Influence of the bordering shelves on nutrient distribution in the Arctic halocline inferred from water column nitrate isotopes

Abstract

The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ18O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ15N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the 15N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual 15N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

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

“…Nevertheless, the δ 18 O NO3 associated with PWW in the Beaufort Gyre suggests that NO 3 − is predominantly remineralized (Granger et al, ). This inference echoes parallel interpretations inferred from δ 18 O NO3 measurements in PWW off the eastern Chukchi slope (Brown et al, ) and the East Siberian shelf (Fripiat et al, ).…”

“…The δ 15 N NO3 maximum and corresponding δ 18 O NO3 minimum associated with the nutrient peaks in the Pacific halocline (S p ~ 33.1) are the most salient features of the NO 3 − isotope depth profiles. These features pervade PWW in the western Arctic Basin, having also been observed off the slope of the eastern Chukchi shelf and off of the East Siberian shelf (Brown et al, ; Fripiat et al, ). At our stations, the δ 15 N NO3 increases to 8‰ in PWW, compared to 5.6‰ in the Atlantic halocline below, which could be interpreted as signaling NO 3 − consumption at the subsurface, from assimilation or water‐column denitrification, processes that discriminate equivalently against the heavier N (and O) isotopologues of NO 3 − (Granger et al, , ).…”

An Investigation Into the Origin of Nitrate in Arctic Sea Ice

“…Concentrations for each end‐member and water isotopes were the only parameters held constant between curve scenarios (Figure ). Values for seawater end‐member based on work by Granger et al () and Fripiat et al ().…”

Global trends in marine nitrate N isotopes from observations and a neural network-based climatology

“…Nitrate δ 15 N observations ( Fig. 2; references and region of measurement in Appendix) were compiled from studies dating from 1975 (Cline and Kaplan, 1975) to 2018 (Fripiat et al, 2018), including data from the GEOTRACES intermediate data product (Schlitzer et al, 2018). Whenever possible, the data were acquired via the original author, but in other cases the data were estimated from the publication directly.…”