Abstract
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.
Highlights
While the long-term variations in our present-day climate are paced by the waxing and waning of the Northern Hemispheric ice sheet, Earth’s surface was entirely icefree at the beginning of the Cenozoic Era, 65.5 Ma
First the model was adapted to paleoclimatic boundary conditions representing those of the Eocene climate system
A CO2 concentration was sought for which the land ice would disappear entirely, while not exceeding the upper-limit of 5 times pre-industrial atmospheric levels (PAL) as set by Pagani et al (2005)
Summary
While the long-term variations in our present-day climate are paced by the waxing and waning of the Northern Hemispheric ice sheet, Earth’s surface was (almost) entirely icefree at the beginning of the Cenozoic Era, 65.5 Ma. DeConto and Pollard (2003a) ran a global climate model coupled to a dynamic ice-sheet model, forcing it with a CO2 decline from 4 to 2 times PAL in 10 Myr. It has been proposed that, once some CO2 threshold (∼750 ppm, Pearson et al, 2009) was reached, feedbacks related to snow/ice-albedo and ice-sheet height/massbalance could have initiated rapid ice-sheet growth (DeConto and Pollard, 2003a,b; Coxall and Pearson, 2007). DeConto and Pollard (2003a) ran a global climate model coupled to a dynamic ice-sheet model, forcing it with a CO2 decline from 4 to 2 times PAL in 10 Myr They found that relatively small ice caps form due to high levels of winter precipitation, which start to grow rapidly beyond some threshold CO2 concentration and eventually coalesce to form one continental scale ice-sheet.
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