Mechanisms of glacial/interglacial changes during the late Oligocene
Final Report Abstract
Within the framework of an international consortium of Expedition 342 scientists, the proposed study aimed to document and understand the dynamics and evolution of obliquity-modulated glacial/interglacial cycles at a resolution that has previously rarely been applied to foraminiferal geochemistry studies of pre-Pliocene sediments (temporal resolution of <1.000 years). In particular, it focused on the question to what extent bottom-water temperatures (BWT) and/or icevolume change contributed to the observed d18O cycles of the Late Oligocene – a question of fundamental importance given the amount of published datasets that rely on sea-level/ice-sheet estimates based on d18O without dividing both influences. In addition to this research question, it was planned to generate (sub)surface-water records of stable isotopes and Mg/Ca based on planktic foraminifera for the study interval to allow for a detailed insight into the evolution of the (sub)surface ocean during the time intervals of the Late Oligocene that are characterized by fundamentally different glacial cycles and large-scale ice-sheet variations. Overall, the project followed the proposed framework but some modifications to the overall objectives and working program as outlined in the original proposal were necessary. In addition to further sampling to increase sample resolution, changes were necessary in terms of species selection for geochemical analyses. It was originally planned to use the surface-dwelling foraminiferal species Globigerina prebulloides. In contrast to the preliminary work on short time intervals, however, this species turned out to be very rare in large parts of the study interval. Therefore, the focus of the geochemical work on planktic foraminifera was shifted to the only species that was abundant in a large part of the samples across the entire study interval, Catapsydrax dissimilis, a subthermocline-dwelling planktic foraminifera. The generated geochemical datasets on benthic foraminifera (stable isotopes and Mg/Ca) allowed for a detailed reconstruction of Antarctic ice-volume for the studied time interval. A striking result of the ice-volume reconstructions generated in the framework of this project has been a highly dynamic Late Oligocene Antarctic ice sheet (AIS) that was mainly triggered by obliquity forcing. This main result of the project reveals that currently available ice-volume reconstructions for the Late Oligocene, that are based on d18O records alone, miss a significant component, which is bottom-water temperature. The reconstructed BWT fluctuations revealed by this project clearly show that they are a significant driver of the d18O signal and therefore necessary to reconstruct reliable ice-volume estimates. The last part of the project was designed to investigate whether the reconstructed dynamics of the AIS affected the thermal dynamics of the North Atlantic. The geochemical temperature and salinity records based on the subthermocline-dwelling foraminiferal species C. dissimilis clearly show a response of heat and salt transport into the high-latitude North Atlantic driven by the amount of ice on the Antarctic continent. Interestingly, these records further reveal a clear asymmetry between both hemispheres. Increases in AIS volume in the Southern Hemisphere, displayed as positive maxima in benthic δ18Osw, thereby coincide with low subthermocline temperatures and salinities as well as low sea-surface temperatures in the North Atlantic. This suggests a thermal decoupling of the Southern Hemisphere from the Northern Hemisphere. Such a distinct interhemispheric asymmetry in the surface and subsurface ocean of the Northern Hemisphere to glacial dynamics in the Southern Hemisphere points to considerable changes in the oceanic and atmospheric circulation affecting the interhemispheric heat balance.
Publications
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2020. Dynamics of the Late Oligocene Antarctic ice sheet. PhD thesis, Heidelberg University, 175 pp.
Brzelinski, S.
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2020. Expedition 378 Preliminary Report: South Pacific Paleogene Climate. International Ocean Discovery Program
Thomas, D.J., Röhl, U., Childress, L.B., and the Expedition 378 Scientists (including S. Brzelinski)
