Reconstructing Dysoxia In The Gulf Of Alaska And The Eastern North Pacific: Severity, Duration, And Drivers

Abstract

High resolution paleoenvironmental studies in the Gulf of Alaska (GoA) were previously restricted to the last ∼17,000 years, limiting our knowledge of oxygenation in the high latitude North Pacific. In this work, I develop a ∼54,000 year-long record of co-registered benthic foraminiferal assemblages and redox sensitive metal concentrations (Mo/Al and U/Al) at Site U1419 in the upper OMZ of GoA to reconstruct the history of OMZ extent and intensity at multi-centennial resolution. Using multivariate analyses of total benthic foraminiferal assemblages, I develop quantitative dissolved oxygen estimates that are robust against differences in the benthic foraminiferal size fraction analyzed, replicate modern oxygenation patterns in the GoA, and are cross validated by redox sensitive metal concentrations. Two dysoxic events of comparable severity to the Bolling-Allerod were found during MIS 3. On comparing taxonomically standardized benthic foraminiferal data across four sites in the North Pacific, I found that dysoxic events occurred basinwide around similar time intervals across the North Pacific and may have been linked to global climatic changes. In addition, the differences in the relative severities of dysoxia at different sites were enhanced by regional factors. GoA which is generally a better oxygenated site did become as dysoxic as the southern sites which are known for their lower oxygen. These studies are important because if happened in the past it may occur in the future of GoA as well. Using paleoenvironment proxies to understand the relationship between different drivers and oxygen we found that dysoxia in the GoA during the glacial interval may have been associated with both surface productivity and warming/meltwater. The relationship between dysoxia and drivers during the deglacial was confounded by the influence of glacial-interglacial changes. However, it is possible that the similar drivers of dysoxia may have led to dysoxia during the deglacial as in the glacial interval. The Holocene in GoA has high relative abundances of opportunistic taxa which represent high surface productivity, if sufficient meltwater is introduced severe dysoxia can develop here. Quantitative estimates of paleo-oxygenation, such as those possible with benthic foraminiferal assemblages, are important for forecasting future oxygenation changes in OMZs and their potential impacts on the marine ecosystems.