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1.
The hydrogeochemical and carbon isotope characteristics of the Krka River, Slovenia, were investigated to estimate the carbon transfer from the land ecosystem in the watershed. During the 3-year sampling period (2008–2010), temperature, pH, electrical conductivity, major ion content, dissolved inorganic carbon (DIC) and dissolved organic carbon content, and the isotopic composition of DIC (δ 13C DIC) were monitored in the main stream of the Krka River and its tributaries. The major solute composition of analysed waters is dominated by an input of HCO 3 ?, Ca 2+ and Mg 2+ originating from carbonate dissolution. The Mg 2+/Ca 2+ and Mg 2+/HCO 3 ? molar ratio values ranging from 0.24 to 0.71 and 0.05 to 0.30, respectively, indicate a high degree of dolomite dissolution relative to calcite. Dissolved CO 2 concentrations in the river were up to tenfold supersaturated relative to the atmosphere, resulting in supersaturation with respect to calcite and degassing of CO 2 downstream. The δ 13C values in river water range from ?15.6 to ?9.4 ‰ and are controlled by the input of tributaries, exchange with atmospheric CO 2, degradation of organic matter, and dissolution of carbonates. The mass balance calculations for riverine DIC suggest that the contribution from carbonate dissolution and degradation of organic matter have major influence, whereas the exchange with atmospheric CO 2 has minor influence on the inorganic carbon pool in the Krka River. 相似文献
2.
Among the risks of CO 2 storage is the potential of CO 2 leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO 2 can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO 2 storage site is essential for assessing the impact of CO 2 leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO 2 partial pressures are rare. Here, the first study for column experiments is presented applying CO 2 partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N 2-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi: 10.2475/ajs.278.2.179, 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO 2 leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO 2 leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m. 相似文献
3.
This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species ( Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO 2 over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH 4+ as an N source, and H 2PO 4− as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H 2O-CO 2-CaCO 3 system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH 4+ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H 2CO 3 generated by dissolution of atmospheric CO 2 (H 2CO 3 + CaCO 3 → Ca 2+ + 2HCO 3−) and H + released during NH 4+ uptake (H + + CaCO 3 → Ca 2+ + HCO 3−). Reaction with H 2CO 3 and H + supplied ∼45% and 55% of the total Ca 2+ and ∼60% and 40% of the total HCO 3−, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH 4+ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H 2CO 3. In lactate bearing reactors, most H + generated by NH 4+ uptake reacted with HCO 3− produced by lactate oxidation to yield CO 2 and H 2O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H 2CO 3 at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH 4+ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions. 相似文献
4.
An investigation was conducted to assess the hydrogeochemical processes of an alluvial channel aquifer located in a typical Karoo Basin of Southern Africa. The investigation was aimed at identifying and describing the groundwater chemistry evolution and its contribution to the overall groundwater quality. X-ray fluorescent spectrometry (XRF) and X-ray diffractometry (XRD) analyses were performed on geological samples to identify and quantify the major element oxides and minerals. The study utilises the conventional Piper diagram, bivariate plots and PHREEQC hydrogeochemical model to analyse groundwater chemistry data obtained during the wet (February and May) and dry seasons (August and December) of 2011. The XRF and XRD results show that the channel deposits are dominated by SiO 2 element oxides and quartz minerals, thus elevated concentrations of silicon (Si 4+) were found in the groundwater. Dolomite and calcite minerals were also detected in the unconsolidated aquifer sediments. The detailed study of the alluvial aquifer system has shown that dissolution of dolomite and calcite minerals and ion exchange are the dominant hydrogeochemical processes influencing the groundwater quality. The groundwater evolves from Ca 2+–Mg 2+–HCO 3 ? recharge water that goes through ion exchange with Na + in the clay-silt sediment to give a Na +–HCO 3 ? water type. The groundwater is supersaturated with respect to quartz, dolomite and calcite minerals. The study shows the potential usefulness of simple bivariate plots as a complimentary tool to the conventional methods for analyzing groundwater hydrogeochemical processes. 相似文献
5.
