Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Abstract

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and d 13C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of 13 C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in 13 C in relation to the d 13 C CH 4 signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH 4 between 0.005 and 0.02 Tg yr À1 . This is not a large contribution but the fluxes from the polynyas are 20-200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change. r

CO<sub>2</sub> and CH<sub>4</sub> in sea ice from a subarctic fjord under influence of riverine input

“…Such high concentrations, up to 15 nmol L −1 , suggest that the top layer of water column was somehow affected by nearby sources (i.e., sediment degassing, riverine input). These measurements are similar to the values (5-55 nmol L −1 water) reported by Damm et al (2007) in Storfjorden (Svalbard archipelago) and in the fjords of Spitsbergen (Damm et al, 2005). The observed concentrations are, however, lower than the concentration in the East Siberian Arctic Shelf (ESAS) measured by Shakova et al (2010).…”

Sensitivity of the carbon cycle in the Arctic to climate change

“…The flux to the atmosphere, which does not include potentially higher fluxes associated with river plumes or polynyas, must at least be supported by a corresponding seabed flux into the bottom water. Damm et al (2007) recently provided an Arctic-wide estimate for CH 4 evasion from polynyas of 0.005-0.020 Tg/yr, which does not affect the estimated range that we present here. Methane bubbles, which bypass flux estimates based on concentration gradients, are likely an important vertical transport mechanism in the shelf water of the Arctic Ocean (e.g., Shakhova et al 2005, Paull et al 2007), but perhaps less so in deeper water where dissolution feeds the deep-ocean reservoir (Damm and Bude´us 2003).…”

Distribution of methane in the Lena Delta and Buor-Khaya Bay, Russia

“…Also this analysis did not reveal significant results (Kruskal-Wallis rank sum test). Thus, methane loss seems not to be the result of mixing methane-rich river water with methane-poor marine water, as described for other estuaries (de Angelis and Lilley, 1987). This may be due to the very strong stratification of the water column (Fig.…”

“…From the literature, fractionation factors for microbial methane oxidation range from 1.02 in fresh water (Bastviken et al, 2002) to 1.017 for Arctic marine water (Damm et al, 2007). When only diffusion is assumed, an α of 1.0009 is reported (Happell et al, 1995).…”