B43B-0144 1340h
Carbon Isotopic Signatures in Living Benthic Foraminifera from Methane-Soaked Gas-Hydrate-Bearing Sediments in Guaymas Basin, Gulf of California
There is an ongoing discussion on the role of methane outgassing from costal basins to explain the rapid warmings in the Northern Hemisphere during the last Glacial cycle. The recurring anomalous depletions in the benthic foraminifera carbon isotopic composition, observed in a few high resolution cores in carbon rich coastal basins, has been interpreted as an indication of the assimilation of methane derived carbon by these organisms. The magnitude and timing of these depletions, coincident with rapid warmings in the Northern hemisphere, have been interpreted in terms of large methane releases from these carbon loaden sediments into the atmosphere. However, it is still poorly known whether benthic foraminifera do in fact assimilate methane derived carbon and if at all the observed carbon isotopic depletions are related to periods of methane release. Here we report on some recent results from an extensive ROV-based exploration along the NE transform margin of the Guaymas Basin in the Gulf of California with the objective of characterizing the patterns of carbon isotopic assimilation into the benthic foraminiferal tests along known gradients of present methane venting. Cores were retrieved from beds of living calyptogenid clams, tubeworms, and white bacterial mats, commonly found on exposed strata on the flanks of the basin; from a methane venting site visible by a continuous emanation of gas bubbles from the seafloor in 1,582 m of water; and from background sites at the same depth in Guaymas Basin. Rose bengal-stained specimens of Uvigerina peregrina (Up), Planulina w?llerstorfi (Pw), Globobulimina pacifica (Gp), and Bulimina mexicana (Bm) were hand-picked and their carbon and oxygen isotopic compositions were determined in 117 samples. The mean carbon isotopic values (relative to PDB) in any group of these benthic foraminifera are not distinctly lighter than expected for these sites (-1.14% Up, -0.28% Pw, -1.71% Gp, and -0.5% Bm). Their range of values are small (1.26% Up, 1.14% Pw, 2.17% Gp, and 0.81% Bm) and can be fully explained in terms of the pore water carbon isotopic composition, that results from the oxidation of organic carbon in the upper few cm of the sediment column further mirrored in the strong sulphate reduction gradients with depth. DIC isotopic composition measured in several pore water sample range from -2.9% to -35.85%. The most depleted values indicate that while some methane derived carbon has entered the authigenic DIC pool within the seafloor sediments, there is no record of the assimilation of this carbon on the calcitic shells of living benthic foraminifera. These results question the extent to which strong carbon isotopic depletion signals in the geological record are features recorded by living foraminifera or the result of a posterior diagenetical imprint.
B43B-0145 1340h
Large Methane Plumes Formed by Hydrate Coated Gas Bubbles from a Deep-Sea Submarine Mud Volcano
Submarine mud volcanoes (SMVs) are major methane sources in the marine environment. Worldwide 10$^{3}$-10$^{5}$ SMVs are believed to release in total about 27 Mt CH$_{4}$ yr$^{-1}$. Results of the last decade $\'{ }$ s research suggest, that most of this methane, primarily released diffusively from deep-sea SMVs is immediately oxidised and, thus, has only little climatic impact. Recent hydroacoustic, visual and geochemical observations performed at Haakon Mosby Mud Volcano (HMMV, 1250 meters water depth) reveal that a considerable amount of methane is released by discharge of bubbles and gas hydrate flakes. Gas bubbles withstand dissolution due to the formation of a gas hydrate skin and, thus, are able to ascend several hundred meters through the water column until leaving the temperature-pressure field of gas hydrate stability. Besides, microbial water column methane oxidation was found to be extremely slow, so that methane potentially escapes to the atmosphere, especially during deep winter mixing. We thus propose a much higher environmental relevance of SMVs than previously assumed.
B43B-0146 1340h
The Influence of Methane Venting on Benthic Foraminiferal Assemblages in Guaymas Basin, Gulf of California
Fossil foraminifera are critical for paleoenvironmental reconstructions including the study of past episodes of methane venting from gas hydrate reservoirs. However, the use of benthic foraminifera as indicators of methane release remains controversial and more modern analog data is needed to understand the ecology and isotopic signatures of foraminifera in methane seeps. The objective of this investigation was to characterize the species composition and vertical distribution of living benthic foraminifera (rose Bengal stained) along known gradients of present methane venting in order to gain insight into the ecological tolerances and preferences of benthic foraminifera in methane seeps. Vertical distribution patterns are also important in determining carbon isotope variability. Samples were retrieved along the NE transform margin of the Guaymas Basin in the Gulf of California (about 1,582 m). Suites of ROV cores were collected from beds of living calyptogenid clams, tubeworms, and bacterial mats; from a methane venting site evidenced by a continuous stream of gas bubbles; and from control sites. Our data shows that foraminiferal abundance is lower in the methane-influenced sites than in the control sites. Lowest foraminiferal abundance occurs at the bacterial mats, probably caused by higher levels of sulfide. The assemblage is dominated by calcareous species that are characteristic of other organic-rich, oxygen-poor environments (e.g., {\it Uvigerina peregrina, Bulimina mexicana, Buliminella tenuata, Globobulimina pacifica}). The vertical distributions of several species are different from those of conspecifics observed in previous studies of non-seep habitats, with deeper and broader depth ranges for some species at the methane-influenced habitats in this study. Of special interest is the occurrence of {\it Planulina wuellerstorfi}, traditionally considered an epifaunal species, at sediment depths of 6 cm and with density maxima between 1 and 3 cm. This may result from deeper and more widespread microbial food sources at methane-influenced sites compared to non-seep habitats.
