Deep Sea Coral Skeleton-Bound δ15N as a Proxy for Late Holocene variability in the Subpolar North Atlantic hydrography

The subpolar North Atlantic is a crucial part of Atlantic Meridional Overturning Circulation. Here, the cyclonic North Atlantic Subpolar Gyre (NASPG) dominates surface circulation, redistributing heat and salt at the loci of deep water formation. Observations over recent decades demonstrate that the upwelling strength and spatial extent of the eastern part of the gyre, between Greenland and Scotland, is governed by the configuration of regional winds, and, in more general sense, the status of North Atlantic Oscillation with implications for weather and climate in Europe. Yet, because of the limited time span of oceanographic and climatological observations, the dynamics of this system are still not well understood. We present a palaeoceanographic analysis of deep-sea scleractinian coral (DSC) skeleton-bound δ15N from the Reykjanes Ridge and Rockall Trough, spanning the time period from the Little Ice Age (LIA) through present. The DSC from the two study sites record conditions across the central part of the NASPG and under the eastern boundary of the gyre, respectively. Upwelling within the gyre results in incomplete nitrate consumption, which is manifested in the spatial gradient of the δ15N of exported organic matter across the NASPG that is in turn recorded in DSC-bound δ15N. We found that the DSC-bound δ15N at both locations was 1.5-2 permil lower during LIA than in the last ~150 years. These observations can be explained by a stronger upwelling within the gyre and, possibly, eastward extension of the NASPG. We hypothesize that a zonally expanded gyre, perhaps combined with enhanced (southward) recirculation in the Rockall Trough, could have modified the northward transport of the Atlantic water and thus contributed to the colder European climate during the LIA.