THE MECHANISMS, TIMING AND IMPACT OF CORDILLERAN ICE SHEET RETREAT ON THE COASTAL WATERS OF THE NORTHEAST PACIFIC OCEAN

At its maximum extent the Cordilleran Ice Sheet (CIS) filled the fiords of British Columbia, transporting significant quantities of glacial sediment to the usually sediment-starved continental shelf off Western Canada. A detailed record of glaciomarine sedimentation is preserved in the well dated ~50-kyr jumbo piston core MD02-2496 (48˚58.47’ N, 127˚02.14’ W, water depth of 1243 m), collected ~75 km off Vancouver Island. To understand the history of the CIS and its relationship to climatic and oceanographic change, we have created high resolution, multi-proxy geochemical records from the interval that captures the Fraser Glaciation (~30-10 ka). These proxies include Mg/Ca-based ocean temperatures from planktonic foraminifera, bulk geochemistry of sediments and K/Ar dating of the <63µm fraction. A detailed reconstruction of the deglaciation of the CIS has been generated based on the source of glaciomarine sediments and ice rafted debris (IRD), as well as evidence for processes such as lake outburst flooding (GLOF) events and iceberg discharge.

At the initiation of the Fraser Glaciation (~30 ka) fine-grained glaciomarine sediments deposited at MD02-2496 had a ~100 Ma volcanic rock source. At ~20 ka, the CIS passed over the Vancouver Island continental shelf at Tofino dramatically increasing sedimentation at the site. From ~19 to 17.3 ka GLOFs created cyclic (~80 year) sedimentary packages of ~300 Ma (εNd of ~-8) shale associated with terrestrial organic carbon, and ~150 Ma felsic volcanic sediment and ~100 Ma (εNd of ~-3) volcanic sediment associated with marine organic carbon. The GLOFs were likely to be associated with glacial lake Missoula outburst flooding, occurring during the interval of the coolest ocean temperatures (2-4°C) and most depleted ^δ18O_seawater. At 17.3 ka, IRD deposition was greater at the site, terminating abruptly at 16.2 ka as ocean temperatures increased by ~3°C. For the next ~1000 years fine-grained sediment was sourced from an ~200 Ma mafic volcanic source, likely the Karmutsen Basalt indicating the deglaciation of Vancouver Island. At the Bølling-Allerød, ocean temperatures rose by > 3°C to 10-12°C in association with an additional IRD event at ~14.8 ka sourced from a ~75 Ma felsic volcanic source, likely the Southern Coast Plutonic Complex. Glaciomarine sedimentation ceased at ~10 ka.