B43B-0147 1340h
Methane Driven Microbial Ecosystems At Mud Volcanoes And Other Types Of Cold Seeps
Ocean margin research of the last decade has provided evidence for distinct microbial habitats fueled by methane, which harbor high biomasses but low diversities of bacteria and archaea. Such microbial ecosystems are found above gas hydrates and at mud volcanoes, pockmarks and other methane-driven cold seep systems. A key process in the emission of methane and sulfide at such seeps, in the turnover of elements and in the formation of carbonate is the anaerobic oxidation of methane (AOM). AOM is the major biological sink of methane in the ocean and crucial in balancing the emission of this important greenhouse gas into the atmosphere. The microbial oxidation of methane with sulfate as electron acceptor provides sulfide as an energy source to chemosynthetic communities of similar biomass and diversity as those of hydrothermal vents. We are just beginning to understand microbial methane cycling in anoxic habitats and its regulation of gas emission, its link to the sulfur cycle, and its relevance in transforming subsurface energy to support benthic communities at the seafloor. A priority of future research is to investigate and visualize the interaction between geological structures forming habitats for anoxic life and microbial communities which shape these structures through their activities such as gas production and consumption, petroleum degradation, microbial calcification. Systems like the Haakon Mosby mud volcano of the Barents Sea and the newly discovered mud volcanoes of the E. Mediterranean Nile fan represent distinct geological structures on continental margins, and are excellent natural laboratories for investigation of microbial ecosystems and cold seep communities. Mud volcanoes represent a window to the deep biosphere because they expel subsurface sediments together with fluids and gases from great depth to the surface of the seafloor. As a consequence, a natural gradient develops from the center of the volcano to its outer rim, representing a succession from the freshly emitted subsurface sediments devoid of surface communities, to pioneer microbial communities and finally to high biomasses of cold seep chemosynthetic populations, and associated fauna immigrating from the surrounding slope environment. Based on observational, experimental and theoretical data, this presentation will discuss the questions: Which are the key microorganisms and biogeochemical pathways depending on methane flux? On which timescales do methane-driven microbial ecosystems develop, change and leave imprints in the geosystem? In which ways do geological structures shape the evolution of microbial and cold seep ecosystems?
http://www.mpi-bremen.de/deutsch/biogeo/mumm2.html
B43B-0148 1340h
Methane Sources, its Fate, and Output From Cold Seeps in Jaco Scarp, an Embayment Caused by Seamount Subduction Offshore Costa Rica
Methane concentrations and their stable carbon isotopic composition were measured in a scarp created by massive landslides caused by the subduction of a seamount offshore Costa Rica in order to gain more knowledge of the venting activity of these structures and to estimate its CH$_{4}$ flux into the ocean. In this scarp (Jaco Scarp) we revealed a number of vent sites. Great amounts of methane escaped from the eastern rim of the scarp and from a sedimentary sequence in the northwest corner where concentrations reached up to 1500 nmol L$^{-1}$. Repeated sampling of these sites indicates continuous venting over time even though the internal structure of the methane plume changed, which is possibly due to opening and closing of pathways of the fluids in the sedimentary sequence. Several small sources located at sites of high accumulation of organic matter e.g. above the talus were confirmed by their light isotopic ratios. The $\delta$$^{13}$C$_{CH4}$ values of all discovered sources range between -50 $\permil$ and -62 $\permil$ pointing to bacterial methane production. The vent-derived CH$_{4}$ becomes distributed and diluted in the scarp by mixing with background water. Part of the CH$_{4}$ is oxidized in the center of the main plume where concentrations of methane are high and along the walls of the scarp. This is probably due to diminished current flow and the availability of different components. The water flow is directed mainly along the isobaths towards the north-west transporting CH$_{4}$ out of the scarp. The CH$_{4}$ flux was estimated to be 58 - 65 Mg yr$^{-1}$, comparable with the flux from hydrothermal vent sites, mud extrusions, and other cold seep areas worldwide. This illustrated that scarps are important seepage sites of natural methane, because the mass wasting associated with seamount subduction can open up deep sedimentary layers from which accumulated methane can discharge. However, the extent of methane seepage from scarps on global scale remains unknown to date.
B43B-0149 1340h
Global Change Simulations Affect Potential Methane Oxidation in Upland Soils
Atmospheric concentrations of methane (CH4) are higher now than they have ever been during the past 420,000 years. However, concentrations have remained stable since 1999. Emissions associated with livestock husbandry are unlikely to have changed, so some combination of reduced production in wetlands, more efficient capture by landfills, or increased consumption by biological CH4 oxidation in upland soils may be responsible. Methane oxidizing bacteria are ubiquitous in upland soils and little is known about how these bacteria respond to anthropogenic global change, and how they will influence - or already are influencing - the radiative balance of the atmosphere. Might ongoing and future global changes increase biological CH4 oxidation? Soils were sampled from two field experiments to assess changes in rates of CH4 oxidation in response to global change simulations. Potential activities of CH4 oxidizing bacterial communities were measured through laboratory incubations under optimal temperature, soil moisture, and atmospheric CH4 concentrations (~18 ppm, or 10x ambient). The ongoing 6-year multifactorial Jasper Ridge Global Change Experiment (JRGCE) simulates warming, elevated precipitation, elevated atmospheric CO2, elevated atmospheric N deposition, and increased wildfire frequency in an annual grassland in a Mediterranean-type climate in central California. The ongoing 1-year multifactorial Merriam Climate Change Experiment (MCCE) simulates warming, elevated precipitation, and reduced precipitation in four different types of ecosystems along an elevational gradient in a semi-arid climate in northern Arizona. The high desert grassland, pinyon-juniper woodland, ponderosa pine forest, and mixed conifer forest ecosystems range in annual precipitation from 100 to 1000 mm yr-1, and from productivity being strongly water limited to strongly temperature limited. Among JRGCE soils, elevated atmospheric CO2 increased potential CH4 oxidation rates (p=0.052) and wildfire decreased rates (p=0.014). These responses may be explained by improved soil aggregate stability in the first case, and reduced aggregate stability in the latter case. No effects of warming, elevated precipitation, elevated N deposition, or multifactor interactions were found. Among MCCE soils, similarly, no effects of elevated or reduced precipitation were found. While warming did not affect low elevation ecosystems, it did significantly decrease rates in the highest elevation mixed conifer forest (p=0.004). This suggests a vulnerability of cold-adapted CH4 oxidizing bacteria to elevated temperature. However, bacterial communities in all sampled ecosystems appear to be resistant to drier conditions and unaffected by wetter conditions. If biological oxidation is responsible for the current stability in atmospheric CH4 concentrations, then the improved function of this global CH4 sink is likely driven by indirect plant effects under elevated atmospheric CO2. Improved function, however, may be absent or reversed in future ecosystems that experience increased wildfire frequency and in high altitude and latitude ecosystems that experience rapid warming.
