Local and remote forcing on the interannual variations of the sedimentary δ15N in Santa Barbara Basin during the past 80 years

Sedimentary nitrogen isotope (δ 15 N sed ) in Santa Barbara Basin (SBB) has been mostly interpreted as the record of the eastern tropical North Pacific (ETNP) intermediate water denitrification process. Nevertheless, debate remains regarding sources and control mechanisms of δ 15 N sed signal in SBB. Multi-proxy analyses including δ 15 N sed , total organic carbon (TOC), total nitrogen (TN), C/N ratio, and marine biomarkers were performed on a 46-cm sediment core (SBB-190629) collected from SBB in 2019. The core was dated with varve counting and 210 Pb dating method, showing a depositional history of 1938–2019 CE with a sedimentation rate of 0.564 cm/year. The findings show that the δ 15 N sed record (at ~0.25-year resolution) ranges from 6.24‰ to 7.43‰, which was affected by both local and remote forcing. The long-term variations of the SBB δ 15 N sed signature show a general decreasing trend from 1940 to the late 1980s, low values during 1980~2000, and an increase afterward, which is thought to reflect changes in ETNP denitrification induced by the strength of tropical trade winds. Our results also reveal a series of abrupt annual to multiannual changes, superimposed on the long-term variation mentioned above. The SBB local δ 15 N signal (Δδ 15 N SBB ) is accessed by using the deviation from the mean δ 15 N (Δδ 15 N) of SBB-190629 to subtract the Δδ 15 N of the ETNP. The Δδ 15 N SBB record compares well with redox-sensitive proxies (Re/Mo ratio and C 29 stanol/stenol ratio) from the SBB bottom water and with the OC marine content calculated based on the C/N ratio mixing model, indicating that the Δδ 15 N SBB is mainly controlled by bottom water denitrification, which was induced by the change of upwelling intensity and marine productivity. Since various climatic factors (e.g., El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Pacific Gyre Oscillation (NPGO)) have different impacts on the upwelling intensity (hence the marine productivity and denitrification) in SBB on different timescales, the influence of combined climatic factors on SBB denitrification is time dependent.