Warm deep-sea temperatures across Eocene Thermal Maximum 2 from clumped isotope thermometry

Lucas J Lourens,Martin Ziegler,Niels J De Winter,Tobias Agterhuis

doi:10.1038/s43247-022-00350-8

Lucas J Lourens, Martin Ziegler + Show 2 more

Open Access

https://doi.org/10.1038/s43247-022-00350-8

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Abstract

The early Eocene hothouse experienced highly elevated atmospheric CO2 levels and multiple transient global warming events, so-called hyperthermals. The deep ocean constitutes an assumed setting to estimate past global mean temperatures. However, available deep-sea temperature reconstructions from conventional benthic foraminiferal oxygen isotopes and magnesium/calcium ratios rely on uncertain assumptions of non-thermal influences, associated with seawater chemistry and species-specific physiological effects. Here we apply the carbonate clumped isotope thermometer, a proxy not governed by these uncertainties, to evaluate South Atlantic deep-sea temperatures across two hyperthermal events in the early Eocene (Eocene Thermal Maximum 2/H1 and H2; ~54 Myr ago). Our independent reconstructions indicate deep-sea temperatures of 13.5 ± 1.9 °C (95% CI) for the background conditions and average hyperthermal peak temperatures of 16.9 ± 2.3 °C (95% CI). On average, these absolute temperatures are three degrees warmer than estimates from benthic oxygen isotopes. This finding implies a necessary reassessment of (1) the Eocene seawater isotope composition and (2) pH changes in the deep ocean and its potential influence on benthic foraminiferal oxygen isotope records.

Highlights

The early Eocene hothouse experienced highly elevated atmospheric CO2 levels and multiple transient global warming events, so-called hyperthermals
The abundance of N. truempyi and O. umbonatus is too low in the samples at all sites to take sufficient measurements for the precise reconstruction of deep-sea temperature change across ETM2 and H2
Our study shows that robust marine temperature reconstructions from foraminiferal oxygen isotopes, and Mg/Ca as well, can only be achieved with independent constraints on non-thermal effects

Summary

Introduction

The early Eocene hothouse experienced highly elevated atmospheric CO2 levels and multiple transient global warming events, so-called hyperthermals. Our independent reconstructions indicate deep-sea temperatures of 13.5 ± 1.9 °C (95% CI) for the background conditions and average hyperthermal peak temperatures of 16.9 ± 2.3 °C (95% CI) On average, these absolute temperatures are three degrees warmer than estimates from benthic oxygen isotopes. In comparison to the sea surface, the deep ocean is spatially uniform in temperature and is considered a relatively stable component in the climate system because of its high heat capacity[11,12,13]. Negative excursions in high-resolution stable oxygen and carbon isotope records from deep marine sediments have revealed the periodic occurrence of multiple transient (10–100 kyr) episodes of global warming and ocean acidification (hyperthermal events; e.g. PETM, ETM2, and ETM3) Negative excursions in high-resolution stable oxygen and carbon isotope records from deep marine sediments have revealed the periodic occurrence of multiple transient (10–100 kyr) episodes of global warming and ocean acidification (hyperthermal events; e.g. PETM, ETM2, and ETM3) (refs. 12,16–21), generally linked to massive release of isotopically light carbon into the ocean-atmosphere system[4,18,22,23]

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