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1.
Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10-20% of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.  相似文献   

2.
Both chemical weathering rates and fluid flow are difficult to measure in natural systems. However, these parameters are critical for understanding the hydrochemical evolution of aquifers, predicting the fate and transport of contaminants, and for water resources/water quality considerations. 87Sr/86Sr and (234U/238U) activity ratios are sensitive indicators of water-rock interaction, and thus provide a means of quantifying both flow and reactivity. The 87Sr/86Sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate. Similarly, the (234U/238U) ratio of natural ground waters is a balance between the flow rate and the dissolution of solids, and α-recoil loss of 234U from the solids. By coupling these two isotope systems it is possible to constrain both the long-term (ca. 100’s to 1000’s of years) flow rate and bulk dissolution rate along the flow path. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes (87Sr/86Sr) are combined with a model for (234U/238U) to constrain the infiltration flux and dissolution rate for a 70-m deep vadose zone core from Hanford, Washington. The coupled model for both (234U/238U) ratios and the 87Sr/86Sr data suggests an infiltration flux of 5 ± 2 mm/yr, and bulk silicate dissolution rates between 10−15.7 and 10−16.5 mol/m2/s. The process of α-recoil enrichment, while primarily responsible for the observed variation in (234U/238U) of natural systems, is difficult to quantify. However, the rate of this process in natural systems affects the interpretation of most U-series data. Models for quantifying the α-recoil loss fraction based on geometric predictions, surface area constraints, and chemical methods are also presented. The agreement between the chemical and theoretical methods, such as direct measurement of (234U/238U) of the small grain size fraction and geometric calculations for that size fraction, is quite good.  相似文献   

3.
The goal of this study is to explain the origin of 234U–238U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3He/4He ratios, to gain insight regarding the evolution of groundwater in the region. (234U/238U) activity ratios, or (234U/238U)act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (234U/238U)act, 1.14 ± 0.01, was measured in Ca–HCO3-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234U–238U isotopic fractionation. The (234U/238U)act increases to 6.07 ± 0.14 in Na–HCO3–Cl-type groundwater. Preferential migration of 234U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (234U/238U)act and 3He/4He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (234U/238U)act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (14C ages of 6.6 kyrs), with (234U/238U)act of ≥6.07 and large amounts of radiogenic 4He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (234U/238U)act and the addition of excess 4He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4He and 234U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4He excesses measured in groundwater.  相似文献   

4.
A mathematical model to calculate the234U/238U activity ratio (AR) in an aqueous phase in contact with rock/soil is presented. The model relies on the supply of238U by dissolution and that of234U by dissolution and preferential release from radiation damaged regions (recoil tracks). The model predicts that values of234U/238U AR>1 in the aqueous phase can be obtained only from weathering “virgin” surfaces. Thus, to account for the observed steady-state supply of234U excess to the oceans by the preferential leaching model, ‘virgin’ rock/soil surfaces would have to be continually exposed and weathered. The238U concentration and234U/238U AR in continental waters allow us to estimate the exposure rates of “virgin” rock/soil surfaces.  相似文献   

