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1.
Dissolution of natural hydrate cores was measured using time-lapse photography on the seafloor at Barkley Canyon (850 m depth and 4.17 °C). Two types of hydrate fabrics in close contact with one another were studied: a “yellow” hydrate stained with condensate oil and a “white” hydrate. From thermogenic origins, both fabrics contained methane as well as heavier hydrocarbons. These multi-component hydrates were calculated to be well within p-T stability conditions (<200 m water depth needed at 4.17 °C). While stable in pressure and temperature, the hydrates were bathed in under-saturated seawater, which promoted dissolution. The flux of gas from the shrinking yellow hydrate core was 0.15 ± 0.01 mmol gas/m2 s, while the white hydrate dissolved faster at 0.25 ± 0.02 mmol gas/m2 s. To determine the controlling mechanism for the observed changes in the hydrate cores, experimental results were compared with an engineering correlation for convective mass transfer. Using water velocity as a fitting parameter, the correlation agreed well with results from a previous dissolution experiment on well-characterized synthetic hydrates. Even with a number of other unknowns, when applied to the natural hydrate, the mass transfer correlation predicted the dissolution rate within 20%. This seafloor-based experiment, along with visual observations of seafloor hydrate dissolution over a 3-day period, were used to further understand the fate of natural seafloor hydrates exposed on the seafloor. By showing that mass transfer is the rate-controlling mechanism for dissolution of these natural hydrate outcrops, proper hydrodynamic calculations can be employed to give a refined estimate on hydrate dissolution rates.  相似文献   

2.
《Applied Geochemistry》1995,10(4):461-475
The storage of CO2(liquid) on the seafloor has been proposed as a method of mitigating the accumulation of greenhouse gases in the Earth's atmosphere. Storage is possible below 3000 m water depth because the density of CO2(liquid) exceeds that of seawater and, thus, injected CO2(liquid) will remain as a stable, density stratified layer on the seafloor. The geochemical consequences of the storage of CO2(liquid) on the seafloor have been investigated using calculations of chemical equilibrium among complex aqueous solutions, gases, and minerals. At 3000 m water depth and 4°C, the stable phases are CO2(hydrate) and a brine. The hydrate composition is CO2·6.3H2O. The equilibrium composition of the brine is a 1.3 molal sodium-calcium-carbonate solution with pH ranging from 3.5 to 5.0. This acidified brine has a density of 1.04 g cm−3 and will displace normal seawater and react with underlying sediments. Seafloor sediment has an intrinsic capacity to neutralize the acid brine by dissolution of calcite and clay minerals and by incorporation of CO2 into carbonates including magnesite and dawsonite. Large volumes of acidified brine, however, can deplete the sediments buffer capacity, resulting in growth of additional CO2(hydrates) in the sediment. Volcanic sediments have the greatest buffer capacity whereas calcareous and siliceous oozes have the least capacity. The conditions that favor carbonate mineral stability and CO2(hydrates) stability are, in general, mutually exclusive although the two phases may coexist under restricted conditions.The brine is likely to cause mortality in both plant and animal comunities: it is acidic, it does not resemble seawater in composition, and it will have reduced capacity to hold oxygen because of the high solute content. Lack of oxygen will, consequently, produce anoxic conditions, however, the reduction of CO2 to CH4 is slow and redox disequilibrium mixtures of CO2 and CH4 are likely. Seismic or volcanic activity may cause conversion of CO2(liquid) to gas with potentially catastrophic release in a Lake Nyos-like event. The long term stability of the CO2(hydrate) may be limited: once isolated from the CO2(liquid) pool, either through burial or through depletion of the CO2 pool, the hydrate will decopose, releasing CO2 back into the sediment-water system.  相似文献   

3.
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.  相似文献   

4.
天然气水合物成因探讨   总被引：18，自引：0，他引：18  
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。  相似文献   

5.
We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2+ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure.  相似文献   

6.
The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of 3H-CH4.Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths <370 m), where methane concentration was low (3–10 nM). Methane turnover time averaged 1.5 yr in the deep water but was on the order of decades in the upper portion of the water column. The depth-integrated water column methane oxidation rates for the deep water averaged 5.2 mmol CH4 m−2 yr−1, whereas the upper portion of the water column averaged only 0.14 mmol CH4 m−2 yr−1; the depth-integrated oxidation rate for deep water in the 25-km2 area encompassing the venting field averaged 2 × 106 g CH4 yr−1. Stable isotope values (δ13C-CH4) for individual samples ranged from −34 to −52‰ (vs. PDB, Peedee belemnite standard) in the region. These values are isotopically enriched relative to hydrates in the region (δ13C-CH4 about −57 to −69‰), further supporting our observations of extensive methane oxidation in this environment.  相似文献   

