Nitrogen cycling in the suboxic Arabian Sea: Implications for atmospheric chemistry and climate

Abstract

An acute oxygen deficiency at mid-depths in the Arabian SEa leads to large-scale microbial conversion (denitrification) of the oxidised combined nitrogen species to molecular nitrogen (N sub(2)) at an estimated rate of 25-30 x 10 sup(10) g N y sup(-1). Denitrification in the region appears to be decoupled from the overhead primary production as evident from the chemical, biochemical and microbiological data. This anomaly requires inputs of organic matter to the subsurface layers through some hitherto poorly understood processes (e.g., vertical migration of mesopelagic fish and transport of dissolved or suspended matter including the transparent exopolymer particles). The reducing conditions greatly affect the abundance of stable isotopes in dissolved nitrogen species. Large enrichment of sup(15)N relative to sup(14)N in nitrate (NO sup(-) sub(3)) and a corresponding depletion in N sub(2) occur within the denitrifying layer. The isotopic fractionation factor during denitrification estimated using simple advection-reaction and diffusion-reaction models ranges between 22 and 25 per mil. The near-surface isotopic composition of NO sup(-) sub(3) is, however, remarkably close to the oceanic average value, presumably due to the input of isotopically light nitrogen through nitrogen fixation. Nitrogen isotope records in sediment cores reveal large cyclic changes in sup(15)N/ sup(14)N, evidently caused by the variability in denitrification in the past associated with the climatic cycles. This reinforces the view that the ocean oscillates between a net source and a net sink of combined nitrogen on geological time scales and that the resultant changes in the combined nitrogen on geological time and that the resultant changes in the combined nitrogen inventory could modulate climatic changes through the carbon dioxide link.