Tracing denitrification in the Canada Basin: N2 loss to the atmosphere on the Chukchi Shelf and benthic inputs in deep waters

The global marine fixed nitrogen budget acts as a strong control on oceanic primary productivity, and the Arctic plays a disproportionately large role in the sink terms for this budget. This paper aims to quantify the impact of nitrogen cycling on the Canada Basin, utilizing two tracers of denitrification: N-2/Ar, a dissolved gas tracer, and N*, a nutrient ratio tracer. In the Pacific Winter Water (PWW), which forms in the Chukchi Sea, we observe a disconnect between N-2/Ar and N*, where the excess N-2 expected from N* observations is far larger than the N-2 excess we measure. We show that loss of N-2 to the atmosphere through ventilation on the Chukchi Shelf likely accounts for this disparity, highlighting the importance of using N-2/Ar as a denitrification tracer only in isolated water masses. We additionally observe increasing N-2/Ar and decreasing N* in the old deep waters of the Canada Basin, suggesting benthic denitrification has been operating in the deep sediments over the 500-year age of this water mass. We use a one-dimensional vertical reaction-diffusion model to estimate denitrification rates of 0.0053-0.0130 mmol-N m(-2) d(-1), or 0.04-0.1 Tg N y(-1) integrated over the whole basin, which is about half the rates estimated for other deep basins, in-line with lower remineralization rates in the deep Canada Basin. Further measurements of these tracers in the Arctic, particularly directly in the Chukchi Sea, will help constrain the relative importance of physical vs. biological processes on N-2 in this region.