Reconstructing the Holocene Variability of the Strength and Radiogenic Isotope Composition of Labrador Sea Water

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Oceanic deep water masses formed in the Labrador Sea (LSW) are an important component of the Atlantic Meridional Overturning Circulation, which contribute significantly to the formation of North Atlantic Deep Water (NADW). Vast decreases of NADW production rates competing with advection of southern source deep waters that for example occurred during the last deglaciation have played a critical role in regulating Northern Hemisphere climate on different time scales in the Late Quaternary and are also predicted for the near future. These changes were tightly linked to LSW but its exact role and past variability is still unclear. To solve this problem radiogenic neodymium (Nd) isotope signatures of deep water masses extracted from marine sediments will be applied to constrain past water mass mixing and ocean circulation in the North Atlantic and Labrador Sea given that water masses are labeled with distinct signatures originating from weathering inputs in their source areas. A shallower and weaker overturning circulation has been reconstructed for the last glacial maximum while during so-called Heinrich events NADW formation was even strongly inhibited, both resulting in the deep North Atlantic being bathed by deep waters from the Southern Ocean. In contrast, the early Holocene was marked by pronouncedly unradiogenic (low) Nd isotope values suggesting an even more vigorous overturning circulation mode than today, likely driven by enhanced convection of unradiogenic LSW. A major prerequisite for these interpretations is, however, that the Nd isotope of the distinct end-member water masses including LSW can be constrained reliably over time, which is the goal of this project. If significant continental input changes into the Labrador Sea indeed modified the LSW signal over time this would have fundamental consequences for the interpretation of all available paleoceanographic records based on Nd isotopes in the North Atlantic obtained thus far. Here a detailed study is proposed in the key areas Canadian Arctic Archipelago, Lancaster Sound, Nares Strait and Hudson Strait to constrain the variability of the water mass end-member signatures for the northwestern Atlantic Ocean since the early Holocene. These investigations will be complemented by radiogenic hafnium (Hf) isotope signatures that have proven to be highly sensitive water mass tracers in the present day Labrador Sea. The study will be performed on uniquely well-characterized sediment cores containing well preserved CaCO3 microfossils and molluscs that allow high resolution dating.