and GDGTs (glycerol dialkyl glycerol tetraether lipids; Super et al, 2018), deep ocean temperature from clumped isotopes (Modestou et al, 2020), and seawater δ 18 O from coupled clumped isotope-δ 18 O measurements of benthic foraminifera. SLE denotes sea level equivalent, using the Miocene δ 18 O-sea level relationship of Langebroek et al (2010).…”

“…The derivation of Figure 1 is described in Text S1 and S2. The data shown in Figure 2 are available from Super et al (2018), Sosdian et al (2018), Shevenell et al (2004), and Modestou et al (2020). The author would like to thank an anonymous reviewer and the editor for providing feedback that substantially improved this contribution, and Will Gray for discussion of benthic foraminifera geochemistry.…”

“…Here, I explain the importance of their data and place them into the broader context of proxy development and mid Miocene climate records, as well as providing an outlook for paleoclimate research in view of the many tools now at our disposal. Modestou et al (2020) present deep sea temperature reconstructions based on the geochemistry of benthic foraminifera, but with a key difference. Rather than 'traditional' isotopic or trace metal analysis, the authors measured the clumped isotopic composition of the foraminifera shells.…”