Direct observation of North Atlantic nutrient transport and biological pump variability linked to the Meridional Overturning Circulation

Abstract The ocean biological carbon pump (BCP) plays a pivotal role in the global carbon cycle. The BCP magnitude is determined by the fraction of nutrients utilised in biological production and remineralised at depth, with the remainder being subducted into the interior unused as ‘preformed’ nutrients. This fraction is currently around 50% and subject to the interaction of biological processes and global scale circulation. Consequently, changes in circulation can potentially impact biological carbon storage. Here we provide observational evidence that the reduction in the Atlantic Meridional Overturning Circulation (AMOC) that occurred over the 2004-2018 period has been accompanied by substantial changes in nutrient transports and associated carbon storage. Persistent southward net nutrient transport across 26.5°N exceeded nutrient sources, except by the end of the period when the system approached balance. This transient net loss of nutrients from the North Atlantic was accompanied by increases in the ratio of remineralized to preformed nutrients, indicating an increasing BCP efficiency (and carbon storage). Our results thus demonstrate observable transient changes in large scale nutrient transports linked to AMOC changes over interannual - decadal timescales, with implications for future ocean carbon storage.