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1.
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   

2.
Inorganic gases are commonly seen in eastern China and occasionally in southern China from the shallow water columns above hot and cold springs. The gases contain 68% to nearly 100% CO2, with δ13CCO2 and δ13C1 values in the range of −1.18‰ to −6.00‰ and −19.48‰ to −24.94‰, respectively. All of the 34 large inorganic CO2 and one inorganic methane accumulations discovered in China are distributed in eastern parts of the country, from both onshore and continental shelf basins. No commercial inorganic gas accumulation has been found in central and western China. This is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in Chinese sedimentary basins. A detailed study of gas samples collected from four representative inorganic CO2 pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show δ13C1 values greater than −10‰ versus PDB (mostly −30‰), with a positive stable carbon isotope sequence of δ13C1 < δ 13C2 < δ13C3 < δ 13C4. In contrast, the δ13C1 values of biogenic alkane gases are lighter than −30‰, with a negative isotope sequence (i.e. δ13C1 > δ13C2 > δ 13C3 > δ13C4). Inorganic gases also tend to show less negative δ13CCO2 values (−10‰) than biogenic gases (<−10‰).  相似文献   

3.
Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ13C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C5, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ13C1: −51.1 to −48.3‰, δ13C2: −37.9 to −25.3‰, δ13C3: −26.0 to −19.2 ‰, respectively. The δD1 values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.  相似文献   

4.
Chemical and isotopic compositions have been measured on 62 microbial gases from Tertiary hemipelagic sediments in the Middle America Trench off Guatemala and from decaying kelp and surf grass currently accumulating in Scripps Submarine Canyon off southern California. Gases from the Middle America Trench have been generated primarily by the reduction of carbon dioxide; methane δ13C varies from −84‰ to −39‰, methane δD varies from −208‰ to −145‰, and carbon dioxide δ13 C varies from −27‰ to +28‰. Gases from Scripps Submarine Canyon have been generated primarily by acetate dissimilation; methane δ13 C varies from −63‰ to −43‰, methane δD varies from −331‰ to −280‰, and carbon dioxide δ13C varies from −17‰ to +3‰.Methane δ13C values as heavy as −40‰ appear to be uncommon for gases produced by carbon dioxide reduction and, in the Middle America Trench, are associated with unusually positive carbon dioxide δ13C values. However, based on the 25‰ intramolecular fractionation between acetate car☐yl carbon and methyl carbon estimated from the Scripps Submarine Canyon data, methane produced by acetate dissimilation may commonly have heavy δ13C values. The δD of methane derived from acetate is more negative than natural methanes from other origins. Microbial methane δD values appear to be controlled primarily by interstitial water δD and by the relative proportions of methane derived from carbon dioxide and acetate.The chemical and isotopic compositions of microbial gas and thermogenic gas overlap, making it difficult to determine the origins of many commercial natural gases from methane δ13C and C2+ hydrocarbon concentrations alone. Measurements of methane δD and carbon dioxide δ13C can provide useful additional information, and together with ethane δ13C data, help identify gases with mixed microbial and thermogenic origins.  相似文献   

5.
The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ18O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ13C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ13C = − 8‰) and raised precipitation temperatures (δ18O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.  相似文献   

6.
Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high δ13C values of C1–C4 gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the δ13CiC4 value is higher than that of the δ13CnC4, the dominant ranges for the δ13C1, δ13C2, and δ13C3 values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The δ13C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the δ13C1, δ13C2 and δ13C3 values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.  相似文献   

