Oxidation of methane in the water column of Lake Baikal

The rate of aerobic oxidation of methane was calculated based on average profiles of the tritiumhelium age of the Baikal waters and concentrations of the dissolved methane in the water column. In the deep lake zone (>200 m), the intensity of oxidation vertically decreases and is (2–0.3) × 10^?2 nl CH₄l^?1 days^?1 in southern and central Baikal and (2.8–1.0) × 10^?2 nl CH₄ l^?1 days^?1 in northern Baikal. The effective coefficient of the oxidation rate in the lake depressions is 3.6 × 10^?4, 3.3 × 10^?4, and 3.7 × 10^?4 days^?1, respectively. At current methane concentrations in the water column, about 80 t of methane is oxidized per year. Oxidation of the dissolved methane in the water column was estimated at a possible increase of its concentration.     

天然气水合物成因探讨

天然气水合物是未来的能源资源。其分布于极地地区、深海地区及深水湖泊中。在海洋里,天然气水合物主要分布于外大陆边缘和洋岛的周围,其分布与近代火山的分布范围具有一致性。同位素组成表明天然气水合物甲烷主要是由自养产甲烷菌还原CO₂形成的。典型的大陆边缘沉积物有机碳含量低（＜0．5％～1．0％）,不足以产生天然气水合物带高含量的甲烷。赋存天然气水合物的沉积物时代主要为晚中新世-晚上新世,具有一定的时限性,并且天然气水合物与火山灰或火山砂共存,表明其形成与火山-热液体系有一定联系。火山与天然气水合物空间上的一致性表明,天然气水合物甲烷的底物可能主要是由洋底火山喷发带来的CO2。由前人研究结果推断 HCO^－₃在脱去两个O原子的同时,可能发生了亲核重排,羟基 H原子迁移到 C原子上,形成了甲酰基（HCO^－）,使甲烷的第一个 H原子来源于水。探讨了甲烷及其水合物的形成机制,提出了天然气水合物成因模型。     

Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge)

Marine sediment sequences with CH₄ hydrate are characterized by an atypical depth profile in dissolved Cl⁻ squeezed from pore space: a shallow subsurface Cl⁻ maximum overlies a lengthy and pronounced Cl⁻ minimum. This pore water Cl⁻ profile represents a combination of multiple processes including glacial–interglacial variations in ocean salinity, advection and diffusion of ions that are excluded during gas hydrate formation at depth, and release of fresh water from dissociation of hydrate during core recovery. In situ quantities of gas hydrate can be determined from a measured pore water Cl⁻ profile provided the in situ pore water signature prior to core recovery can be separated. Ocean Drilling Program (ODP) Site 997 was drilled into a large CH₄ hydrate reservoir on the Blake Ridge in the western Atlantic Ocean. Previously we have constructed a high-resolution pore water Cl⁻ profile at this location; here we present a `coupled chloride-hydrate' numerical model to explain basic trends in the Cl⁻ profile and to isolate in situ Cl⁻ concentrations. The model is based on thermodynamic and ecological considerations, and uses established equations for describing chemical behavior in marine sediment–pore water systems. The model incorporates four key concepts: (1) most gas hydrate is formed immediately below the SO₄²⁻ reduction zone; (2) fluid, dissolved ions and gas advect upward through the sediment column; (3) CH₄ hydrate dissociates at the base of hydrate stability conditions; and (4) seawater salinity fluctuates during glacial–interglacial cycles of the late Pliocene and Quaternary. Rates of upward advection in the model are sufficient to account for measured Br⁻ and I⁻ concentrations as well as CH₄ oxidation at the base of the SO₄²⁻ reduction zone. In situ pore water Cl⁻ inferred from the model is similar to that determined by limited direct sampling; in situ CH₄ hydrate amounts inferred from the model (an average of about 4% of porosity) are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques. The model also predicts production of substantial quantities of free CH₄ gas bubbles (>2.5% of porosity) at a depth immediately below the lowest accumulation of CH₄ hydrate in the sediment column. Our explanation for the pore water Cl⁻ profile at Site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water Cl⁻, gas hydrate, and free gas in a marine sediment column.     

