Nitrite isotopes as tracers of marine N cycle processes

Nitrite (NO2-) is a key intermediate in the marine nitrogen (N) cycle. It is produced and consumed throughout the ocean by the dominant processes driving the distribution, availability and speciation of N. However, the accumulation of nitrite is typically confined to depths near the base of the sunlit euphotic zone and in oxygen-deficient zones. These features are known as the primary and secondary nitrite maximum (PNM and SNM), respectively. The processes controlling nitrite accumulation in these features are not fully understood, but are thought to depend on the microbial community composition and its response to environmental conditions. A variety of approaches have been applied to understanding these features since their discovery, with the stable N and oxygen (O) isotope measurements of nitrite being added to this toolkit most recently. Large variations in nitrite N isotope ratios (N-15/N-14) and dramatic depletions in N-15 contrast with more consistent nitrite O isotope ratios (O-18/O-16) in the SNM. These signals provide unique information about the mechanisms of nitrite consumption in the SNM. By contrast, nitrite in the PNM shows less variation in N-15/N-14, but variations in O-18/O-16 that provide insight into the mechanisms and rates of N cycling there. This review presents a synthesis of nitrite isotopemeasurements in the marine environment, highlighting the insights that have been gained from these measurements. This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.