B42B-01 10:20h
Estimates of Biogenic Methane Production Rates in Deep Marine Sediments
Much of the methane in natural gas hydrates in marine sediments is made by methanogens. Current models used to predict hydrate distribution and concentration in these sediments require estimates of microbial methane production rates. However, accurate estimates are difficult to achieve because of the bias introduced by sampling and because methanogen activities in these sediments are low and not easily detected. To derive useful methane production rates for marine sediments we have measured the methanogen biomass in samples taken from different depths in Hydrate Ridge (HR) sediments off the coast of Oregon and, separately, the minimal rates of activity for a methanogen in a laboratory reactor. For methanogen biomass, we used a polymerase chain reaction assay in real time to target the methanogen-specific mcr gene. Using this method we found that a majority of the samples collected from boreholes at HR show no evidence of methanogens (detection limit: less than 100 methanogens per g of sediment). Most of the samples with detectable numbers of methanogens were from shallow sediments (less than 10 meters below seafloor [mbsf]) although a few samples with apparently high numbers of methanogens (greater than 10,000 methanogens per g) were from as deep as 230 mbsf and were associated with notable geological features (e.g., the bottom-simulating reflector and an ash-bearing zone with high fluid movement). Laboratory studies with {\it Methanoculleus submarinus} (isolated from a hydrate zone at the Nankai Trough) maintained in a biomass recycle reactor showed that when this methanogen is merely surviving, as is likely the case in deep marine sediments, it produces approximately 0.06 fmol methane per cell per day. This is far lower than rates reported for methanogens in other environments. By combining this estimate of specific methanogenic rates and an extrapolation from the numbers of methanogens at selected depths in the sediment column at HR sites we have derived a maximum estimate of 6 x 10$^{-6}$ nmol methane produced per g sediment per day for samples in which methanogens could not be detected. Rates are likely lower than this if methanogens are not actually present in these samples. Where methanogen numbers are higher in the HR samples rates may be 6 x 10$^{-4}$ nmol methane produced per g sediment per day or higher. Previous reports of higher methanogenic rates in hydrate-bearing sediments (e.g., up to 10$^{3}$ nmol methane produced per g sediment per day in Blake Ridge sediments) may indicate that those samples contain more methanogenic biomass and activity. Our revised estimates of in situ methanogenesis rates will help to improve models intended to predict the location and distribution of hydrates in marine sediments.
B42B-02 INVITED 10:35h
Permafrost And Vegetation Dynamics In Peatlands: Implications For CO$_{2}$ and CH$_{4}$ Exchange
Ecosystems along the 0 degree C mean annual isotherm are arguably among the most sensitive to changing climate. When the temperature goes from below to above 0 degree C for more than two subsequent years permafrost moves out of equilibrium with the climate and starts thawing. This in itself provide an early warning of a warming climate while the permafrost melting at the same time has serious implications for hydrology, vegetation composition and ultimately ecosystem functioning. Peatlands underlain by permafrost emit significant amounts of the important greenhouse gas methane (CH$_{4}$) to the atmosphere and act as a moderate CO$_{2}$ sink. These CH$_{4}$ emissions are intimately related to temperature and hydrology, and alterations in permafrost coverage, which affect both of those, have dramatic impacts on the emissions. Using a variety of data and information sources from the same region in subarctic Sweden we show that mire ecosystems are subject to dramatic recent changes in the distribution of permafrost and vegetation. These changes are most likely caused by a warming, which has been observed during recent decades. The derived vegetation maps of the central part of the Stordalen mire (15 ha) clearly show a decadal change in vegetation composition between 1970-2000. The areal extension of dry elevated ombrotrophic areas has decreased by 11-36% or expressed in hectares, an approximate loss of 1 to 3 hectares. Accordingly dwarf shrub vegetation with a high abundance of bare peat and lichens in the bottom layer and associated species in the field layer are now less abundant on the mire. During the same time period the total CO$_{2}$ and CH$_{4}$ flux changed by 1-11% (sink) and 19-66% (source) respectively. When calculating the flux as total C-equivalents, using IPCC's GWP (Global Warming Potentials) for CH$_{4}$ at the 100-year timescale, it shows that the mire in 2000 has a 14-31% greater radiative forcing on the atmosphere.
