On the Properties of the Arctic Halocline and Deep Water Masses of the Canada Basin from Nitrate Isotope Ratios

Abstract

Nitrogen is a limiting nutrient for primary production in the western Arctic Ocean. Measurements of the nitrogen (15N/14N) and oxygen (18O/16O) isotope ratios of nitrate in the southeastern Beaufort Sea provide insight into biogeochemical cycling of nitrogen in the western Arctic Ocean. Nitrate O isotope ratios in the Pacific halocline evidence a highly regenerated reservoir. Coincident peaks in nutrient concentrations and reduced dissolved oxygen concentrations suggest that nitrate accrues from organic matter remineralization in bottom waters of the Chukchi shelf and that these ventilate the basin predominantly in summer, when isolated from the atmosphere. Preformed nitrate in Pacific Winter Water lacks 18O/16O elevation from nitrate assimilation, contrasting with preformed nitrate in other ocean regions. A reactive N deficit and elevated nitrate N isotope ratios in the Pacific halocline further indicate substantial N loss to coupled nitrification‐denitrification in shelf sediments upstream. In the Atlantic Water below, nitrate isotope ratios identify two distinct waters entering the Arctic at Fram Strait, from (1) the surface West Spitsbergen Current, bearing isotopic signatures akin to North Atlantic waters, and (2) deeper inflows of waters ventilated in the Nordic Seas, transporting nitrate O isotope ratios indicative of regenerated nitrate. Poorly ventilated Canada Basin Deep Water shows evidence of nominal accrual of remineralized products, and nitrate isotope ratios suggest an influence of slow benthic denitrification on the sea floor. The observations reveal that shelf processes have a disproportionate influence on tracer properties of the Pacific halocline, while those in Atlantic Water are dominated by processes in the Nordic Seas.

Remote Western Arctic Nutrients Fuel Remineralization in Deep Baffin Bay

“…The low δ 18 O NO3 (0.8‰) of the upper Baffin Bay halocline is within the range observed in the upper halocline of the western Arctic (~0.0‐1.1‰; Brown et al, ; Granger et al, ), where it is diagnostic of a highly remineralized NO 3 ‐ reservoir from regeneration on the western Arctic shelves upstream (Granger et al, ). The low δ 18 O NO3 signal in the upper halocline of Baffin Bay may thus be entrained from the western Arctic and may also derive from subsurface remineralization in the CAA and in situ in Baffin Bay itself.…”

“…The N* signal associated with Pacific‐derived nutrients in Baffin Bay, however, is less pronounced than upstream in the upper halocline of the Canada Basin (‐10 to ‐13 μmol/L). Similarly, δ 15 N NO3 is concurrently lower than in the Canada Basin (7.7‐8.0‰; Brown et al, ; Granger et al, ), presumably due to vertical and tidal mixing in the area of eastern Barrow Strait (Hughes et al, ; Melling et al, ; Prinsenberg & Bennett, ), promoting exchange between western Arctic water and underlying Atlantic‐derived water that has a characteristic δ 15 N of 5‰ (Figure g), as well as potential horizontal mixing with the shallow component of the WGC within Baffin Bay as part of the general cyclonic circulation in the basin (e.g., Hamilton & Wu, ; Münchow et al, ; Tang et al, ).…”

“…Mean nutrient concentrations at BB2 (Table ) are comparable to peak values associated with the WGC recorded at Davis Strait (Torres‐Valdés et al, ). Given the lower nutrient content of the western Arctic end‐member (~12 and ~0.8 μmol/L in the Canada Basin lower halocline; Granger et al, ) and evidenced mixing between Arctic and Atlantic waters, we would expect the nutrient concentrations at BB2 to reflect an intermediate between the two end‐members. The relatively high nutrient concentrations in the central bay thus suggest some additional nutrient input from remineralization in situ or in transit from the Canada Basin to Baffin Bay.…”

“…The low δ 18 O NO3 values in the Baffin Bay halocline contrast with more elevated values of ≥2‰ typical of the North Atlantic thermocline (e.g., station LS2; Figure h). The latter is raised due to partial NO 3 ‐ assimilation in the Southern Ocean (i.e., pre‐formed NO 3 ‐ ) and also reflects a higher δ 18 O of water incorporated in NO 3 ‐ remineralized in the Atlantic compared to NO 3 ‐ remineralized in the western Arctic, where the subsurface δ 18 O H2O is relatively low (Figure i; Marconi et al ; Granger et al, ).…”

“…These coincident signals are largely acquired from benthic denitrification on the western Arctic continental shelf area (Brown et al, ; Fripiat et al, ; Granger et al, ), and possibly during the eastward propagation through the shallow Archipelago (Jones, ). Specifically, the elevated δ 15 N and low δ 18 O of NO 3 ‐ in Pacific‐origin waters derive from benthic coupled nitrification‐denitrification on the Bering, Chukchi, and east Siberian shelves, which increases δ 15 N NO3 and lowers δ 18 O NO3 compared to the open Pacific end‐member (Brown et al, ; Fripiat et al, ; Granger et al, , ). In keeping with the cyclonic circulation in Baffin Bay and the intrusion of Arctic water (Figure ), the Pacific NO 3 ‐ signature is most pronounced in the southern LS (CAA3; ‐10.5 μmol/L for N* and ~7.0 ± 0.2‰ for δ 15 N NO3 ; Figure S1 in the supporting information), indicating the eastward transport through the CAA.…”