Aspects of the marine nitrogen cycle of the Chukchi Sea shelf and Canada Basin

As a highly productive, seasonally ice-covered sea with an expansive shallow continental shelf, the Chukchi Sea fuels high rates of sedimentary denitrification. This contributes to its fixed nitrogen (N) deficit relative to phosphorus (P), which is among the largest in the global ocean, making the Chukchi Sea severely N-limited during the phytoplankton growth season. Here, we examine aspects of the N cycle on the Chukchi Sea shelf and the downstream Canada Basin using nutrients, dissolved oxygen (O2), and the stable isotopes of nitrate (NO3−). In the northward flow path across the Chukchi shelf, bottom waters experienced strong O2 drawdown, from which we calculated a nitrification rate of 1.3 mmol m−2 d−1. This nitrification was likely primarily in sediments and directly fueled sedimentary denitrification, historically measured at similar rates. We observed significant accumulations of ammonium (NH4+) in bottom waters of the Chukchi shelf (up to >5 µM), which were inversely correlated with δ15NNO3, indicating a sediment source of 15N-enriched NH4+. This is consistent with a process of coupled partial nitrification-denitrification (CPND), which imparts significant 15N enrichment and 18O depletion to Pacific-origin NO3−. This CPND mechanism is consistent with a significant decrease in δ18ONO3 relative to Bering Sea source waters, indicating that at least 58% of NO3− populating the Pacific halocline was regenerated during its transit across the North Bering and Chukchi shelves, rather than arriving preformed from the Bering Sea slope. This Pacific-origin NO3− propagates into the Canada Basin and towards the North Atlantic, being significantly 15N-enriched and 18O-depleted relative to the underlying Atlantic waters.