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 (NO 3 − ) 15 N/ 14 N and 18 O/ 16 O associated with denitrification in the oxygen‐deficient thermocline waters of the eastern tropical North Pacific. The increase in δ 18 O is up to 3‰ greater than in δ 15 N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO 3 − δ 18 O and δ 15 N during NO 3 − consumption. Moreover, the maximum in NO 3 − δ 18 O is somewhat shallower than the maximum in NO 3 − δ 15 N. These two observations can be summarized as an “anomaly” from the 1:1 δ 18 O‐to‐δ 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‐δ 15 N NO 3 − to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO 3 − and NO 2 − (coupled NO 3 − reduction and NO 2 − oxidation) in the suboxic zone.