Central Arctic Ocean methane emission and carbon dioxide uptake constrained using floating chamber flux and gas transfer velocity measurements

<p>The Central Arctic Ocean&#8217;s (CAO) current and future role in the exchange of climate-forcing trace gases with the atmosphere is highly uncertain due to sparse observations of dissolved gas concentrations and sea-air fluxes the poorly understood role of sea ice in the exchange, and the rapidly reducing sea-ice cover. The highest rates of gas exchange in pack ice regions occur through open water areas such as leads. These appear on scales from < 1m to > 10km, but are highly variable in form, size, and existence.</p><p>Estimates of polar region trace gas fluxes are highly dependent on the choice of gas transfer parameterisation. Differences in Arctic Ocean CO₂ uptake estimates resulting from this choice may be as large as 50 Tg C yr^-1, ~30 % of the total (Yasunaka et al., 2018). Direct gas exchange observations suggest that gas transfer rates will be reduced in the presence of sea ice (Prytherch and Yelland, 2021).</p><p>There are no previously reported direct sea-air methane flux estimates from the CAO. Indirect flux rates estimated from water column measurements of dissolved methane are variable but low, typically < 10 mol m^-2 day^-1. Much higher fluxes, 125 mol m^-2 day^-1, have been derived from aircraft-based atmospheric profile measurements (Kort et al., 2012). Sea-air CO₂ fluxes determined from direct, eddy covariance measurements in areas adjacent to ice or from lead water surfaces in both central and outlying areas of the Arctic Ocean are on the order of 10 mmol m^-2 day^-1 (Prytherch and Yelland, 2021).</p><p>Here we present direct measurements of the sea-air flux of CH₄and CO₂, as well as ice-air fluxes of CO₂ in the summertime CAO, obtained during the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker <em>Oden</em> in 2021. Measurements of sea-air CH₄ and CO₂ flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden. Fluxes and dissolved gas concentrations from surface water were used to determine gas transfer velocities. The dependence of the exchange on wind speed, buoyancy, lead width and ice condition was investigated in order to develop improved parameterisations of polar sea-air gas exchange. The directly measured fluxes provide an improved constraint on the regional sea-air exchange of CH₄ and CO₂, and in particular show that CH₄ emissions in the summertime CAO are substantially lower than suggested by previous estimates.</p><p>&#160;</p><p>Kort, E. A. et al: Atmospheric observations of Arctic Ocean methane emissions up to 82&#176; north, Nature Geosci, 5, 318&#8211;321, https://doi.org/10.1038/ngeo1452, 2012.</p><p>Prytherch, J. and Yelland, M. J.: Wind, Convection and Fetch Dependence of Gas Transfer Velocity in an Arctic Sea&#8208;Ice Lead Determined From Eddy Covariance CO2 Flux Measurements, Global Biogeochem Cycles, 35, https://doi.org/10.1029/2020gb006633, 2021.</p><p>Yasunaka, S., et al: Arctic Ocean CO2 uptake: an improved multiyear estimate of the air&#8211;sea CO2 flux incorporating chlorophyll a&#160;concentrations, Biogeosciences, 15, 1643&#8211;1661, https://doi.org/10.5194/bg-15-1643-2018, 2018.</p>