No abstract

Abstract-Methane concentrations and oxidation rates were measured in the water column and sediments of the Black Sea at a central station during leg 5 of the 1988 U.S.-Turkey Black Sea Expedition. Methane concentrations were 10 nM in the upper 100 m, increased to 11,uM at 550 m, and were uniform to the bottom. Water column methane oxidation rates were measured using two independent radiotracer techniques: tracer level additions of 3H_C~and non-tracer level additions of 14C-CH4.The methods agree within a factor of two. Methane oxidation rates were low in the surface 100 m and increased to relatively uniform values of 0.6,uM y-I below 500 m.Sediment methane concentration and oxidation rate distributions showed that shelf and slope sediments were methane sources, while deep basin sediments were methane sinks.These measurements were used to construct a methane budget for Black Sea waters. Microbially mediated anaerobic methane oxidation is the dominant water column methane sink, followed by evasion to the atmosphere, abyssal plain sediment consumption and outflow at the Bosporus. The source of methane appears to be anoxic, high deposition rate shelf and slope sediments. The water column oxidation rate measurements suggest a short (5-20 year) residence time for methane in the Black Sea, indicating a higher geochemical activity than previously believed. The quantity of carbon participating in the Black Sea methane cycle is equivalent to about 0.5% of the primary production.

Methane oxidation rates observed in a topsoil covering a retired landfill are the highest reported (45 g m-2 day-') for any environment. This microbial community had the capacity to rapidly oxidize CH4 at concentrations ranging from <1 ppm (microliters per liter) (first-order rate constant [k] =-0.54 h-) to > 104 ppm (k =-2.37 h-'). The physiological characteristics of a methanotroph isolated from the soil (characteristics determined in aqueous medium) and the natural population, however, were similar to those of other natural populations and cultures: the Qlo and optimum temperature were 1.9 and 31°C, respectively, the apparent half-saturation constant was 2.5 to 9.3 ,uM, and 19 to 69% of oxidized CH4 was assimilated into biomass. The CH4 oxidation rate of this soil under waterlogged (41% [wt/vol] H20) conditions, 6.1 mg liter-' day-, was near rates reported for lake sediment and much lower than the rate of 116 mg liter-l day-l in the same soil under moist (11% H20) conditions. Since there are no large physiological differences between this microbial community and other CH4 oxidizers, we attribute the high CH4 oxidation rate in moist soil to enhanced CH4 transport to the microorganisms; gas-phase molecular diffusion is 104-fold faster than aqueous diffusion. These high CH4 oxidation rates in moist soil have implications that are important in global climate change. Soil CH4 oxidation could become a negative feedback to atmospheric CH4 increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture.

No abstract

Seasonal measurements of net methane flux were made at permanent sites representing important components of arctic tundra. The sites include Eriophorum tussocks, intertussock depressions, moss‐covered areas, and Carex stands. Methane fluxes showed high diel, seasonal, intra site, and between site variability. Eriophorum tussocks and Carex dominated methane release to the atmosphere, with mean annual (± 1σ) net methane fluxes of 8.05 ± 2.50 g CH4 m−2 and 4.88 ± 0.73 g CH4 m−2, respectively. Methane fluxes from the moss sites and intertussock depressions were much lower (0.47 ± 0.16 and 0.62 ± 0.28 g CH4 m−2 yr−1. Over 90% of the mean annual methane flux from the Eriophorum, intertussock depressions, and Carex sites occurred between thaw and freeze‐up. Some 40% of the mean annual methane flux from the moss sites occurred during winter. Composite methane fluxes for tussock tundra and Carex‐dominated wet meadow tundra environments were produced by weighting measured component fluxes according to areal coverage. Tussock and wet meadow tundra account for an estimated global methane emission of 19–33 Tg yr−1.