Wood decomposition in Amazonian hydropower reservoirs: An additional source of greenhouse gases

Amazonian hydroelectric reservoirs produce abundant carbon dioxide and methane from large quantities of flooded biomass that decompose anaerobically underwater. Emissions are extreme the first years after impounding and progressively decrease with time. To date, only water-to-air fluxes have been considered in these estimates. Here, we investigate in two Amazonian reservoirs (Balbina and Petit Saut) the fate of above water standing dead trees, by combining a qualitative analysis of wood state and density through time and a quantitative analysis of the biomass initially flooded. Dead wood was much more decomposed in the Balbina reservoir 23 years after flooding than in the Petit Saut reservoir 10 years after flooding. Termites apparently played a major role in wood decomposition, occurring mainly above water, and resulting in a complete conversion of this carbon biomass into CO₂ and CH₄ at a timescale much shorter than reservoir operation. The analysis of pre-impounding wood biomass reveals that above-water decomposition in Amazonian reservoirs is a large, previously unrecognized source of carbon emissions to the atmosphere, representing 26–45% of the total reservoir flux integrated over 100 years. Accounting for both below- and above-water fluxes, we could estimate that each km² of Amazonian forest converted to reservoir would emit over 140 Gg CO₂-eq in 100 years. Hydropower plants in the Amazon should thus generate 0.25–0.4 MW h per km² flooded area to produce lower greenhouse gas emissions than gas power plants. They also have the disadvantage to emit most of their greenhouse gases the earliest years of operation.     

The case for human causes of increased atmospheric CH4 over the last 5000 years

We propose that humans significantly altered atmospheric CH₄ levels after 5000 years BP and that anthropogenic inputs just prior to the industrial revolution accounted for up to 25% of the CH₄ level of 725 ppb (parts per billion). We base this hypothesis on three arguments: (1) the 100 ppb increase in atmospheric CH₄ that occurred after 5000 years BP follows a pattern unprecedented in any prior orbitally driven change in the ice-core record; (2) non-anthropogenic explanations for this increase (expansion of boreal peat lands or tropical wetlands) are inconsistent with existing evidence; and (3) inefficient early rice farming is a quantitatively plausible means of producing anomalously large CH₄ inputs to the atmosphere prior to the industrial revolution. If the areas flooded for farming harbored abundant CH₄-producing weeds, disproportionately large amounts of CH₄ would have been produced in feeding relatively small pre-industrial populations.     

Diurnal Variations of Carbon Dioxide, Methane, and Nitrous Oxide Vertical Fluxes in a Subtropical Estuarine Marsh on Neap and Spring Tide Days

Measuring fluxes of greenhouse gases (GHGs) is fundamental to estimating their impact on global warming. We examined diurnal variations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) vertical fluxes in a tidal marsh ecosystem. Measurements were recorded on neap and spring tide days in April and September 2010 in the Shanyutan wetland of the Min River estuary, southeast China. Here, we define a positive flux as directing into the atmosphere. CH₄ fluxes on the diurnal scale were positive throughout, and CH₄ emissions into the atmosphere on neap tide days were higher than on spring tide days. CH₄ releases from the marsh ecosystem on neap tide days were higher in the daytime; however, on spring tide days, daily variations of CH₄ emissions were more complex. The marsh ecosystem plays a twofold role in both releasing and assimilating CO₂ and N₂O gases on the diurnal scale. Average CO₂ fluxes were positive on the daily scale both on neap and spring days and were greater on the neap tide days than on spring tide days. Diurnal variations of N₂O fluxes fluctuated more. Over the diurnal period, soil temperature markedly controlled variations of CH₄ emissions compared to other soil factors, such as salinity and redox potential. Tidal water height was a key factor influencing GHGs fluxes at the water–air interface. Compared with N₂O, the diurnal course of CO₂ and CH₄ fluxes in the marsh ecosystem appeared to be directly controlled by marsh plants. These results have implications for sampling and scaling strategies for estimating GHGs fluxes in tidal marsh ecosystems.     

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt,China

Natural gas, consisting primarily of methane(CH₄), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep-sourced abiotic CH₄ in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH₄ flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiot...     

