OBSERVATIONS OF PERSISTENT BENTHIC HYPOXIA ON THE PACIFIC NORTHWEST MARGIN: AN EARLY HOLOCENE ANALOGUE FOR OUR NEAR-FUTURE

Anthropogenic activity is causing warming climates and increased nutrient loading in coastal waters, leading to declines in ocean oxygenation concentrations and the expansion of mid-depth oxygen minimum zones (OMZs). Coastal OMZs occur naturally with upwelling systems in Eastern Boundary Currents, making these regions especially susceptible to further deoxygenation with added human influences. As deoxygenation has intensified in recent decades, marginal waters offshore of the Pacific Northwest (PNW) have developed seasonally permanent hypoxia, although the prehistoric context for these events is uncertain due to the limited paleoceanographic reconstructions available for the Oregon and Washington margins. Our research aims to fill in these gaps using a suite of sediment cores collected during the Cascadia H.O.P.S. Cruises in 2017 and 2020 from the continental slope of northern Oregon to northern Washington, focusing on the early Holocene as a comparative time for modern climate change. Initial core observations and Computed Tomography (CT) scans show preservation of finely laminated lithofacies in early Holocene aged sediment cores from the upper slope (~640m – 900m water depth). Preliminary radiocarbon-based age models indicate sediment accumulation rates in this period were similar to overlying sediments with strong bioturbation, and faunal analyses indicate the prevalence of low-oxygen tolerant benthic foraminifera. We will explore the origins, expression, and duration of these events in the paleo record using XRF and bulk solution geochemistry, foraminiferal faunal assemblages, and radiocarbon geochronology.