ANALYZING VARIABILITY IN SEDIMENT PROVENANCE TO UNDERSTAND THE RETREAT OF THE SOUTHERN CORDILLERAN

During the Last Glacial Maximum (LGM, ~20 ka), the southern margin of the Cordilleran Ice Sheet extended as far south as the Puget Lowlands of Washington State with glacial runoff draining into Columbia River tributaries in Washington, Oregon, Idaho, and Montana. Over the following five millennia the ice sheet retreated north to the mountainous refugia in Alaska and Canada before vanishing entirely. This retreat follows a timeline that remains ill-defined due to temporal constraints of terrestrial records. Offshore marine sediments that capture the major drainages of the Southern Lobe may provide the necessary information to understand drivers and timings of the last deglaciation.

Here we examine three offshore sediment cores collected during the Cascadia H.O.P.S. cruises of 2017 and 2020: OC1706B-11JC (near the mouth of the Columbia River), OC2006A-18JC (near the southern drainage of the Puget Lobe/proglacial Lake Russell), and OC2206A-12JC (near the mouth of the Strait of Juan De Fuca). These cores are sensitive to material transported through three major drainage channels of the Southern Cordilleran. We use X-ray Fluorescence (XRF) core scanning to examine the variability in provenance of the eroded sediments, within and across core sites. This data will then be compared with physical properties and interpreted on a preliminary chronology informed by paleomagnetic secular variation and radiocarbon dates of planktonic foraminifera.

By evaluating the variations in lithogenic sediment supply within and between core sites, we hope to further interpret and develop site provenance and physical properties as a geochemical tool. A tool which can then be used to help understand the timing and velocity of the final retreat of the Southern Cordilleran.