Isotopic ratios of nitrite as tracers of the sources and age of oceanic nitrite

Nitrite is a central intermediate in the marine nitrogen cycle, and is generally found at low concentrations in the ocean. However, nitrite accumulates at the base of the sunlit surface ocean. The origin of this ubiquitous feature, known as the primary nitrite maximum, is debated and has been difficult to resolve through short-term isotope tracer incubations. Here, we use measurements of the dual isotopic composition of nitrite to evaluate the sources, sinks and average rates of nitrite turnover in the primary nitrite maximum in the Arabian Sea. We determined the rate of abiotic oxygen isotope exchange between nitrite and water, as well as equilibrium isotope effects, under a variety of conditions in a series of laboratory experiments. We used these data to model nitrite isotope data from several sites in the Arabian Sea. The results suggest that ammonia oxidation is the primary source of nitrite at most sites. Steady-state nitrite turnover times of 33-178 days correspond to ammonia oxidation rates of 9-30 nM per day at these sites. We suggest that a similar approach could be used to determine the rates of nitrite turnover in other environments, including the secondary nitrite maximum in low-oxygen regions.