Was the discharge of thick Arctic sea-ice to the North Atlantic a trigger for the Younger Dryas?

The sudden release of freshwater previously stored in glacial lakes at the margin of the Laurentide Ice Sheet is commonly thought to have triggered the onset of the Younger Dryas cold episode. In this typical scenario, freshwater is rapidly advected across the northern North Atlantic Ocean and disrupts the climate system by weakening deep convection in the Greenland and Labrador Seas. Here we present an alternative hypothesis that this pulse of freshwater was delivered to the North Atlantic, not from a glacial lake outburst (i.e. as liquid freshwater), but from a dramatic increase in the discharge of thick (up to 50-m) sea-ice in the Arctic that formed during the Last Glacial Maximum.

Using a high-resolution (1/6 degree, 18 km) eddy permitting, ocean sea-ice numerical model (MITgcm), we examined the pathways of the thick sea-ice from the Arctic to the North Atlantic, and quantified the amount of freshwater delivered to the deepwater formation regions. We then undertook a second suite of experiments using the same model, but discharged the freshwater to the ocean entirely as liquid at the mouth of the Mackenzie River (Arctic) and Gulf of St. Lawrence (west Atlantic) - the two regions most frequently attributed to triggering the Younger Dryas.

In our ice experiments, the thick sea-ice was rapidly transported from the Arctic to the North Atlantic, via the Fram Strait, and quickly covered both the Labrador and Greenland Seas convective regions. As the ice melted it released enormous amounts of freshwater to the ocean, resulting in the development of a freshwater 'cap' that inhibited deep convection. However, when we released freshwater to the ocean entirely as liquid via the Mackenzie River and Gulf of St Lawrence, we found that convection in the Greenland and Labrador Seas was considerably less disrupted, although the Mackenzie (Arctic) source was substantially more effective than the Gulf of St. Lawrence at delivering freshwater to both of these climatically sensitive regions. These simulations indicate that more consideration needs to be given to the effects of thick sea-ice, as well as to freshwater transported through the Arctic Ocean, on deep- and intermediate water formation in the North Atlantic. Although our focus has been on the Younger Dryas episode, other cold anomalies seen in the paleoclimate record may also have been driven by these factors.