B43B-0150 1340h
Assessment of Microbial Methane Oxidation Above a Petroleum-Contaminated Aquifer Using Gas Push-Pull Tests, Stable Carbon Isotope Fractionation and Profile Modeling
Microbial methane oxidation is an important process reducing methane emissions from different environments such as landfills, peat bogs and contaminated aquifers. We applied the recently developed gas push-pull test (GPPT) to quantify methanotrophic activity and stable carbon isotope fractionation in-situ above a methanogenic petroleum-contaminated aquifer in Studen, Switzerland. The GPPT consists of the injection of a gas mixture of reactants methane and oxygen and non-reactive tracers neon and argon into the vadose zone followed by its extraction together with soil air from the same location. Rate constants of methane oxidation are then calculated from breakthrough curves of extracted methane and neon. We performed six GPPTs, four at 2.7m depth, directly above the groundwater table and two at 1.1m depth. Methane injection concentrations ranged from 17 ml/L to 195 ml/L. At 2.7m depth, rate constants decreased with increasing injection concentration. This indicated that methane oxidation followed Michaelis-Menten kinetics. Rate constants measured at low or medium methane concentrations in the GPPTs were in good agreement with a rate constant calculated from a previously measured gas profile. During the GPPTs we also quantified stable carbon isotope fractionation in methane. The computed fractionation factor $\epsilon$ ranged from 5.3 $\permil$ to 29.5 $\permil$ with stronger fractionation during the tests with higher methane concentrations. The strongest fractionation occurred during the tests at 1.1m depth. At this depth calculated rate constants were more than three times lower than in the zone directly above the aquifer while methane concentrations remained higher throughout the tests. Thus, while GPPTs and gas profile modeling allowed direct quantification of microbial methane oxidation, the use of stable carbon isotope analysis for this purpose may be more complicated due to the observed variability in isotopic fractionation.
B43B-0151 1340h
Non-methane Hydrocarbon Emissions from a Subarctic Mire in Northern Sweden
Northern peatlands in the Abisko region of northern Sweden are now subjected to climatic warming. In addition to carbon dioxide (CO$_{2}$) and methane (CH$_{4}$), which are intensively studied regarding carbon cycling and climate, there are non-methane hydrocarbons (NMHCs) emitted by plants. NMHC emissions may be a substantial part of the carbon emissions from an ecosystem. They can play a role in tropospheric radiation balance, oxidation chemistry and biogenic aerosol formation. The dynamics of NMHC emissions are not well understood, but impacts of light, temperature and photosynthetic rate and stress have been documented. There has been very little research conducted on NMHCs from vegetation in high latitude ecosystems. We have made high frequency automated chamber measurements of CO$_{2}$ and total hydrocarbon (THC) fluxes from different subhabitats in a subarctic mire underlain by discontinuous permafrost in northern Sweden. Methane fluxes were also measured regularly but less frequently by manual grab sampling from the same chambers. During the summer of 2003-2004, a detailed study of the THC and methane flux rates was conducted in order to resolve patterns of NMHC fluxes. Four different sub-ecosystems are involved in the study. Manual sampling of methane was conducted three times a week during June to August in order to compare the THC to the methane flux. The THC fluxes range from near 0 to greater than 650 mg m$^{-2}$ d$^{-1}$ (calibrated as CH$_{4}$). The highest fluxes are from the wettest sites. NMHCs are often more than 10% of the THC fluxes with a greater proportion of the THC flux from the drier sites with the lowest THC fluxes. The largest emissions were recorded from the wet, minerotrophic sites (E. angustifolium, E. vaginatum) followed by a semiwet site (Sphagnum mosses, C. rotundata). The dry ombrotrophic sites with several types of herbs and dwarf shrubs have much lower emission rates during clear chamber measurements but a distinct dark chamber effect has been detected from this vegetation, showing high rates of NMHC emission when darkened. Preliminary results from the HC analyses will be presented.
B43B-0152 1340h
Spatial And Temporal Variation Of Methane Fluxes In A Small Humid Temperate Forest Watershed In Japan
\qquad It is generally thought that forest soils function as a sink for methane (CH$_{4}$). However, in forests subject to a humid climate, wetlands often occur in riparian zones around streams and ponds. It is possible that CH$_{4}$ emissions in these wetlands exceed the uptake in forest soil. It is still not fully understood whether forest catchments act as net sinks or sources of CH$_{4}$. We routinely monitored CH$_{4}$ fluxes at three different sites in a wetland and three dry hillslopes in a small forested catchment in central Japan. CH$_{4}$ emission was measured using the static chamber technique. At the same time, several environmental factors were measured, too. The study site is covered with mixed stands of secondary broad-leaved deciduous trees and planted coniferous trees. Several wetlands are located along the main stream of the catchment. The area of the entire catchment is 59,900 m$^{2}$ and that of the wetlands is 300 m$^{2}$. CH$_{4}$ was emitted almost throughout the year, with clear seasonality, at the monitored points in the wetland. The CH$_{4}$ emission rates in wetland sites were strongly dependent on soil temperatures, and were highest in summer and lowest in winter. Annual average CH$_{4}$ emissions in the wetland ranged from 13.8 to 92.4 mg CH$_{4}$ m $^{-2}$ d $^{-1}$. Although CH$_{4}$ emissions increased markedly every summer, emissions were constrained by depletion of the water table in the summer of 2002. Most of the forest floor acted as a small sink (annual average fluxes ranged from -0.54 to -0.02 mg CH$_{4}$ m $^{-2}$ d $^{-1}$.), but CH$_{4}$ emissions were occasionally observed in wet parts in summer. Consumption rates in the forest floor did not significantly vary through the year, except for wet areas. \qquad It was found that the water table and soil water content dictated redox conditions and affected CH$_{4}$ production and oxidation in several different parts of the watershed. While the CH$_{4}$ emission rate increased with increased soil temperature, the CH$_{4}$ uptake rate was less affected by fluctuations in soil temperature. Average CH$_{4}$ emission rates from wetland sites were 3 orders of magnitude greater than the uptake rate at the forest floor. Our results suggest that forest watersheds of this type may function as a source of CH$_{4}$, especially when seasonal temperatures are high, in spite of the area of wetland being relatively small compared to the entire watershed area.
