Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry

Evans, David, Sagoo, Navjit, Renema, Willem, Cotton, Laura J., Müller, Wolfgang, Todd, Jonathan A., Saraswati, Pratul Kumar, Stassen, Peter, Ziegler, Martin, Pearson, Paul N., Valdes, Paul J. and Affek, Hagit P. (2018) Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry. Proceedings of the National Academy of Sciences, 115 (6), 1174-1179. (doi:10.1073/pnas.1714744115).

Abstract

Past greenhouse periods with elevated atmospheric CO₂ were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO₂, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Ca_sw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

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Contributors

Author: David Evans

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