CMIP6 Earth System Models (ESMs) project dramatic slowing (28%-42% by 2100) of Atlantic Meridional Overturning Circulation (AMOC) and Southern Meridional Overturning Circulation (SMOC) across a range of climate scenarios, with a complete shutdown of SMOC possible by year 2300.  Slowing MOC differentially impacts the ocean biological and solubility carbon pumps, leaving the net impact on ocean carbon uptake uncertain. We show with a suite of ESMs that slowing MOC reduces anthropogenic carbon uptake by the solubility pump, but increases deep ocean storage of carbon and nutrients by the biological pump. The net effect reduces ocean uptake of anthropogenic CO₂. The deep-ocean nutrient sequestration will increasingly depress global-scale, marine net primary production (NPP) over time. MOC slowdown represents a positive feedback that could extend or intensify peak-warmth climate conditions on multicentury timescales.