5.
The daughter to parent (234U/238U) activity ratio in natural waters is often out of secular radioactive equilibrium. The major reason for this disequilibrium is related to the energetic α-decay of 238U and differential release of 234U relative to 238U. This disequilibrium originates from (1) preferential release of more loosely bound 234U from damaged mineral lattice sites or; (2) direct recoil of 234Th into surrounding media from near mineral surface boundaries, however, it is unclear which of the two mechanisms is most important in nature. To better quantify the effects of preferential release of 234U, two continuous laboratory granite leaching experiments conducted over 1100 h were performed. The leachates were characterized by declining U concentrations with time and (234U/238U) initially greater than unity (up to 1.15), which changed to below unity during leaching (∼0.95). The early elevated (234U/238U) suggests that additional 234U is released into solution by preferential release of 234U from mineral phases. However, the excess 234U constitutes a finite pool of easy leachable 234U and the (234U/238U) values become lower than unity when this pool is used up. A model based on first-order kinetics, dissolution rates and preferential release of 234U from damaged lattice sites was developed and is able to quantitatively predict the observed pattern of (234U/238U) values and U concentrations for the two granite leaching experiments. Extending the modeling to longer time scales more comparable to natural systems shows that the production of waters with high (234U/238U) ratios can be achieved in two distinct regimes (1) slow weathering where the rate of directly recoiled 234U near mineral surfaces into waters is high; (2) fast weathering where the role of incipient chemical weathering and preferential release of loosely bound 234U are important. The model is able to explain apparent opposite correlations between physical erosion rates and (234U/238U) in waters and it provides a new framework that will be useful for examining weathering regimes, their timescales and their coupling with physical erosion.  相似文献   

6.
Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (δ44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of δ44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, δ44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the δ44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.  相似文献   

7.
Stilbite from Malmberget and Svappavara is part of hydrothermal mineral assemblages occupying regionally occurring open Palaeoproterozoic fractures in northern Sweden. At these locations, stilbite is characterized by Pbrad excess relative to U and by activity ratios of [234U]/[238U] > 1 and [230Th]/[238U] > 1. The activity disequilibrium requires a disturbance of the U-Th systematics within the last one million years. Leaching and infiltration experiments on Malmberget stilbite demonstrate: (i) preferential leaching in the order Pb >U >Th and uptake in the order Pb > U, and (ii) isotopic fractionation of U by preferential mobilization of 238U and 235U relative to 234U. Stepwise-leaching further indicates that the bulk of U is hosted in the channel sites of stilbite. The Th-U disequilibrium systematics observed in untreated Malmberget and Svappavara stilbite can be explained by: (1) addition of U with [234U]/[238U] > 1 from a fluid, or alternatively (2) loss of U from a two-component system, consisting of a component that is “open” or accessible and a component that is “closed” or inaccessible to mobilization. U addition requires a multistage history involving multiple gain or loss of U and/or Pb. In contrast, U loss does not necessarily require multistage processes but can also be explained by preferential removal of 238U (and 235U) relative to recoiled daughter isotopes such as 234U, 230Th, and 206Pb (and 207Pb) during a single event. Such a behavior could be obtained if the recoiled daughter isotopes of channel-sited uranium are implanted into the crystal lattice and, in such a way, become less mobile than their parent isotopes. This case implies an open-system behavior for ions in the channel sites and a closed-system behavior for ions in the silicate framework of stilbite. Each α-recoil directly or indirectly, i.e., through its recoil cascade, damages the silicate framework. Subsequent (continuous) low-temperature annealing of the damaged stilbite lattice could trap the recoiled daughter isotopes in the repaired crystal lattice or sealed-off channels. Such immobile recoiled material can, in part, represent the “closed” component of the system. This model can account for all observations regarding the Th-U-Pb systematics, including the Th-U disequilibrium systematics, the similarity in Th/U as deduced from Th-U disequilibrium and Pb isotope data, and the excess of radiogenic Pb (208Pb-parents also had been multiply recoiled). These two contrasting explanations involve either multistage or multicomponent systems. They do not permit the derivation of an accurate age.  相似文献   

