Evolution of the oceanic circulation in the subtropical Atlantic across the Mid-Pleistocene Transition

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Existing conceptual models fail to convincingly explain the rapid advance of northern hemisphere ice sheets during the mid-Pleistocene Transition (MPT), between approx. 1.2-0.7 Ma. We aim to investigate whether alterations in the North Atlantic Subtropical Gyre circulation played a key role for initiating and sustaining the growth of large glacial ice sheets during this critical interval. The Subtropical Gyre modulates heat and moisture transport into high latitudes, which is essential for the growth of ice sheets under a cooling climate. Proxy and model data indeed suggest dynamic fluctuations of the Subtropical Gyre’s strength and position, which might have fueled glacial ice shield growth with the onset of Marine Isotope Stage (MIS) 22 onwards. However, the existing data is controversial and the reasons for the observed fluctuations remained elusive so far. Within the framework of the proposed project, the role of the Subtropical Gyre for the glacial northward heat and moisture transport during the MPT will be constrained by testing two scenarios that link explain ice sheet growth to alterations in the subtropical oceanic circulation:(1) Climate changes in the source region of the Gulf Stream modify its salinity and temperature and thereby alter the northward heat and moisture advection.(2) Ice sheet growth causes a southward displacement of the Subtropical Gyre causing a warming in the western Atlantic and enhanced atmospheric moisture production. This moisture is transported into central Europe by intensified Westerlies and, as a positive feedback, further increases snow accumulation.Therefore, we constrain the temperature and salinity evolution in the surface waters of the North Atlantic Subtropical Gyre during the mid-Pleistocene, by investigating two sites by means of geochemical methods (Mg/Ca and stable oxygen isotopes on foraminiferal calcite), covering the interval 1.21 – 0.54 Ma (MIS 35-15). One Site will be ODP Site 1058 located on Blake Ridge within the main path of the Gulfstream. Further, IODP Site U1313 from the central Atlantic at 41°N will be investigated. The latter site is ideally located to capture the latitudinal displacement of Gulfstream and Subtropical Gyre. As methods we will apply the combined analyses of d18O and Mg/Ca on surface dwelling foraminifera (Globigerinoides ruber), which allows for the computation of sea surface temperature and salinity variations by means of the ice-volume corrected d18Oseawater. For Site U1313 we will further generate d18O and Mg/Ca records based on the deep dwellers Globorotalia truncatulinoides (Site 1058) Globorotalia crassaformis (Site 1313) to reconstruct the thermocline structure, which reflects the intensity of the Subtropical Gyre circulation. The newly obtained data will help to disentangle feedbacks between climatic processes in high and low latitudes and provide novel insights into their role during the MPT.