B43B-0153 1340h
Phospholipid-Derived Fatty Acids and Their Stable and Radiocarbon Isotope Values as Indicators of Bacterial Methane Oxidation at a Thermogenic Methane Seep
The importance of aerobic methanotrophy as a filter for biogenic methane emissions is well documented for environments such as natural wetlands, landfills, and rice paddies, but less is known about methane oxidation in soils overlying thermogenic methane seeps. We are utilizing phospholipid-derived fatty acids (PLFAs) extracted from soils overlying a high-rate, coal-bed methane seep in Southwestern Colorado to investigate the location and extent of bacterial methane consumption within the soil column. PLFAs have been widely used as indicators of both quantities and types of viable bacterial populations. Two specific PLFAs, 16:1$\omega$8 and 18:1$\omega$8, appear to be unique to type I and type II methanotrophs, respectively. We have detected higher abundances of these methanotroph biomarkers in surface soils ($\sim$0-30 cm) under wetter soil conditions and near the water table ($\sim$150 cm) under drier conditions. Maximum concentrations of both type I and type II methanotroph PLFA biomarkers were greatest in the shallow soils during wetter conditions with the type I maximum located just above the type II maximum. This is consistent with pure culture studies that have shown type I methanotrophs to prefer higher oxygen, lower methane conditions and type II methanotrophs to prefer lower oxygen, higher methane conditions. Soil gas methane concentrations during this wetter period were approximately 30$%$ at 20 cm depth and 80$%$ at 100 cm depth. During a drier period a type II methanotroph biomarker maximum was observed near the water table but no type I maximum was observed. Soil gas methane concentrations at this time were less than 1$%$ at 20 cm and 25$%$ at 100 cm. These data suggest that methanotrophs may consume a significant fraction of the methane as it rises through the soil column. Greater saturation of soil pore spaces during wetter conditions may inhibit atmospheric oxygen diffusion into deeper soils forcing methanotrophs to reside in the shallow soils and resulting in larger amounts of methane being released to the atmosphere. Both stable and radiocarbon values of detected PLFAs suggest that methane accounts for a significant fraction of the microbial carbon source in soils overlying the methane seep, especially in the deeper soils where organic matter carbon sources are more scarce.
B43B-0154 1340h
Compared chemistry of natural gas hydrates from different oceanic environments
Natural gas hydrates are generally associated with pockmarks or mud volcanoes on margins. They occurred both in deep sedimentary structures, and sometimes as outcrops on the seafloor. The gas hydrates studied here were collected from gravity sediment cores and occur as small fragments and massive crystal aggregates, mostly disseminated irregularly in the sediment. In most cases, they escape in the overlying deep seawater creating CH4-rich plumes which extend 100-150 m above the seafloor. These plumes are also enriched in particles, manganese, iron related to discharges of high turbid fluids issued from sediments and in all cases methane is rejected from sediment as free gas or as a consequence of the decomposition of gas hydrates. Specific equipments are necessary for collecting, and storing these gas hydrates in good conditions to avoid decomposition. The crystalline structure of solid gas hydrates is studied by Raman Spectrometry and Synchrotron techniques and show mainly methane gas hydrate of cubic Structure I. Analyses of hydrate water show variations (depletions or enrichments) of mineral elements compared to ambient deep seawater. Gas analysis shows that CH4 is the major component but CO2 and heavier gases (C2H4, C2H6, H2S) are also present as traces. In addition, many families of organic compounds detected by chromatography-mass spectrometry are present as traces. In most cases, the carbon and hydrogen isotopic data indicate a primarily microbial origin for the CH4 which is generated through bacterial CO2 reduction. All these chemical data contribute to understand the origin, formation and stability of gas hydrates trapped in sediments on oceanic margins. Specimens of natural gas hydrates recently collected on the African and Norvegian margins will be discussed.
