Isotopic Insight Into Nitrate Sources and Cycling in Polar Sea Ice

Abstract

Sea ice alters nitrate (NO ₃ ⁻ ) availability in polar oceans, yet the influence of sea‐ice type on NO ₃ ⁻ sources and cycling remains poorly constrained. We analyzed NO ₃ ⁻ concentrations and isotopic compositions ( δ ¹⁵ N, δ ¹⁸ O, Δ ¹⁷ O) in Antarctic one‐year and Arctic multi‐year sea ice to investigate NO ₃ ⁻ sources and transformation processes. Mean NO ₃ ⁻ concentrations were substantially higher in Antarctic one‐year sea ice (3.89 ± 17.75 μmol L ⁻¹ , with the large standard deviation driven by a few bottom‐layer samples with elevated concentrations) than in Arctic multi‐year sea ice (0.50 ± 0.22 μmol L ⁻¹ ). Δ ¹⁷ O‐based source apportionment showed that atmospheric NO ₃ ⁻ contributed 37% ± 11.64% in Arctic multi‐year sea ice, compared with 4% ± 2.69% in Antarctic one‐year sea ice. Despite much lower total NO ₃ ⁻ concentrations, Arctic multi‐year sea ice exhibited a higher atmospheric NO ₃ ⁻ contribution, indicating better preservation of atmospheric NO ₃ ⁻ . Non‐atmospheric NO ₃ ⁻ , derived from in situ nitrification and/or underlying seawater, accounted for 96% ± 2.69% and 63% ± 11.64% of the NO ₃ ⁻ pool in Antarctic and Arctic sea ice, respectively. Isotopic compositions and nutrient concentrations indicate the occurrence of microbial NO ₃ ⁻ cycling in both Antarctic one‐year and Arctic multi‐year sea ice, with more pronounced cycling in Antarctic one‐year sea ice. Collectively, these results suggest that sea‐ice type influences NO ₃ ⁻ concentrations, sources, and cycling, highlighting the role of sea ice in NO ₃ ⁻ availability in polar marine environments. We acknowledge that nitrite (NO ₂ ⁻ ) in sea‐ice samples could introduce a degree of uncertainty into isotope interpretations; however, given the relatively low NO ₂ ⁻ /(NO ₂ ⁻  + NO ₃ ⁻ ) fractions, this uncertainty is unlikely to have affected our main conclusions.