Mechanisms of glacial/interglacial changes during the late Oligocene

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The principal objective of this project is to obtain a mechanistic understanding of the processes that led to the differences in the morphology of glacial/interglacial (G/I) cycles during the Late Oligocene of IODP Site U1406 (off Newfoundland, IODP Expedition 342). An existing sub-orbital resolution benthic foraminiferal oxygen-isotope dataset from this site for the time interval ~25.6 to 23.9 Ma reveals two morphologies of G/I cycles. The prominent morphology is U-shaped ("control interval") and well-known from glacials before the Middle-Pleistocene Transition (MPT). The other morphology is characterized by a slow increase of benthic foraminiferal oxygen isotope values followed by a rapid decrease (“target interval”). This pattern is fundamentally different from the U-like shape of other Oligocene glacial cycles, but conspicuously reminiscent of the sawtooth nature of Late Pleistocene glacial/interglacial cycles. In light of the above, benthic foraminiferal δ18O and Mg/Ca records for the „target“ and "Control" intervals have been generated and used to constrain the magnitudes and durations of ice-volume and sea-level fluctuations for the observed G/IG cycles. For both the "target" and the "Control" intervals, BWT are surprisingly constant, with values that range generally between 2 and 4 °C. Even more surprisingly, both intervals lack a clear G/IG cycle in BWT, which suggests that the contribution of BWT fluctuations to the observed G/IG cycles in the benthic delta18O data is small. This becomes even more evident when the δ18O of seawater is calculated and converted to sea level. The observed difference in sea-level estimates between the two study intervals argues for a substantial ice-volume loss on Antarctica. If compared to obliquity, sea-level/ice-volume change during the "control interval" seems to respond nearly linearly to obliquity. Sea-level/ice-volume change during the “target interval”, in contrast, occurred during a time of weaker obliquity. This suggests a potential non-linear threshold response of Antarctic ice volume to the underlying obliquity forcing during the study interval. Whether such a threshold truly existed and what the exact climatic conditions for this potential threshold behavior were, however, cannot be answered with the records at hand so far. Therefore, it is proposed to expand the new Site U1406 high-resolution records to cover the time interval 26–24.5 Ma (i.e. the time interval characterized by significant "steps" in the available delta18O record. Following the approach successfully employed during the first project phase, this will be achieved via the investigation of benthic foraminiferal Mg/Ca and stable isotopes.