Deep-sea coral recording of the neodymium isotope signature of Labrador Sea intermediate water at sporadic intervals over the last two climatic cycles

Live and sub-fossil deep-sea solitary scleractinian corals (Desmophyllum dianthus) were collected at Orphan Knoll and Flemish Cap, in the southern Labrador Sea (LS), at depths ranging 1600–2200 m, thus encompassing the maximum convection depth of the modern Labrador Sea Water (LSW) and slightly above the modern Northeast Atlantic Deep-Water mass, the two upper components of the North Atlantic Deep Water (NADW). U-series and ¹⁴C ages allowed the assignment of the fossil specimens to the mid-to late Holocene, the Marine Isotope Stages (MIS) 5c, and 7 (likely 7a). The research objective was to document the specificity of the neodymium isotope recordings of water masses occupying the Labrador Sea during these time windows, with special attention to possible convection in this basin, thus to production of the intermediate Labrador Sea Water (LSW). Live-collected specimens and ambient seawater fit with a εNd value of – 14, assigned to the modern NADW, indicating that the coral skeletons reliably capture the isotopic composition of dissolved Nd. During the mid-Holocene, they recorded an εNd-shift from ∼ −18 to ∼ −14, this latter value since remaining practically unchanged, pointing to a late attainment of modern like conditions in the Labrador Sea. A similar value (∼−14) was also recorded by a Bølling-Allerød pseudo-colony. The MIS 7a specimens yielded a lesser radiogenic value, close to that of the early Holocene (εNd ∼ −18 vs −19), recording the influence of Greenland and Canadian shield lithogenic unradiogenic sources on the open North Atlantic water masses bearing a more radiogenic εNd-signature. With εNd-values ranging from −20 to −26, the MIS 5c cluster illustrates a drastically distinct and unstable situation that we associate with sporadically-enhanced overflows of Baffin Bay water and/or the partial dissolution of dolostones linked to hypopycnal sporadic plumes deposited along the Labrador Slope. So far, only the late Holocene and Bølling-Allerød corals (eNd ∼ −14) point to a modern-like situation with convection down to about 2000 m in the basin, thus LSW production, linked to the advection of more salty and more radiogenic εNd-bearing North Atlantic water masses, i.e., a full integration of the Labrador Sea into the Atlantic Meridional Overturning Circulation.