Methane Observations in the Air and Surface Waters of the Eastern Arctic Ocean during SWERUS-C3

Abstract:

The broad shelf areas of the Russian Arctic Ocean are purported to be substantial and increasing sources of atmospheric methane that are particularly sensitive to warming climate. I report here an overview of an extensive continuous record of ambient CH₄ mixing ratios and surface water concentrations made during the SWERUS-C3 expedition and how these observations might fit into the conversation about Arctic CH₄ emissions.

During Jul and Aug 2014 continuous high-resolution measurements of CH₄ were made in the atmosphere and the surface ocean as the Swedish I/B Oden crossed the middle and outer shelf areas of the Laptev and East Siberian Seas and into the Chukchi Sea. Air and water samples were analyzed with independent CRDS laser systems. Air was continuously sampled from four heights 9 to 35 m above the sea surface. Heights were switched every 2 minutes and target gases of ca. 1.800 and 4.000 ppm were run every two hours. Surface seawater was collected from 8 m below the surface. The counter flow of the equilibration sampler was analyzed with a CRDS laser. Two standards of 15 and 150 ppm were used in addition to the ambient air targets.

In the Laptev Sea the air burden ranged from 1.865 to 1.908 ppm and the water 1.976 to 105 ppm. In the ice covered East Siberian Sea the range was 1.874 to 1.996 ppm in air and 2.132 to 210 ppm in water (83% were <10 ppm). Calculated fluxes ranged from 1.6 to 7.7 mg m^-2d^-1 (including regions with sea ice where flux calculations are problematic).

The air and water data were combined with wind data to calculate wind driven fluxes. Our mean flux of 3.1 mg m^-2d^-1 leads to a shelf-wide extrapolated flux of ca. 2.4 Tg yr^-1. This is much higher than reported fluxes from many shelf seas but consistent with previous reports from these regions from monitoring, shipboard and aircraft data. These observations, integrated with terrestrial data, contribute to an emerging quantitative picture of Arctic CH₄ dynamics and its role in the global budget.