Neodymium isotopic composition of deep-sea corals from the Labrador Sea: implications for NW Atlantic deep-water masses circulation during the Holocene, MIS 5 and 7

English Abstract:

The Neodymium isotopic composition (εNd) of aragonitic deep-sea corals (Desmophyllum dianthus) collected off Belle Isle Strait and Newfoundland, at Orphan Knoll (1740 m water depth) and Flemish Cap (1550 m water depth), was investigated to identify changes in the seawater εNd and thus changes in the Northwestern Atlantic deep-water masses circulation during the late Quaternary. These sites, located at depths corresponding approximately to the maximum convection depth of the Labrador Sea Water (LSW) just above the North East Atlantic Deep Water (NEADW) mass, are under the influence of the intermediate to deep-water masses re-circulating in the North Atlantic subpolar gyre. ¹⁴C and U-Th dating performed on unaltered specimens selected on the basis of geochemical and mineralogical criterions indicate that they developed during warm periods MIS 1/Holocene, the Bølling-Allerød, MIS 5c and MIS 7a. Comparison between εNd values of 4 modern corals and of ambient seawater at both sites yields a good agreement around a value of – 13.5, indicating that these corals have the potential to reliably record changes in seawater εNd. First results obtained on subfossil corals indicate seawater εNd of around -14 during the Holocene and Bølling-Allerød, -25 during MIS 5c and -17 during MIS 7a. The strongly negative seawater εNd during MIS 5c and MIS 7a reveal an increased imprint of unradiogenic lithogenic sources of Nd derived from Greenland and the Canadian Shield via slope sediment remobilizations and/or an increased Baffin Bay overflow of relatively dense waters, through Davis Strait. Nonetheless, this implies the absence of deep-water formation in the Labrador Sea through the advection of more radiogenic north Atlantic waters during these time periods.