An Investigation Into the Origin of Nitrate in Arctic Sea Ice

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 Δ17O and δ18O (Δ17ONO3 = δ17O − 0.52 × δ18O = 27.1–33.5‰ versus Vienna Standard Mean Ocean Water (VSMOW); δ18ONO3 = 70.8–87.8‰), and low δ15NNO3 (−5.9–2‰ versus N2). In contrast to the atmospheric samples, the sea ice δ15NNO3 was typically higher (−0.3–15.0‰) and the δ17ONO3 and δ18ONO3 much lower (Δ17ONO3 = 0–12.4‰; δ18ONO3 = 23.3–67.5‰). The presence of Δ17ONO3 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 (Δ17ONO3 = 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.

Significant Latitudinal Gradient of Nitrate Production in the Marine Atmospheric Boundary Layer of the Northern Hemisphere

“…However, the Clark et al. (2020) work and this study were conducted during mid to late summer, when chlorine chemistry is likely much weaker than in spring (Brough et al., 2019). Thus, further year‐round observations of NO 3 − are needed to characterize the role of sea ice in atmospheric oxidative capacity over the polar oceans.…”

“…is associated with δ 15 N of sea ice NO 3 − and the isotope fractionation during snow/ice NO 3 − photolysis. δ 15 N‐NO 3 − of Arctic surface sea ice is about 5‰ during summer (Clark et al., 2020), and nitrogen isotopic fractionation of snow NO 3 − photolysis is about −48‰ (Berhanu et al., 2014, 2015). If it is assumed that the fraction of photolytic loss of NO 3 − in surface sea ice is ∼10% (Burkhart & Hutterli, 2004), δ 15 N‐NO 3 − (ice.)…”

Physical and Biogeochemical Influences on Nutrients Through the Canadian Arctic Archipelago: Insights From Nitrate Isotope Ratios

“…The greater increase in δ 18 O NO3 relative to δ 15 N NO3 , in turn, likely signals the concurrent production of NO 3 − by nitrification, a dynamic that is expected to manifest as a steeper increase in δ 18 O NO3 than δ 15 N NO3 (DiFiore et al., 2010; Granger et al., 2004; Wankel et al., 2007). Inputs of exogenous reactive N through N 2 fixation, DON degradation, river discharge, atmospheric deposition, and/or glacial meltwater (Alkire et al., 2017; Bhatia et al., 2021; Blais et al., 2012; Clark et al., 2020; Sipler et al., 2017; Tremblay et al., 2014; Xie et al., 2012), with the potential to lower δ 15 N NO3 relative to δ 18 O NO3 , are deemed minor.…”

Subzero, saline incubations of Colwellia psychrerythraea reveal strategies and biomarkers for sustained life in extreme icy environments

“…4, cluster 3; Fig. S6), suggests Cp 34H modulates its need for nitrogen through the directed acquisition of extracellular nitrate, a predominant and readily available form of nitrogen shown to drive biogeography and trophic status in Arctic surface waters and sea ice (Ardyna and Arrigo, 2020; Clark et al ., 2020; Henley et al ., 2020). Notable nitrate transporters included NrtA, an ABC transporter responsible for high‐affinity nitrate acquisition at low concentrations (Nagore et al ., 2006; Akhtar et al ., 2015), and GlnG, a component of the NtrB/NtrC system that responds to nitrogen limitation and plays an important role in the regulation of transcription (Pahel et al ., 1982).…”