Nitrite isotopes as tracers for marine N cycle processes

Nitrite is a key intermediate in the marine nitrogen (N) cycle, being produced and consumed in many of the processes driving the speciation, availability, and distribution of N in the ocean. However, nitrite accumulates only in distinct features in the euphotic zone or oxygen-deficient regions of the water column, known as primary and secondary nitrite maxima (PNM and SNM), respectively. The processes controlling these features are varied, and depend on microbial communities and their responses to environmental conditions. Recently, the stable N and oxygen (O) isotopes of nitrite have been introduced as tools for understanding the processes controlling nitrite accumulation in various PNM and SNM features. Large variations in N isotope ratios (¹⁵N/¹⁴N) and dramatic depletions in ¹⁵N contrast with more consistent O isotope ratios (¹⁸O/¹⁶O) in the SNM. Nitrite in the PNM and surface waters shows regional variations that may be indicative of different biogeochemical processing. Here we present a synthesis of nitrite isotopes in the marine environment, including isotope effects used to interpret processes involved with nitrite production and consumption, as well as a compilation of nitrite isotope data from the three main oceanic oxygen deficient zones and their overlying PNMs.