Rate measurements and detection of gas microseepage to the atmosphere from an enhanced oil recovery/sequestration project,Rangely, Colorado,USA

One of the proposals for large-scale sequestration of fossil fuel-derived CO₂ is deep geologic disposal in depleted oil/gas reservoirs or deep aquifers. Previously published scenarios for this inadequately proven technology have either ignored or dismissed the possibility of vertical migration of gases caused by overpressure. Overpressuring of a reservoir or aquifer will be necessary in order to have acceptable rates for dispersal of injected CO₂. This research describes methodology and the results of measurement of microseepage of CO₂ and CH₄ at a large-scale CO₂-enhanced oil recovery (EOR) operation at Rangely, Colorado, USA. Shallow and deep soil gas concentrations, and direct transport of CO₂ and CH₄ into the atmosphere were measured. The interpretation of the measurements was complemented by both stable and radiogenic isotopic measurements of C. The results have demonstrated an estimated microseepage to the atmosphere of approximately 400 metric tonnes of CH₄/a from the 78 km² area of the Rangely field. Preliminary estimates of deep-sourced CO₂ losses are <3800 tonnes/a, based on stable isotope measurements of soil gases. Several holes up to 10 m deep were drilled on, and off the field for nested gas sampling of composition and stable C isotopic ratios for CO₂ and CH₄. Carbon-14 measurements on CO₂ from these holes indicate that deep-sourced CO₂ microseepage losses were approximately 170 tonnes/a.     

Computer modeling of methanotrophic oxidation of hydrocarbons in the unsaturated zone from an enhanced oil recovery/sequestration project, Rangely, Colorado, USA

One of the proposals for large-scale sequestration of fossil fuel-derived CO₂ is deep geologic disposal in depleted oil/gas reservoirs or deep aquifers. Previously published scenarios for this inadequately proven technology have either ignored or dismissed the possibility of vertical migration of gases caused by overpressure. Overpressuring of a reservoir or aquifer will be necessary in order to have acceptable rates for dispersal of injected CO₂. This research describes methodology and the results of measurement of microseepage of CO₂ and CH₄ at a large-scale CO₂-enhanced oil recovery (EOR) operation at Rangely, Colorado, USA. Shallow and deep soil gas concentrations, and direct transport of CO₂ and CH₄ into the atmosphere were measured. The interpretation of the measurements was complemented by both stable and radiogenic isotopic measurements of C. The results have demonstrated an estimated microseepage to the atmosphere of approximately 400 metric tonnes of CH₄/a from the 78 km² area of the Rangely field. Preliminary estimates of deep-sourced CO₂ losses are <3800 tonnes/a, based on stable isotope measurements of soil gases. Several holes up to 10 m deep were drilled on, and off the field for nested gas sampling of composition and stable C isotopic ratios for CO₂ and CH₄. Carbon-14 measurements on CO₂ from these holes indicate that deep-sourced CO₂ microseepage losses were approximately 170 tonnes/a.     

Flooding of the continental shelves as a contributor to deglacial CH4 rise

The transition from the last glacial and beginning of Bølling–Allerød and Pre‐Boreal periods in particular is marked by rapid increases in atmospheric methane (CH₄) concentrations. The CH₄ concentrations reached during these intervals, ～650–750 ppb, is twice that at the last glacial maximum and is not exceeded until the onset of industrialization at the end of the Holocene. Periods of rapid sea‐level rise as the Last Glacial Maximum ice sheets retreated and associated with ‘melt‐water pulses’ appear to coincide with the onset of elevated concentrations of CH₄, suggestive of a potential causative link. Here we identify and outline a mechanism involving the flooding of the continental shelves that were exposed and vegetated during the glacial sea‐level low stand and that can help account for some of these observations. Specifically, we hypothesize that waterlogging (and later, flooding) of large tracts of forest and savanna in the Tropics and Subtropics during the deglacial transition and early Holocene would have resulted in rapid anaerobic decomposition of standing biomass and emission of methane to the atmosphere. This novel mechanism, akin to the consequences of filling new hydroelectric reservoirs, provides a mechanistic explanation for the apparent synchronicity between rate of sea‐level rise and occurrence of elevated concentrations of ice core CH₄. However, shelf flooding and the creation of transient wetlands are unlikely to explain more than ～60 ppb of the increase in atmospheric CH₄ during the deglacial transition, requiring additional mechanisms to explain the bulk of the glacial to interglacial increase. Similarly, this mechanism has the potential also to play some role in the rapid changes in atmospheric methane associated with the Dansgaard–Oeschger cycles. Copyright © 2012 John Wiley & Sons, Ltd.     

