Warm, salty surface water incursions and destabilization of the Cordilleran Ice Sheet

Ocean temperature change has the potential to destabilize tide-water glaciers and ice shelves. Here we investigate the potential impact of changing North Pacific sea surface temperatures (SST) on the stability of the Cordilleran Ice Sheet during the last deglaciation. Stable isotope values and trace metal ratios were generated on the planktonic foraminifera Neogloboquadrina pachyderma and Globigerina bulloides from core MD02-2496, (1243 m water depth; 48°58N, 127°02W), British Columbia. The site is located where the North Pacific Current bifurcates in the modern climate system, transporting water northward into the Alaskan Gyre and southward to the California Current system. In addition as the site is ~35 km from the coast of Vancouver Island, it is ideally located to detect changes in Cordilleran Ice Sheet behavior. The region is also affected by plumes from the Columbia River deflected north by the Coriolis effect, making it possible to monitor Glacial Lake Missoula Outburst Flooding. The high-resolution (50-200cm kyr-1) reconstruction of SST and δ18Oseawater (salinity) reveals cool (4-7°C), relatively fresh and stratified surface waters occupied the region between 20 and 16.5 ka. Frequent incursions of warm (>10°C), relatively saline water on decadal to centennial timescales began ~18.8 kyr, persisting until ~14.7 kyr. Reconstructed warm and salty waters from 18.5-17.9 kyr are associated with cyclic (~80 year) sedimentation of terrigenous organic carbon-rich, >300 Ma shale-like sediments, which may be evidence of Lake Missoula outburst floodwaters. These sediments contrast with the typical ~100 Ma volcanic sediments typically deposited during deglaciation. A step-wise warming of ~2-4°C occurs at ~16.6 ka and both planktonic foraminiferal species record identical SSTs until ~14.7 ka. During this interval the Vancouver Margin surface waters were relatively more saline and very well mixed. The warmest (14.5-16°C) incursion of saline water occurs at ~16.5 ka. At this time the Cordilleran Ice Sheet (CIS) was at its maximum extent and grounded on the continental shelf. A rapid retreat identified at 16.2 by ice rafted debris (IRD) occurs directly after these extreme SSTs. CIS calving may have been induced by warm ocean water thinning the terminus of tide-water ice lobes/glaciers as the warmest deglacial SSTs in MD02-2496 directly precede IRD events in the core. A further ~2°C warming of surface waters occurred at 14.7 ka and the two planktonic foraminiferal SST reconstructions decouple during the Bølling-Ållerød.