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 O-18 is up to 3 parts per thousand greater than in delta N-15, whereas our experiments with denitrifier cultures in seawater medium indicate a 1: 1 increase in NO3- delta O-18 and delta N-15 during NO3- consumption. Moreover, the maximum in NO3- delta O-18 is somewhat shallower than the maximum in NO3- delta N-15. These two observations can be summarized as an "anomaly'' from the 1: 1 delta O-18- to- delta N-15 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.