A new interpretation of 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 indicative of a greenhouse gas control on climate. Previous 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 did not address the potential role of changes in ocean circulation in the E–O transition. 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 represents 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. This shift can be initiated by a small density perturbation in the model, although there is also a parameter regime for which the shift occurs spontaneously. The second step in the δ18O profile occurs due to a rapid glaciation of the Antarctic continent. This new interpretation is a robust outcome of our model and is in good agreement with proxy data.