In situ measured microprofiles of Ca 2+, pCO 2, pH and O 2 were performed to quantify the CaCO 3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m −2 d −1 and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m −2 a −1. The metabolic CO 2, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca 2+ microprofiles were 0.6 mmol m −2 d −1 for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO 2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O 2, pH and Ca 2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw −1 a −1 (mol per kg water and year), which equals 95% d −1. This rate constant is at the upper end of reported in situ values. 相似文献
6.
The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 h equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA57 (1993), 3505; Pokrovsky et al., Langmuir16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO 3Fe + and >CO 3FeCO 3H 0. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system ( t1/2 = 145 ± 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite ( t1/2 = 41 ± 1 min and t1/2 = 100 ± 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO 3FeCO 3H 0, for which the species specific rate constant was estimated. 相似文献
7.
The geochemical effects of microbially mediated degradation of aromatic hydrocarbons were observed as changes in solution composition of an artificial groundwater in packed-sand laboratory columns. Benzene, toluene, and xylene, both individually and in a combined fashion, were used as substrates in biodegradation experiments conducted under oxygenated and anoxic conditions in columns filled with quartz, calcite, or Fe 3+-coated quartz sand. Typically, column effluent had increased concentrations of dissolved inorganic C, decreased pH, and decreased concentrations of NO 3 and dissolved O 2 relative to column influent. Efficiency of CO 2 generation was similar for the three different substrates, ranging from 22.5 to 26.6% organic C converted to CO 2. When all three substrates were combined, the percentage of CO 2 produced fell within the range observed in the single substrate experiments. Nitrate disappearance was more varied as a function of substrate identity, with greatest amounts lost when toluene was the substrate. Calcite dissolved as a result of CO 2 generated during the biodegradation reactions, and empirically calculated dissolution rates varied between 1.9 and 4.0 x 10 −9 mmol cm −2 s −1. The calcite dissolution rate was slower than the biodegradation rate, as evidenced by excess generation of CO 2 relative to Ca 2+ production. The decrease in pH was less in experiments with calcite present than in those with quartz sand present due to buffering by calcite dissolution. Dissolution of Fe oxyhydroxides was not observed under any experimental conditions. 相似文献
8.
The hydrogeochemical processes that took place during an aquifer storage and recovery (ASR) trial in a confined anoxic sandy aquifer (Herten, the Netherlands) were identified and quantified, using observation wells at 0.1, 8 and 25 m distance from the ASR well. Oxic drinking water was injected in 14 ASR cycles in the period 2000–2009. The main reactions consisted of the oxidation of pyrite, sedimentary organic matter, and (adsorbed) Fe(II) and Mn(II) in all aquifer layers (A–D), whereas the dissolution of carbonates (Mg-calcite and Mn-siderite) occurred mainly in aquifer layer D. Extinction of the mobilization of SO 4, Fe(II), Mn(II), As, Co, Ni, Ca and total inorganic C pointed at pyrite and calcite leaching in layer A, whereas reactions with Mn-siderite in layer D did not show a significant extinction over time. Iron(II) and Mn(II) removal during recovery was demonstrated by particle tracking and pointed at sorption to neoformed ferrihydrite. Part of the oxidants was removed by neoformed organic material in the ASR proximal zone (0 – ca. 5 m) where micro-organisms grow during injection and die away when storage exceeds about 1 month. Anoxic conditions during storage led to increased concentrations for a.o. Fe(II), Mn(II) and NH 4 as noted for the first 50–200 m 3 of abstracted water during the recovery phase. With a mass balance approach the water–sediment reactions and leaching rate of the reactive solid phases were quantified. Leaching of pyrite and calcite reached completion at up to 8 m distance in layer A, but not in layer D. The mass balance approach moreover showed that Mn-siderite in layer D was probably responsible for the Mn(II) exceedances of the drinking water standard (0.9 μmol/L) in the recovered water. Leaching of the Mn-siderite up to 8 m from the ASR well would take 1600 more pore volumes of drinking water injection (on top of the realized 460). 相似文献
9.
