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

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Abstract
Dissolved nitrate in the subsurface water (200–650 m) off southern California is unusually high in δ15N compared with waters from the same depth in the central and western Pacific, which have δ15N 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 δ15N of nitrate. The mean δ15N (± 1 SD) of deep waters (>1,500 m) is 5.7±0.7‰. The mean δ15N in the 15N‐anomaly zone off southern California is 9.0±0.7‰. Maxima of δ15N were found to coincide with salinity maxima, which occurred in the same isopycnal levels as the 15N‐enriched denitrifying zone in the eastern tropical North Pacific (ETNP). Using the correlation between δ15N 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 15N‐enriched nitrate.

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