8.
Current models using U- and Th-series disequilibria to study radioisotope transport in groundwater systems mostly consider a steady-state situation. These models have limited applicability to the vadose zone (UZ) where the concentration and migratory behavior of radioisotopes in fluid are often transitory. We present here, as a first attempt of its kind, a model simulating the non-steady state, intermittent fluid transport in vadose layers. It provides quantitative constraints on in-situ migration of dissolved and colloidal radioisotopes in terms of retardation factor and rock-water interaction (or water transit) time. For uranium, the simulation predicts that intermittent flushing in the UZ leads to a linear relationship between reciprocal U concentration and 234U/238U ratio in percolating waters, with the intercept and slope bearing information on the rates of dissolution and α-recoil of U isotopes, respectively. The general validity of the model appears to be borne out by the measurement of uranium isotopes in UZ waters collected at various times over a period during 1995-2006 from a site in the Peña Blanca mining district, Mexico, where the Nopal I uranium deposit is located. Enhanced 234U/238U ratios in vadose-zone waters resulting from lengthened non-flushing time as prescribed by the model provide an interpretative basis for using 234U/238U in cave calcites to reconstruct the regional changes in hydrology and climate. We also provide a theoretical account of the model’s potential applications using radium isotopes.  相似文献   

9.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.  相似文献   

10.
The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements.  相似文献   

11.
U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas (234U/238U) and (230Th/232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between (230Th/232Th) activity ratios and 87Sr/86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by (234U/238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/86Sr than the rocks. Moreover, (230Th/232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10-35 and from 1 × 104 to 2 × 105 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km2), a characteristic migration time from recharge to riverine discharge of 200-600 yr for U and 0.2-3.7 Myr for Th was obtained.  相似文献   

12.
Uranium (U) concentrations and activity ratios (δ234U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids (“cold seeps”) at Hydrate Ridge, off the coast of Oregon, USA. The low U concentrations (mean: 1.3 ± 0.4 μg/g) and high δ234U values (165-317‰) of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor. Their 230Th/234U ages span a time interval from 0.8 to 6.4 ka and cluster around 1.2 and 4.7 ka. In contrast, chemoherm carbonates precipitate from marine bottom water marked by relatively high U concentrations (mean: 5.2 ± 0.8 μg/g) and a mean δ234U ratio of 166 ± 3‰. Their U isotopes reflect the δ234U ratios of the bottom water being enriched in 234U relative to normal seawater. Simple mass balance calculations based on U concentrations and their corresponding δ234U ratios reveal a contribution of about 11% of sedimentary pore water to the bottom water. From the U pore water flux and the reconstructed U pore water concentration a mean flow rate of about 147 ± 68 cm/a can be estimated. 230Th/234U ages of chemoherm carbonates range from 7.3 to 267.6 ka. 230Th/234U ages of two chemoherms (Alvin and SE-Knoll chemoherm) correspond to time intervals of low sealevel stands in marine isotope stages (MIS) 2, 4, 5, 6, 7 and 8. This observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments. The δ18OPDB ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times. Deviations of the chemoherm δ18O values from the marine δ18O record can be interpreted as to reflect temporally and spatially varying bottom water and/or vent fluid temperatures during carbonate precipitation between 2.6 and 8.6°C.  相似文献   

13.
Activity profiles of excess 234Th, excess 210Pb, 232Th, 230Th, 234U and 238U, and 228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of 228Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and 210Pb in this coastal zone.Comparison of excess 234Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess 234Th have no excess 228Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of 228Th due to loss of soluble 228Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. 232Th and 230Th activities and 230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic 230Th. Steady state 210Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved 210Pb in sea waters adverting into Puget Sound.Excess 234Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess 210Pb profiles. At least six of eight 234Th profiles show that mixing within the 210Pb-defined surface mixed layer is depth dependent. In three profiles, 234Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the 234Th data are consistent with 210Pb profiles determined for the same sediments, strongly suggesting that 234Th and 210Pb are mixed equivalently and in a multilayered manner.  相似文献   

