HOLOCENE VARIATIONS IN HYPOXIA AND PALEOPRODUCTIVITY ALONG THE OREGON MARGIN

Recent hypoxic events off the Oregon coast within the last decade have been especially severe, occupying 3000 km² of the water column and resulting in mass mortalities throughout the marine ecosystem. In an effort to determine whether these occurrences are part of a naturally variable system or due to anthropogenic influences, this paleoceanographic study examined a sediment core collected along the Oregon margin at a depth of 360 m. Methods included geochemical analyses of trace metals (Re, Mo), minor elements (Mn, Al), biogenic silica, as well as carbon, nitrogen, and their associated isotopes. Metals data indicate a likely upwelling-induced hypoxic event occurred at 20-80 cm. The carbon isotope composition ranged between -22.8 ‰ and -24.8‰ with slightly more enriched values at the top of the core, which may reflect recent increases in temperature. Nitrogen isotope composition ranged from 6.5‰ to 5.5‰, with a cyclical enrichment/depletion pattern noted between 75 cm and 175 cm, perhaps suggesting regular changes in the N cycle. Terrestrial input was noted at several locations in the core, particularly, at 145 cm, 205 cm, and 255 cm. Shifts in the source of organic matter appear to occur independently of the hypoxic events. A preliminary age model suggests that a hypoxic event occurred in the middle Holocene (2-4 ka). Thus, these hypoxic events have occurred in the past at this location due to natural climate variability; consequently, it is believed that the system may be susceptible to and exacerbated by current anthropogenic climate change.