Neodymium isotopic composition of deep-sea corals from the NE Atlantic: implications for past hydrological changes during the Holocene

Abstract

The Nd isotopic composition of deep-sea corals ( Lophelia pertusa and Madrepora oculata) located on the Rockall Bank at a depth of 747 m was investigated to reconstruct past hydrological changes of the North Atlantic. The coral ɛ Nd values ranged from −15.4 to −11.9 and demonstrate strong and systematic changes during the Holocene, most likely reflecting changes in seawater ɛ Nd. The ɛ Nd record displays a general trend, with a small decrease from −14.1 to −15.2 during the time interval from 9400 to 6000 yrs followed by an abrupt increase in ɛ Nd towards more radiogenic ɛ Nd values, ranging from −13.9 to −11.9. These variations imply strong changes in the relative proportions of upper-intermediate waters originating in the subtropical and subpolar Atlantic, which are characterised today by ɛ Nd of −10.4/−11.5 and −15, respectively. Therefore, we suggest that the ɛ Nd of reef frameworks forming deep-sea corals reveals strong evidence of systematic mid-Holocene changes in mid-depth subpolar gyre circulation. A wider eastward extension of the subpolar gyre, induced by stronger westerlies, and a less intense deep-water formation in the Labrador Sea occurred during the early-mid-Holocene. Supposedly, larger amounts of freshwater from the subpolar gyre were carried eastward underneath the Nordic seas. Similar to today, a freshening of the subpolar gyre may have been offset by a reinforcement of the NAC and higher salinities of water masses contributing to the Nordic inflow. Beyond 6000 yrs, a weakening of the subpolar gyre triggered an extension of saline subtropical water masses to the eastern North Atlantic, increasing the potential for Labrador Sea deep convection and reducing the contribution of subpolar Atlantic waters to the basin-scale re-circulation at the upper-intermediate depth and to the Nordic seas inflow. Such modifications could have affected deep-water convective activity in the northern hemisphere through a redistribution of freshwater from the early-Holocene melting of northern-hemisphere ice sheets.