7.
Secondary reactions occurring in pyrite-containing sediments from open cut coal mines are complex and not fully understood. In this study, the changes in seepage water composition in a column experiment with a sediment containing pyrite (5.6 g kg−1) were evaluated using a chemical equilibrium model. A column experiment with artificial irrigation (730 mm water yr−1) was carried out for 2 yr with a sediment from the open pit mine Garzweiler, Germany, at the Institute of Applied Geology. Tracer (LiCl) was added to the sediment. Seepage water composition at 52 cm depth was sampled weekly. Redox potential and the water potential were also recorded weekly. Sulphate and Fe(II) were the dominant ions in the seepage water with concentration maxima of 500 and 350 mmol l−1 after 50 days (0.7 pore volumes (PV)). Minimum pH values were around 0.8 after 100 days (1.4 PV), but increased subsequently and reached 2.4 after 700 days (9.5 PV). Ion activity product calculations indicated the intermediate formation of gypsum (19th–480th day of the experiment). Solutions were undersaturated with respect to alunite, jarosite, jurbanite, schwertmannite, melanterite, gibbsite and goethite during the whole experiment. The model of coupled equilibria which included inorganic complexation, precipitation/dissolution of gypsum and multiple cation exchange was tested. Pyrite oxidation and pH-dependent silicate weathering were considered using simple input functions. Transport was modelled using a field capacity cascade submodel. Model results showed satisfactory agreement with measured values for pH and concentrations of SO4, Fe, Mg, Ca and Al. Correlation coefficients lay between 0.7 and 0.9 and linear regression coefficients (modelled against measured) were 1.5 (Ca), 1.0 (Fe, SO4), 0.8 (Mg), 0.7 (pH) and 0.6 (Al). The results showed that the protons produced during pyrite oxidation (94 mmolc H+ kg−1) were mainly released into seepage water (as HSO4− and H+). Cation exchange reactions buffered 20 mmolc of H+ kg−1 sediment, and Al released by silicate weathering accounted for 3.6 mmolc H+ kg−1. Modelling was useful to further understand the significance of different pH buffer reactions.  相似文献   

8.
The molecular and carbon isotopic compositions of polar isopranyl glycerol ether lipids, which are direct indicators of viable archaea, and neutral isopranyl glycerol ether lipids, which are derived from polar lipids via hydrolysis, in near-surface sediments from a methane seep in the Nankai Trough (off central Japan) were investigated. Procedures for extracting, separating and derivatizing polar and neutral ether lipids for detection using gas chromatography were first examined with one sediment sample and a cultivated methanogen. For all sediment samples, archaeol and hydroxyarchaeol were detected in both the polar and neutral ether lipid fractions. Acyclic and cyclic biphytanes were also detected in both types of lipid fractions after treatment with HI/LiAlH4 for ether cleavage and alkylation. The δ13C values of archaeol, sn-2-hydroxyarchaeol, and sn-3-hydroxyarchaeol in the sample from 0.82 m below the seafloor were lower than −100‰ relative to PDB, indicating that diverse living methanotrophic archaea are present in the seep sediments. Biphytanes released from polar ether lipids in the same sample were less depleted in δ13C (−71‰ to −36‰). The wide range of δ13C values suggests that the biphytanes were derived not only from methanotrophic but also from non-methanotrophic archaea, and that the relative contributions of the methanotrophic and non-methanotrophic archaea differed, depending on the biphytane compound. The vertical profiles and δ13C values of the neutral ether lipids were similar to those of the intact polar ether lipids, suggesting that neutral ether lipids derived from fossil archaea in the samples had mainly been lost by the time of sampling.  相似文献   

9.
Gas hydrate is a recently-found new source of energy that mostly exists in marine sediments. In recent years, we have conducted gas hydrate exploration in the South China Sea. The Xisha trough, one of the promising target areas for gas hydrate, is located in the northern margin of the South China Sea, adjacent to several large oil and gas fields. The Xisha trough extends 420 km long with the water depth of 1 500 m in the west part and 3 400 m in the east part and deposits thick sediments with organic matter content of 0.41%–1.02%. Previous studies on topographical features, geological P-T conditions, structural geology, sedimentary geology and geophysical bottom simulating reflectors (BSR) in the Xisha trough suggest that this area is favorable for the formation and accumulation of gas hydrate. In this paper, we present geochemical analyses for the sediment and pore water from a piston core at Site XS-01 in the Xisha trough. Seven pore water samples were analyzed for their anion (Cl−, SO4 2−, Br−, I−) contents, cation (Na, K, Ca, Mg) contents and trace element (Li, B, Sr, Ba, Rb, Mn) contents. Eight sediment samples were analyzed for stable carbon and oxygen isotopic compositions. A number of geochemical anomalies such as anions (e.g. Cl−, SO4 2−), cations (e.g. Ca, Mg) and trace elements (e.g. Sr, Ba, B) were found in this study. For example, the concentrations of Cl− and SO4 2− in pore water show a decreasing trend with depth. The estimated sulfate/methane interface (SMI) is only 18 m, which is quite similar to the SMI value of 23 m in the ODP164 Leg 997 at Blake Ridge. The Ca, Mg and Sr concentrations of pore water also decrease with depth, but concentrations of Ba, and Mg/Ca and Sr/Ca ratios increase with depth. These geochemical anomalies are quite similar to those found in gas hydrate locations in the world such as the Blake Ridge and may be related to the formation and dissociation of gas hydrates. The salt exclusion effect during the gas hydrate formation will cause an increase in major ion concentrations in the pore waters that diffused upward such as Cl. The anaerobic methane oxidation (AMO) may lead to the change of SO4 2− and other cations such as Ca, Mg, Sr and Ba in pore water. Low δ 13C value of authigenic carbonates is a good indicator for gas hydrate occurrence. However, the bulk sediment samples we analyzed all show normal δ 13C values similar to biogenic marine carbonates, and this may also suggest that no gas hydrate-related authigenic carbonates exist or their amount is so small that they are not detectable by using this bulk analytical method. In conclusion, we suggest that the Site XS-01 in the Xisha trough of the northern margin of the South China Sea is a potential target for further gas hydrate exploration. Translated from Quaternary Sciences, 2006, 26(3): 442–448 [译自: 第四纪研究]  相似文献   