7.
Presented is an improved model for the prediction of phase equilibria and cage occupancy of CH4 and CO2 hydrate in aqueous systems. Different from most hydrate models that employ Kihara potential or Lennard-Jones potential with parameters derived from experimental phase equilibrium data of hydrates, we use atomic site-site potentials to account for the angle-dependent molecular interactions with parameters directly from ab initio calculation results. Because of this treatment, our model can predict the phase equilibria of CH4 hydrate and CO2 hydrate in binary systems over a wide temperature-pressure range (from 243-318 K, and from 10-3000 bar for CH4 hydrate; from 253-293 K, and from 5-2000 bar for CO2 hydrate) with accuracy close to experiment. The average deviation of this model from experimental data is less than 3% in pressures for a given temperature. This accuracy is similar to previous models for pressures below 500 bar, but is more accurate than previous models at higher pressures. This model is also capable of predicting the cage occupancy and hydration number for CH4 hydrate and CO2 hydrate without fitting any experimental data. The success of this study validates the predictability of ab initio intermolecular potentials for thermodynamic properties.  相似文献   

8.
The Shenhu gas hydrate drilling area is located in the central Baiyun sag, Zhu Ⅱ depression, Pearl River Mouth basin, northern South China Sea. The gas compositions contained in the hydrate-bearing zones is dominated by methane with content up to 99.89% and 99.91%. The carbon isotope of the methane (δ13C1 ) are 56.7‰ and 60.9‰, and its hydrogen isotope (δD) are 199‰ and 180‰, respectively, indicating the methane from the microbial reduction of CO2 . Based on the data of measured seafloor temperature and geothermal gradient, the gas formed hydrate reservoirs are from depths 24-1699 m below the seafloor, and main gas-generation zone is present at the depth interval of 416-1165 m. Gas-bearing zones include the Hanjiang Formation, Yuehai Formation, Wanshan Formation and Quaternary sediments. We infer that the microbial gas migrated laterally or vertically along faults (especially interlayer faults), slump structures, small-scale diapiric structures, regional sand beds and sedimentary boundaries to the hydrate stability zone, and formed natural gas hydrates in the upper Yuehai Formation and lower Wanshan Formation, probably with contribution of a little thermogenic gas from the deep sedments during this process.  相似文献   

9.
Blake Ridge hosts an extensive gas hydrate system where escaping CH4 is consumed through anaerobic oxidation of methane (AOM) at a sulfate–methane transition (SMT) in shallow sediment. Previous geochemical work on ridge crest sediment has documented Ba fronts above the SMT, and has suggested that these horizons can be used to constrain the evolution of the SMT and AOM over time. We expand on this concept and further test it by determining the labile Ba contents of sediment and the dissolved Ba2+ concentrations of pore waters at four ODP sites on Blake Ridge (on the crest at Sites 994, 995 and 997, and on the southern flank at Site 1059). Labile Ba contents are fairly low at all four sites (0.44 and 1.32 mmol/kg), except within 3 m above the SMT at Sites 994, 995 and 997, where they typically exceed 1.24 mmol/kg and can reach 11.3 mmol/kg. These Ba fronts have a diagenetic origin, and SEM analyses show them to be composed of microcrystalline barite. Site 1059 lacks a prominent Ba front. The lowest labile Ba contents generally underlie the Ba fronts and correlate to the base of the SMT. Dissolved Ba2+ concentrations are low (< 1–4 μM) from the seafloor to within 2 m above the main Ba front. Below this depth, they rapidly increase at Sites 994, 995, and 1059, reaching peak concentrations (to 57 μM) at the base of the SMT. By contrast, a rapid rise in dissolved Ba2+ is not observed at Site 997. Dissolved Ba2+ concentrations are only moderately high (10–25 μM) below the SMT at all four sites. Collectively, this information supports a diagenetic model where barite passing into the SMT dissolves, and some of the dissolved Ba2+ then migrates up to form an authigenic barite peak. The contrasting signatures at the different sites indicate non-steady-state differences in the overall process. The size of the peaks on the crest of Blake Ridge necessitates that the recycling of Ba across the SMT has been operating at the current sub-bottom depths for > 100 kyr. Thus, CH4 escaping through the AOM has likely been fairly constant over this time. It is possible that the SMT is currently rising toward the seafloor at Site 1059.  相似文献   

