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No extreme bipolar glaciation during the main Eocene calcite compensation shift


Major ice sheets were permanently established on Antarctica approximately 34 million years ago1,2,3, close to the Eocene/Oligocene boundary, at the same time as a permanent deepening of the calcite compensation depth in the world’s oceans4. Until recently, it was thought that Northern Hemisphere glaciation began much later, between 11 and 5 million years ago1,2,3,5. This view has been challenged, however, by records of ice rafting at high northern latitudes during the Eocene epoch6,7 and by estimates of global ice volume that exceed the storage capacity of Antarctica8 at the same time as a temporary deepening of the calcite compensation depth 41.6 million years ago9. Here we test the hypothesis that large ice sheets were present in both hemispheres 41.6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial Atlantic Ocean. These records allow, at most, an ice budget that can easily be accommodated on Antarctica, indicating that large ice sheets were not present in the Northern Hemisphere. The records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled, ocean temperatures increased and carbon isotope values decreased in the equatorial Atlantic. The nature of these changes around 41.6 million years ago implies common links, in terms of carbon cycling, with events at the Eocene/Oligocene boundary4 and with the ‘hyperthermals’ of the Early Eocene climate optimum3,10,11. Our findings help to resolve the apparent discrepancy between the geological records of Northern Hemisphere glaciation6,7,8 and model results12,13 that indicate that the threshold for continental glaciation was crossed earlier in the Southern Hemisphere than in the Northern Hemisphere.

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Figure 1: Palaeoceanographic records showing changes in ocean chemistry and global climate.
Figure 2: Measured increases in δ18O in benthic foraminifera (Δδ18Obenthic) and resulting global continental ice volumes for a range of values for the mean δ18O of stored ice (δ18Oice).
Figure 3: Palaeoceanographic records from ODP Site 1260 showing the run-up to CAE-3.

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This work used samples provided by the Ocean Drilling Program (ODP). The ODP (now IODP) is sponsored by the US National Science Foundation and participating countries under management of the Joint Oceanographic Institutions (JOI), Inc. We thank the shipboard party of ODP Leg 207, M. Bolshaw, M. Cooper and D. Spanner for help with laboratory work, H. Pälike for making data available; S. Gibbs, K. Moriya, H. Pälike, A. Roberts and T. Tyrrell for discussions and comments on an earlier draft and R. Zeebe for his constructive review. Financial support was provided by NERC in the form of a UK IODP grant to P.A.W. and P.F.S and by NERC and Perkin Elmer in the form of a CASE studentship to K.M.E.

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Correspondence to Paul A. Wilson.

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Edgar, K., Wilson, P., Sexton, P. et al. No extreme bipolar glaciation during the main Eocene calcite compensation shift. Nature 448, 908–911 (2007).

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