10.
A maturity indexing procedure based on the isotopic difference between the total accumulated methane produced by exhaustive pyrolysis and the kerogen (Δ13C) and the mole ratio of methane to kerogen carbon (CMR), has been tested by applying a standardized technique, i.e. exhaustive pyrolysis (600°C for 120 hr) of extracted-powdered samples and measurement of the amounts and isotopic composition of the methane and kerogen carbon, on a suite of 15 Bakken shale samples.A linear relation was found between the carbon mole ratio of pyrolysis-derived methane and total organic carbon and the δ13C difference between the pyrolysis-derived methane and total organic carbon (r = −0.79); and between the amount of CH4 generated from exhaustive pyrolysis and H/C atomic ratios (r = +0.91).  相似文献   

11.
The isotopic composition (δD and δ18O) and chloride concentration (Cl−) of pore waters from the northern Cascadia continental margin offshore Vancouver Island were measured to characterize the relations between the water flow regime and the distribution, formation and dissociation of gas hydrates. The δD values of pore waters in gas hydrate-bearing sediments are slightly higher ( 1‰) than those of seawater as the result of gas hydrate dissociation during core recovery and handling. Within the seismic blanking zone, the δD values were slightly lower (− 1‰) than values measured from sites outside the blanking area (0‰). We attribute these differences to 1) distillation of D-rich water during hydrate formation in the center of the blanking zone and 2) limited migration of pore water between inside and outside of the blanking zone due to different fluid fluxes. In contrast, the δ18O values and Cl− concentrations do not show significant spatial variation due to decreased isotopic fractionation of oxygen and small fraction of chloride relative to hydrogen isotope during gas hydrate formation. The δD value of pore water, therefore, appears to be a sensitive indicator of gas hydrate occurrence. We estimate that gas hydrate occupied at least 2.0 to 6.3% of sediment pore space using δD distribution in this area.  相似文献   

12.
Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ13C values are found in the Tarim and Qaidam basins (δ13C1: − 19.0 to − 29.9‰; δ13C2: − 18.8 to − 27.1‰), and those with lowest δ13C values occur in the Turpan-Harmi and Junggar basins (δ13C1: − 40.1 to − 44.0‰; δ13C2: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ13C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.  相似文献   

13.
This study provided an overview of the geological setting and geochemical characteristics of the Pleistocene shallow gas accumulations in the eastern Qaidam Basin, NW China. The five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km2 and 7.9 trillion cubic feet (tcf) of proven gas reserves. The dominance of methane (mostly more than 99.9%) and the δ13C and δD values of methane (−68.51 to −65.00‰ and −227.55 to −221.94‰, respectively) suggest that these gases are biogenic, derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures (<75 °C). A sufficient supply and adequate preservation of organic matter in the Pleistocene sediments was made possible by the lake basin’s high altitude (2600–3000 m), high water salinity (>15% TDS) and strong stratification. The deposition and extensive lateral occurrence of lacustrine – shoreline sands/silts in beach sand sheets and sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks. Effective but dynamic gas seals are provided by a combination of factors, such as the intermittent vertical variation in the sediment lithologies, hydraulic trapping due to the mudstone water saturation, the hydrocarbon gradient created as the result of gas generation from potential caprocks, and the presence of a regional caprock consisting of 400–800 m of muds and evaporites. It appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen, and the prolific gas pools are often those large gentle anticlines with little faulting complication.  相似文献   