Gas flux to the atmosphere from mud volcanoes in eastern Romania

Gas flux measurements have for the first time been taken from vents and soil of eastern Romania mud volcanoes, the largest geological structures in Europe releasing methane into the atmosphere. In the quiescent phase, the methane emission from single vents is up to 28 t yr^?1. Diffuse soil microseepage is of the order of 10²?10⁵ mg m^?2 day^?1. A total output of at least 1200 tonnes of CH₄ per year can be conservatively estimated over the area investigated alone (～ 2.3 km²). Helium fluxes are up to five orders of magnitude higher than the average flux in a stable continental area, pointing to a close link between mud volcanoes and crustal degassing through faults crossing the deep hydrocarbon reservoirs. These data represent a key contribution towards refining global CH₄‐emission estimates, which indicate mud volcanoes as a significant and unavoidable source of greenhouse gases for the atmosphere.     

Eddy covariance based methane flux in Sundarbans mangroves,India

We report the initial results of the methane flux measured using eddy covariance method during summer months from the world’s largest mangrove ecosystem, Sundarbans of India. Mangrove ecosystems are known sources for methane (CH₄) having very high global warming potential. In order to quantify the methane flux in mangroves, an eddy covariance flux tower was recently erected in the largest unpolluted and undisturbed mangrove ecosystem in Sundarbans (India). The tower is equipped with eddy covariance flux tower instruments to continuously measure methane fluxes besides the mass and energy fluxes. This paper presents the preliminary results of methane flux variations during summer months (i.e., April and May 2012) in Sundarbans mangrove ecosystem. The mean concentrations of CH₄ emission over the study period was 1682 ± 956 ppb. The measured CH₄ fluxes computed from eddy covariance technique showed that the study area acts as a net source for CH₄ with daily mean flux of 150.22 ± 248.87 mg m^?2 day^?1. The methane emission as well as its flux showed very high variability diurnally. Though the environmental conditions controlling methane emission is not yet fully understood, an attempt has been made in the present study to analyse the relationships of methane efflux with tidal activity. This present study is part of Indian Space Research Organisation–Geosphere Biosphere Program (ISRO–GBP) initiative under ‘National Carbon Project’.     

Potential of microbial methane formation in a high-temperature hydrocarbon seep

Hydrocarbon seepage is a surface expression where fluids mixed with sediments and hydrocarbons are expelled through fracture systems that potentially tap into gas–petroleum reservoirs. Hydrocarbons released from most seeps appear to be thermogenic on the basis of their relative abundance and isotopic composition. The potential for subsurface microbial processes modifying these geochemical fingerprints remains poorly constrained. In this study, microcosm incubations were conducted on mud slurries supplied with/without various methanogenic precursors at temperatures ranging from ambient conditions to 90 °C, in order to assess microbial CH₄ formation in the subsurface beneath hydrocarbon seeps. The analyses indicated that CH₄ production was positive at ?80 °C, regardless of whether or not or which precursors were added. However, the pattern of CH₄ production rates varied with the precursor and temperature. In general, the optimum CH₄ production from H₂/CO₂ and formate occurred over a wide range of temperatures (?40 °C), whereas that from acetate, methanol and methylamine was restricted to relatively lower temperatures (40–50 °C). The CH₄ recoveries, together with the C isotopic compositions of CH₄, further indicated that the quantities of CH₄ produced could not completely account for the quantities of precursor consumed, suggesting that a complex metabolic network was involved in the transformation of the added precursor and organic C inherited from inoculated sediments. Microbial CH₄ was estimated to constitute 7–61% of the CH₄ observed using experimentally-derived apparent isotope fractionations as the end member compositions. This illustrates the possibility that microbial CH₄ produced at shallower depths could quantitatively and isotopically alter deeply-sourced thermogenic CH₄ in hydrocarbon seep environments.     

Chemical and isotope compositions of shallow groundwater in areas impacted by hydraulic fracturing and surface mining in the Central Appalachian Basin,Eastern United States

