VARIATION IN ERODED MATERIAL AND ACCUMULATION OF COLUMBIA RIVER SEDIMENTS OFF THE OREGON CONTINENTAL MARGIN SINCE THE LAST GLACIAL MAXIMUM

The advance and retreat of the Cordilleran ice sheet during the late Pleistocene shaped the modern Columbia River basin, with multiple catastrophic flooding events from glacial Lake Missoula influenced regional sediment transport and accumulation on the Washington/Oregon continental margin. Likewise, general melting of the Cordilleran ice sheet supplied sediment to the river and continental margin. Although the Columbia River holds great cultural and economic significance for the Pacific Northwest, relatively little is known about changes in regional erosional history and sediment accumulation from the Last Glacial Maximum (LGM, ~26-19 ka) to the present.

To better understand changes in glacial activity, meltwater runoff, flood events, and erosive processes active in the Pacific Northwest during the LGM and last deglaciation, sediment cores were collected from the Columbia River continental margin during coring cruise OC1706B on the R/V Oceanus in 2017. Two sites, one on a bathymetric rise near the Willapa Canyon submarine fan (02JC; 46.26°N, 125.71°W, 2263 m depth) and another on the upper continental slope south of Astoria Canyon (06JC; 45.89°N, 124.87°W, 761 m depth) were selected for core-scanning X-ray fluorescence (XRF) analysis at Oregon State University at 0.2 mm resolution. Benthic foraminifera oxygen isotopes from the core indicate that these sediments were deposited during and since the LGM.

Sediment elemental ratios in the cores point towards at least three distinct chemostratigraphic units corresponding to changes in provenance and erosional activity of the Columbia River basin. Geochemical provenance tracers obtained from XRF analysis of the cores indicate a change from a mafic source for the oldest sediments that were deposited as a series of turbidite layers to hemipelagic sediments from a felsic source, before transitioning to a Holocene regime. Further geochemical analysis of source samples collected throughout the Columbia River basin will help constrain past changes in provenance of sediments that have accumulated in the offshore environment, as well as provide additional insight on the history of the southern Cordilleran ice sheet and drainage of glacial Lake Missoula.