An annual laminated stalagmite from northern Spain recorded two outstanding oxygen isotope (δ18O) pulses during the 8.2 ka event. Most of the oxygen isotope record variability is related to the amount of rainfall, although other factors affect the signal. The lamina thickness record, which is also related to amount of rainfall, does not show any significant anomaly at the time of oxygen isotope spikes. Considering the factors affecting the isotope signal, the two prominent falls in δ18O during the event are interpreted to be caused by the release of large amounts of fresh waters into the North Atlantic. Thus, 18O-depleted ocean surface waters shifted the rainfall δ18O composition for Europe and Greenland. Our precise chronology provides the timing of the outbursts that caused the δ18O anomalies at 8350–8340 and 8221–8211 ± 34 yr B.P., most probably generated by the drainage of proglacial lakes Agassiz and Ojibway. Therefore, in the North Atlantic region δ18O records during the 8.2 ka event trace important hydrological modifications in the ocean, not just local climate. As a consequence, paleoclimate reconstructions from Europe and Greenland using the δ18O proxy that does not take this into account would be overestimating the magnitude of the anomalies during the 8.2 ka event.