Sediment dynamic sortable silt suggest variable Labrador Current and Labrador Sea Water strength during the Younger Dryas and 8,200-year meltwater outburst event

Detailed structure of the Younger Dryas cooling event (12.9 -11.7 ka) in the Labrador Sea provides new constraints on its origin. Meltwater outbursts from the Laurentide ice-sheet have been proposed as the initiation mechanism for such cooling, by perturbing the Atlantic Meridional Overturning Circulation (AMOC). However, the extent to which two components of the AMOC, i.e., the Labrador Current (LC) and Labrador Sea Water (LSW), were affected by meltwater outburst is poorly known. Here we use a sediment core that was retrieved from the flow-path of these currents to reconstruct their past strength using the sortable silt flow speed proxy and geochemical data.

Sediment Rb/Zr abruptly increases at 12.9±0.1 ka at the beginning of the YD whereas sortable silt and Hudson Strait detrital carbonate (DC) increase at 12.2 ka and 11.6 ka, respectively. Further, the Rb/Zr sharply decreases at 12.1 ka whereas the sortable silt decreases at 11.8 ka. These data suggest that the sediments were discharged from somewhere on the SE sector of the LIS, probably Labrador, during the first phase (12.9-12.2 ka) of the YD. The LC flow speed abruptly intensified at 12.2 ka due to enhanced meltwater supply and it decelerated at 11.9 ka. In this interval, there was enhanced supply of red sediment probably from the NE Newfoundland Shelf. From 11.9-11.7 ka, when LC flow speed intensity was low, the DC signature from Hudson Strait is predominant, although red sediment supply also continued. A high sediment discharge event between 9.2 ka and 6.4 ka that brackets a series of short-duration events on the Cartwright Saddle (Jennings et al., 2015) was identified by Rb/Zr and minor DC increases that coincide with the gradual decline of the LC flow speed. It appears that the 8.4 ka Lake Agassiz and Ojibway outburst event missed the core site, consistent with the proposition that the Holocene freshwaters in the Labrador Sea would flow southward along the eastern continental Canadian shelf (Condron and Winsor, 2011; Lewis et al., 2012). Our sortable silt data are the first NW Atlantic record that uses the sediment dynamic proxy to assess the impact of freshwater in modifying the AMOC during the YD and 8,200-year events. These preliminary data suggest that there is no simple relationship between size of outburst events and their impact on LC flow speed.