10.
Most submarine gas hydrates are located within the two-phase equilibrium region of hydrate and interstitial water with pressures (P) ranging from 8 to 60 MPa and temperatures (T) from 275 to 293 K. However, current measurements of solubilities of methane in equilibrium with hydrate in the absence of a vapor phase are limited below 20 MPa and 283.15 K, and the differences among these data are up to 30%. When these data were extrapolated to other P-T conditions, it leads to large and poorly known uncertainties. In this study, in situ Raman spectroscopy was used to measure methane concentrations in pure water in equilibrium with sI (structure one) methane hydrate, in the absence of a vapor phase, at temperatures from 276.6 to 294.6 (±0.3) K and pressures at 10, 20, 30 and 40 (±0.4%) MPa. The relationship among concentration of methane in water in equilibrium with hydrate, in mole fraction [X(CH4)], the temperature in K, and pressure in MPa was derived as: X(CH4) = exp [11.0464 + 0.023267 P − (4886.0 + 8.0158 P)/T]. Both the standard enthalpy and entropy of hydrate dissolution at the studied T-P conditions increase slightly with increasing pressure, ranging from 41.29 to 43.29 kJ/mol and from 0.1272 to 0.1330 kJ/K · mol, respectively. When compared with traditional sampling and analytical methods, the advantages of our method include: (1) the use of in situ Raman signals for methane concentration measurements eliminates possible uncertainty caused by sampling and ex situ analysis, (2) it is simple and efficient, and (3) high-pressure data can be obtained safely.  相似文献   

11.
《Chemical Geology》1999,153(1-4):53-79
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.  相似文献   

12.
Presented here are halogen concentrations (Cl, Br and I) in pore waters and sediments from three deep cores in gas hydrate fields of the Nankai Trough area. The three cores were drilled between 1999 and 2004 in different geologic regions of the northeastern Nankai Trough hydrate zone. Iodine concentrations in all three cores increase rapidly with depth from seawater concentrations (0.00043 mmol/L) to values of up to 0.45 mmol/L. The chemical form of I was identified as I−, in accordance with the anaerobic conditions in marine sediments below the SO4 reduction depth. The increase in I is accompanied by a parallel, although lesser increase in Br concentrations, while Cl concentrations are close to seawater values throughout most of the profiles. Large concentration fluctuations of the three halogens in pore waters were found close to the lower boundary of the hydrate stability zone, related to processes of formation and dissociation of hydrates in this zone. Generally low concentrations of I and Br in sediments and the lack of correlation between sediment and pore water profiles speak against derivation of I and Br from local sediments and suggest transport of halogen rich fluids into the gas hydrate fields. Differences in the concentration profiles between the three cores indicate that modes of transportation shifted from an essentially vertical pattern in a sedimentary basin location to more horizontal patterns in accretionary ridge settings. Because of the close association between organic material and I and the similarity of transport behavior for I− and CH4, the results suggest that the CH4 in the gas hydrates also was transported by aqueous fluids from older sediments into the present layers.  相似文献   

13.
The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH4, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH4 in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH4 at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of 129I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH4 together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones.  相似文献   

14.
It is a typical multiphase flow process for hydrate formation in seeping seafloor sediments. Free gas can not only be present but also take part in formation of hydrate. The volume fraction of free gas in local pore of hydrate stable zone (HSZ) influences the formation of hydrate in seeping seafloor area, and methane flux determines the abundance and resource of hydrate-bearing reservoirs. In this paper, a multiphase flow model including water (dissolved methane and salt)-free gas hydrate has been established to describe this kind of flow-transfer-reaction process where there exists a large scale of free gas migration and transform in seafloor pore. In the order of three different scenarios, the conversions among permeability, capillary pressure, phase saturations and salinity along with the formation of hydrate have been deducted. Furthermore, the influence of four sorts of free gas saturations and three classes of methane fluxes on hydrate formation and the resource has also been analyzed and compared. Based on the rules drawn from the simulation, and combined information gotten from drills in field, the methane hydrate(MH) formation in Shenhu area of South China Sea has been forecasted. It has been speculated that there may breed a moderate methane flux below this seafloor HSZ. If the flux is about 0.5 kg m−2 a−1, then it will go on to evolve about 2700 ka until the hydrate saturation in pore will arrive its peak (about 75%). Approximately 1.47 × 109 m3 MH has been reckoned in this marine basin finally, is about 13 times over preliminary estimate.  相似文献   

15.
The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m2 in the study area utilizing the BSR’s observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.  相似文献   