This paper gives an account of the implementation of hydrochemical and isotopic techniques to identify and explain the processes that govern solute exchange in two groundwater-dependent shallow lakes in the Southeastern Pampa Plain of Argentina. Water samples (lakes, streams, spring water and groundwater) for hydrochemical and stable isotopic determination were collected and the main physical–chemical parameters were measured. The combination of stable isotope data with hydrogeochemical techniques was used for the identification of sources and preferential recharge areas to these aquatic ecosystems which allowed the explanation of the lake water origin. The hydrochemical processes which explain Los Padres Lake water chemistry are evaporation from groundwater, CO 2 input, calcite dissolution, Na + release by Ca 2+ and Mg 2+ exchange, and sulfate reduction. The model that best aligns with La Brava Lake hydrochemical constraints includes: mixing, CO 2 and calcite dissolution, cationic exchange with Na + release and Mg 2+ adsorption, and to a lesser extent, Ca/Na exchange. This model suggests that the fractured aquifer contribution to this water body is greater than 50 %. An isotopic-specific fingerprint for each lake was identified, finding a higher evaporation rate for La Brava Lake compared to Los Padres Lake. Isotopic data demonstrate the importance of these shallow lakes as recharge areas to the regional aquifer, becoming areas of high groundwater vulnerability. The Tandilia Range System, considered in many hydrogeological studies as the impermeable bedrock of the Pampean aquifer, acts as a fissured aquifer in this area, contributing to low salinity waters and with a fingerprint similar to groundwater isotopic composition. 相似文献
10.
The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate. 相似文献
11.
Using groundwater quality data from the Lusatian post-mining district a hydrogeochemical model is derived for the evolution of mining affected groundwaters in pyrite-rich dumps which consist mainly of silicates and variable amounts of calcite. Pyrite oxidation paralleled by buffer processes leads to gypsum saturation in a significant portion of the water. Gypsum precipitation controls SO 4 and Ca concentrations in groundwaters above an ionic strength ( I) of 60 mM. It has been found that there is always a clear relationship between I, SO 4 and Ca concentrations. In particular, there is a tendency that Ca concentrations decrease with increase in ionic strength above I = 60 mM and a striking rareness of samples with SO 4 concentrations between 20 and 30 mM above an ionic strength of 100 mM. These observations are explained by a genetic model. This model also explains the observed relationship between the c(Fe)/c(SO 4)-ratio, the ionic strength, and the observed pH-values. Based on the field data and supported by geochemical equilibrium calculations, it is shown that silicate weathering along with calcite dissolution must be a significant buffering process at least in some areas. 相似文献
12.
The Wilcox aquifer is a major groundwater resource in the northern Gulf Coastal Plain (lower Mississippi Valley) of the USA, yet the processes controlling water chemistry in this clastic aquifer have received relatively little attention. The current study combines analyses of solutes and stable isotopes in groundwater, petrography of core samples, and geochemical modeling to identify plausible reactions along a regional flow path ~300 km long. The hydrochemical facies evolves from Ca-HCO 3 upgradient to Na-HCO 3 downgradient, with a sequential zonation of terminal electron-accepting processes from Fe(III) reduction through SO 4 2? reduction to methanogenesis. In particular, decreasing SO 4 2? and increasing δ 34S of SO 4 2? along the flow path, as well as observations of authigenic pyrite in core samples, provide evidence of SO 4 2? reduction. Values of δ 13C in groundwater suggest that dissolved inorganic carbon is contributed both by oxidation of sedimentary organic matter and calcite dissolution. Inverse modeling identified multiple plausible sets of reactions between sampled wells, which typically involved cation exchange, pyrite precipitation, CH 2O oxidation, and dissolution of amorphous Fe(OH) 3, calcite, or siderite. These reactions are consistent with processes identified in previous studies of Atlantic Coastal Plain aquifers. Contrasts in groundwater chemistry between the Wilcox and the underlying McNairy and overlying Claiborne aquifers indicate that confining units are relatively effective in limiting cross-formational flow, but localized cross-formational mixing could occur via fault zones. Consequently, increased pumping in the vicinity of fault zones could facilitate upward movement of saline water into the Wilcox. 相似文献
13.
