Antarctic weathering and carbonate compensation at the Eocene-Oligocene transition

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 oceans(1) and perturbations in atmospheric carbon dioxide concentrations(2,3). These changes may have been linked to continentalweathering 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 Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 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 Pb-206/Pb-204 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 Pb-206/Pb-204 trend, and could have contributed to the observed deepening of the carbonate compensation depth through contributions to ocean alkalinity.