Holocene shifts in sub-surface water circulation of the North-East Atlantic inferred from Nd isotopic composition in cold-water corals

摘要

Variations of the Sub-Polar Gyre (SPG) and the Sub-Tropical Gyre (STG) circulation during the Holocene are believed to be related to regional and global climate over this time period. To improve our understanding of these phenomena we provide new constraints on variations in surface circulation patterns using neodymium isotopes (epsilon Nd) on precisely U-Th dated coral fragments of L. pertusa. The fragments were retrieved from two sediment cores taken from cold-water coral (CWC) mounds at similar to 127-134 m water depth in the Mingulay Reef Complex located on the Western British continental shelf. The results have been combined with epsilon Nd analyzed on seawater samples from two stations located on the continental shelf and margin in order to establish whether epsilon Nd is a reliable proxy of the ocean circulation variations and notably of the relative contribution of water originating from the SPG and STG. epsilon Nd values in CWCs from the Mingulay Reef Complex range from -14.5 +/- 0.4 to -11.8 +/- 0.3, highlighting two major variations. Unradiogenic epsilon Nd values (-14.5 +/- 0.4) indicate a higher contribution of water from the SPG around 2.8 ka. Conversely, more radiogenic values at 3.4 ka (-11.8 +/- 0.3) point to a declining SPG strength, accompanied by stronger northward penetration of STG water along the western European margin transported by the Shelf Edge Current (SEC) and/or cooler and fresher waters from the interior Seas. The eastward extension of the SPG at 2.8 ka is associated with lower C-14 reservoir age (200 yrs) compared to periods associated with a higher contribution of STG waters. This indicates that C-14 reservoir ages are mainly a function of vertical mixing of the sub-surface of the ocean. As stronger vertical ventilation is not associated with a higher proportion of local radiogenic surface water, we hypothesize it could represent vertical ventilation in the North-Eastern Atlantic. Active SPG is associated with a better ventilation of the water masses within the SPG and warmer climatic conditions in Northern Europe and in the Eastern Norwegian Sea linked to an intensification of the surface limb of the AMOC.