14.
Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH4). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH4 and its hydrological implications, stable carbon isotope ratios (δ13C) and radiocarbon (14C) ages of exsolved CH4, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ13CCH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ13CCH4=38.2‰) from the Ilan plain, all CH4 samples analyzed were produced via microbially mediated CO2 reduction. Many δ13CDIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO2 reduction. (2) Almost all the 14C ages of CH4 samples from the shallow aquifer (I) (<60 m depth) are greater than the 14C ages of coexisting DIC and sediments, suggesting the presence of CH4 from underlying aquifers. (3) The 14C ages of coexisting CH4, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH4 and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH4 and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH4 sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO2 and CH4 from underlying strata.  相似文献   

15.
The possible contamination of a groundwater system with industrial wastewater originating from a paper mill factory has been investigated in Piteå, N. Sweden. Six samples were collected from the wastewater in the waste dump and twelve samples from the adjacent groundwater were analyzed for chemistry and sulfur isotopes. The industrial wastewater is a saline water consisting mainly of Na–HCO3–SO4, having a high pH and showing δ34S values between 7‰ and 9‰ affected by bacterial sulfate reduction. The groundwaters are relatively dilute, dominated by Na+, Ca2+ and HCO3−, but with varying concentrations as exemplified by sulfate with concentrations varying between 3 and 69 mg L− 1 while the δ34S values range from − 0.5‰ to 14.3‰. The data suggest that the main S sources in the waters are the bedrock sulfides and/or atmospheric deposition, which, sometimes, are overlapped by bacterial sulfate reduction. Contamination from the waste dump does not occur.  相似文献   

16.
The Mapocho river, which crosses downtown Santiago, is one of the most important rivers in contact with a population of about six million inhabitants. Anthropogenic activities, industrialization, farming activities, transport, urbanization, animal and human excretions, domestic wastes and copper mining have affected the river, contaminating it and its sediments with heavy metals. Concentration and distribution of Cu, Zn, Pb and Cd were studied with the purpose of determining their bioavailability and their relation with the characteristics of the sediments. Freshly deposited seasonal sediments were collected from 0–8 cm depths from 6 locations (S1 to S6) along the 30-km long channel length, in the four seasons of year on the following dates: May 2001 (D1, autumn); August 2001 (D2, winter); October 2001 (D3, spring) and January 2002 (D4, summer). The dried samples were sifted to obtain the < 63-μm sediment fraction, since it has been shown that large amounts of heavy metals are bound in the fine-grained fraction of the sediment. Cu and Zn were analyzed by atomic absorption spectrophotometry and Pb and Cd by square wave anodic stripping voltammetry. The highest concentrations of Cu (2850 μg g− 1) were found in the northern part of the river (S1, average D1–D4), near the mountains and a copper mine, and then decreased downstream to 209 μg g− 1 (S6). Total Zn showed an irregular variation, with higher values at S1 (1290 μg g− 1) and high values in some winter sampling (1384 μg g− 1 S4, S5–D2). Pb showed different trends, increasing from S1 to S6 (17 to 61 μg g− 1), with the highest values in the summer samples (83 μg g− 1, S4–S6, D4), and total Cd increased slightly from mean values of 0.2 and 0.5 μg g− 1. Partition into five fractions was made using Tessier's analytical sequential extraction technique; the residue was treated with aqua regia for recovery studies, although this step is not part of the Tessier procedure. The results show that Cu, Zn and Pb in the sediments were dependent on the sampling places along the river, and variation in two years was low (D1–D4). The highest values of total organic matter, carbonate and conductivity were found in S6, which has the smallest size particles, while at S1 the sediments were predominantly sand and contain larger amounts of silica. Cu associated with carbonate decreased gradually from 58% (1771 μg g− 1, S1) to 16% (32 μg g− 1, S6); Cu bonded to reducible fraction was almost constant (33% to 37%), and Cu associated with oxidizable fraction increased from 7% (S1) to 34% (S6), but copper content was lower (214 to 68 μg g− 1). Zn had a similar fractionation profile. However, Pb bound to oxidizable fraction did not show significant percent variation along the river (20% to 19%), but the amount bounded was 4 to 12 μg g− 1. The residual fraction increased from 24% to 41% (5 to 25 μg g− 1, S1 to S6). The distribution of Cd in the sediment was almost independent of the sampling stations and was bound to carbonate, reducible and residual fraction in similar proportion. Cu and Zn at S1 were mainly bound to carbonates and reducible phases with 91% and 73% (2779 and 965 μg g− 1, respectively), and with a change in the pH and/or the redox potential of the sediment–water system, these contaminants could easily enter the food chain. In S6 the amount of Cu and Zn in these phases was 50% and 53% (100 to 313 μg g− 1, respectively).  相似文献   