Methane in aging hydrothermal plumes

Methane concentration and the stable carbon isotopic composition (δ ¹³C-CH₄) were measured in aging hydrothermal plume waters originating at the Endeavour Segment, Juan de Fuca Ridge. CH₄ concentrations decreased systematically from 600 nM at on-axis stations to <11 nM at a distal station 15 km off axis; background CH₄ concentrations are <2 nM. CH₄ behaves nonconservatively in plume waters and does not correlate with conservative parameters such as temperature anomaly (Δθ), but is highly correlated with NH₄⁺ and total dissolvable Mn. δ ¹³C-CH₄ values for plume depth samples varies inversely and significantly (R² = 0.89) with methane concentrations normalized to Δθ. Some δ ¹³C-CH₄ values (+1.8 and +10.9 ‰) measured at the 15 km off-axis station are among the heaviest yet reported from a natural marine environment. Less than 5% of original hydrothermal methane remains in the plume at this station. The data are consistent with extensive microbial methane oxidation. A narrow range of fractionation factors (r_c ≈1.0072 to 1.0077) was calculated for the Endeavour samples. These fractionation factors are less than those reported by Coleman et al. (1981), but fall near the trend line of their r_c versus temperature data when extrapolated to plume water temperature (∼2 °C).     

Sources and flux of natural gases from Mono Lake,California

The ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present. Four primary sources of methane have been identified in Mono Lake. Two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary; the two other gas sources result from current microbiological processes. In the natural gas seeps, we observed flow rates as high as 160 moles CH₄ day⁻¹, and estimate total lakewide annual seep flux to be 2.1 × 10⁶ moles CH₄. Geochemical parameters (δ¹³CH₄,δDCH₄,CH₄/[C₂H₆+ C₃H₈]) andδ¹⁴CH₄measurements revealed that most of the seeps originate from a paleo-biogenic (δ¹³CH₄ = about −70%.). natural gas deposit of Pleistocene age which underlies the current and former lakebed. Gas seeps in the vicinity of hot springs had, in combination with the biogenic gas, a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter.Current microbiological processes responsible for sources of natural gas in the lake included pelagic meth- anogenesis and decomposition of terrestrial grasses in the littoral zone. Methanogenesis in the pelagic sediments resulted in methane saturation (2–3 mM at 50 cm; δ₁₃CH₄ = about −85%.). Interstitial sulfate decreased from 133 mM at the surface to 35 mM by 110 cm depth, indicating that sulfate-reduction and methanogenesis operated concurrently. Methane diffused out of the sediments resulting in concentrations of about 50 μM in the anoxic bottom waters. Methane oxidation in the oxic/anoxic boundry lowered the concentration by >98%, but values in surface waters (0.1–1.3μM) were supersaturated with respect to the atmosphere. The δ¹³CH₄ (range = −21.8 to −71.8%.) of this unoxidized residual methane was enriched in ¹³C relative to methane in the bottom water and sediments. Average outward flux of this methane was 2.77 × 10⁷ moles yr⁻¹. A fourth, but minor source of methane (δ¹³CH₄ = −55.2%.) was associated with the decomposition of terrestrial grasses taking place in the lake's recently expanded littoral zone.     

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.     

Natural seabed gas seeps as sources of atmospheric methane

Microbial and thermogenic methane migrates towards the seabed where some is utilised during microbially-mediated anaerobic oxidation. Excess methane escapes as gas seeps, which occur in a variety of geological contexts in every sea and ocean, from inter-tidal zones to deep ocean trenches. Some seeps are localised, gentle emanations; others are vigorous covering areas of >1 km²; the most prolific seeps reported (offshore Georgia) produce ~40 t CH₄ per year. Gas bubbles lose methane to the water as they rise, so deep water seeps are unlikely to contribute to the atmosphere. However, bubbles break the surface above some shallow water seeps. Estimates of the total methane contribution to the atmosphere are poorly constrained, largely because the data set is so small. 20 Tg yr^–1 is considered a realistic first approximation. This is a significant contribution to the global budget, particularly as methane from seeps is ¹⁴C-depleted. A seep measurement programme is urgently required.GEM     

Enrichment of nitrogen and 13C of methane in natural gases from the Khuff Formation,Saudi Arabia,caused by thermochemical sulfate reduction

Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N₂/CH₄, CO₂/CH₄ and H₂S/CH₄ suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ¹³C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ¹³C between −3‰ and 0‰ VPDB. As H₂S/CH₄ increases, methane δ¹³C increases to as much as −3‰ and carbon dioxide δ¹³C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide.Khuff reservoir temperatures, which locally exceed 150 °C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N₂/CH₄ of the residual gas and leave δ¹⁵N unaltered. δ¹⁵N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N₂/CH₄ in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ¹⁵N (0.3–0.5‰) is trivial.Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ¹³C versus log(N₂/CH₄). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.     

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.     

Spatial variation in fluid flow and geochemical fluxes across the sediment-seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment-seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34-180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr⁻¹ at the apex of the MV but they rapidly decrease to 0.4 cm yr⁻¹ at the periphery. The associated fluxes of boron, lithium and methane vary between 7-301, 0.5-6 and 0-806 mmol m⁻² yr⁻¹, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy.     

