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

S E Modestou,A Fernandez,C H Lear,A N Meckler,T J Leutert

doi:10.1029/2020pa003927

S E Modestou, A Fernandez + Show 3 more

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https://doi.org/10.1029/2020pa003927

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Journal: Paleoceanography	Publication Date: Nov 1, 2020
Citations: 35	License type: CC BY 4.0

Affiliation: University of Bergen, Cardiff University

Abstract

AbstractThe 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.

Highlights

The middle Miocene is a period of strategic importance as a future warm climate analogue: It is the most recent period in geological history with atmospheric CO2 concentrations relevant to elevated future CO2 scenarios
This allowed for the most direct comparison possible between Mg/Ca and Δ47 temperatures, we could not use the same species of foraminifera
The rest of the Δ47 temperatures calculated with the largest discrete bin size do not differ greatly from those calculated using the Gaussian window filter (GWF); we focus on the GWF‐based temperature record when comparing with the Mg/Ca‐based temperatures

Summary

Introduction

The middle Miocene is a period of strategic importance as a future warm climate analogue: It is the most recent period in geological history with atmospheric CO2 concentrations relevant to elevated future CO2 scenarios. The warm MCO was followed by the middle Miocene Climate Transition (MCT; ~13–15 Ma) This subsequent global cooling is associated with a drop in CO2 concentrations (Badger et al, 2013; Greenop et al, 2014; Kürschner et al, 2008; Sosdian et al, 2018; Super et al, 2018; Zhang et al, 2013), major Antarctic ice sheet (AIS) expansion (see Shevenell and Kennett, 2007, and references therein) and a corresponding ~1‰ increase in benthic foraminiferal δ18O (δ18Ob; Cramer et al, 2009; Mudelsee et al, 2014; Zachos et al, 2008)

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Conclusion