17.
A detailed model is presented of methane photochemistry in the primitive terrestrial atmosphere along with speculation about its interpretation. Steady-state CH4 mixing ratios of 10−6–10−4 could have been maintained by a methane source of about 1011 cm−2 s−1, which is comparable to the modern biogenic methane production rate. In the absence of a source, methane would have disappeared in <104 years, being either oxidized, or polymerized into more complex hydrocarbons. The source strength needed to maintain a steady CH4 mixing ratio and the degree to which methane could have polymerized to form higher hydrocarbons depend upon the amount of CO2 present in the early atmosphere. The dependence on H2 is much weaker. Infrared absorption by methane, and especially by other hydrocarbon species, may have supplemented the greenhouse warming due to carbon dioxide. A radiative model is needed to establish this effect quantitatively. The destruction of the methane greenhouse early in the Proterozoic may have triggered the Huronian glaciation.These calculations also suggest that atmospheres rich in both CO2 and CH4 may be photochemically unstable with respect to conversion to CO.  相似文献   

18.
Organic geochemical data was collected from a KP-2 piston core sample obtained from a mud volcano in the Nankai Trough, off the southeast coast of Japan. The Nankai Trough was investigated geologically and was found to have a large amount of methane hydrates. Biomarkers indicating anaerobic oxidation of methane (AOM) were expected to be found in the sediment of the mud volcano collected by KP-2 core. But the gas chromatography-mass spectrometry (GC-MS) analysis of the samples showed only one biomarker related to the methane-oxidizing Archaea, namely 2,6,10,15,19-pentamethyleicosane (PME), could be detected throughout the KP-2 core. Phytane was predominant in the upper part of the KP-2 core; however, a series of 2,2-dimethylalkanes (2,2-DMAs) were characterized in the lower part of KP-2 core. The individual δ13C values of 2,2-DMAs, which were − 29.1 to − 27.3‰ indicate that the origin of 2,2-DMAs was not related to methane-oxidizing Archaea.  相似文献   

19.
The occurrence and distribution of polycyclic musks in the Lippe River system (a tributary of the Rhine River, Germany) was investigated in order to observe the dynamic transport and partitioning of these compounds between aqueous and particulate phases after their discharge into the river by sewage effluents. 1,3,4,6,7,8-Hexahydro-4,6,6,7,8,8-hexamethylcyclopenta[g]-2-benzopyrane (HHCB), 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene (AHTN), 6-acetyl-1,1,2,3,3,5-hexamethylindane (AHMI) and 4-acetyl-1,1-dimethyl-6-tert.-butylindane (ADBI) concentrations were determined in 19 water and surface sediment samples which were taken from a longitudinal section of the river. HHCB and AHTN were present in each of the water samples at concentrations ranging from <10 to 180 ng l−1 and <10 to 70 ng l−1, respectively. The load of dissolved HHCB and AHTN was calculated on the basis of compound concentrations in water and the corresponding river runoff data and ranged from 3 to 293 g day−1 and from 1 to 108 g day−1, respectively. Increasing loads of HHCB and AHTN along the river reflect a high input of sewage effluents to the densely populated areas along the central part of the river. Decreasing loads at the lower reaches indicate that in the corresponding river sections the rate of removal of musks was higher than the rate of input. Degradation and/or adsorption to particulate matter are processes that might explain this phenomenon. Consequently, high concentrations of HHCB and AHTN were detected in surface sediments from the Lippe River (from 5 to 191 μg kg−1 and from 2 to 1399 μg kg−1, respectively). HHCB/AHTN ratios in sediment samples were lower (average 1.2) than in water samples (average 2.9), suggesting the preferential adsorption of AHTN to particulate matter.  相似文献   

20.
The local and geometrical structure around gold (III) e.g., Au3+ ions in aqueous solution with different OH−/Cl− molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCln(OH)4−n]− solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl− ligands by OH− ligands. A square–planar geometry for [AuCln(OH)4−n]− with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH−/Cl− molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.  相似文献   

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