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1.
Stable isotope ratios of oxygen and carbon were determined for CO2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO2 concentrations detected previously by McCarthy et al. (1986). Values of δ13C in soil gas CO2 from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ13C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C3 (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C3 and C4 (Hatch-Slack) plants, together with atmospheric CO2 are the likely sources of carbon in soil gas CO2. Values of δ18O in soil-gas CO2 range from 32 to 38‰ (SMOW). These δ18O values are intermediate between that calculated for CO2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO2. The δ18O data indicate that atmospheric CO2 has been incorporated by mixing or diffusion. Any CO2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO2 taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ13C value of soil-gas CO2 was observed, contrary to what would be expected if the anomalous CO2 were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C3 plant material were responsible for most CO2 generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO2 anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO2 concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.  相似文献   

2.
The CO2 gas reservoir sandstones in the Hailaer Basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of CO2. The origins of dawsonite and CO2 in these sandstones were studied by petrographic and isotopic analysis. According to the paragenetic sequence of the sandstones, dawsonite grew later than CO2 charging at 110–85 Ma. The dawsonite δ18O value is 7.4‰ (SMOW), and the calculated δ18O values of the water present during dawsonite growth are from −11.4‰ to −9.2‰ (SMOW). This, combined with the NaHCO3-dominated water linked to dawsonite growth, suggests meteoric water being responsible for dawsonite growth. The δ13C values of gas phase CO2 and the ratios of 3He/4He of the associated He suggest a mantle magmatic origin of CO2-rich natural gas in Hailaer basin. Dawsonite δ13C values are −5.3‰ to −1.5‰ (average −3.4‰), and the calculated δ13C values of CO2 gas in isotopic equilibrium with dawsonite are −11.4‰ to −7.3‰. These C isotopic values are ambiguous for the dawsonite C source. From the geological context, the timing of events, together with formation water conditions for dawsonite growth, dawsonite possibly grew in meteoric-derived water, atmospherically-derived CO2 maybe, or at least the dominant, C source for dawsonite. It seems that there are few relationships between dawsonite and the modern gas phase of CO2 in the Hailaer basin.  相似文献   

3.
《Applied Geochemistry》2002,17(12):1503-1513
The interaction of CO2 with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.  相似文献   

4.
The stable carbon isotopic compositions of free and sulfur (S)-bound biomarkers derived from algae, (cyano)bacteria, archaea and higher plants and total organic carbon (TOC) during the first phase of the late Cenomanian/Turonian oceanic anoxic event (OAE) were measured in black shales deposited in the southern proto-Atlantic Ocean in the Cape Verde basin (DSDP Site 367) to determine the response of these organisms to this major perturbation of the global carbon cycle resulting from widespread burial of marine organic matter. The average positive isotope excursions of TOC and biomarkers varied from 5.1‰ to 8.3‰. The δ13C values were cross correlated to infer potential common sources of biomarkers. This revealed common sources for C31 and C32 hopanes but no 1:1 relationship for pristane and phytane. The correlation of δ13CTOC with the δ13C value of sulfur (S)-bound phytane is the strongest. This is because S-bound phytane is derived from phytol that originates from all marine primary producers (algae and cyanobacteria) and thus represents a weighted average of their carbon isotopic compositions. The δ13C values of S-bound phytane and C35 hopane were also used to estimate pCO2 levels. Before the OAE burial event, pCO2 levels are estimated to be ca. 1300 ppmv using both biomarkers and the independent maximum Rubisco fractionation factors. At times of maximum organic carbon burial rates during the OAE, reconstructed pCO2 levels are estimated to be ca. 700 ppmv. However, compared to other C/T OAE sections the positive isotope excursion of S-bound phytane is also affected by an increased production during the OAE. When we compensate for this, we arrive at pCO2 levels around 1000 ppmv, a reduction of ca. 25%. This indicates that burial of organic matter can have a large effect on atmospheric CO2 levels.  相似文献   