Interaction of Biogeochemical Sulfur and Carbon Cycles in Marine Basins by the Example of the Black Sea

The interaction of biogeochemical sulfur and carbon cycles in water bodies has been studied by the different authors and methods, including the use of ³⁵S and ¹⁴C radioactive tracers. The main reactions of sulfur and carbon interaction by the example of the Black Sea (sulfate reduction, methanogenesis, aerobic and anaerobic methane oxidation) are considered. Based on the experimental radioisotope data, the annual microbial production of H₂S and CH₄ in the water column of the Black Sea is estimated. The average annual production of CH₄ is 3.8% of the annual production of H₂S. The share of migration methane from the bottom sediments (seeps, mud volcanoes, etc.) is approximately 2% of the total dissolved methane pool.

     

Abiotic methane seepage in the Ronda peridotite massif,southern Spain

Abiotic methane in serpentinized peridotites (MSP) has implications for energy resource exploration, planetary geology, subsurface microbiology and astrobiology. Once considered a rare occurrence on Earth, reports of MSP are increasing for numerous localities worldwide in low temperature, land-based springs and seeps. We report the discovery of six methane-rich water springs and two ponds with active gas bubbling in the Ronda peridotite massif, in southern Spain. Water is hyperalkaline with typical hydrochemical features of active serpentinization (pH: 10.7 to 11.7, T: 17.1 to 21.5 °C, Ca–OH facies). Dissolved CH₄ concentrations range from 0.1 to 3.2 mg/L. The methane stable C and H isotope ratios in the natural spring and bubbling sites (δ¹³C_CH4: −12.3 to −37‰ VPDB; δ²H_CH4: −280 to −333‰ VSMOW) indicate a predominant abiotic origin. In contrast, springs with manmade water systems, i.e., pipes or fountains, appear to have mixed biotic-abiotic origin (δ¹³C_CH4: −44 to −69‰; δ²H_CH4: −180 to −319‰). Radiocarbon (¹⁴C) analyses show that methane C in a natural spring is older than ca. 50,000 y BP, whereas dissolved inorganic carbon (DIC) analysed in all springs has an apparent ¹⁴C age ranging from modern to 2334 y BP. Therefore most, if not all, of the CH₄ is allochthonous, i.e., not generated from the carbon in the hyperalkaline water. Methane is also released as bubbles in natural ponds and as diffuse seepages (∼10¹–10² mg CH₄ m⁻²d⁻¹) from the ground up to several tens of metres from the seeps and springs, albeit with no overt visual evidence. These data suggest that the gas follows independent migration pathways, potentially along faults or fracture systems, physically isolated from the hyperalkaline springs. Methane does not seem to be genetically related to the hyperalkaline water, which may only act as a carrier of the gas. Gas-bearing springs, vents and invisible microseepage in land-based peridotites are more common than previously thought. In addition to other geological sources, MSP is potentially a natural source of methane for the troposphere and requires more worldwide flux measurements.     

Fluid flow, methane fluxes, carbonate precipitation and biogeochemical turnover in gas hydrate-bearing sediments at Hydrate Ridge, Cascadia Margin: numerical modeling and mass balances

A numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate-bearing surface sediments from a cold vent site situated at Hydrate Ridge, an accretionary structure located in the Cascadia Margin subduction zone. Steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below. The model results underline the dominance of advective fluid flow that forces a large inflow of methane from below (869 μmol cm⁻² a⁻¹) inducing high oxidation rates in the surface layers. Anaerobic methane oxidation is the major process, proceeding at a depth-integrated rate of 870 μmol cm⁻² a⁻¹. A significant fraction (14%) of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite. The total rate of carbonate precipitation (120 μmol cm⁻² a⁻¹) allows for the build-up of a massive carbonate layer with a thickness of 1 m over a period of 20,000 years. Aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane-charge fluids is high enough (≥10 cm a⁻¹) to maintain super-saturation with respect to this highly soluble carbonate phase. It precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments, suggesting microbial catalysis. The investigated station is characterized by high carbon and oxygen turnover rates (≈1000 μmol cm⁻² a⁻¹) that are well beyond the rates observed at other continental slope sites not affected by fluid venting. This underlines the strong impact of fluid venting on the benthic system, even though the flow velocity of 10 cm a⁻¹ derived by the model is relative low compared to fluid flow rates found at other cold vent sites. Non-steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow. Moreover, they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities (<80 cm a⁻¹). Only at flow rates exceeding approximately 100 cm a⁻¹, does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 μmol CH₄ cm² a⁻¹ into the overlying bottom water.     

