Neodymium Isotopic Composition of Deep-sea Corals from Rockall Bank, Implications for Variations of Eastern North Atlantic Intermediate Water Hydrology During the Past 10,000 years

The main proxies for reconstructing past circulation of the ocean are stable C isotope ratios (d13C) of dissolved inorganic carbon (DIC) and Cd/Ca ratios as recorded by benthic foraminifers. Both proxies vary as a function of nutrient content of ambient deep water and it is difficult to make quantitative estimates of mixing between water masses. In contrast to those classical ocean tracers, Nd-isotope ratios have been shown to be a promising new proxy to trace the water mass provenance and mixing. It has the advantage to be independent of fractionation induced by biological processes in the water column. Neodymium isotope ratios have been analyzed at high resolution on deep-sea corals (Lophelia pertusa) from core MD01-2454 (55°31.17N - 15°39.08W, water depth : 747 m) located on the Rockall bank in order to trace the eastern North Atlantic intermediate water hydrology during Holocene. To obtain this record rigorous cleaning techniques have been investigated to remove the authigenic fraction of Nd that can be precipitated on deep-water corals by iron-manganese oxide precipitation (Frank, 1998). In addition, a Nd-Oxyde technique for thermal ionization mass spectrometry has been used to precisely determine eNd of corals, which is mandatory as Nd is incorporated in the coralline aragonite solely in small traces (~5-10 ng/g). Deep-sea corals have been dated precisely by means of U-series dating. Core MD01-2454 provides a sedimentary record extending down to the last 10,000 years. eNd(0) values of the deep-sea corals from core MD01-2454G exhibit a range from -12 to -15.5 indicating a significant changes in water mass composition on Rockall Bank over the past 10,000 years. Moreover, several modern deep-water corals, collected from Gibraltar straight to the Norwegian Sea, have been analyzed and confirm that the Nd isotopic signature of deep-sea corals reflect those of the different modern intermediate water masses of the eastern North Atlantic (LSW, MOW, NAC, etc...). For the Holocene, the eNd(0) variations of deep-sea corals from core MD01-2454G permit us to reconstruct past changes of water mass mixing between the SPG and STG that are characterized by different eNd values, -15 and -10/-11 respectively.