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

Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO₃^-) ¹⁵N/¹⁴N and ¹⁸O/¹⁶O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ¹⁸O is up to 3%o greater than in δ¹⁵N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO₃ ^- δ¹⁸O and δ¹⁵N during NO₃^- consumption. Moreover, the maximum in NO₃^- δ¹⁸O is somewhat shallower than the maximum in NO₃^- δ¹⁵N. These two observations can be summarized as an "anomaly" from the 1:1 δ¹⁸O-to-δ¹⁵N 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-δ ¹⁵N 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.