B43B-0155 1340h
Reduced Atmospheric CH4 Consumption by Temperate Forest Soils Under Elevated CO2
Models project that atmospheric CO$_2$ concentrations, by the end of the present century, will exceed the preindustrial concentration by up to 250%. The regional and global impact of this projected concentration increase on other biogeochemical cycles is uncertain. We recently reported in a two year study a 17 (year 2) to 30% (year 1) decrease in atmospheric CH$_4$ consumption by soils in CO$_2$-enriched plots in a temperate loblolly pine (Pinus taeda) forest, although the reason for the decline was unclear. Consumption by upland soils is the only terrestrial sink for atmospheric CH$_4$, which is second only to CO$_2$ in terms of radiative forcing. Forest ecosystems occupy about half of the Earth's terrestrial surface. A sustained CO$_2$-induced negative feedback on forest soil CH$_4$ consumption could lead to a 25% reduction (7.5 Tg CH$_4$ yr$^-1$) in the current upland soil sink of $\sim$30 Tg yr$^-1$. However, CO$_2$-enriched tundra ecosystems showed down regulation in at least the photosynthetic response after 3 yr of fertilization and it is uncertain whether decreased atmospheric CH$_4$ consumption represents a transient or sustained response of forest-soil systems to elevated CO$_2$. We report here the early results of our efforts to determine the duration and underlying causes for the decline in atmospheric CH$_4$ consumption in a CO$_2$-enriched forest. Reduced CH$_4$ consumption persisted in elevated CO$_2$ plots, which showed declines of 13% (year 3) and 34% (year 5, to date), relative to unenriched controls. This decline may be related to the rate of supply of CH$_4$ to the subsurface zone of oxidation, as soil moisture was significantly higher in CO$_2$-enriched plots. A single experiment to date showed that changes in the chemical composition of leachate from aboveground plant material had no impact on the CH$_4$ oxidizing community, as rates of CH$_4$ consumption by soil samples amended with throughfall from CO$_2$-enriched and control plots were not significantly different.
B43B-0156 1340h
Effects of Acetate Competition, pH and Soil Structure on the Rates and Pathways of Methane Formation in Tropical Rain Forest Soils
The C isotopic composition of CH$_{4}$ emissions are strongly influenced by the pathway of CH$_{4}$ formation. Contrary to data from other freshwater systems, soil gas and surface flux measurements made in the tropical rain forests of Puerto Rico strongly suggest that CH$_{4}$ produced in these environments was derived from CO$_{2}$ reduction, rather than from acetate consumption. This study explored the effects of bacterial competition for acetate, pH, and soil structure on the pathways of CH$_{4}$ formation in tropical rain forest soils. Our goal was to test two principal hypotheses: (1) ferric iron-reducing bacteria out-competed methanogens for acetate, resulting in greater CO$_{2}$ reduction rather than aceticlastic methanogenesis, and (2) the low pH of tropical rain forest soils favors CO$_{2}$ reduction rather than aceticlastic methanogenesis. In addition, this study also investigated the effect of destroying soil aggregate structure on the pathways and rates of CH$_{4}$ production. Ferric iron-reducing bacteria out-competed methanogens for acetate, reducing CH$_{4}$ production rates by a factor of 3. However, competition for acetate with iron-reducing bacteria did not alter the partitioning of C between different methanogenic pathways, challenging our first hypothesis. Approximately 58 % of the CH$_{4}$ produced in the first 10 days of incubation was derived from acetate in all treatments. Lowering soil pH to 4 significantly reduced CH$_{4}$ production and total acetate utilization, while increasing soil pH to 6 doubled CH$_{4}$ production and increased total acetate utilization. Disruption of soil aggregate structure through slurry conversion negatively affected CH$_{4}$ production. Methanogenesis declined by a factor of 17 after intact, well-aggregated soils were converted to slurries. Slurry conversion also negatively affected the ability of methanogens to compete with other bacteria for acetate. At least 9 times more $^{13}$C-tracer was recovered in CO$_{2}$ compared to CH$_{4}$ after soils were converted to slurries, suggesting that less of the $^{13}$C-labeled acetate was consumed by methanogens after slurry conversion. Lastly, the isotopic data indicate that the relative partitioning of C between aceticlastic and hydrogentrophic pathways was unchanged after conversion of soils to slurries, regardless of the other effects of slurry formation on methanogenesis. The discrepancy between our field and laboratory data suggest that most of the $^{13}$C-depleted CH$_{4}$ was produced from deeper in the soil. Surface emissions were probably dominated by the upward advection of more $^{13}$C-depleted material, rather than by CH$_{4}$ production from the 0-15 cm layer.
B43B-0157 1340h
Insight Into West Siberian Gas and Wetland Methane Emissions From $\delta$$^{13}$C Studies of Ambient Air
The Ob River region of West Siberia is home to some of the largest known gasfields and wetlands, and a source of around 2.5% of the total global methane emissions. It is also a major source region of CO$_{2}$ emissions. Carbon isotopes of ambient air and emission sources provide an important tool for understanding these poorly constrained sources. Tank samples of ambient air were collected overnight for $\delta$$^{13}$C analysis of methane during the summer (August-September) campaigns of 1999 and 2000, as part of INTAS-funded projects. The main sampling centres were Nadym and Urengoy, with samples collected above the known gas reserves, near pumping stations and by boat on the Nadym River, downstream toward Salekhard. CH$_{4}$ of up to 900 ppb above background was recorded and $\delta$$^{13}$C depletions of up to 3$\permil$ relative to background. West Siberian gas has measured $\delta$$^{13}$C values of -51.5 to -49.5$\permil$ based on well samples and supplies in St. Petersburg and Germany. Implications from aircraft flights (Sugawara et al. 1996) and measurements along the Trans-Siberian railroad (Bergamaschi et al., 1998) are that the wetland signature is around -67$\permil$. The ambient air samples give a range of calculated source inputs from -67.3 to -49.3$\permil$, the end members corresponding to sampling areas expected to contain only wetland or gas emissions and confirmed by back trajectory analysis for sampling times. Using the end members, the average excess methane from the wetland source was 62 ppb around Nadym and 61 ppb around Urengoy. The average excess from the gasfields was 30 ppb around Nadym but 82 ppb around Urengoy, reflecting the closer proximity to the gasfields. The maximum excess recorded from wetlands was 160 ppb and from gasfields was 710 ppb. Experiments during summer 2004 have focussed on bag sampling on the tower of a super deep well at Korotchaevo, SE of the Urengoy gas field. Samples collected at 20, 30 and 60m heights will be analysed for $\delta$$^{13}$C of both CH$_{4}$ and CO$_{2}$ and apportioned to gas and wetland sources. Bergamaschi P.et al. Isotope analysis based source identification for atmospheric CH$_{4}$ and CO sampled across Russia using the Trans-Siberian railroad. J. Geophys. Res., 103 (D), 8227-8235, 1998. Suguwara S. et al. Aircraft measurements of the stable carbon isotopic ratio of atmospheric methane over Siberia. Global Biogeochem. Cycles, 10, 223-231, 1996.
