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Antarctic weathering and carbonate compensation at the Eocene–Oligocene transition

Abstract

During the Eocene–Oligocene transition about 34 million years ago, permanent ice cover developed on Antarctica. This pronounced climate transition was accompanied by the deepening of the carbonate compensation depth in the oceans1 and perturbations in atmospheric carbon dioxide concentrations2,3. These changes may have been linked to continental weathering on Antarctica, but reconstructing which rock types were subject to weathering and the intensity of that weathering has proved challenging. Here we compare the lead (Pb) isotope values of seawater as recorded by extractions from decarbonated bulk sediments and those of silicate detrital fractions from deep-sea sediments from sites in the Southern Ocean that span the Eocene–Oligocene transition. These comparisons allowed us to assess local weathering inputs of Pb from Antarctica. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios suggest high rates of chemical weathering in the late Eocene, which would have helped draw down atmospheric CO2 to levels necessary for glacial initiation. Mechanical weathering and the introduction of newly exposed material was enhanced during the establishment of the Antarctic ice sheet. We also observe a divergence of seawater 206Pb/204Pb from detrital values during the Eocene–Oligocene transition, which implies an additional source of weathered material. We argue that the weathering of carbonate basement rock from Antarctica could explain the 206Pb/204Pb trend, and could have contributed to the observed deepening of the carbonate compensation depth through contributions to ocean alkalinity.

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Figure 1: Seawater and detrital silicate Pb and Nd isotopes versus age for sites 738 and 689.
Figure 2: Proxy responses to the Eocene/Oligocene glaciation of Antarctica.

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Acknowledgements

We thank G. Kamenov for technical support regarding Nd and Pb isotopic analyses on the Nu Plasma MC-ICPMS at the University of Florida and P. Muller for scientific discussions. Funding for this research was provided by NSF grant OCE0926474 to E.E.M.

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Authors

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C.B. and E.E.M. conceived the study. C.B. analysed the Nd and Pb isotope data. Both authors contributed towards writing the manuscript.

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Correspondence to Chandranath Basak.

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The authors declare no competing financial interests.

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Basak, C., Martin, E. Antarctic weathering and carbonate compensation at the Eocene–Oligocene transition. Nature Geosci 6, 121–124 (2013). https://doi.org/10.1038/ngeo1707

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