Reply on RC2

Abstract. In the Arctic Ocean region, methane (CH4) concentrations are higher than the global average, with particularly high concentrations of dissolved CH4 observed along many subarctic and Arctic continental shelf margins. Despite this, the Arctic Ocean emits only minimal methane fluxes to the atmosphere across the air-sea interface, suggesting that water column oxidation of methane may be an important process. In this study, we paired thermohaline, chemical, and biological data collected during the Northwest Passage Project transit through the Canadian Arctic Archipelago (CAA) waters in the summer of 2019 with in-situ and in-vitro methane data. Our findings suggested that the most elevated in-situ concentration of dissolved methane was present in the near-surface waters of the Pacific, particularly in meltwater regions. The highest methane concentrations were observed within shallow waters, averaging at 5.8 ± 2.5 nM within the upper 30 m depth. Furthermore, the methane distribution showed a distinct pattern from east to west, with higher concentrations and oxidation rate potential in the western region. In our study, we observed generally low methane oxidation rate constants, averaging at 0.006 ± 0.002 d−1. However, surface waters from Wellington Channel and Croker Bay exhibited relatively higher methane oxidation rates, averaging at 0.01 ± 0.0004 d−1. These regions were distinguished by a significant proportion of meltwater, including both meteoric water and sea ice meltwater, mixed with water of Pacific origin. We identified microbial taxa of Pacific-origin likely associated with methane oxidation, including Oleispira (γ-proteobacteria) and Aurantivirga (Flavobacteria), in the Pacific and meteoric waters. In contrast, deeper layers (> 200 m depth) showed lower methane concentrations (av. 3.1 ± 1.1 nM) and lower methane oxidation rate constants (av. 0.005 ± 0.001 d−1). Within the sea ice, dissolved methane concentrations were found to be higher than the concentrations at equilibrium with atmospheric capacity, with an average of [CH4] = 9.2 ± 5 nM. The sea ice temperature data (Table S2) indicated the presence of ice permeability, which likely facilitated the release of dissolved methane that was either trapped or produced since the previous freezing period. Notably, methane concentrations were 25 % higher in waters collected in the western CAA in comparison to the ice-free waters (eq. S1). The overall picture suggested supersaturation of in-situ methane in shallow waters, coupled with faster oxidation rates in meltwater and Pacific dominant layers, suggesting rapid seasonal cycling of methane and prevention of the methane migration into the atmosphere.

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