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The 8.2 ka event was the last deglacial abrupt climate event. A reduction in the Atlantic meridional overturning circulation (AMOC) attributed to the drainage of glacial Lake Agassiz may have caused the event, but the freshwater signature of Lake Agassiz discharge has yet to be identified in delta O-18 of foraminiferal calcite records from the Labrador Sea, calling into question the connection between freshwater discharge to the North Atlantic and AMOC strength. Using Mg/Ca-paleothermometry, we demonstrate that similar to 3 degrees C of near-surface ocean cooling masked an similar to 1.0 parts per thousand decrease in western Labrador Sea delta O-18 of seawater concurrent with Lake Agassiz drainage. Comparison with North Atlantic delta O-18 of seawater records shows that the freshwater discharge was transported to regions of deep-water formation where it could perturb AMOC and force the 8.2 ka event.