14.
《Applied Geochemistry》2002,17(6):751-779
Uranium concentrations and 234U/238U ratios in saturated-zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of Yucca Mountain, a potential high-level radioactive waste disposal site. The U data were obtained by thermal ionization mass spectrometry on more than 280 samples from the Death Valley regional flow system. Large variations in both U concentrations (commonly 0.6–10 μg l−1) and 234U/238U activity ratios (commonly 1.5–6) are present on both local and regional scales; however, ground water with 234U/238U activity ratios from 7 up to 8.06 is restricted largely to samples from Yucca Mountain. Data from ground water in the Tertiary volcanic and Quaternary alluvial aquifers at and adjacent to Yucca Mountain plot in 3 distinct fields of reciprocal U concentration versus 234U/238U activity ratio correlated to different geographic areas. Ground water to the west of Yucca Mountain has large U concentrations and moderate 234U/238U whereas ground water to the east in the Fortymile flow system has similar 234U/238U, but distinctly smaller U concentrations. Ground water beneath the central part of Yucca Mountain has intermediate U concentrations but distinctive 234U/238U activity ratios of about 7–8. Perched water from the lower part of the unsaturated zone at Yucca Mountain has similarly large values of 234U/238U. These U data imply that the Tertiary volcanic aquifer beneath the central part of Yucca Mountain is isolated from north-south regional flow. The similarity of 234U/238U in both saturated- and unsaturated-zone ground water at Yucca Mountain further indicates that saturated-zone ground water beneath Yucca Mountain is dominated by local recharge rather than regional flow. The distinctive 234U/238U signatures also provide a natural tracer of downgradient flow. Elevated 234U/238U in ground water from two water-supply wells east of Yucca Mountain are interpreted as the result of induced flow from 40 a of ground-water withdrawal. Elevated 234U/238U in a borehole south of Yucca Mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic Tertiary volcanic aquifer where it becomes diluted by regional flow in the Fortymile system.  相似文献   

15.
Abundances of 238U, 234U, 232Th, 226Ra, 228Ra, 224Ra, and 222Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that 232Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of 232Th and 230Th on aquifer grains. Uranium in the aquifer waters has very high [234U/238U] ∼ 9 and low 238U concentrations. These levels can be explained by low weathering rates in the aquifer (w238U ∼ 2 × 10−18 to 2 × 10−17s−1) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The 222Rn and 228Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [234U/238U] and high 238U concentrations. The flow model shows that it is not possible to obtain the high [234U/238U] and low [238U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.  相似文献   

16.
Factors controlling the groundwater transport of U, Th, Ra, and Rn   总被引：1，自引：0，他引：1  
A model for the groundwater transport of naturally occurring U, Th, Ra, and Rn nuclides in the238U and232Th decay series is discussed. The model developed here takes into account transport by advection and the physico-chemical processes of weathering, decay, α-recoil, and sorption at the water-rock interface. It describes the evolution along a flowline of the activities of the238U and232Th decay series nuclides in groundwater. Simple sets of relationships governing the activities of the various species in solution are derived, and these can be used both to calculate effective retardation factors and to interpret groundwater data. For the activities of each nuclide, a general solution to the transport equation has been obtained, which shows that the activities reach a constant value after a distance ϰi, characteristic of each nuclide. Where ϰi is much longer than the aquifer length, (for238U,234U, and232Th), the activities grow linearly with distance. Where gKi is short compared to the aquifer length, (for234Th,230Th,228Th,228Ra, and224Ra), the activities rapidly reach a constant or quasi-constant activity value. For226Ra and222Rn, the limiting activity is reached after 1 km. High δ234U values (proportional to the ratioɛ234Th/W238U) can be obtained through high recoil fraction and/or low weathering rates. The activity ratios230Th/232Th,228Ra/226Ra and224Ra/226Ra have been considered in the cases where either weathering or recoil is the predominant process of input from the mineral grain. Typical values for weathering rates and recoil fractions for a sandy aquifer indicate that recoil is the dominant process for Th isotopic ratios in the water. Measured data for Ra isotope activity ratios indicate that recoil is the process generally controlling the Ra isotopic composition in water. Higher isotopic ratios can be explained by different desorption kinetics of Ra. However, the model does not provide an explanation for228Ra/226Ra and224Ra/226Ra activity ratios less than unity. From the model, the highest222Rn emanation equals 2ɛ. This is in agreement with the hypothesis that222Rn activity can be used as a first approximation for input by recoil (Krishnaswamiet al 1982). However, high222Rn emanation cannot be explained by production from the surface layer as formulated in the model. Other possibilities involve models including surface precipitation, where the surface layer is not in steady-state.  相似文献   

