The eastern tropical Pacific as a source of 15N‐enriched nitrate in seawater off southern California

Dissolved nitrate in the subsurface water (200–650 m) off southern California is unusually high in δ 1 5 N compared with waters from the same depth in the central and western Pacific, which have δ 1 5 N values similar to the deep oceans. More than 150 seawater samples from the sea off southern California and the North Atlantic were analyzed for the δ 1 5 N of nitrate. The mean δ 15 N (± 1 SD) of deep waters (>1,500 m) is 5.7±0.7‰. The mean δ 15 N in the 15 N‐anomaly zone off southern California is 9.0±0.7‰. Maxima of δ 15 N were found to coincide with salinity maxima, which occurred in the same isopycnal levels as the 15 N‐enriched denitrifying zone in the eastern tropical North Pacific (ETNP). Using the correlation between δ 15 N and salinity, we can show the southern California waters to be of mixed origin with the denitrifying water from the ETNP as one end member. Our observations support the notion that the oxygen‐deficient waters in the eastern tropical Pacific are important sources of 15 N‐enriched nitrate.