B43B-0158 1340h
High Precision Stable Isotope Analysis of Methane and Carbon Dioxide using Trace Gas CF-IRMS: Instrumentation Development, London Diurnal Studies and Measurements of Emissions from Irish Wetlands
Continuous flow isotope ratio mass spectrometry coupled with the Trace Gas preparation system (GV Instruments) has been used to analyse air samples with a precision of better than 0.1 $\permil$ for $\delta$$^{13}$C of CH$_{4}$ and $\delta$$^{13}$C and $\delta$$^{18}$O of CO$_{2}$. 75cm$^{3}$ of air is required, with a 16-minute analysis time. This precision makes the technique ideal for source studies, although it is also approaching the precision required to identify annual variations in measurements at some background stations. Diurnal studies have been carried out from the sampling site at Royal Holloway, west of London, measuring either $\delta$$^{13}$C of CH$_{4}$ or $\delta$$^{13}$C and $\delta$$^{18}$O of CO$_{2}$ at 30-minute intervals. Source calculations for methane showed the local sources had $\delta$$^{13}$C values of between -37.9$\permil$ (typical of gas leaks) and -54.0$\permil$ (landfill emissions). The methane was most enriched in $^{13}$C in the early evening, during the rush hour, suggesting that at this time vehicle exhaust emissions are a measurable local source of methane. Methane and carbon dioxide mixing ratios and isotopes were measured on samples collected from Irish wetlands. Samples were collected in early summer and autumn to investigate variations in emissions from blanket bogs, raised bogs and fens. Air samples were collected upwind, downwind and in the centre of the bogs and were supplemented with soil air samples and chamber collection above waterlogged areas. Results were compared with the Atlantic background record of methane $\delta$$^{13}$C at Mace Head for which measurements have been carried out using a conventional cryogenic extraction line since 1995, to investigate whether Irish wetland emissions affect the Mace Head record for trajectories passing over the southwestern corner of Ireland. Preliminary results from the summer campaign indicate that blanket bogs, which are the prevalent type of peatland along the western coast of Ireland, did not emit significant amounts of methane. Hence Atlantic blanket bog is unlikely to affect the methane isotopes measured at Mace Head. However raised bogs and fens were found to have much larger methane emissions. Source $\delta$$^{13}$C isotope values of emitted methane ranged from -55 $\permil$ at a fen (Pollardstown fen) to -80 $\permil$ at a raised bog (Clara bog).
B43B-0159 1340h
Seasonal and Inter-annual Variations of Methane Flux at Wet Tundra in North Slope, Alaska
Methane (CH4) is a very effective greenhouse gas, and its major source in nature is anaerobic ecosystem. As high latitude ecosystems such as tundra and wetland pool large amount of organic carbon in the soil layer, which may enhance the CH4 emission in relation to the current arctic warming. However, we have a little information of potential feedback mechanisms on climate change and greenhouse gas emission, thus model parameterization of CH4 emission is insufficient and the future estimation includes large uncertainties. We have measured CH4 flux at inland wet tundra (Happy Valley) in 1995 & 96 and at coastal tundra (Barrow) from 1999. We applied aerodynamic method to determine CH4 flux, which provides continuous flux as hectare scale under natural condition. CO2 and energy fluxes were measured by eddy correlation method. Hourly CH4 flux showed unclear daily variation with soil temperature nor solar radiation but the daily flux levels (seasonal trend) changed with seasonal change of soil temperature at 10-cm depth. Seasonal cumulative CH4 flux at inland wet tundra at happy valley was 8.1 gCH4m-2 in 1995 and that at moist tundra (non-acidic) was 3.3 gCH4m-2 in 1996 growing season. At Barrow, CH4 flux increased rapidly after snowmelt and the peak flux occurred in mid July with yearly different levels 60-120mgCH4m-2d-1. Cumulative CH4 flux during the growing period varied 4.2-6.6 gCH4m-2 from 1999-2003. The carbon ratio of emitted CH4 to the NEP of the vegetation was around 3% over the measured period.
B43B-0160 1340h
Methanotrophy in London, UK, Landfill Topsoil: Microbiology, Stable Carbon Isotopes, Seasonal Variation and Laboratory Model Study
Landfill is a major source of methane emissions into the atmosphere. Aerobic soil is also a good sink of methane, as it is inhabited by methane consuming bacteria, methanotrophs. Methanotrophic bacteria were cultured from landfill soil samples. Three genera of methanotrophs were cultured: {\it Methylocaldum}, {\it Methylosinus} and {\it Methylomonas}. Interestingly, the only established members of the {\it Methylocaldum} genus are all thermophilic, whilst those isolated in this study are mesophilic. This suggests that those {\it Methylocaldum} methanotrophs found in landfills may have migrated from hot spring natural settings. Representatives of each genera were inoculated into a simple topsoil model and subjected to variations in temperature, methane concentration and incubation periods. As expected, temperature greatly affected methane oxidation, but methane concentration affected the rate of oxidation far more than expected. The model study implies that the complete combustion of methane to carbon dioxide is greatly affected by temperature and methane availability, whilst the effect on the uptake of methane is not as great. Seasonal variations in methane concentrations within the topsoil were monitored over a one year period from November 2002 to October 2003 and show that methane flow through the topsoil, and consequently methanotrophy, is strongly controlled by meteorology, mainly air temperature and pressure. Generally, methanotrophy was low during colder months and higher at during warmer months, but changes in air pressure complicate this by controlling the rate of flow of methane through the topsoil. $\delta$$^{13}$C analyses of methane and carbon dioxide emitted from landfill topsoil showed that there was a great deal of methanotrophic activity during the warmer months of 2003, with most fractionation of residual methane occurring during August. During the heat wave experienced in the UK in August 2003, the $\delta$$^{13}$C from borehole samples of methane in the anaerobic zone shifted from -57$\permil$ to -16$\permil$ at a depth of 5cm. Results derived from this study indicate that with careful engineering of landfill sites and other sources of methane emissions, methanotrophic bacteria could be employed more intensively as biological methane removal systems. This would alleviate greenhouse gas emissions.
