William S. Reeburgh

Methane consumption in Cariaco Trench waters and sediments

Detailed measurements of CH4 in the water column and sediments of the Cariaco Trench show that CH4 is non-conservative in both environments. Concentration differences between the sediments and adjacent overlying water suggest that the sediments are the source of the water column CH4. Co-metabolism of CH4 by sulfate reducers appears to be the CH4 sink in anoxic environments.

Anaerobic mineralization of marine sediment organic matter: Rates and the role of anaerobic processes in the oceanic carbon economy

Abstract This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy? Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments. A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the a...

Ecology of sea ice biota - 2. Global significance

SummaryThe sea ice does not only determine the ecology of ice biota, but it also influences the pelagic systems under the ice cover and at ice edges. In this paper, new estimates of Arctic and Antarctic production of biogenic carbon are derived, and differences as well as similarities between the two oceans are examined. In ice-covered seas, high algal concentrations (blooms) occur in association with several types of conditions. Blooms often lead to high sedimentation of intact cells and faecal pellets. In addition to ice-related blooms, there is progressive accumulation of organic matter in Arctic multi-year ice, whose fate may potentially be similar to that of blooms. A fraction of the carbon fixed by microalgae that grow in sea ice or in relation to it is exported out of the production zone. This includes particulate material sinking out of the euphotic zone, and also material passed on to the food web. Pathways through which ice algal production does reach various components of the pelagic and benthic food webs, and through them such top predators as marine mammals and birds, are discussed. Concerning global climate change and biogeochemical fluxes of carbon, not all export pathways from the euphotic zone result in the sequestration of carbon for periods of hundreds of years or more. This is because various processes, that take place in both the ice and the water column, contribute to mineralize organic carbon into CO2 before it becomes sequestered. Processes that favour the production and accumulation of biogenic carbon as well as its export to deep waters and sequestration are discussed, together with those that influence mineralization in the upper ice-covered ocean.

Anaerobic methane oxidation: Rate depth distributions in Skan Bay sediments

Abstract A radiotracer method that measures rates of oxidation of methane to carbon dioxide has been applied to anoxic marine sediments. The results confirm the occurrence of anaerobic methane oxidation and agree with model predictions of a zone of intense anaerobic methane oxidation at the base of the sulfate-reducing zone.

Ecology of sea ice biota

SummaryPolar regions are covered by extensive sea ice that is inhabited by a variety of plants and animals. The environments where the organisms live vary depending on the structure and age of the ice. Many terms have been used to describe the habitats and the organisms. We here characterize the habitats and communities and suggest some standard terms for them. We also suggest routine sampling methods and reporting units for measurements of biological and chemical variables.

Interannual variations in tundra methane emission: A 4‐year time series at fixed sites

Abstract. This paper summarizes 4 years (1987-1990) of weekly net CH4 flux measurements at permanent sites representing important plant components of Arctic tundra. The data coincide with variations in precipitation and temperature of interest in regional and global modeling efforts and are useful in placing bounds on the role of tundra in the global CH4 budget. Precipitation in the study area during the summer emission period ranged from twice to half the long-term mean, and air temperature anomalies were about +2 °C. This data set also permits consideration of temporal (seasonal to interannual) and spatial variability in CH4 flux. We studied the relationship between the net CH4 flux and subsurface properties (water table depth, thaw depth, soil temperature, /pCH4 distributions) at these permanent sites during the 1988 and 1989 emission periods. Net CH4 emission and subsurface properties are largely unrelated. Relationships between soil temperature (or any single variable) and emission are site specific and are of little value as flux predictors. Parameters that integrate conditions influencing flux appear to be the best flux predictors over the emission period. We estimate that Arctic wet meadow and tussock:shrub tundra presently emit about 42 ± 26 Tg CH4 yr−1 to the atmosphere. This estimate has a North American bias, but it is supported by measurements in a range of locations, transect studies, and model calculations.

Water column methane oxidation adjacent to an area of active hydrate dissociation, Eel River Basin

Abstract The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of 3H-CH4. Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths

Ecology of sea ice biota - 1. Habitat, terminology, and methodology

SummaryPolar regions are covered by extensive sea ice that is inhabited by a variety of plants and animals. The environments where the organisms live vary depending on the structure and age of the ice. Many terms have been used to describe the habitats and the organisms. We here characterize the habitats and communities and suggest some standard terms for them. We also suggest routine sampling methods and reporting units for measurements of biological and chemical variables.

Black Sea methane geochemistry

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 μM 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−CH4, 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 0.6μM y−1 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.

Moisture and temperature sensitivity of CH4 oxidation in boreal soils

Abstract We used laboratory experiments to evaluate CH4 uptake kinetics and the influence of soil moisture and temperature on rates of CH4-oxidation by boreal soils at in situ CH4 concentrations. Two upland forest sites (AS2 and BS2) were atmospheric CH4 sinks; a bog site (LB) was an atmospheric CH4 source characterized by distinct depth zonation of CH4 production and consumption. Apparent half-saturation constants (Ks) for CH4-oxidation showed relatively well-adapted communities. The Ks for the high CH4-source soil (LB) was 1.1 μ m , about 10-fold higher than values for CH4-sink soils (AS2 and BS2), 37 and 124 n m . Experiments assessing the individual effects of moisture and temperature on CH4-oxidation indicated that moisture was the primary control in CH4-sink soils at AS2 and BS2, while temperature was more important in CH4-source soil at LB. A combination of the highest moisture content and lowest temperature for each soil gave the lowest CH4-oxidation rates in experiments evaluating the interactive effects of these two variables. Conversely, a soil moisture content close to the optimum identified in moisture dependence experiments combined with the highest soil temperature consistently gave the highest CH4-oxidation rate.