Investigating N cycle processes in the Arabian Sea oxygen deficient zone using paired N and O isotope measurements of nitrate and nitrite

The Arabian Sea is home to one of three main oxygen deficient zones (ODZ), responsible for approximately 20% of the fixed nitrogen (N) loss from the ocean. A unique feature of these ODZs is that nitrite (NO₂^-) accumulates in a secondary nitrite maximum (SNM) within the anoxic section of the water column. The accumulation of NO₂^- where nitrate (NO₃^-) is also present allows for a unique opportunity to use these two N species in tandem to better understand the N cycle processes that are occurring. In this study we explore the N cycle processes in the Arabian Sea using paired stable isotope analysis of NO₃^- and NO₂^- based on samples collected during a cruise in the fall of 2007. We employ both N and oxygen (O) isotopes of NO₃^- and NO₂^- in our study, which allows for more complete and complex modeling of the N cycle processes. Our results show evidence for nitrite oxidation within the ODZ due to depletion in N and O isotopes of NO₂^-. We also employ an optimum multi-parameter (OMP) analysis to examine how water mass mixing plays a role in distributing NO₂^- and NO₃^- isotopic signals within the Arabian Sea ODZ.