5.
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   

6.
The global rise in atmospheric greenhouse gas concentrations calls for practicable solutions to capture CO2. In this study, a mineral carbonation process was applied in which CO2 reacts with alkaline lignite ash and forms stable carbonate solids. In comparison to previous studies, the assays were conducted at low temperatures and pressures and under semi-dry reaction conditions in an 8 L laboratory mixing device. In order to find optimum process conditions the pCO2 (10-20%), stirring rate (500-3000 rpm) and the liquid to solid ratio (L/S = 0.03-0.36 L kg−1) were varied. In all experiments a considerable CO2 uptake from the gas phase was observed. Concurrently the solid phase contents of Ca and Mg (hydr)oxides decreased and CaCO3 and MgCO3 fractions increased throughout the experiments, showing that CO2 was stabilized as a solid carbonate. The carbonation reaction depends on three factors: Dissolution of CO2 in the liquid phase, mobilization of Ca and Mg from the mineral surface and precipitation of the carbonate solids. Those limitations were found to depend strongly on the variation of the process parameters. Optimum reaction conditions could be found for L/S ratios between 0.12 and 0.18, medium stirring velocities and pCO2 between 10% and 20%.Maximum CO2 uptake by the solid phase was 4.8 mmol g−1 after 120 min, corresponding to a carbonation efficiency for the alkaline material of 53% of the theoretical CO2 binding capacity. In comparison to previous studies both CO2 uptake and carbonation efficiencies were in a similar range, but the reaction times in the semi-dry process were considerably shorter. The proposed method additionally allows for a more simple carbonation setup due to low T and P, and produces an easier to handle product with low water content.  相似文献   

7.
H2S and CO2 are found in elevated concentrations in the reservoirs near the Carboniferous–Ordovician unconformity in the Hetianhe Field of the Tarim Basin, NW China. Chemical and isotopic analyses have been performed on produced gases, formation waters and reservoir rocks to determine the origin of CO2 and H2S and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids. It is unlikely that H2S and CO2 had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source. Instead, H2S and CO2 are probably the result of sulphate reduction of the light hydrocarbon gases (LHG). Increasing H2S concentrations and CO2/(CO2+ΣC1–4) values to the west of the Hetianhe Field occur commensurately with increasingly heavy hydrocarbon gas δ13C values. However, thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low, no H2S or rare pyrite was detected in deeper reservoirs (where more TSR should have occurred) and inferred δ34S values of H2S (from late-stage pyrite in the Carboniferous and Ordovician reservoirs) are as low as −24.9‰. Such low δ34S values discount the decomposition of organic matter as a major source of H2S and CO2. Bacterial sulphate reduction of the light hydrocarbon gases in the reservoir, possibly coupled indirectly with the consumption of organic acids and anions is most likely. The result is the preferential oxidation of 12C-rich alkanes (due to the kinetic isotope effect) and decreasing concentration of organic acids and anions. Modern formation water stable isotope data reveal that it is possible that sulphate-reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion-related unconformity. This may account for the apparently stronger influence of bacterial sulphate reduction to the west of the Hetianhe Field, and the consequent greatest decrease of the δ13C-CO2 values and the greatest increase in δ13C values of the alkane gases.  相似文献   

8.
Large-scale atmospheric circulation patterns determine the quantity and seasonality of precipitation, the major source of water in most terrestrial ecosystems. Oxygen isotope (δ18O) dynamics of the present-day hydrologic system in the Palouse region of the northwestern U.S.A. indicate a seasonal correlation between the δ18O values of precipitation and temperature, but no seasonal trends of δ18O records in soil water and shallow groundwater. Their isotope values are close to those of winter precipitation because the Palouse receives  75% of its precipitation during winter. Palouse Loess deposits contain late Pleistocene pedogenic carbonate having ca. 2 to 3‰ higher δ18O values and up to 5‰ higher carbon isotope (δ13C) values than Holocene and modern carbonates. The late Pleistocene δ18O values are best explained by a decrease in isotopically light winter precipitation relative to the modern winter-dominated infiltration. The δ13C values are attributed to a proportional increase of atmospheric CO2 in soil CO2 due to a decrease in soil respiration rate and 13C discrimination in plants under much drier paleoclimate conditions than today. The regional climate difference was likely related to anticyclonic circulation over the Pleistocene Laurentide and Ice Sheet.  相似文献   

9.
The oxygen (δ18Oc) and carbon (δ13Cc) isotope compositions of the structural carbonate group (CO3) in apatites from lateritic profiles were investigated. The weathering profiles, located in southern Brazil and in western Senegal, are developed on three different types of apatite-rich parent rock: carbonatite, metamorphosed marine phosphorite and sedimentary marine phosphorite. The parent rock apatites are of magmatic, hydrothermal, metamorphic and sedimentary origins. The in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically.The overall range of measured δ18Oc and δ13Cc values of apatites of weathering origin (22 to 27 SMOW for δ18Oc and −15 to −10 PDB for δ13Cc) is much smaller than the range of measured and/or published isotope compositions of parent rock apatites (4–35 for δ18Oc and −11 to +1 for δ13Cc). In any profile, the apatites of weathering origin can exhibit lower, similar or higher δ18Oc values than parent rock apatites. In contrast, their δ13Cc values are systematically and significantly lower than those of the parent rock apatites. Apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition.Assuming that apatite CO3 fractionates O in a way similar to calcite CO3, the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions. The very low δ13Cc values exhibited by these apatites indicate that the dominant sources of dissolved CO2 in the soil water are organic. The isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies.  相似文献   

