Extreme (sub)Tropical Eocene oceanic warmth: Clumped isotope temperatures of shallow-dwelling large Benthic Foraminifera

Abstract:

The response of the tropical surface oceans to greater than modern atmospheric carbon dioxide is poorly constrained. Eocene climate modelling broadly indicates that the tropical surface ocean was 8-10°C warmer compared to pre-industrial simulations, at odds with much of the currently available proxy information which suggests low latitude sea surface temperatures (SST) no more than a few degrees warmer than at present. However, the accuracy of some of this proxy information, particularly the δ¹⁸O and Mg/Ca ratio of biogenic marine carbonates, is hampered by uncertainties regarding the secular evolution of seawater chemistry. Here, we present clumped isotope temperatures of modern and Eocene shallow-dwelling benthic foraminifera, a palaeothermometer independent of seawater isotopic composition. These organisms have photosymbionts and therefore inhabit the photic zone, within the depth range of planktic species considered to be surface dwelling. Specimens collected from the modern ocean precipitate calcite in agreement with the clumped isotope-temperature calibration of Zaarur et al. [2013]. Based on 11 tropical to mid-latitude localities from across the globe we demonstrate that the Eocene ocean was significantly warmer than suggested by much of the previous proxy data. Exceptionally-preserved samples from the mid-Eocene of Java indicate the West Pacific was characterised by mean annual SST of 34-37°C at this time, whilst mid-latitude northern hemisphere SST (from localities in the UK, France and Belgium) were 24-30°C throughout the Eocene. These data bring (sub)tropical SST in a high-CO₂ world into much better agreement with climate models, indicating low-mid latitudinal SST gradients similar to modern.