17.
Extreme U and Pb isotope variations produced by disequilibrium in decay chains of 238U and 232Th are found in calcite, opal/chalcedony, and Mn-oxides occurring as secondary mineral coatings in the unsaturated zone at Yucca Mountain, Nevada. These very slowly growing minerals (mm my−1) contain excess 206Pb and 208Pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206Pb/238U geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect 207Pb/235U ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages.Opal and calcite from outer (younger) portions of coatings have 230Th/U ages from 94.6 ± 3.7 to 361.3 ± 9.8 ka and initial 234U/238U activity ratios (AR) from 4.351 ± 0.070 to 7.02 ± 0.12, which indicate 234U enrichment from percolating water. Present-day 234U/238U AR is ∼1 in opal/chalcedony from older portions of the coatings. The 207Pb/235U ages of opal/chalcedony samples range from 0.1329 ± 0.0080 to 9.10 ± 0.21 Ma, increase with microstratigraphic depth, and define slow long-term average growth rates of about 1.2-2.0 mm my−1, in good agreement with previous results. Measured 234U/238U AR in Mn-oxides, which pre-date the oldest calcite and opal/chalcedony, range from 0.939 ± 0.006 to 2.091 ± 0.006 and are >1 in most samples. The range of 87Sr/86Sr ratios (0.71156-0.71280) in Mn-oxides overlaps that in the late calcite. These data indicate that Mn-oxides exchange U and Sr with percolating water and cannot be used as a reliable dating tool.In the U-poor calcite samples, measured 206Pb/207Pb ratios have a wide range, do not correlate with Ba concentration as would be expected if excess Ra was present, and reach a value of about 1400, the highest ever reported for natural Pb. Calcite intergrown with opal contains excesses of both 206Pb and 207Pb derived from Rn diffusion and from direct α-recoil from U-rich opal. Calcite from coatings devoid of opal/chalcedony contains 206Pb and 208Pb excesses, but no appreciable 207Pb excesses. Observed Pb isotope anomalies in calcite are explained by Rn-produced excess Pb. The Rn emanation may strongly affect 206Pb-238U ages of slow-growing U-poor calcite, but should be negligible for dating fast-growing U-enriched speleothem calcite.  相似文献   

18.
We have measured U in benthic incubation chambers, sediment pore waters, and in sediments along the California continental margin. Sedimentary U uptake rates, based on a combination of sediment pore water profiles and benthic incubation chambers, generally agree with those predicted from sediment accumulation rate data. This agreement supports the view that most of the continental margin sedimentary U is delivered by diffusion across the seawater-sediment boundary. The average rate of authigenic U accumulation for all the sites examined here is ∼−0.2 nmol cm-2 y-1, which is consistent with published global estimates of sedimentary U uptake. In addition, the accumulation rate of U in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate. These relationships highlight the potential utility for the U accumulation rate as a proxy for these processes.  相似文献   

19.
This study presents the temporal and spatial variability of 234U/238U activity ratios in the Shu River and provides interpretation to explain the downstream changes of uranium and the 234U/238U activity ratios in the study area. The positive linear correlation (R 2 = 0.98, p < 0.001) between uranium concentration and specific electrical conductance is consistent with rock weathering and leaching as the major contributor of dissolved uranium in the studied area of the river. The 234U/238U activity ratio ranged between ~1.6 in the upper reaches of the river to ~1.15 furthest downstream. Activity ratios at specific sampling points do not show significant seasonal variability.  相似文献   

20.
The calcium isotopic compositions (δ44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (αs−f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of αs−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ44Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.  相似文献   

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