Evidence for persistent benthic hypoxia on the Pacific Northwest continental margin during the Holocene Thermal Maximum

Benthic hypoxic events, driven by invigorated late-summer coastal upwelling during periods of low river discharge, have been increasing in frequency on the Pacific Northwest continental margin of North America over the past two decades (Peterson et al., 2013; Barth et al., 2018). The 2018 season proved among the worst on record, with detrimental consequences for local ecology and economically significant fisheries. It is hypothesized that these events are driven by the direct and indirect (e.g. changes in wind patterns, upwelling, and ocean currents) warming of the regional surface ocean, and may soon become an annual occurrence as global temperatures rise through the 21st century (Barth et al., 2018). Highly resolved and well-dated environmental reconstructions from marine sediments offer an opportunity to extend our understanding of the interplay between climate variability and oceanographic conditions through a period when orbital forcing resulted in Northern Hemisphere temperatures ~1-2 ° C warmer than today: the Holocene Thermal Maximum, at its peak in the Pacific Northwest between 11,000-9,000 cal ybp. Here we review early results from jumbo piston core OC1706b-11JC (45 ° 5.9492' N, 124 ° 51.278' W, 828 m), recovered from the upper continental slope of the northern Oregon margin. The uppermost 350 cm below seafloor (cmbsf) consist of bioturbated hemipelagic mud typical of the modern environment as determined from multicores taken at the site. However CT-scan images reveal the preservation of over 200 laminae between 350-450 cmbsf. An initial evaluation of Mn/Ti and S/Ti ratios generated from X-ray fluorescence (XRF) scanning indicate these laminations may be preserved during a prolonged interval of persistent benthic hypoxia. A preliminary chronology based on planktonic foraminiferal radiocarbon dates suggest that these events occurred during the early Holocene, potentially offering paleo-insights for a warming world where benthic hypoxia could be the norm rather than the exception.