Reconstructing the Plio-Pleistocene evolution of the water mass exchange and climate variability in the Nordic Seas and North Atlantic Ocean

Global climate of the Plio-Pleistocene has been largely impacted by changes of the Atlantic Meridional Overturning Circulation (AMOC), which has most likely to a large extent been controlled by the formation of North Atlantic Deep Water (NADW) ventilating the entire Atlantic Ocean. The main areas contributing to the formation of NADW have been the Labrador Sea, the Nordic Seas, and North Atlantic Ocean. The circulation pattern in these areas underwent dramatic changes during the Plio – and Pleistocene, in particular during the onset and intensification of the Northern Hemisphere Glaciation (NHG) at 2.7 Ma and the Mid Pleistocene Transition (MPT) 1.5 – 0.6 Ma. The main goal of this study is to improve the understanding of the role of changes in ocean circulation in these key areas and their relationships to glaciations controlling and responding to climate. To investigate the past water mass mixing and erosional input between the climatically important areas of Nordic Seas and North Atlantic Ocean, the radiogenic isotope signatures of neodymium (Nd), lead (Pb) and strontium (Sr) of seawater-derived ferromanganese coatings on sediment particles, as well as of detrital material have been analyzed. Sediments obtained from six ODP (Ocean Drilling Program) Sites covering intermediate to deep waters in the Nordic Seas and North Atlantic Ocean are the basis of this study. Two Sites are located in the North Atlantic Ocean on Bjorn Drift (Site 984) in 1648 m water depth and on Rockall Plateau (Site 982) in 1135 m water depth. The sites in the Nordic Seas are located in the region influenced by Arctic Intermediate Water on Iceland Plateau (Site 907, 1800 m water depth) and of the inflowing Atlantic waters on Voring Plateau (Site 644, 1220 m water depth), on the Svalbard margin (Site 986, 2051 m deep) and on Yermak Plateau in the Fram Strait (Site 911, 906 m water depth). The extraction of seawater Nd and Pb isotope compositions from authigenic ferromanganese coatings, as well as Nd isotope composition from the calcitic foraminiferal shells from locations in the western part of the study area (Sites 907 and 984) gave no reliable results as compared to the present seawater signatures in this area, mainly as a consequence of partial dissolution of basaltic ash particles during the extraction procedures. In contrast, for the sites (982, 644, 986, 911) in the eastern part of the Nordic Seas and in the North Atlantic Ocean reliable past bottom water signatures were extracted using different leaching methods and allow paleoreconstructions for the past 3 - 5 Ma. The Nd and Pb isotope records of ferromanganese coatings of Site 911 on the Yermak Plateau were extracted with a leach method based on studies without a preceding decarbonation step. These records show only small variations prior to 2.7 Ma pointing to a continuous Atlantic inflow under generally warmer climatic conditions. In contrast, since the intensification of the Northern Hemisphere Glaciation the Nd isotope composition has shown a high variability as a consequence of the waxing and waning of ice sheets on Svalbard, the Barents Sea and Eurasia. The Pb record at the same time reveals a trend to more radiogenic values as a result of increasing glacial weathering inputs from old continental landmasses after 2 Ma. A similar evolution is evident from the Nd and Pb isotope records of Sites 986, 644 and 982, which were extracted from seawater derived ferromanganese coatings applying the leaching method with a preceding decarbonation step. Comparison of the Nd isotope records of these three ODP sites shows periods of a warmer climate, moderate glacial conditions and an enhanced Atlantic inflow into the Nordic Seas prior to the onset of the NHG at 2.7 Ma and between 2.2 Ma and 1.5 Ma. In contrast, at the beginning of the MPT between 1.5 and 1.2 Ma the Atlantic inflow was significantly reduced, expressed as marked differences between the Nd isotope records in the Nordic Seas and North Atlantic Ocean. Sediment provenance obtained from Nd and Pb isotope compositions of the detrital material shows a dominant influence of Svalbard, Barents Sea and Eurasian shelf at the sites on the Svalbard margin and Yermak Plateau. The Voring Plateau has been dominated by sediment input from Scandinavia to which material from the Barents Sea via icebergs and sea ice was added during glaciations. In contrast, the Rockall Plateau shows the lowest erosional influence due to its location distant from land. However, the Pb isotope evolution obtained from ferromanganese coating of sediment of Site 982 nevertheless points to a slightly increased erosional input from Greenland and Iceland after 1.2 Ma. The results of this study provide a deeper insight into the overall climate control mechanisms of the past 5 Myr and will contribute to improve the understanding of future climatic change.