Global ocean heat content over the past 3 million years

The Pleistocene epoch was characterized by global cooling and an increase in the intensity and duration of glacial cycles. Regional surface and subsurface ocean temperature records follow distinct trends over this interval, suggesting dynamic changes in zonal and meridional heat transport and ocean circulation. These differing trends also complicate efforts to determine the evolution of total ocean heat content. Here we provide a record of mean ocean temperature over the past 3 million years from noble gas (Xe/Kr) measurements in shallow ice cores recovered in the Allan Hills blue ice area, Antarctica¹. The stratigraphically complex records preclude reconstruction of individual glacial cycles and probably represent a weighted averaging of glacial and interglacial conditions². Nonetheless, we find pronounced cooling roughly coincident with the Plio-Pleistocene Transition (around 2.7 million years ago), and steady temperatures across the Mid-Pleistocene Transition (1.2 to 0.8 million years ago). Comparisons with a recent global sea surface temperature compilation³ show broad consistency in long-term cooling but important differences at the Plio-Pleistocene and Mid-Pleistocene transitions. We suggest that the different trends in surface temperature and mean ocean temperature during these intervals are related to a redistribution of heat between the surface and subsurface via changes in deep water formation and upwelling. Our temperature record also permits an estimate of global ice volume changes between 3 and 0.5 million years ago through a deconvolution of the benthic foraminiferal δ¹⁸O record and points to a period of enhanced ice sheet growth around the time of the Mid-Pleistocene Transition.