Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary delta N-15 Record

Decades of observations show that the world's oceans have been losing oxygen, with far-reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary delta N-15. We combine two delta N-15 records from the Santa Barbara Basin (SBB), a 24-year-long, biweekly sediment trap time series, and a 114-year, high-resolution sediment core together spanning the years 1892-2017. These records allow for the examination of delta N-15 variability on seasonal to centennial timescales. Seasonal variability in SBB delta N-15 is consistent in timing with the poleward advection of a high delta N-15 signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Nino events result in variable delta N-15 signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary delta N-15. Seasonal and interannual variability in sediment trap delta C-13(org) is consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary delta N-15 in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in delta N-15 reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional delta N-15 records our results further suggest that SBB is uniquely situated to record long-term change in the Eastern Tropical North Pacific.