Possible groundwater quality changes related to pyrite oxidation during artificial groundwater recharge and its storage in the Tertiary sands of the London Basin are investigated. Pyrite textures in the Tertiary sands are examined by scanning electron microscopy while an experimental approach is used to study mechanisms of pyrite oxidation and of some associated chemical reactions. In the Tertiary sands of the London Basin aquifer, pyrite occurs as aggregates made of discrete individual crystals 0.5–5 μm in size or, in a cryptocrystalline form, often as pseudomorphs of biogenic debris. It can expose a very large specific surface area to porefluids. Although ferric iron, which can be an oxidising agent of pyrite, is abundant in the solid phase of the Tertiary sands, it does not appear to take a significant part in this case. Pyrite oxidation seems to rely on a supply of oxygen. Leaching experiments using a 0.001 M H 2SO 4 solution were carried out to examine interactions between mildly acidic groundwater resulting from pyrite oxidation at a moderate rate and the host-sediment. In the presence of CaCO 3 in the solid phase, H + is rapidly buffered by CaCO 3 dissolution. Oscillations of this reaction around equilibrium appear to trigger cation-exchange reactions on clay mineral surfaces, resulting in the release of major cations (e.g. K and Mg) into solution. In the absence of CaCO 3 in the solid phase, H + buffering occurs less efficiently solely through exchange of cations for H + on clay minerals surfaces. If the rate of pyrite oxidation in the Tertiary sands becomes high enough for the buffering capacity of the system to be exceeded, the groundwater pH begins to fall. Interactions between low pH (2) groundwaters and the host sediments were examined by leaching solid material in 0.01 M and 0.1 M H 2SO 4 solutions. Concentrations of Fe, Mg and K increase in solution throughout the experiment, indicating partial dissolution of clay minerals. The composition of the porefluid thus depends on the geochemical composition and surface area of the different clay minerals present. 相似文献
14.
Hydrothermal alteration of organic-rich diatomaceous sediment by seawater was modelled experimentally at 350°C, 500 bars and seawater/sediment mass ratio of 3. The experiment was performed to assess the effect of organic matter reactivity on solution speciation and sediment alteration processes at an elevated temperature and pressure and provide requisite data to better understand the chemistry of hydrothermal fluids issuing from vents in the Guaymas Basin, Gulf of California.Seawater chemistry changed greatly during the experiment. In particular, Na, Mg and SO 4 decreased, while ∑ CO 2, ∑ NH 3, ∑ H 2S, SiO 2, Ca, K, H 2, CH 4 and heavy and base metals increased. Moreover, owing to the thermal alteration of sediment organic matter, organic acids, phenolic derivatives and phthlate were released to solution. Examination of solid alteration products revealed the effects of extensive dissolution and precipitation processes characterized by total elimination of diatoms and formation of cristobalite, quartz (?), pyrite, pyrrhotite, mixed layer chlorite/smectite and calcite. Plagioclase feldspar (An 40) recrystallized to a more albitic form owing to Na fixation and Ca cycling to calcite. A graphitic residue was also present in the products of the experiment.Mg and Na fixation reactions during the experiment generated significant H +, although the pH measured at 25°C was approximately 6.2. SO 4 reduction and thermal alteration and dissolution of organics, however, consume H + and are chiefly responsible for the near neutral pH for the overall reaction. Speciation calculations including ammine and acetate protonation reactions give a pH at experimental conditions of approximately 5.1, while mineral solubility relations involving virtually all alteration phases require a pH of 5.57 to 5.94. A near neutral pH at experimental conditions constrains the mobility of Fe, Mn, Zn, Cu and Ni, which existed in solution as chloro-complexes. Dissolved concentrations of Pb and Al, in contrast, covaried with dissolved organics, especially acetate, suggesting organo-metallic complex formation. 相似文献
15.
The systematic sampling of the chemical composition of the groundwater from five karst springs (including an overflow spring) and one outflowing borehole have permitted to determine distinctive chemical changes in the waters that reflect the geochemical processes occurring in a carbonate aquifer system from southern Spain. The analysis of the dissolution parameters revealed that geochemical evolution of the karst waters basically depends on the availability of the minerals forming aquifer rocks and the residence time within the aquifers. In the three proposed scenarios in the aquifers, which include the preferential flow routines, the more important geochemical processes taking place during the groundwater flow from the recharge to the discharge zones are: CO 2 dissolution and exsolution (outgassing), calcite net dissolution, calcite and dolomite sequential dissolution, gypsum/anhydrite and halite dissolution, de-dolomitization and calcite precipitation. A detailed analysis of the hydrochemical data set, saturation indices of the minerals and partial pressure of CO 2 in the waters joined to the application of geochemical modelling methods allowed the elaboration of a hydrogeochemical model of the studied aquifers. The developed approach contributes to a better understanding of the karstification processes and the hydrogeological functioning of carbonate aquifers, the latter being a crucial aspect for the suitable management of the water resources. 相似文献
16.