10.
Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO2. There is preferential dissolution of Ca12CO3 during hydrolysis, resulting in δ13C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as εcarb. εcarb is greater on average for glacial sediments (−17.4‰) than for calcium carbonate (−7.8‰) for the < 63 μm size fraction, a sediment concentration of 5 g L−1 and closed system conditions at 5°C. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on δ13C-DIC at higher CaCO3:water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO2 at sediment concentrations of 5 g L−1. Atmospheric CO2 dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least ∼ 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the δ13C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO2 signatures in DIC from δ13C-DIC data.  相似文献   

11.
Detailed records of δ18O, δ13C, percentage and mass accumulation rate of CaCO3, and eolian percentage, mass accumulation rate, and grainsize generated for core RC11-210 from the equatorial Pacific reveal the timing of paleoclimatic events over the past 950,000 yr. The CaCO3 percentage record shows the standard Pacific correlation of high CaCO3 content with glacial periods, but displays a marked change of character about 490,000 yr ago with older stages showing much less variability. The carbonate mass flux record, however, does not show such a noticeable change. Sedimentation rates vary from about 0.5 to 3.0 cm/1000 yr and, during the past 490,000 yr, sections with enhanced sedimentation rates correspond to periods of high CaCO3 percentage. Eolian mass accumulation rates, an indication of the aridity of the source region, are usually higher during glacial times. Eolian grainsize, an indication of the intensity of atmospheric circulation, generally fluctuates at a higher frequency than the 100,000-yr glacial cycle. The mid-Brunhes climatic event centered at 300,000 yr ago appears as a 50,000-yr interval of low intensity and reduced variability of atmospheric circulation. Furthermore, the nature of this entire record changes then, with the younger portion indicating less variation in wind intensity than the older part of the record. The late Matuyama increase in amplitude of paleoclimatic signals begins 875,000 yr ago in the eolian record, 25,000 yr before the δ18O and CaCO3 percentage amplitude increases about 850,000 yr ago.  相似文献   

12.
Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ18O and δ13C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ13C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3− (aq) reservoir during degassing of 13C-depleted CO2.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in 18O relative to equilibrium values. δ18O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ13C, δ18O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO2 hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO3− (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in 18O enrichment in the HCO3− (aq) reservoir and ultimately in the precipitated CaCO3.  相似文献   

13.
This study describes the biogeochemical cycling of seston in Grand Traverse Bay, Lake Michigan. Seston was characterized by carbon and nitrogen elemental and isotopic abundances. Fluorescence, temperature, light transmittance, and concentrations of dissolved inorganic nitrogen were also determined. PCBs were analyzed from surface (10 m) seston and ΣPCB was calculated by summing all of the congeners quantified in each sample. The vertical and seasonal trends in the δ13C values of seston exhibited a broad range from −30.7 to −23.9‰. Low δ13C values that occur concurrently with a peak in fluorescence below the thermocline reflect uptake of 13C depleted respiratory CO2 and/or the accumulation of 13C depleted lipids by phytoplankton. High δ13C values late in the season likely result from a reduction in photosynthetic fractionation associated with a decrease in the CO2 pool. Seasonal δ15N values of seston were high in the spring and declined through August. The δ15N values of seston reflect a balance between fractionation during assimilation of NH4+ or NO3− and degradative processes. The seston ΣPCB and fluorescence were both high in the spring and subsequently declined, suggesting that the concentrations of PCBs in seston were associated with labile material derived from primary productivity. The strong seasonal trends in the organic geochemical characteristics of seston and concentrations of PCBs emphasize the complex nature of particle cycling in aquatic environments.  相似文献   

