06 Jul 2020

06 Jul 2020

Review status: a revised version of this preprint is currently under review for the journal CP.

Deep Ocean Temperatures through Time

Paul J. Valdes1,, Christopher R. Scotese2, and Dan J. Lunt1 Paul J. Valdes et al.
  • 1School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK
  • 2NorthwesternUniversity, Dept Earth & Planetary Sci, Evanston, IL USA
  • Invited contribution by Paul J. Valdes, recipient of the EGU Milutin Milankovic Medal 2015.

Abstract. Benthic oxygen isotope records are commonly used as a proxy for global mean surface temperatures during the late Cretaceous and Cenozoic, and the resulting estimates have been extensively used in characterising major trends and transitions in the climate system, and for analysing past climate sensitivity. However, some fundamental assumptions governing this proxy have rarely been tested. Two key assumptions are: (a) benthic foraminiferal temperatures are geographically well mixed and are linked to surface high latitude temperatures, and (b) surface high latitude temperatures are well correlated with global mean temperatures. To investigate the robustness of these assumptions through geological time, we performed a series of 109 climate model simulations using a unique set of paleogeographical reconstructions covering the entire Phanerozoic at the stage-level. The simulations have been run for at least 5000 model years to ensure that the deep ocean is in dynamic equilibrium. We find that the correlation between deep ocean temperatures and global mean surface temperatures is good for the Cenozoic and thus the proxy data are reliable indicators for this time period, albeit with a standard error of 2 K. This uncertainty has not normally been assessed and needs to be combined with other sources of uncertainty when, for instance, estimating climate sensitivity based on using δ18O measurements from benthic foraminifera. The correlation between deep and global mean surface temperature becomes weaker for pre-Cenozoic time periods (when the paleogeography is significantly different than the present-day). The reasons for the weaker correlation includes variability in the source region of the deep water (varying hemispheres but also varying latitudes of sinking), the depth of ocean overturning (some extreme warm climates have relatively shallow and sluggish circulations weakening the link between surface and deep ocean), and the extent of polar amplification (e.g. ice albedo feedbacks). Deep ocean sediments prior to the Cretaceous are rare, so extending the benthic foram proxy further into deeper time is problematic, but the model results presented here would suggest that the predicted deep ocean temperatures would probably be unreliable.

Paul J. Valdes et al.

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Paul J. Valdes et al.

Paul J. Valdes et al.


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Short summary
Deep ocean temperatures are widely used as a proxy for global mean surface temperature in the past but the underlying assumptions have not been tested. We use a unique set of 109 climate model simulations for the last 545 million years to show that the relationship is valid for approximately the last 100 million years but breaks down for older time periods when the continents (and hence ocean circulation) are in very different position.