Coupled nitrification-denitrification in sediment of the eastern Bering Sea shelf leads to super(15)N enrichment of fixed N in shelf waters

Benthic processes influence the fixed N burden of the water column Sediment remobilization contributes to seasonal nutrient recharge under sea ice Benthic denitrification imprints super(15)N-enrichment to shelf fixed N We studied the nitrogen biogeochemistry of the ice-covered eastern Bering Sea shelf using the isotope ratios ( super(15)N/ super(14)N and super(18)O/ super(16)O) of NO super(-) sub(3) and other N species. The super(15)N/ super(14)N of late winter NO super(-) sub(3) on the shelf decreases inshore and is inversely correlated with bottom water [NH super(+) sub(4)], consistent with an input of low- super(15)N/ super(14)N NO super(-) sub(3) from partial nitrification of NH super(+) sub(4) remineralized from the sediments. An inshore super(15)N/ super(14)N increase in total dissolved N (TDN) suggests that (1) the sediment-derived NH super(+) sub(4) is elevated in super(15)N due to the same partial nitrification that yields the low- super(15)N/ super(14)N NO super(-) sub(3), and (2) super(15)N-deplete NO super(-) sub(3) from partial nitrification within the sediments is denitrified to N sub(2). The proportion of newly nitrified NO super(-) sub(3) on the shelf, evidenced by an inshore decrease in NO super(-) sub(3) super(18)O/ super(16)O, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO super(-) sub(3) into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the super(15)N/ super(14)N of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO super(-) sub(3) isotope data indicating that the fixed N transported through Bering Strait has a super(15)N/ super(14)N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6-8 ppt, is at odds with previous assumptions regarding the global ocean's N isotope budget.