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the
lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration
products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine
and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine
facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within
the marine units. The δ 13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰).
This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite
in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon.
The δ 34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial
seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high
(+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ 34S and δ 13C data which deviate from a negative correlation line toward higher δ 13C values suggest methanogenic CO 2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated
by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions
of organic matter, producing CH 4 and CO 2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U 6+ to U 4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low
Eh in which nearly equal concentrations of CH 4 and HCO 3
− existed and reduced sulfur species (H 2S, HS −) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by
a predominance of SO 4
2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and
measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment
in which U 4+ is stable has been maintained since precipitation of uranium minerals.
Received: 9 February 1996 / Accepted: 11 April 1997 相似文献
17.
Pyrite dissolution and interaction with Fe (II), Co (II), Eu (III) and U (VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S] aq/[Fe] aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm −2. Concentration isotherms for Eu (III) and U (VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu (III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10 −6 mol g −1. In the U (VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10 −8 mol g −1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO 2(s).Scanning electron microscopy and micro-Raman spectrometry of U (VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO 2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe (II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U (VI). 相似文献
18.
A highly reproducible seeded growth technique was used to study calcite crystallization from calcium bicarbonate solutions at 25°C and fixed carbon dioxide partial pressures between 0.03 and 0.3 atm. The results are not consistent with empirical crystallization models that have successfully described calcite growth at low PCO2 (< 10 ?3 atm). Good agreement was found between observed crystallization rates and those calculated from the calcite dissolution rate law and mechanism proposed by Plummer et al. (1978). 相似文献
19.
Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO 2 components—atmospheric CO 2 and CO 2 from in situ oxidation of organic matter. The δ 13C values measured for the Fe(CO 3)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(CO 3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO 2 components. δ 13C 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 Pco 2 value of about 1 × PAL, but the scatter in the measured δ 13C values of organic matter permits a calculated maximum Pco 2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO 3)OH in MCGoeth and SAP correspond to soil CO 2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO 2 concentrations are similar to modern soils in warm, wet environments.The average δ 13C 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 δ 13C value for all profiles of −5.4‰. Thus, the value of Δ 13C 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 Pco 2 and organic matter δ 13C values were the same for both paleosol types. Furthermore, the atmospheric Pco 2 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 Pco 2 (permitted by one standard deviation of the organic matter δ 13C value) of ∼5 × PAL.If, however, measured average δ 13C 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 Pco 2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco 2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO 2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO 2.The Fe(CO 3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO 3)OH in goethites from soils in which there are mixtures of two isotopic CO 2 components. Field-relationships and the δ 13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO 2 components at the time of goethite crystallization. The three components were probably atmospheric CO 2, CO 2 from in situ oxidation of organic matter and CO 2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO 2 components, as recorded by Fe(CO 3)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 Pco 2. 相似文献
20.
This paper reports detailed O 2 measurements of pyrite bearing sediments in a column study and their interpretation based on a hydrogeochemical modelling approach. The research focuses on the quantitative effects of effective diffusion and microbiologic activity on pyrite weathering and acidification. A column experiment was set up and O 2 saturation and moisture contents were monitored over 100 days. The anoxic material used for the column experiment was taken from a sediment core of a mining waste dump in the southern periphery of the Lohsa storage system in the Lusatia region of Germany. The measured O 2 breakthrough curves were modelled using the simulator SAPY, a one-dimensional reactive transport code which considers the kinetics of chemical reactions and the delivery of O 2 into the sediment. The simulation yielded a strong dependence of pyrite oxidation on the moisture content which was quantified by an empirical equation. It was shown that the oxidation rate was catalysed by microbial activity exceeding the rate of diffusive O 2 delivery. In order to develop a management tool for predictive issues the results have already been applied to natural environments in another study using the adapted model. 相似文献
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