Hydraulic fracturing of shale deposits has greatly increased the productivity of the natural gas industry by allowing it to exploit previously inaccessible reservoirs. Previous research has demonstrated that this practice has the potential to contaminate shallow aquifers with methane (CH₄) from deeper formations. This study compares concentrations and isotopic compositions of CH₄ sampled from domestic groundwater wells in Letcher County, Eastern Kentucky in order to characterize its occurrence and origins in relation to both neighboring hydraulically fractured natural gas wells and surface coal mines. The studied groundwater showed concentrations of CH₄ ranging from 0.05 mg/L to 10 mg/L, thus, no immediate remediation is required. The δ¹³C values of CH₄ ranged from −66‰ to −16‰, and δ²H values ranged from −286‰ to −86‰, suggesting an immature thermogenic and mixed biogenic/thermogenic origin. The occurrence of CH₄ was not correlated with proximity to hydraulically fractured natural gas wells. Generally, CH₄ occurrence corresponded with groundwater abundant in Na⁺, Cl⁻, and HCO₃⁻, and with low concentrations of SO₄²⁻. The CH₄ and SO₄²⁻concentrations were best predicted by the oxidation/reduction potential of the studied groundwater. CH₄ was abundant in more reducing waters, and SO₄²⁻ was abundant in more oxidizing waters. Additionally, groundwater in greater proximity to surface mining was more likely to be oxidized. This, in turn, might have increased the likelihood of CH₄ oxidation in shallow groundwater.     

Comparison study of methane emissions from landfills with different landfill covers

Atmospheric methane, a more effective heat-trapping gas than CO₂ that may affect climate change, has its greatest man-made source in the US from municipal solid waste (MSW) landfills. Consequently, the wise management of landfills can reduce these greenhouse gas emissions to the atmosphere. Methane from modern MSW landfills built with composite covers is frequently vented directly to the atmosphere. Biofiltration of landfill gas could oxidize CH₄ to CO₂ and water. Methane oxidation in old landfills with conventional soil covers can be effective in reducing the amount of CH₄ emitted. In this study, comparison of methane emissions from three different landfill covers was conducted. Methane emissions from old landfills constructed with conventional soil covers, modern MSW landfills constructed with composite covers, and modern MSW landfills constructed with composite covers plus biofilters were calculated using the calculated CH₄ oxidation rates. The results showed that an average of only 14% of the generated CH₄ was emitted from landfills with modern composite covers plus biofilters, and an average of 85% of the generated CH₄ was emitted from landfills with conventional covers when 100% of the generated CH₄ emissions to the atmosphere from landfills with modern composite covers was assumed. By comparing the CH₄ emission rates from three different landfill types, the use of a properly sized biofilter should be an effective technique to reduce CH₄ emissions from landfills across the USA and potentially in many other areas of the world.     

Spatial and temporal distribution of methane in an extensive shallow estuary, South India

Sedimentary methane (CH₄) fluxes and oxidation rates were determined over the wet and dry seasons (four measurement campaigns) in Pulicat lake, an extensive shallow estuary in south India. Dissolved CH₄ concentrations were measured at 52 locations in December 2000. The annual mean net CH₄ flux from Pulicat lake sediments was 3.7 × 10⁹ g yr^-1 based on static chamber measurements. A further 1.7 × 10⁹g yr_-1 was estimated to be oxidized at the sediment-water interface. The mean dissolved concentration of CH₄ was 242nmol |^-1 (ranging between 94 and 501 nmol |^-1) and the spatial distribution could be explained by tidal dynamics and freshwater input. Sea-air exchange estimates using models, account only for ∼13% (0.5 × 10⁹ g yr-1) of the total CH₄ produced in sediments, whereas ebullition appeared to be the major route for loss to the atmosphere (∼ 63% of the net sediment flux). We estimated the total atmospheric source of CH₄ from Pulicat lake to be 0.5 to 4.0 × 10⁹g yr^-1.     

Methane microseepage from different sectors of the Yakela condensed gas field in Tarim Basin,Xinjiang, China

Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in commercial petroleum fields. While the role of microseepage as a pathfinder in petroleum exploration has been known for about 80 a, its significance as an atmospheric CH₄ source has only recently been studied, and flux data are currently available only in the USA and Europe. With the aim of increasing the global data-set and better understanding flux magnitudes and variabilities, microseepage is now being extensively studied in China. A static flux chamber method was recently applied to study microseepage emissions into the atmosphere in four different sectors of the Yakela condensed gas field in Tarim Basin, Xinjiang, China, and specifically in: (a) a faulted sector, across the Luntai fault systems; (b) an oil–water interface sector, at the northern margin of the field; (c) an oil–gas interface sector, in the middle of the field; (d) an external area, outside the northern gas field boundaries. The results show that positive CH₄ fluxes are pervasive in all sectors and therefore, only part of the CH₄ migrating from the deep oil–gas reservoirs is consumed in the soil by methanotrophic oxidation. The intensity of gas seepage seems to be controlled by subsurface geologic settings and lateral variabilities of natural gas pressure in the condensed gas field. The highest CH₄ fluxes, up to ∼14 mg m⁻² d⁻¹ (mean of 7.55 mg m⁻² d⁻¹) with higher spatial variability (standard deviation, σ: 2.58 mg m⁻² d⁻¹), occur in the Luntai fault sector. Merhane flux was lower in the oil–water area (mean of 0.53 mg m⁻² d⁻¹) and the external area (mean of 1.55 mg m⁻² d⁻¹), and at the intermediate level in the gas–oil sector (mean of 2.89 mg m⁻² d⁻¹). These values are consistent with microseepage data reported for petroleum basins in the USA and Europe. The build-up of methane concentration in the flux chambers is always coupled with an enrichment of ¹³C, from δ¹³C₁ of −46‰ to −42.5‰ (VPDB), which demonstrates that seeping methane is thermogenic, as that occurring in the deep Yakela reservoir. Daily variations of microseepage are very low, with minima in the afternoon, corresponding to higher soil temperature (and higher methanotrophic consumption), and maxima in the early morning (when soil temperatures are lowest). A preliminary and rough estimate of the total amount of CH₄ exhaled from the Yakela field is in the order of 10² tonnes a⁻¹.     

Global Nitrogen Cycle: Pre-Anthropocene Mass and Isotope Fluxes and the Effects of Human Perturbations

We present a nitrogen cycle model for pre-industrial times based on an extensive literature database. The model consists of 18 reservoirs in the domains of the atmosphere, land, and ocean. The biotic reservoirs on land and in the ocean (N-fixing plants, non-N-fixing plants, and marine biota) interact with atmospheric N₂ and dissolved inorganic nitrogen (DIN, consisting of N₂, NO₃ ^?, and NH₄ ⁺) in the ocean and soil waters. Marine DIN is taken up by marine biota and transformed from ocean particulate organic matter to dissolved organic nitrogen and the ocean sediment. The atmosphere, the largest nitrogen reservoir, supplies N₂ to the system by N fixation, deposition, and dissolution, and these input fluxes are balanced by denitrification and volatilization back to the atmosphere. The land and ocean domains are linked by river transport, which carries both dissolved and particulate nitrogen to the oceanic coastal zone. The isotope–mass balances of the N reservoirs are calculated from the isotopic composition of the reservoirs and the fractionation factors accompanying the fluxes between the reservoirs based on reported values from different natural conditions. The model sensitivity was tested for different biouptake rates and was run with various human perturbations, including fertilization, nitrous oxide emissions, population-related sewage disposal, land-use changes, and temperature-dependent rate kinetics. The new N mass–isotope cycle model provides the basis for assessment of the impact of artificial fertilization between 1700 and 2050. The perturbation experiments in this study suggest that land-use change is the key factor altering the N mass cycle since industrialization.     

Methane flux from mangrove sediments along the Southwestern coast of Puerto Rico

Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH₄ m^?2 d^?1, 42 mg CH₄ m^?2 d^?1, and 82 mg CH₄ m^?2 d^?1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH₄ flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.     

Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters,Butte, Montana

Samples of mine water from Butte, Montana were collected for paired geochemical and stable isotopic analysis. The samples included two sets of depth profiles from the acidic Berkeley pit lake, deep groundwater from several mine shafts in the adjacent flooded underground mine workings, and the acidic Horseshoe Bend Spring. Beginning in July-2000, the spring was a major surface water input into the Berkeley pit lake. Vertical trends in major ions and heavy metals in the pit lake show major changes across a chemocline at 10–20 m depth. The chemocline most likely represents the boundary between pre-2000 and post-2000 lake water, with lower salinity, modified Horseshoe Bend Spring water on top of higher salinity lake water below. Based on stable isotope results, the deep pit lake has lost approximately 12% of its initial water to evaporation, while the shallow lake is up to 25% evaporated. The stable isotopic composition of SO₄ in the pit lake is similar to that of Horseshoe Bend Spring, but differs markedly from SO₄ in the surrounding flooded mine shafts. The latter is heavier in both δ³⁴S and δ¹⁸O, which may be due to dissolution of hypogene SO₄ minerals (anhydrite, gypsum, barite) in the ore deposit. The isotopic and geochemical evidence suggests that much of the SO₄ and dissolved heavy metals in the deep Berkeley pit lake were generated in situ, either by leaching of soluble salts from the weathered pit walls as the lake waters rose, or by subaqueous oxidation of pyrite on the submerged mine walls by dissolved Fe(III). Laboratory experiments were performed to contrast the isotopic composition of SO₄ formed by aerobic leaching of weathered wallrock vs. SO₄ from anaerobic pyrite oxidation. The results suggest that both processes were likely important in the evolution of the Berkeley pit lake.     

