The evolution of ice-sheets in the late Eocene; reconstructions from coupled Hf-Nd isotopes in authigenic and lithogenic sediments

Large ice sheets at both poles may be the namesakes for the present-day climate state—the icehouse—however our understanding of when these massive accumulations of ice first appeared is far from complete. The continental crust beneath the Antarctic and Greenland ice sheets is ancient, and processes of glacial weathering contribute distinct and easily recognizable isotopic fingerprints from these sources to the ocean. We have used radiogenic isotopes in sedimentary record from the Southern Ocean surrounding Antarctica and from Newfoundland Ridge in the North Atlantic to figure out the timing of major glacial weathering pulses. Here we present evidence that precursor glaciations preceded the widespread development of ice-sheets on Antarctica. We will also present results from the North Atlantic (Expedition 342, Newfoundland drifts) that indicate an episode of glacial weathering on Greenland in the earliest Oligocene. The paradox implied by these results—that there was continental ice in the northern hemisphere when CO₂ levels were above the bipolar glaciation threshold—may be resolved in the future with a better understanding of past CO₂ levels and paleo-elevation in the Northern Hemisphere.