B42B-03 INVITED 11:00h
Determination of Methane Emissions by Region and Generating Process Using Inverse Methods
Methane is a chemically and radiatively important trace gas with a wide range of geographically and temporally varying biogenic and anthropogenic sources and sinks. A powerful method for determining the net methane fluxes due to these sources and sinks involves solution of an inverse problem in which the observed concentrations are effectively lagrangian line integrals and the unknown fluxes are contained in the integrands. The general method calculates optimal estimates in the Bayesian sense using an eulerian or lagrangian atmospheric transport model and global atmospheric methane measurements. We review the results of previous studies of regional and global methane fluxes using these methods. We also present the results of a recently completed study using a Kalman filter and the global 3D Model for Atmospheric Transport and Chemistry (MATCH) driven by NCEP analysed observed meteorology at T62 (1.8o x 1.8o) resolution. Monthly fluxes are optimally estimated for three large wetland/bog regions, rice agriculture (globally aggregated), and three large biomass burning regions. The study uses AGAGE, CMDL and other methane observations for 1996-2001 and also estimates average annual emissions from coal, gas, animals and waste sites. Deduced seasonal cycles for the biogenic sources (including rice) are qualitatively similar to prior estimates from site measurements but show differences in peak amplitudes and phases and significant year-to-year variability. Emissions from rice-growing regions are greater than prior estimates, while emissions from fossil sources are less. Enhanced emissions from northern wetland/bog regions are inferred to be the dominant contributor to the large 1998 increases in methane.
B42B-04 11:35h
Methane Emissions and the Greenhouse Gas Budget in Alaska for the Past and 21st Centuries
We used a biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to study the net methane (CH4) fluxes between Alaskan ecosystems and the atmosphere. We estimated that the current net emissions of CH4 (emissions minus consumption) from Alaskan soils are about 3 Tg CH4 yr-1. Wet tundra ecosystems are responsible for 75% of the region's net emissions, while dry tundra and upland boreal forests are responsible for 50% and 45% of total consumption over the region, respectively. In response to climate change over the 21st century, our simulations indicate that CH4 emissions from wet soils will be enhanced more than consumption by dry soils of the tundra and boreal forests. As a consequence, we project that net CH4 emissions will almost double by the end of the century in response to high-latitude warming and associated climate changes. When we placed these CH4 emissions in the context of the projected carbon budget (carbon dioxide, CO2, and CH4) for Alaska at the end of the 21st century, we estimated that Alaska will be a net source of greenhouse gases to the atmosphere of 33 T g CO2-Eq. yr-1; that is, a balance between the net methane emissions of 69 T g CO2-Eq. yr-1 and the carbon sequestration of 10 T g C yr-1 (36 T g CO2-Eq. yr-1).
B42B-05 11:50h
West Siberian peatlands: A global methane source since the early Holocene
Tropical wetlands are thought to have been a major driver of early Holocene fluctuations in atmospheric methane, in part because high-latitude peatlands were not extensively developed in North America by ~11 ka, a period of peak methane concentration. However, the timing of peatland expansion in Russia, which contains nearly half of the world's peat, is virtually unknown. We determine the age, spatial evolution, and carbon stock of the world's largest peatland complex, the West Siberian Lowland (WSL). Radiocarbon dates taken throughout the region reveal broad and rapid peatland expansion 11.5 - 9 ka, significantly pre-dating comparable development in North America. This same period coincides with peak atmospheric methane concentrations and high interpolar gradient as recorded in Greenland and Antarctic ice cores. GIS inventory of ~30,000 unpublished measurements, satellite imagery and field data finds that WSL carbon stocks are larger than previously thought (70.2 Pg), representing up to ~26% of all terrestrial carbon accumulated since the Last Glacial Maximum. No evidence for catastrophic oxidation is found, suggesting the region has behaved primarily as a net carbon sink and global source of atmospheric methane since the early Holocene.
http://lena.sscnet.ucla.edu
B42B-06 12:05h
CO$^{2}$ and CH$^{4}$ Exchange in Interior Alaska: Interactions Between Fire, Water, Soils and Vegetation.
The Alaskan interior contains large carbon reserves stored in poorly drained ecosystems. With warming, these areas of the boreal forest may experience more frequent or extensive stand replacing fires, and thus change the primary factors controlling carbon emissions. In 2001, a low-lying area of the Tanana Flood Plain adjacent to the Bonanza Creek LTER burned. Historical changes in vegetation, hydrology and fire at this site were tracked through macrofossil, charcoal and diatom analysis of peat cores. Dating the charcoal layers in the peat cores indicate four fire events in the past 800 years. The paleoecological record reveals a pattern of expansion of the bog after fire. After the most recent fire, a 30m transect was established along a moisture gradient from the center of a sphagnum dominated collapse feature into the surrounding burn. Thermocarst and subsiding soils were observed on the margin of the sphagnum bog in the three years since the fire. This has increased the anaerobic fraction of the soil profile. CO$^{2}$ flux data suggest that both the dry and wet ends of the moisture gradient are sinks for CO$^{2}$, with a growing season average daytime NEE of -2 $\mu$mol CO$^{2}$ m$^{-2} s$^{-1}$. The moat is a CH$^{4}$ source with an average growing season flux of 30 mg CH$^{4}$ m$^{-2}$ d$^{-1}$ in the abnormally dry summer of 2004. We hypothesize that, after fire, lowland areas become wetter. This leads to high NEP, greater inputs of labile carbon, and increased CH$^{4}$ efflux. However, if interior Alaska experiences more abnormally warm and dry summers like that of 2004, future CH4 production may be suppressed by the changing climate.