Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical North Pacific waters

Abstract

Isotopic analyses of nitrate by the denitrifier method, and indeed by many other analytical methods, do not discriminate between nitrate and nitrite. For samples containing both chemical species, accurate isotopic analysis of nitrate requires either removal of nitrite or independent isotopic analysis of nitrite and subtraction of its contribution to the mixed isotopic signal. This study evaluates the application of a variety of available analytical approaches to the isotopic analysis of mixed nitrate and nitrite solutions, with the goal of producing accurate coupled isotopic analyses of both nitrate and nitrite. These methods are tested on mixtures of standard solutions of nitrate and nitrite, and then applied to the coupled δ 15N and δ 18O analyses of nitrate and nitrite in waters of the Eastern Tropical North Pacific (ETNP). Results from standard mixtures show that even for extreme values of nitrate and nitrite δ 15N and δ 18O, both nitrite removal by ascorbate and nitrite isotopic analysis and subtraction from the mixed isotopic signal yield nitrate δ 15N and δ 18O values that are close to the expected values. Application of these analyses to samples from the ETNP yielded δ 15N NO 3 and δ 18O NO 3 values as high as 21‰ vs. AIR and 19‰ vs. VSMOW, respectively. Conversely, very low δ 15N values were observed in nitrite, with values ranging from − 7.2 to − 18.5‰ vs. AIR. Removal of nitrite from ETNP samples thus revealed differences of up to 5‰ between NO 3 - and NO 2 - + NO 3 - for both δ 15N and δ 18O. Moreover, the δ 15N offset between co-occurring nitrate and nitrite is greater than expected from the action of denitrification alone and may provide a unique constraint on the processes involved in the cycling of nitrite in and around oxygen deficient zones. Finally, subtraction of the nitrite δ 15N and δ 18O from ETNP samples allows the extension of the Δ(15,18) tracer into suboxic regions containing nitrite. The magnitude and distribution of Δ(15,18) in these samples suggests an important role for nitrite reoxidation in nitrate isotope variations.