Short-term waxing and waning of Antarctic ice sheets during the late Oligocene

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Available ice-volume reconstructions from ODP Site 689 (Maud Rise, Weddell Sea) and from North Atlantic IODP Site U1406 show a highly dynamic Antarctic ice sheet (AIS) characterized by orbitally-controlled high-amplitude fluctuations of up to 70% relative to modern ice volume. The principal objectives of the proposed project are to critically test these sea-level/ice-volume reconstructions for the late Oligocene and to obtain a mechanistic understanding of the processes that led to the observed ice-sheet dynamics. These objectives will be reached through the generation of paleoclimate proxy records for a specific time interval of the late Oligocene (magnetochron C8n.2n; 25.304–25.987 Ma) to allow to work on highest possible stratigraphic precision. Proxy records to be generated comprise three independent approaches to have a sensitive test of the available sea-level/ice-volume reconstructions: (i) based on benthic fora¬mini¬fe¬ral geochemistry (Mg/Ca and stable isotopes), (ii) Nd isotopes from the detrital fraction of bulk-sediment samples, and (iii) Pb isotopes from the authigenic fraction of bulk-sediment samples. The first two approaches were already in the focus of the first project phase. For the second project phase, the focus will be on approach (iii), the gene¬rat¬¬ion of an authigenic Pb-isotope record across Chron C8n.2n from Site 689 to decipher provenance changes of the runoff signal resulting from different source rocks that are made available for weathering processes as a function of expanding or diminishing ice-sheet coverage in the late Oligocene (i.e., coastal dominated erosion during large ice volume vs. inland-dominated erosion during intervals of a small AIS) as it is suggested by the newly generated foraminiferal-based ice-volume estimates of the first project phase. The site focused upon in the here proposed project (Site 689) provides the opportunity to work at a high temporal resolution within a robust magnetostratigraphic framework. Moreover, it is located in a strategic position with regard to tracing the influence of changing Antarctic ice sheets on the delivery of detritus and weathering products to the Southern Ocean. The proposed records will allow detailed insight into the evolution of the cryosphere for a critical interval of Cenozoic climate evolution (i.e., the Oligocene rise of the icehouse climate). Moreover, the data will allow a sophisticated comparison of the driving mechanisms behind glacial/interglacial cycles during two fundamentally different intervals of Earth’s history with respect to their climatic processes: the unipolar-glaciated Oligocene and the bipolar-glaciated Late Pleistocene.