Climate of the Past | 2021
Deep ocean temperatures through time
Abstract
Abstract. Benthic oxygen isotope records are commonly used as a proxy for global mean\nsurface temperatures during the Late Cretaceous and Cenozoic, and the\nresulting estimates have been extensively used in characterizing major\ntrends and transitions in the climate system and for analysing past climate\nsensitivity. However, some fundamental assumptions governing this proxy have\nrarely been tested. Two key assumptions are (a)\xa0benthic foraminiferal\ntemperatures are geographically well mixed and are linked to surface\nhigh-latitude temperatures, and (b)\xa0surface high-latitude temperatures are well\ncorrelated with global mean temperatures. To investigate the robustness of\nthese assumptions through geological time, we performed a series of 109\nclimate model simulations using a unique set of paleogeographical\nreconstructions covering the entire Phanerozoic at the stage level. The\nsimulations have been run for at least 5000 model years to ensure that the\ndeep ocean is in dynamic equilibrium. We find that the correlation between\ndeep ocean temperatures and global mean surface temperatures is good for the\nCenozoic, and thus the proxy data are reliable indicators for this time\nperiod, albeit with a standard error of 2\u2009K. This uncertainty has not\nnormally been assessed and needs to be combined with other sources of\nuncertainty when, for instance, estimating climate sensitivity based on\nusing δ18O measurements from benthic foraminifera. The\ncorrelation between deep and global mean surface temperature becomes weaker\nfor pre-Cenozoic time periods (when the paleogeography is significantly\ndifferent from the present day). The reasons for the weaker correlation\ninclude variability in the source region of the deep water (varying\nhemispheres but also varying latitudes of sinking), the depth of ocean\noverturning (some extreme warm climates have relatively shallow and sluggish\ncirculations weakening the link between the surface and deep ocean), and the\nextent of polar amplification (e.g. ice albedo feedbacks). Deep ocean\nsediments prior to the Cretaceous are rare, so extending the benthic foraminifera\nproxy further into deeper time is problematic, but the model results\npresented here would suggest that the deep ocean temperatures from such time\nperiods would probably be an unreliable indicator of global mean surface conditions.\n