Abstract

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.