The origin of high sulfate concentrations in a coastal plain aquifer,Long Island,New York

Ion-exchange batch experiments were run on Cretaceous (Magothy aquifer) clay cores from a nearshore borehole and an inland borehole on Long Island, NY, to determine the origin of high SO₄²⁻ concentrations in ground water. Desorption batch tests indicate that the amounts of SO₄²⁻ released from the core samples are much greater (980–4700 μg/g of sediment) than the concentrations in ground-water samples. The locally high SO₄²⁻ concentrations in pore water extracted from cores are consistent with the overall increase in SO₄²⁻ concentrations in ground water along Magothy flow paths. Results of the sorption batch tests indicate that SO₄²⁻ sorption onto clay is small but significant (40–120 μg/g of sediment) in the low-pH (<5) pore water of clays, and a significant part of the SO₄²⁻ in Magothy pore water may result from the oxidation of FeS₂ by dissolved Fe(III). The acidic conditions that result from FeS₂ oxidation in acidic pore water should result in greater sorption of SO₄²⁻ and other anions onto protonated surfaces than in neutral-pH pore water. Comparison of the amounts of Cl⁻ released from a clay core sample in desorption batch tests (4 μg/g of sediment) with the amounts of Cl⁻ sorbed to the same clay in sorption tests (3.7–5 μg/g) indicates that the high concentrations of Cl⁻ in pore water did not originate from connate seawater but were desorbed from sediment that was previously in contact with seawater. Furthermore, a hypothetical seawater transgression in the past is consistent with the observed pattern of sorbed cation complexes in the Magothy cores and could be a significant source of high SO₄²⁻ concentrations in Magothy ground water.     

Methane production in Mississippi River deltaic plain peat

Seasonal methane production in a coastal peat deposit was studied. Dissolved methane content in interstitial water increased with increasing depth. Methane production rate was related to temperature in the 0–40 cm depth, but was relatively constant at depths below 40 cm. Maximal methane content was found in spring and lowest content in November to January. Production rates were pH dependent, with an optimum rate of methanogenesis at pH 7.7. The accumulation of metabolic end products produced within the peat-water matrix appear to be important in limiting methanogenesis in wetland ecosystems. The reserve of methane in the interstitial water of the brackish marsh was estimated to be 15.8 g CH₄ m⁻² or 221 CH₄ per m⁻². Results suggest that the vast peat deposits in the Mississippi deltaic plain could possibly be a potential source of renewable energy if in situ method for removal of the produced methane could be developed.     

Dissolution rates of pure methane hydrate and carbon-dioxide hydrate in undersaturated seawater at 1000-m depth

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH₄ and CO₂ hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10⁻² μm/s for the CO₂ and CH₄ hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO₂/m²s and 0.37 ± 0.03 mmol CH₄/m²s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH₄) or tens of mm (CO₂) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO₂ hydrates in marine carbon sequestration strategies.     

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.     

Iodine diagenesis in pelagic deep-sea sediments

Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10⁻⁴ in excess of previous reported values for planktonic material (~1 × 10⁻⁴). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10⁻⁶ yr⁻¹ which vary with the carbon accumulation rate.Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I⁻ production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I⁻ oxidation where interconversion of I⁻ to IO⁻₃ occurs; 3. a zone of IO⁻₃ reduction where interconversion of IO⁻₃ back to I⁻ occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I⁻ production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10⁻⁸ μmol cm⁻² sec⁻, respectively.In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I⁻ from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10⁻⁴ are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO₂ reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I⁻ production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.     

A large methane plume east of Bear Island (Barents Sea): implications for the marine methane cycle

A pockmark field extending over 35 km² at 74°54N, 27°3E, described by Solheim and Elverhøi (1993), was re-surveyed and found to be covered with more than 30 steep-sided craters between 300 and 700 m in diameter and up to 28 m deep. The craters are thought to have been formed by an explosive gas eruption. Anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today. Methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the Barents Sea shelf. From the characteristic vertical decrease of methane towards the sea surface, it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98% between 300 m depth and the sea surface. Degassing to the atmosphere is minimal based on the shape of the methane concentration gradient. Nevertheless, the net flux of methane from this area of the Barents Sea is about 2.9 × 10⁴ g CH₄ km^–2 yr^–1 and thus in the upper range of the presently estimated global marine methane release. This flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter. The amount of methane consumed in the water column, however, is about 50 times greater and hence should significantly contribute to the marine carbon inventory.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.