The ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth. At present, this depth is ∼4,500 m, with some variation between and within ocean basins. The calcite compensation depth is linked to ocean acidity, which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1. Geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the Eocene/Oligocene boundary (∼ 34 million years ago)2 when significant permanent ice sheets first appeared on Antarctica3,4,5,6, but the relationship between these two events is poorly understood. Here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical Pacific Ocean that cover the Eocene/Oligocene boundary. We find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40,000 years each, synchronous with the stepwise onset of Antarctic ice-sheet growth. The glaciation was initiated, after climatic preconditioning7, by an interval when the Earth's orbit of the Sun favoured cool summers. The changes in oxygen-isotope composition across the Eocene/Oligocene boundary are too large to be explained by Antarctic ice-sheet growth alone and must therefore also indicate contemporaneous global cooling and/or Northern Hemisphere glaciation.
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We thank the Shipboard Party of Ocean Drilling Program Leg 199 for assistance at sea and M. Bolshaw, M. Cooper and H. Birch for laboratory assistance. This work was supported by a NERC UK ODP grant to P.A.W., a Royal Commission for the Exhibition of 1851 fellowship awarded to H.K.C. and by Swedish Research Council (VR) funding to H.P. We thank W. Broecker, R. Hindmarsh, S. D'Hondt, A. Merico, Y. Rosenthal, R. Rickaby, J. Shepherd and T. Tyrrell for discussions and comments on an earlier draft and L. Kump for a constructive review.
The authors declare that they have no competing financial interests.
Contains: Supplementary Data and Supplementary Table 1. New astronomically tuned age calibrations derived from the ODP Site 1218 chronology are presented for magnetic reversals and nannofossil datums close to the Eocene/Oligocene boundary and for the boundary itself. The methods used to obtain these estimates are briefly discussed and the values are compared to an existing age model. (DOC 29 kb)
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