High-temperature fluctuations of the deep ocean: an early Eocene clumped isotope (Δ47) temperature record of ODP Site 1263, Walvis Ridge

Abstract

The continued emissions of carbon-dioxide poses a threat to the global community, of which the magnitude depends on the sensitivity of the climate. However, constraints on the climate sensitivity are still very uncertain, and more pessimistic emission scenarios appear to become more relevant. Studying the climate of the early Eocene (56-47.8 Ma) hothouse presents us with a way to confine these uncertainties, and to investigate the mechanisms that may become relevant at higher carbon-dioxide levels. In this study, deep ocean temperatures and seawater δ18O are reconstructed using ‘clumped isotopes’ (∆47). As opposed to other temperature proxies, such as TEX86, Mg/Ca and the classical δ18O, clumped isotopes are independent of seawater chemistry vital effects, and is purely thermodynamical. The clumped isotopes are measured on the Nuttallides truempyi, Oridorsalis umbonatus, Cibicidoides spp, and Hanzawaia ammophila benthic foraminifera species using isotope ratio mass spectrometry. A total of six reconstructed temperatures spanning the entire early Eocene are presented. Of particular interest is the early Eocene Climate Optimum (EECO, ~51.5-49.5 Ma), a period in which Earth’s surface temperature culminated. A deep ocean warming of ~7.22 ±4.44 °C is recorded between 53 Ma and the EECO, after which a cooling of ~5.29 ±4.37 °C occurred in a few million years span, towards 48 Ma. These temperatures are not recorded by δ18O, and poorly in Mg/Ca, most likely due to problems with those proxies. A potential pH-effect in the δ18O of benthic foraminifera is investigated, but the current understanding is insufficient for taking conclusions. Moreover, sea surface temperatures do not reflect changes in the deep ocean during the early Eocene, exposing problems with the calibrations for the TEX86 temperature proxy, as well as assumptions of surface and deep ocean coupling, usually done based on benthic δ18O alone. The δ18O composition of the seawater (δ18Ow) has been recalculated for the Eocene, revealing much heavier values than conventionally assumed. δ18Ow values as heavy as 0.61 ‰ VSMOW were recorded, which was usually taken to be -0.98 ‰, assuming that the early Eocene was ice-free. This puts forward a series of complications that need explaining with regard to the origin of the δ18Ow and overall ocean circulation, which are explored here. New estimates for climate sensitivity during the EECO reveal a temperature change of 6.0 °C per doubling in carbon-dioxide (CO2), with a maximum of 9.1 °C minimum of 4.1 °C. This is higher than estimates based on benthic δ18O. The new ∆47-derived climate sensitivity is in line with a recent climate model simulation which contains enhanced cloud microphysics, and agrees with a recent Mg/Ca temperatures and boron-based CO2 study. The findings here support a climate-state dependency of climate sensitivity. Altogether, clumped isotope thermometry brings the need for critically re-evaluating long-standing assumptions, which may in the end help in a better understanding of the Earth System, and thus establishing better climate models and predictions for future climate warming.