B43B-0161 1340h
Episodic spring release of CH4 through complete snow cover at a boreal mire as measured with micro-meteorological gradient technique
Methane flux was monitored by the micrometerological gradient method and a tunable diod laser at a boreal mire in northern Sweden from late thaw to late spring (April 17th to May 24ht). The part of the mire monitored is dominated by oligotrophic low sedge lawns, with a limited area of carpets with pools and also one sector dominated by ridges and flarks. The snowmelt period was characterized by high episodic release (hourly values, 5-15 mg CH4 h-1 m-2) with daily averages up to 5 mg CH4 h-1 m-2. High episodic release during snowmelt occurred from lawn areas with ten centimeter or more snow on top of the standing water. At the pool area the snow cover was thin with standing water close to the snow surface, the hourly methane flux rates from this area being <2 mg CH4 h-1 m-2. After snowmelt, a period of about thirty days followed without any episodic release and daily averages normally between 0.6 - <2 mg CH4 h-1 m-2. From May 20 to May 24 the daily average flux increased from <2 mg CH4 h-1 m-2 to 8 mg CH4 h-1 m-2, with hourly values up to 13 mg CH4 h-1 m-2. Not only the flux rates changed during the five week period monitored, but also the diurnal methane flux pattern varied considerably both with time period but also due to the particular mire area monitored by the mast. During snowmelt, the hourly flux rate peaked at night between 20 and 02 hours. During the following periods distinct diurnal variation in methane flux rates occurred from areas dominated by vascular plants while the methane flux from areas with less dense vascular plant vegetation was lower and constant during the day. The diurnal amplitude increased from 1.5 mg CH4 h-1 m-2 (1 mg CH4 h-1 m-2 at night and 2.5 mg CH4 h-1 m-2 during the day) at late April to 7 mg CH4 h-1 m-2 (2 mg CH4 h-1 m-2 at night and 9 mg CH4 h-1 m-2 during the day) in late May.
B43B-0162 1340h
Species-specific Effects of Vascular Plants on Carbon Turnover and Methane Emissions from a Subarctic Wetland
Several aspects of biodiversity have important implications for ecosystem functioning. Species composition in any ecosystem affects carbon and energy exchange as well as greenhouse gas fluxes. In combination with the effects of vascular plants on CO$_{2}$ dynamics through photosynthesis and respiration plants also affect important processes relating to CH$_{4}$ dynamics, e.g., production, consumption and transport. Details of the mechanisms behind these relationships, however, are still poorly known. Here we present results from a field experiment on the subarctic mire Stordalen near Abisko ($68\deg$21'N, $19\deg$00'E) in northern Sweden where we during one growing season (June to September) investigated the effect of three dominating sedges, i.e. {\it Carex rotundata}, {\it Eriophorum vaginatum} and {\it Eriophorum angustifolium}, on ecosystem functioning. Throughout the season we determined CO$_{2}$ fluxes, CH$_{4}$ emission, dissolved CH$_{4}$ in the pore water, active layer, water table depth and substrate availability for methane producing bacteria (determined as the pore water concentration of easily degradable carbon compounds, i.e., organic acids, amino acids and carbohydrates). The results are evaluated in the context of current undergoing changes in vegetation composition and the implications for landscape scale methane emissions.
B43B-0163 1340h
Methane Production In Forearc Sediments At The Costa Rican Convergent Margin
Plate tectonics creates suitable habitats for deep biosphere organisms, affecting the distribution of biological communities on Earth. Subduction zones, where crustal materials return to the planetary interior through plate convergence, expose active microbial communities in subducting seafloor sediments to a fresh chemical inventory as diagenesis, metamorphic reactions, and tectonically-induced fluid flow alter sediments and surrounding porewaters. The plate interface (the decollement) experiences persistent geochemical flux of light hydrocarbon- and metal-bearing fluids from depth. This project (1) examines the habitability of the decollement zone at the Costa Rican convergent margin from a geochemical perspective, (2) uses lipid biomarkers to describe biomass distribution in sediment samples adjacent to and within the decollement, and (3) cites methanogenesis as a likely metabolic strategy employed by the resident microbial community. Sterile plugs of sediment were recovered from cores taken during Leg 205 of the Ocean Drilling Program, in the Middle America Trench off Costa Rica. Samples are from the incoming carbonate section of Site 1253 at 370-437 meters below seafloor (mbsf), in the forearc sedimentary wedge at Site 1255 at 134-145 mbsf, and around an upper fault (153-220 mbsf) and in the decollement zone (305-366 mbsf) at Site 1254. Drilling mud and fluid were sampled to monitor potential microbial contamination. Samples were immediately frozen at -80°C. Prior to analysis, samples were freeze-dried in preparation for serial extraction of DNA and lipids. DNA was identified by fluorometry in 13 of 26 samples tested. The DNA was screened for methanogens by real time polymerase chain reaction (PCR), employing ME1 and ME2 primers that amplify a 0.75-kb region of the alpha-subunit gene for methyl coenzyme M reductase (MCR). Methanogen-specific genes were detected in DNA extracted from one Site 1253 sample (at 436.9 mbsf in the basal carbonates) and four Site 1254 samples (at 161.2 and 197.4 mbsf in the upper prism, and at 326.1 and 330.3 mbsf in the decollement). Analysis of ester-linked phospholipid fatty acids (PLFA) in the samples was carried out via capillary gas chromatography with mass spectrometry (GC/MS) in positive ion electron impact mode. PLFA were detected in 8 of the 26 samples, documenting eubacterial presence in those samples. At Site 1254, the maximum PLFA content was observed in the decollement interval with documented occurrence of hydrocarbons (up to 10 carbon atom chains) in gas and fluids sampled shipboard during Leg 205, implying significant deeply sourced fluid flux in that microenvironment. At Site 1253, PLFA were detected in the sample from the base of the carbonate section only. At Site 1255, the highest overall PLFA content was found in the shallow sedimentary wedge. This is the first analysis of microbial community structure in the deep subsurface near the Costa Rican trench, and one of few studies focused on the deep biosphere associated with convergent margins. Documented viable methanogenic populations exist within the decollement and sedimentary wedge fault zone, both sites of deeply sourced hydrocarbon-bearing and chemically distinct fluid flow. Microbial consortia in these settings may convert thermogenic hydrocarbons into methane and carbon dioxide, enhancing the release of these gases associated with subduction.
