A zona incognita surrounds the secondary nitrite maximum in open-ocean oxygen minimum zones

An analytic gap exists between the lower limit of detection of oxygen (O2) by conventional sensors (LOD, ~1 - 3 µM O2) and the appearance of NO2- from NO3- reduction (~0.05 µM O2), which signifies the secondary nitrite maximum. The near-anoxic milieu where precise O2 measurements could not be made until very recently even by current optodes, favors biogeochemically important redox reactions. The ongoing improvement of optodes might make the gap vanish. Significant O2 levels near the conventional LOD for O2, when accompanied by ~ ≥0.05 µM NO2-, must be erroneous and should be deleted from historical data going back to the 1930s.