The timing of Cordilleran-Laurentide ice-sheet separation and meltwater pulse 1A

The sources of meltwater pulse 1A (MWP-1A; 14.6-14.3 ka) have been debated ever since this abrupt deglacial rise in sea level was identified in 1989. One potential contributor is the rapid deglaciation of the ice saddle thought to have existed between the Cordilleran and Laurentide ice sheets during the last ice age. However, there is currently no direct chronology for the separation of these two ice sheets across the full length of their suture zone. We use cosmogenic surface exposure ages at six locations along a transect spanning the Cordilleran-Laurentide suture zone from ~50° N to ~59° N to directly date the timing of ice-sheet separation and thus the end of ice-saddle meltwater contribution to sea-level rise. We combine our data with another eight previously published exposure ages near our two southern sampling locations that have four or more samples per location. After accounting for effects of isostatic rebound on exposure ages, we find that the southern portion of the suture zone finished deglaciating at 15.2± 0.3 ka (50° N; n=9, 3 outliers) and 14.5± 0.4 ka (52° N; n=8, 2 outliers). The central to northern part of the suture zone became ice free at 14.4± 0.4 ka (54° N; n=4, 1 outlier), 13.6± 0.1 ka (56° N; n=4, 1 outlier), 14.3± 0.5 ka (57.5° N; n=4, 1 outlier) and 14.0± 0.1 ka (59° N; n=4, 1 outlier). The timing of southern ice-sheet separation agrees with observations of increased meltwater input to the Gulf of Mexico while that of the more northern separation aligns with constraints on greater meltwater inflow to the Arctic Ocean. Overall, our results show that the Cordilleran-Laurentide ice-sheet separation did contribute to MWP-1A; data-constrained ice-sheet model simulations will determine what fraction of the MWP came from the ice-sheet separation. Our findings also raise a question over the hypothesis that deglacial meltwater input to the Arctic Ocean slowed the Atlantic meridional overturning circulation (AMOC). The timing of central to northern Cordilleran-Laurentide separation and attendant meltwater discharge to the Arctic Ocean is concurrent with the relatively strong AMOC of the Bølling warm period. This temporal correlation implies that meltwater discharge to the Arctic Ocean may have less of an impact on the AMOC than is commonly simulated by fully-coupled-climate and eddy-permitting-ocean models.