BENTHIC FORAMINIFERA RECORD THE REPEATED EXPANSION AND INTENSIFICATION OF THE OXYGEN MINIMUM ZONE IN THE GULF OF ALASKA AND ASSOCIATED INCREASES IN PHYTODETRIUS (Invited Presentation)

Paleoecological analyses of benthic foraminiferal faunas can reveal past changes in sea floor oxygenation and help evaluate the potential drivers of dysoxic events. The North Pacific contains the largest modern oxygen minimum zone (OMZ) and oxygenation has declined in recent decades, affecting marine ecosystems and carbon cycle processes. Here we reconstruct the severity of dysoxia in the Gulf of Alaska (0-60 ka) at high-resolution (~200-350 yr) to understand past variation in oxygen concentration and its drivers. We use ordination analyses of benthic foraminiferal assemblages that combine data from two core locations (U1419 at 692 m water depth in the upper OMZ and U1418 at 3667 m water depth), which allows us to quantitatively compare the severity and duration of the dysoxic events at each site. We assign each species to an oxygenation tolerance using the modern ecology of identified taxa as well as the covariation of species along an ordination gradient constrained by the concentration of redox-sensitive metals measured from the same sediments as the faunas. This allows us to consider all taxa, including those without measured oxygen tolerances, when we calculate oxygen concentrations.

We identify >10 dysoxic events at the shallow site; many are lower oxygen than observed in the modern Gulf of Alaska indicating intervals with a more intense OMZ. We also observe suboxia at the deeper site during the last deglacial (~17-13 ka) and between 30-50 ka (in MIS 3), demonstrating past expansions of the OMZ. Faunal changes suggest that deglacial dysoxia developed abruptly (within <100 yr) at the shallow site while oxygenation declined more gradually at the deep site. Opportunistic foraminifera sensitive to phytodetrius flux increase in relative abundance after hypoxic events at both sites, but not before the increase in species tolerant of suboxia and dysoxia. This suggests that nutrient availability increased during the dysoxic event, perhaps driven by the remobilization of nutrients in the sediments, but that high productivity and organic flux to the sea floor did not drive the initiation of dysoxia. Instead, other potential factors, such as changes in oxygen solubility, ocean circulation or water column mixing, may have caused the dysoxia whereas productivity may amplify the severity of the event, especially in the upper OMZ.