B43B-0164 1340h
Two Dimensional Distributions of Microbial Methane Oxidation in Marine Gas Seeps Interpreted from $^{13}$C-CH$_{4}$
The Southern California Bight is well known for its \"cold\" hydrocarbon vents, areas of intense leakage of underlying thermogenic natural gas. Isotopic studies of methane in sediments at Coal Oil Point, offshore UC Santa Barbara, were performed around small (~.5 mol of gas h$^{-1}$) marine gas seeps. Seep gas was found to consist of 80% methane, 15% CO$_{2}$, and 5% C$_{2}$-C$_{4}$ hydrocarbons. In order to assess the subsurface bacterial consumption of methane, we devised a two-dimensional, in-situ sampling system to collect gas samples. The system consists of a stainless-steel, square sampling grid that is inserted vertically into the sediment. Samples are collected by syringe through holes in the plate, after sediment has been removed from one side of the plate and the grid has been exposed. Samples are immediately injected into serum bottles, which are crimp-sealed at depth. The $^{13$C-CH$_{4}$ in each sample is analyzed by isotope ratio mass spectrometry upon return to the lab. Progressive enrichments of $^{13$C-CH$_{4}$ in sediments surrounding several gas seeps were observed; methane was enriched by as much as 60% compared to the original seep gas $^{13$C-CH$_{4}$ (typical seep gas $^{13$C-CH$_{4}$ = -51%). The maximum enrichment was typically observed at a lateral distance of 3-4 cm from the seep. The isotopic enrichment is attributed to the preferential utilization of $^{12}$C-CH${_4}$ by methane-oxidizing bacteria and provides an estimate of the extent of methane oxidation - which is up to 95%. This work also provides insights into the manner of subsurface methane flow in gas seeps, and minor channels of seepage can be inferred from plumes of $^{12}$C methane.
B43B-0165 1340h
Constraining the Methane Budget: The use of $\delta$$^{13}$C-CH$_{4}$ data as a constraint for methane source estimates
Methane is the second most potent greenhouse gas and is responsible for nearly 20% of the enhanced greenhouse effect. The most common means of estimating the source budget are through inverse modeling techniques using variations of the atmospheric concentration in space and time and by extrapolation of localized source flux measurements to a global scale. Yet, due to the wide array of methane sources and a relative paucity of measurements, its budget is not well constrained. Carbon-13 measurements offer a means to help clarify the methane budget because different groups of methane sources have distinct isotopic signatures. For example, the isotopic ratio of microbially produced methane is around -60 per mil, while that originating from biomass burning is around -25 per mil. Since 1998, in co-operation with NOAA/CMDL, $\delta$$^{13}$C-CH$_{4}$ has been measured at the University of Colorado's Stable Isotope Lab from samples drawn on a weekly basis from select sites of the NOAA/CMDL Cooperative Air Sampling Network. We use a box model to simulate the effect that different methane source estimates would have on atmospheric CH$_{4}$ and $\delta$$^{13}$C-CH$_{4}$ values at our measurement sites. We then compare the modeled $\delta$$^{13}$C-CH$_{4}$ values with $\delta$$^{13}$C-CH$_{4}$ data to evaluate those source estimates. We focus our analysis on comparison of seasonal variations in methane concentration and $\delta$$^{13}$C-CH$_{4}$ to better understand seasonal source dynamics and how they might be changing over our study timeframe. In addition we compare model and observed latitudinal gradients from 1998 to the present to evaluate changes in source strength and distribution.
B43B-0166 1340h
Novel Instrumentation for Methane Flux Measurements
We describe the development and testing of a new compact, rugged and inexpensive instrument for measurements of methane flux in ambient air. The instrument is based on a new technology called Off-Axis Integrated Cavity Output Spectroscopy (Off-Axis ICOS). This novel instrument measures methane with high sensitivity, accuracy (<2 ppbv at 1 Hz), and specificity in real time (no cross interferences). The instrument combines inexpensive, robust telecommunications-grade room-temperature diode laser operating at 1.65 microns with Off-Axis ICOS to yield an instrument capable of continuously recording data in the field with high precision (better than 0.2% uncertainty at a 10-Hz rate). We will discuss the measurement strategy in detail and present recent results demonstrating real-time measurements without the need for any user intervention. Ongoing efforts to demonstrate the instrument's capabilities to record measurements with high precision and accuracy without calibration over extended periods as well as testing of the instrument at field sites in the AmeriFlux and FLUXNET networks, and NOAA/CMDL will also be discussed. By significantly increasing the accuracy and precision of methane flux measurements, the Off-Axis ICOS instrument will enhance local, regional and global studies of global warming and facilitate controlled multi-year studies and comparisons between field sites. These studies, which could involve using the instruments aboard airplanes to enable coverage over large distances and to correlate with satellite images, will help track and quantify the global carbon cycle on small and large spatial scales, and enable atmospheric chemists to generate more reliable models of climate change and determine environmental impact.
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