Quantitative Comparisons Among North Pacific Paleo-Oxygenation Records Using Multivariate Analyses On Taxonomically Standardized Benthic Foraminiferal Assemblages

Low-oxygen (dysoxic) events are well-recorded in marine sediment records from deglacial and interstadial events throughout the North Pacific. Multiple proxies, including benthic foraminiferal faunal, sedimentological, and geochemical approaches have been used to detect these events, however proxy differences make it difficult to quantify the relative severity of dysoxia among events and across the basin. In this study, we use multivariate analyses of taxonomically standardized benthic foraminiferal assemblages to quantitatively compare the relative severity and duration of dysoxic events on the same scale at several sites from ~500-700 m water depths within oxygen minimum zones (OMZs) in the Gulf of Alaska (GoA), Santa Barbara Basin (SBB), and Baja California Sur (BCS). We develop a quantitative dissolved oxygen index that incorporates the total faunal variation and does not rely on a few index taxa as in previous work. In the GoA record where both foraminifera and redox-sensitive metals (U/Al and Mo/Al) are well preserved, this new benthic foraminiferal index is more closely correlated with the geochemical changes than other faunal methods, suggesting our method tracks similar components of oxygen variation as the redox-sensitive metals. We find that although GoA is usually better-oxygenated than the other sites, however it reaches dysoxic conditions as severe as SBB and BCS during the BllingAllerd and during several brief events in MIS3. However, dysoxic events at the GoA site were shorter than at the southern sites. The BCS site remained dysoxic throughout the ~54,000 years examined here whereas the SBB sites record severe dysoxia with better oxygenation conditions during the Younger Dryas and glacial times. Decreases in oxygenation are contemporaneous across sites and the differences in the severity and duration of dysoxia may have be related to the strength of north and south flowing currents. Relaxation of NPIW formation allows low-oxygen waters from the Eastern Tropical North Pacific OMZ to flow northward via the California Undercurrent, thus briefly affecting GoA. While local productivity and carbon export may also affect the severity of dysoxia, foraminiferal records suggest the highest export in GoA occurs during the Holocene when suboxic, rather than dysoxic, conditions are present.