The magnitude and source of meltwater forcing of the 8.2 ka climate event constrained by relative sea-level data from eastern Scotland

The 8.2 ka climate event is the most significant North Atlantic cooling event during the Holocene. Freshwater pulses from the melting Laurentide Ice Sheet draining into the North Atlantic Ocean are commonly thought to be its cause by perturbing the Atlantic Meridional Overturning Circulation. The timing, magnitude and number of freshwater pulses, however, remain uncertain. This is problematic for predicting future climate scenarios because it prevents rigorous testing of coupled ocean–atmosphere climate models against an otherwise excellent test case of climate effects of meltwater inputs into the North Atlantic. To address this knowledge gap, we present a high-resolution relative sea-level record from the Ythan Estuary, Scotland, spanning the centuries leading into the 8.2 ka climate event. The results show a ‘sea-level event’ with two distinct stages between 8,530 and 8,240 cal yr BP when rates of sea-level rise departed from the background rates of around 2 mm yr-1 and reached around 13 mm yr-1 and 4 mm yr-1, respectively. The maximum probable magnitude of local sea-level rise during the stages was 1.67 and 0.41 m, which equate to barystatic magnitudes of 2.39 and 0.58 m respectively after considering the geographic location relative to the source. For the first time, we demonstrate that Lake Agassiz-Ojibway drainage alone is insufficient to explain the large volumes of North Atlantic freshwater input, and that the collapse of the Hudson Bay Ice Saddle appears to have been the main source of meltwater in to the North Atlantic. By comparing the Ythan sea-level record with other sources of evidence, we hypothesise that an initial thinning of the Laurentide Ice Sheet enabled subglacial drainage of Lake Agassiz and subsequent collapse of the Hudson Bay Ice Saddle. This was followed by the terminal drainage of Lake Agassiz completing a sequence of events that likely forced the shift in the Atlantic Meridional Overturning Circulation and hence the 8.2 ka climate event.