ALBERT

Description: Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea-ice can have a deep impact on ocean circulation dynamics in the North Atlantic modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice-rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29° N, 48.37° W, 3432 m water depth). IRD from the fraction 63–150 µm show higher concentration during the intervals: ~ 1000–1100, ~ 1150–1250, ~ 1400–1450, ~ 1650–1700 and ~ 1750–1800 yr AD. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains (HSG) reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling events during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode through internal feedbacks that reduced convection in the Labrador Sea decreasing its contribution to the Atlantic Meridional overturning circulation and, thus, the amount of heat transported to high latitudes. This mechanism very likely preconditioned the North Atlantic inducing a state in which external forcings (e.g. solar irradiance and volcanic input) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. The outcomes of this work indicate that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.