14.
This study identifies isotope signatures associated with autotrophic and heterotrophic microbial communities that may provide a means to determine carbon cycling relationships in situ for acid mine drainage (AMD) sites. Stable carbon isotope ratios (δ13C) of carbon sources, bulk cells, and membrane phospholipids (PLFA) were measured for autotrophic and heterotrophic microbial enrichment cultures from a mine tailings impoundment in northern Ontario, Canada, and for pure strains of the sulfur oxidizing bacteria Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. The autotrophic enrichments had indistinguishable PLFA distributions from the pure cultures, and the PLFA cyc-C19:0 was determined to be a unique biomarker in this system for these sulfur oxidizing bacteria. The PLFA distributions produced by the heterotrophic enrichments were distinct from the autotrophic distributions and the C18:2 PLFA was identified as a biomarker for these heterotrophic enrichments. Genetic analysis (16S, 18S rRNA) of the heterotrophic cultures indicated that these communities were primarily composed of Acremonium fungi.Stable carbon isotope analysis revealed that bulk cellular material in all autotrophic cultures was depleted in δ13C by 5.6–10.9‰ relative to their atmospheric CO2 derived carbon source, suggesting that inorganic carbon fixation in these cultures is carbon limited. Individual PLFA from these autotrophs were further depleted by 8.2–14.6‰ compared to the bulk cell δ13C, which are among the largest biosynthetic isotope fractionation factors between bulk cell and PLFA reported in the literature. In contrast, the heterotrophic bulk cells were not significantly fractionated in δ13C relative to their carbon source and heterotrophic PLFA ranged from 3‰ enriched to 4‰ depleted relative to the isotopic composition of their total biomass. These distinct PLFA biomarkers and isotopic fractionations associated with autotrophic and heterotrophic activity in this laboratory study provide potential biomarkers for delineating autotrophic and heterotrophic carbon cycling in AMD environments.  相似文献   

15.
Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ13Cmatrix and δ18Omatrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ18Omatrix = 0.61 · δ13Cmatrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO2 into alkaline Ca(OH)2 solutions shows a similar relationship, δ18Ocalcite = 0.67 · δ13Ccalcite − 6.4 (VPDB). Both relationships indicate that the 13C/12C and 18O/16O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO2 from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO2 (δ13Cmatrix ≈  −25‰ and δ18Omatrix ≈ −20‰). As calcite formation continued the remaining gaseous CO2 is subsequently enriched in 13C and 18O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H2O, e.g. evaporation, the source of CO2, e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for 14C dating.  相似文献   

16.
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   

17.
Release of CO2 from surface ocean water owing to precipitation of CaCO3 and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a quotient θ = (CO2 flux to the atmosphere)/(CaCO3 precipitated). θ depends not only on water temperature and atmospheric CO2 concentration but also on the CaCO3 and organic carbon masses formed. In CO2 generation by CaCO3 precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric CO2 concentration (195–375 ppmv), and increasing CaCO3 precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic carbon counteracts the CO2 production by carbonate precipitation and it results in lower CO2 emissions from the surface layer. When atmospheric CO2 increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO2 transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric CO2. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon from land, the calculated CO2 flux to the atmosphere is a function of the CaCO3 and Corg net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO2 flux near the Last Glacial Maximum is 17 to 7×1012 mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time it is 38–42×1012 mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic emissions of CO2 to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total alkalinity owing to the CaCO3 storage and consequent emission of CO2. These results are in agreement with the conclusions of a number of other investigators. As the CO2 uptake in mineral weathering is a major flux in the global carbon cycle, the CO2 weathering pathway that originates in the CO2 produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large imbalances of the atmosphere and land organic reservoir at 102–104-year time scales.  相似文献   

18.
Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum Pco2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments.The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric Pco2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL.If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric Pco2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2.The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco2.  相似文献   

19.
Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ13C of the kerogen (δ13CTOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ13CTOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ13C values of primary production markers. These δ13C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ13CTOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ13CTOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C1-C3 alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ13CTOC relationship reported by Huc et al. (1992).  相似文献   

20.
Turnover of C in soils is the dominant flux in the global C cycle and is responsible for transporting 20 times the quantity of anthropogenic emissions each year. This paper investigates the potential for soils to be modified with Ca-rich materials (e.g. demolition waste or basic slag) to capture some of the transferred C as geologically stable CaCO3. To test this principal, artificial soil known to contain Ca-rich minerals (Ca silicates and portlandite) was analysed from two sites across NE England, UK. The results demonstrate an average C content of 30 ± 15.3 Kg C m−2 stored as CaCO3, which is three times the expected organic C content and that it has accumulated at a rate of 25 ± 12.8 t C ha−1 a−1 since 1996. Isotopic analysis of the carbonates gave values between −6.4‰ and −27.5‰ for δ13C and −3.92‰ and −20.89‰ for δ18O, respectively (against V-PDB), which suggests that a combination of carbonate formation mechanisms are operating including the hydroxylation of gaseous CO2 in solution, and the sequestration of degraded organic C with minor remobilisation/precipitation of lithogenic carbonates. This study implies that construction/development sites may be designed with a C capture function to sequester atmospheric C into the soil matrix with a maximum global potential of 290 Mt C a−1.  相似文献   

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