Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin

[1] Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3-) N-15/N-14 and O-18/O-16 associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in delta(18)O is up to 3 parts per thousand greater than in delta(15)N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1: 1 increase in NO3- delta(18)O and delta(15)N during NO3- consumption. Moreover, the maximum in NO3- delta(18)O is somewhat shallower than the maximum in NO3- delta(15)N. These two observations can be summarized as an "anomaly'' from the 1: 1 delta(18)O- to- delta(15)N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: ( 1) the addition of low-delta(15) N NO3- to the shallow thermocline by the remineralization of newly fixed nitrogen, or ( 2) active cycling between NO3- and NO2- ( coupled NO3- reduction and NO2- oxidation) in the suboxic zone.