Particulate δ¹⁵N in laminated marine sediments as a proxy for mixing between the California Undercurrent and the California Current: A proof of concept

Abstract

Abstract The measurements of particulate δ 15 N in coastal marine laminated sediments provide a high‐resolution proxy for fluctuations in the intensity of denitrification in the water column. In the eastern tropical North Pacific oxygen minimum zone, this denitrification signal is transported northward by the California Undercurrent, thus serving as a tracer of ocean circulation. This is verified through comparisons between salinity in the thermocline off Southern California (Santa Monica Basin) and the difference between δ 15 N sed within age equivalent sediments from a southern (Pescadero Slope) and northern (Santa Monica Basin) site. Trends in this parameter, Δ δ 15 N sed , relate to Pacific Decadal Oscillation (PDO) phase changes between 1900 and 1990. We hypothesize that the decline in Δ δ 15 N sed during warm PDO phases is due to a strengthening of the California Undercurrent transporting 15 N‐enriched nitrate from the eastern tropical North Pacific northward. The deviation from this trend after 1990 suggests recent changes in circulation and/or California Current water nutrient biogeochemistry.