A new mechanism for the two-step δ¹⁸O signal at the Eocene-Oligocene boundary

Abstract. The most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E-O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ¹⁸O record, showing an increase by 1.2–1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ¹⁸O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ¹³C record are suggestive of a greenhouse gas control on climate. Previous modelling studies show that a decline in pCO₂ beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ¹⁸O. Furthermore, they considered changes in the ocean circulation only regionally, or indirectly through the oceanic heat transport. The potential role of global changes in ocean circulation in the E-O transition has not been addressed yet. Here a new interpretation of the δ¹⁸O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ¹⁸O record reflects a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. The second step in the δ¹⁸O profile occurs due to a rapid glaciation of the Antarctic continent. This new mechanism is a robust outcome of our model and is qualitatively in close agreement with proxy data.