Dual-stable isotope constraints on aerobic methane oxidation in the water column of the outer Laptev Sea

Methane (CH₄) oxidation in Arctic shelf waters plays a critical role in regulating subsea CH₄ emissions, yet remains difficult to quantify due to strong spatial heterogeneity and complex transport processes. Here, we investigate CH₄ oxidation in the outer Laptev Sea using vertical and lateral profiles of CH₄ concentration together with dual-stable isotopic compositions (δ¹³C–CH₄ and δD–CH₄). Across the study area, dissolved CH₄ exhibits large concentration gradients accompanied by pronounced enrichment in the heavy isotopes. The observed δ¹³C–CH₄ and δD–CH₄ values increase synchronously, yielding dual-isotope slopes (Λ=7.9–13.7) that fall within the characteristic range of aerobic CH₄ oxidation. Isotopic enrichment is most clearly expressed below the pycnocline, indicating substantial oxidation of sediment-derived CH₄ during its residence in sub-pycnocline waters. For samples that show Rayleigh-type isotope–concentration relationships, we quantify the fraction of CH₄ oxidized (f_ox) using site-specific isotopic source signatures and incubation-derived fractionation factors. Station-integrated results yield regional median f_ox values of 22% (interquartile range, IQR: 11–32%) based on δ¹³C–CH₄ and 36% (IQR: 17–42%) based on δD–CH₄. These estimates likely represent conservative lower bounds as it is difficult to measure CH₄ addition by bubble dissolution and to capture the complete trajectory of CH₄ through the sub-pycnocline waters. The exceptionally high δ¹³C–CH₄ (up to +21‰) and δD–CH₄ values (up to +573‰) occur at relatively low CH₄ concentrations (~80 nM) at stations located between active seep hotspots (~3000 nM), suggesting advanced oxidation under open-system conditions during continued transport. Comparison of isotope systems shows that δD–CH₄ provides a more robust constraint on oxidation than δ¹³C–CH₄ in heterogeneous shelf environments. Overall, our results demonstrate that water-column structure and CH₄ residence time primarily control the extent of CH₄ oxidation in the outer Laptev Sea. This study provides new quantitative and process-based constraints on water-column CH₄ oxidation in Arctic shelf seas and demonstrates the utility of dual-stable isotope approaches for resolving CH₄ cycling in complex marine systems.