16.
14C measurements of CH4 in environmental samples (e.g. soil gas, lake water, gas hydrates) can advance understanding of C cycling in terrestrial and marine systems. The measurements are particularly useful for detecting the release of old C from climate sensitive environments such as peatlands and hydrate fields. However, because 14C CH4 measurements tend to be complex and time consuming, they are uncommon. Here, we describe a novel vacuum line system for the preparation of CH4 and CO2 from environmental samples for 14C analysis using accelerator mass spectrometry (AMS). The vacuum line is a flow-through system that allows rapid preparation of samples (1 h for CH4 and CO2, 30 min for CH4 alone), complete separation of CH4 and CO2 and is an easy addition to multipurpose CO2 vacuum lines already in use. We evaluated the line using CH4 and CO2 standards with different 14C content. For CH4 and CO2, respectively, the total line blank was 0.4 ± 0.2 and 1.4 ± 0.6 μg C, the 14C background 51.1 ± 1.2 and 48.4 ± 1.5 kyr and the precision (based on pooled standard deviation) 0.9‰ and 1.3‰. The line was designed for sample volumes of ca. 180 ml containing 0.5–1% CH4 and CO2, but can be adjusted to handle lower concentration and larger volume samples. This rapid and convenient method for the preparation of CH4 and CO2 in environmental samples for 14C AMS analysis should provide more opportunities for the use of 14C CH4 measurements in C cycle studies.  相似文献   

17.
Gas hydrate measurements at Hydrate Ridge using Raman spectroscopy   总被引：1，自引：0，他引：1  
Oceanic gas hydrates have been measured near the seafloor for the first time using a seagoing Raman spectrometer at Hydrate Ridge, Oregon, where extensive layers of hydrates have been found to occur near the seafloor. All of the hydrates analyzed were liberated from the upper meter of the sediment column near active gas venting sites in water depths of 770-780 m. Hydrate properties, such as structure and composition, were measured with significantly less disturbance to the sample than would be realized with core recovery. The natural hydrates measured were sI, with methane as the predominant guest component, and minor/trace amounts of hydrogen sulfide present in three of the twelve samples measured. Methane large-to-small cage occupancy ratios of the hydrates varied from 1.01 to 1.30, in good agreement with measurements of laboratory synthesized and recovered natural hydrates. Although the samples visually appeared to be solid, varying quantities of free methane gas were detected, indicating the possible presence of occluded gas in a hydrate bubble fabric.  相似文献   

18.
Rising methane gas bubbles form massive hydrate layers at the seafloor   总被引：3，自引：0，他引：3  
Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM.A simple transport-reaction model was developed to reproduce the Cl− observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl− anomaly, high hydrate formation rates (0.15-1.08 mol cm−2 a−1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45-300 cm a−1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water.  相似文献   

19.
《Applied Geochemistry》1993,8(3):207-221
The gases dissolved in Lake Nyos, Cameroon, were quantified recently (December 1989 and September 1990) by two independent techniques: in-situ measurements using a newly designed probe and laboratory analyses of samples collected in pre-evacuated stainless steel cylinders. The highest concentrations of CO2 and CH4 were 0.30 mol/kg and 1.7 mmol/kg, respectively, measured in cylinders collected 1 m above lake bottom. Probe measurements of in-situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible. Total dissolved-gas pressure near the lake bottom is 1.06 MPa (10.5 atm), 50% as high as the hydrostatic pressure of 2.1 MPa (21 atm). Comparing the CO2 profile constructed from the 1990 data to one obtained in May 1987 shows that CO2 concentrations have increased at depths to below 150 m. Based on these profiles, the average rate of CO2 input to bottom waters was 2.6 × 108 mol/a. Increased deep-water temperatures require an average heat flow of 0.32 MW into the hypolimnion over the same time period. The transport rates of CO2, heat, and major ions into the hypolimnion suggest that a low-temperature reservoir of free CO2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the CO2 into the lake from the underlying diatreme. Increased CH4 concentrations at all depths below the oxycline and a high14C content (41% modern) in the CH4 4 m above lake bottom show that much of the CH4 is biologically produced within the lake. The CH4 production rate may vary with time, but if the CO2 recharge rate remains constant, CO2 saturation of the entire hypolimnion below 50 m depth would require ∼140a, given present-day concentrations.  相似文献   

20.
A review of the geochemistry of methane in natural gas hydrate   总被引：7，自引：0，他引：7  
The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO2 from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ13CPDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ13C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd  相似文献   

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