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The oceanic nitrogen cycle is critically important for the partitioning of greenhouse gases between ocean and atmosphere. Baffin Bay connects ocean regions that are major sources (North Atlantic) and sinks (North Pacific and western Arctic) of biologically available nitrogen and further harbors supersaturation of nitrous oxide and a coincident deficit in nitrate in the deep basin. Isotopic tracer profiles of both nitrogen species presented here provide novel insights into the origin and cycling of reactive nitrogen in Baffin Bay, highlighting the connectivity between different Arctic systems and horizontal components of basin‐scale nutrient transport. Baffin Bay bottom water properties are derived from export production in northern Baffin Bay, which is largely fueled by Pacific‐derived nutrients. In situ remineralization at depth gives rise to benthic denitrification, evidenced by a pronounced accumulation of nitrous oxide with a distinctively high site preference (<44‰) in the deep basin. Nutrients supplied to Baffin Bay are hence stripped from surface waters and trapped at depth over long timescales, where sedimentary denitrification further adds to the N removal capacity of the Arctic Ocean. Nitrate isotope ratios indicate substantial remineralization of organic matter in deep Baffin Bay, fueled by Pacific‐derived nutrientsNitrous oxide isotopomers support a predominantly sedimentary source of N2O propagating well into the oxygenated water columnSedimentary denitrification is a significant sink of dissolved inorganic nitrogen in deep Baffin Bay

A third or more of the fixed nitrogen lost from the oceans as N2is removed by anaerobic microbial processes in open ocean oxygen minimum zones. These zones have expanded over the past decades, and further anthropogenically induced expansion could accelerate nitrogen loss. However, in the Bay of Bengal there has been no indication of nitrogen loss, although oxygen levels are below the detection level of conventional methods (1 to 2 μM). Here we quantify the abundance of microbial genes associated with N2production, measure nitrogen transformations in incubations of sampled seawater with isotopically labelled nitrogen compounds and analyse geochemical signatures of these processes in the water column. We find that the Bay of Bengal supports denitrifier and anammox microbial populations, mediating low, but significant N loss. Yet, unlike other oxygen minimum zones, our measurements using a highly sensitive oxygen sensor demonstrate that the Bay of Bengal has persistent concentrations of oxygen in the 10 to 200 nM range. We propose that this oxygen supports nitrite oxidation, thereby restricting the nitrite available for anammox or denitrification. If these traces of oxygen were removed, nitrogen loss in the Bay of Bengal oxygen minimum zone waters could accelerate to global significance.

Measurements of15N/14N in dissolved molecular nitrogen (N2), nitrate (NO3−) and nitrous oxide (N2O) and18O/16O in N2O [expressed as δ15N and δ18O, relative to atmospheric N2and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O2minima observed in the open ocean. Microbially-mediated reduction of NO3−to N2(denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO3−increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ15N of N2concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO3−is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N2O reveals extremely large enrichments of both15N and18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of15N, but slight enrichment of18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N2O production.