HB-1-12

Melting of continental ice sheets generates an excess freshwater influx routed into the North Atlantic, where it may interfere with the formation of deep water and thus affect the global oceanic circulation. Large meltwater discharge is the principal carrier of detritus from the continent into the ocean. The dispersion of this detritus by ocean currents is a measure for the distribution of the spatially focused addition of freshwater in the ocean. Detritus is released from the melting of ice and eroded from the rock/soil substrate by the drainage system. Its composition depends on (1) the substrates sampled and (2) particles included from the atmosphere in the ice sheet over time. The coarse grained fractions are deposited proximal to the source in deltaic environments. The resolution of additions from spatially-compositionally variable sources is high in these rocks. The fine fractions of the detritus can be transported by ocean currents and deposited far from their sources. In contrast to detritus, authigenic minerals in the marine sediment column preserve the signature of the element load that is dissolved in the sea water and could provide information on past long-range water movements. Proxies for water (detritus) discharge from land ice include radiogenic (e.g. Sr, Nd, Hf, Pb) isotope systems, and element abundances. The same elements also occur dissolved in fresh and sea water and provide tracers for moving water masses if their residence time is short (< 2000a; e.g. REE, Hf, Pb) compared with the time needed for mixing different source compositions by exchange of sea water masses (> 5000 – 10000 a). The expertise of the Bremen team is on high precision analyses of non-traditional stable (Li, B, Ca) and radiogenic (Sr, Nd, Pb) isotopes in Earth and Ocean Sciences. Long standing interests are on mineral chemistry and isotope geochemistry for source tracing and isotopic dating metamorphic, magmatic, and sedimentary rocks. Current work includes stable isotope investigation of marine carbonates to reconstruct past records of continental weathering and ocean acidification. An (inter)-disciplinary synergy is given via the expertise of the Canadian partners on radiochemical and traditional stable isotopes in the North Atlantic and ice sheet dynamics and melt supply.

A combined study of elemental and isotope proxies for water discharge in continental detritus, authigenic minerals, and biogenic shell material from the same marine sediments has the potential to identify source regions of continental detritus and hence transport directions (activity of ocean currents), the origin of the transport medium (long-range marine currents), and the compositional impact of fresh and sea water mixing on biota. High resolution sampling of sediment cores around Greenland and Baffin Bay should reveal short-period changes in freshwater generation related to continental ice sheet dynamics. The study will be closely correlated to HB-3 and HB-9 projects on sea ice distribution and water mass properties using biomarker and traditional stable isotope studies. We will work on the same sediment cores with available age constrain. Our first PhD project will focus on radiogenic isotope and elemental compositions at high spatial resolution. Sr, Pb and Nd and Hf isotope signatures constrain continental detritus, (mixed?) sea-water sources, and potentially changing input/weathering through time. These isotopes are relatively well known in marine systems, which enables the characterization/interpretation of our material from point sources within the bigger picture. This will provide the base to study less well understood/established non-traditional stable isotope systems (Li, B, Ca) in inorganic and organic material of the same cores as proxies to quantify continental weathering flux and inform on weathering production of alkalinity within the framework of the second PhD project. The stable isotope systems have a similar behavior in the weathering environment but display different sensitivities to lithologies, biogeochemical cycles and atmospheric input. Such multi-proxy approach on the same sediment and meltwater record provides a powerful means of cross checking the reproducibility and validity of isotopic trends and interpretations. Detailed studies on coherent source - discharge - transport - deposition systems can substantially contribute to validate existing concepts. Ideally, samples from lake deposits on the continental side are included, but these can be substituted by proximal samples on the sea side. Sample characterization starts with the separation of continental detritus in different grain sizes and its isolation from authigenic formations. Existing procedures have to be adapted to the specific samples with the aim to recover all types of material for analyses on radiogenic isotopes and future projects on non-traditional stable isotopes. The material will be characterized by appropriate standard methods (e.g. SEM, EMP, XRD in collaboration with project HB-3, HB-9) and selected for radiogenic isotope analyses. The analytical procedures for radiogenic isotope ratio measurements and elemental abundances are well established and are the core expertise contributed by the isotope geochemistry group in Bremen. This presents the early stage researcher with excellent training and experience in carrying out self-directed and independent work as part of a diverse and skilled interdisciplinary team with the Canadian project partners and in close interaction with the collaborating German projects.