The Intergovernmental Panel on Climate Change has expressed concern over the potential reduction in the Atlantic Meridional Overturning Circulation (AMOC) in the next century, which could cause alteration in regional climate and global carbon cycle. While instrumental observations of the AMOC system are only available for the last decades, confident prediction of future trend must rely on proxy indicators in natural archives to capture the long-term trend of ocean circulation throughout the Industrial Age, and further back in time when there were little anthropogenic carbon emissions. Such a long-term record is particularly important for the Labrador Sea: a region of deep-water formation that constitutes a fundamental component of AMOC.

Utilizing a sediment core with exceptionally high sedimentation rates (up to 60 cm/1000 year), this study has reconstructed time-series of temperature and deep Labrador current speed from 1000 CE to 2017 CE, with the foraminiferal magnesium-to-calcium ratio and sortable silt mean grain size proxies respectively. Ground truthing of the temperature proxy reconstructions are carried out with objective analyses of instrumental observations (Met Office EN4 gridded product) that are available over the past 120 years.

Results reveal a pronounced warming of the Labrador Sea and a substantial decline in the deep Labrador current speed during the last century. Today’s (2017 CE) Labrador Sea deep water mass is the warmest and slowest in the past millennium. The inferred weakening of deep-water formation and transport in the Labrador Sea is thought to contribute to the recent slowdown of AMOC, and has important implications for the oceanic uptake of atmospheric carbon dioxide in the North Atlantic.