An Investigation Into the Origin of Nitrate in Arctic Sea Ice

Abstract

Abstract Atmospheric deposition has been suggested to be an important source of reactive nitrogen stored in Northern Hemisphere land‐fast ice, in contrast to Antarctic sea ice, where bulk nutrients originate predominantly from underlying seawater. A paucity of sea ice studies in the open Arctic Ocean limits our understanding of the potential for melting ice to contribute to primary production in N‐deplete waters of the Arctic. As part of the U.S. western Arctic GEOTRACES 2015 expedition, samples of pack ice, overlying snow, atmospheric aerosols, and underlying seawater were collected between 82°N and 89°N. To identify the provenance of N in sea ice, we measured a suite of tracers including the isotopic composition of nitrate, ammonium, water, and particulate N. Relatively low concentrations of nitrate and ammonium were detected in sea ice (0.1–8.2 and 0.6–1.2 μmol L −1 , respectively), and in atmospheric samples (1.1–3.7 and 0.8–1.2 μmol L −1 , respectively). Atmospheric nitrate in snow had characteristically high Δ 17 O and δ 18 O (Δ 17 O NO3 = δ 17 O − 0.52 × δ 18 O = 27.1–33.5‰ versus Vienna Standard Mean Ocean Water (VSMOW); δ 18 O NO3 = 70.8–87.8‰), and low δ 15 N NO3 (−5.9–2‰ versus N 2 ). In contrast to the atmospheric samples, the sea ice δ 15 N NO3 was typically higher (−0.3–15.0‰) and the δ 17 O NO3 and δ 18 O NO3 much lower (Δ 17 O NO3 = 0–12.4‰; δ 18 O NO3 = 23.3–67.5‰). The presence of Δ 17 O NO3 in the sea ice indicated that 0–40% of the nitrate is sourced from the atmosphere, while the majority of the nitrate is non‐atmospheric (Δ 17 O NO3 = 0‰). Based upon concentration, isotopic observations, and dynamic box modeling with atmospheric deposition and biological processes, we find that the majority of nitrate can be explained by in‐situ biological nitrate production.

Topics & Concepts