The eastern tropical Pacific as a source of 15Nenriched nitrate in seawater off southern California. Limnology and Oceanography 34

• Main Authors: Kon-kee Liu, Isaac R. Kaplan
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1989
• Subjects: Pacific; North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.508.1413; http://www.aslo.org/lo/toc/vol_34/issue_5/0820.pdf

Dissolved nitrate in the subsurface water (200-650 m) off southern California is unusually high in 615N compared with waters from the same depth in the central and western Pacific, which have 6r5N values similar to the deep oceans. Marc than 150 seawater samples from the sea off southern California and the North Atlantic were analyzed for the 615N of nitrate. The mean 615N ( & 1 SD) of deep waters (> 1,500 m) is 5.7 40.7%. The mean iY5N in the “N-anomaly zone off southern California is 9.0*0.7%~. Maxima of 615N 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 ti15N 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 arc important sources of 15N-enriched nitrate. Sweeney et al. (1978) studied the isotopic composition of particulate organic matter (POM) in sediments from the southern Cal-ifornia borderland and showed that the av-crage 615N of marine organic nitrogen was higher than that of terrigenous organic ni-trogen by 7%~. Wada et al. (1975) suggested that the high 615N of nitrogenous com-pounds in the sea, and in planktonic pro-teins, is mainly attributable to the isotopic composition of nitrate in seawater, which is enriched in 15N due to preferential removal of 14N03- during denitrification. Saino and Hattori (1987) reported geographic varia-tion of the 615N value of POM and attrib-uted the difference primarily to isotopic al-tcration in nitrate. This paper demonstrates geographic inhomogeneity in the nitrogen isotopic composition of nitrate and offers some explanations for the variation. The