The crucial role of deep-sourced methane in maintaining the subseafloor sulfate budget

Methane (CH₄) is a powerful greenhouse gas and its largest reservoir on Earth is held in marine sediments. CH₄ in marine sediments is mainly stored in gas-hydrate reservoirs and deep sedimentary strata along continental margins, where large amounts of deep-sourced CH₄ ascend to different degrees toward the seafloor. However, the amount of deep-sourced CH₄ and its role in subseafloor carbon and sulfur cycling remains poorly constrained. We analyzed sulfate (SO₄^2?) profiles of 157 sites along with previous published 85 sites to determine the regional distribution and amount of SO₄^2? reduction for an area of 1.23 × 10⁵ km² of the northern South China Sea. Then we compared these obtained results with estimates based on sedimentation rates from the same area. Significantly higher regional SO₄^2? flux estimates based on SO₄^2? profiles (4.26 × 10^?3 Tmol a^?1), compared to lower estimates based on sedimentation rates (1.23 × 10^?3 Tmol a^?1), reflect abundant ascending deep-sourced CH₄. The difference of the regional SO₄^2? flux estimates (3.03 × 10^?3 Tmol a^?1) represents the amount of SO₄^2? reduced by CH₄ through the anaerobic oxidation of CH₄ (AOM). Deep-sourced CH₄ contributes 71% to total SO₄^2? consumption in the study area, largely exceeding SO₄^2? consumption by organoclastic sulfate reduction. Our findings substantiate that deep-sourced CH₄ governs subseafloor carbon and sulfur cycling to a previously underrated extent, fueling extensive chemosynthesis-based ecosystems along continental slope and rise.     

Temporal change of C-isotope signatures and methanogenic pathways in rice field soil incubated anoxically at different temperatures

Production of CH₄ and CO₂ was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37°C. The δ¹³C of the accumulated CO₂, CH₄ and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH₄ production. The calculations were further constrained by the determination of ¹⁴CH₄ production from ¹⁴CO₂ at steady state. At 50°C, CH₄ was exclusively produced from CO₂, indicating a fractionation factor of α_CO2/CH4 = 1.073. Between 10 and 37°C, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO₂-derived fraction of CH₄ production in the beginning was followed by a phase in which contribution of CO₂-derived CH₄ decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of ¹³C during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation.     

Biodegradation of CO2, CH4 and volatile organic compounds (VOCs) in soil gas from the Vicano–Cimino hydrothermal system (central Italy)

We investigated the effect of microbial activity on the chemistry of hydrothermal fluids related to the Vicano–Cimino system, central Italy. The database included the composition and δ¹³C CO₂ and δ¹³C CH₄ values for soil gas from an area characterized by intense degassing of fluids having a deep origin. The δ¹³C CH₄ values along vertical profiles in the soil indicated that CH₄ was controlled by microbial oxidation occurring at shallow (< 50 cm) depth, where free O₂ was available. This was consistent with the vertical gradients of CH₄, H₂S and O₂ concentrations. The δ¹³C CO₂ values in soil gas, characterized by a composition similar to that of the hydrothermal fluids, were not significantly influenced by biodegradation. On the contrary, gas strongly affected by air contamination showed a significant δ¹³C CO₂ fractionation. Microbial activity caused strong consumption of hydrothermal alkanes, alkenes, cyclics and hydrogenated halocarbons, whereas benzene was recalcitrant. Oxygenated compounds from hydrocarbon degradation consisted of alcohols, with minor aldehydes, ketones and carboxylic acids. A predominance of alcohols at a high rate of degassing flux, corresponding to a short residence time of hydrothermal gas within the soil, indicated incomplete oxidation. N-bearing compounds were likely produced by humic substances in the soil and/or related to contamination by pesticides, whereas α-pinene traced air entering the soil. The study demonstrates that microbial communities in the soil play an important role for mitigating the release to the atmosphere of C-bearing gases, especially CH₄, through diffuse soil degassing, a mechanism that in central Italy significantly contributes to the discharge of CO₂-rich gas from deep sources.