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1.
This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (234U/238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.  相似文献   

2.
The Rio Solimões/Amazonas (Amazon River) and its major tributaries have been analyzed for U-series nuclides. 238U-234U-230Th-226Ra disequilibria have been measured in the dissolved (<0.2 μm) and suspended loads (>0.2 μm) as well as bed sands. U-series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering. Moreover, while the dissolved load records present-day weathering, suspended particles integrate the erosion history over much longer time scales (>100 ka). Lowland rivers are characterized by long time scales of chemical erosion (?100 ka) resulting in a high weathering intensity. Moreover, exchange between suspended particles and the dissolved load may explain the U-series signature for these rivers. By combining U-series and Pb isotopes in suspended particles, we show that erosion in the Rio Madeira basin occurred as a multi-step process, whereby the pristine continental crust was eroded several hundreds of Ma ago to produce sediments that have then been integrated in the Cordillera by crustal shortening and are currently eroded. In contrast, recent erosion of a pristine crust is more likely for the Rio Solimões/Amazonas (<10 ka). The suspended particles of the rivers draining the Andes (Solimões/Amazonas, Madeira) suggest time scales of weathering ranging between 4 and 20 ka. This indicates that suspended particles transported by those rivers are not stored for long periods in the Andean foreland basin and the tropical plain. The sediments delivered to the ocean have resided only a few ka in the Amazon basin (6.3 ± 1 ka for the Rio Amazonas at Óbidos). Nevertheless, a large fraction of the sediments coming out from the Andes are trapped in the foreland basin and may never reach the ocean. Erosion in the Andes is not operating in steady state. U-series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady-state erosion and that is a consequence of soil destruction greater than production. By relating this observation to the short time scales of weathering inferred for the Andes (a few ka), it appears that the erosion regime has been recently perturbed, resulting in high denudation rates. A possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies. Our results indicate that erosion responds rapidly to high-frequency climatic fluctuations.  相似文献   

3.
Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.  相似文献   

4.
U-series radioactive disequilibria in basaltic lavas have been used to infer many important aspects of melt generation and extraction processes in Earth’s mantle and crust, including the porosity of the melting zone, the solid mantle upwelling rate, and the melt transport rate. Most of these inferences have been based on simplified theoretical treatments of the fractionation process, which assume equilibrium partitioning of U-series nuclides among minerals and melt. We have developed a numerical model in which solid-state diffusion controls the exchange of U-series nuclides among multiple minerals and melt. First the initial steady-state distribution of nuclides among the phases, which represents a balance between diffusive fluxes and radioactive production and decay, is calculated. Next, partial melting begins, or a foreign melt is introduced into the system, and nuclides are again redistributed among the phases via diffusion. U-series nuclides can be separated during this stage due to differences in their diffusivity; radium in particular, and possibly protactinium as well, can be strongly fractionated from slower-diffusing thorium and uranium. We show that two distinct processes are not required for the generation of 226Ra and 230Th excesses in mid-ocean ridge basalts, as has been argued previously; instead the observed negative correlations of the (226Ra/230Th) activity ratio with (230Th/238U) and with the extent of trace element enrichment may result from diffusive fractionation of Ra from Th during partial melting of the mantle. Alternatively, the (226Ra/230Th) disequilibrium in mid-ocean ridge basalts may result from diffusive fractionation during shallow-level interaction of mantle melts with gabbroic cumulates, and we show that the results of the interaction have a weak dependence on the age of the cumulate if both plagioclase and clinopyroxene are present.  相似文献   

5.
The paper presents a synthesis of the approach recently developed to constrain the nature and the age of recent element mobility in weathering profiles. The approach relies on a comparison of variations of trace elements and U-series disequilibria in weathering profiles. It is exemplified in the case of old lateritic profiles from the Kaya toposequence, Burkina Faso. Trace elements and 238U---234U---230Th disequilibria have been analysed in whole rock samples from two pits located in very contrasted topographical positions. Trace element data show that the whole toposequence is marked by an intense chemical remobilization, including uranium, from the cap to the lower part of the profiles. 238U---234U---230Th disequilibrium data outline that all the levels of the profiles are affected by recent U---Th fractionations, and that each level of the toposequence is marked by U gains and losses. The chronological approach developed in this paper leads to an age of about 400 kyr for the dismantling of the iron cap, and ages ranging from 0 to 400 kyr for U accumulation in the pink clay horizon of the profiles. The depth repartition of U accumulation in the profile implies that the remobilisation processes in this toposequence varied through time, on time constants similar to those of climatic variations. To cite this article: F. Chabaux et al., C. R. Geoscience 335 (2003).  相似文献   

6.
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   

7.
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   

8.
The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. 234U/238U and 230Th/234U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.  相似文献   

9.
Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total 232Th was gradually removed during weathering. Although the whole-rock 228Th232Th activity ratios were in equilibrium, there were large excesses of 228Th in the leachable fraction of both parent rock (228Th232Th = 2.06) and partially weathered saprolite (228Th232Th = 3–6.5), due to alpha recoil and release of daughter 228Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and 228Th232Th = 1, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and 234U238U were in equilibrium in both the whole-rock and leachable fractions. 230Th was deficient relative to 234U and 226Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.  相似文献   

10.
Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO2 m−2 year−1) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO2 m−2 year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22-15.0 mmol CO2 m−2 year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5-20 Tmol CO2 year−1 is as high as the global rate of continental silicate weathering.  相似文献   

11.
Silicate weathering of soil-mantled slopes in an active Alpine landscape   总被引：1，自引：0，他引：1  
Despite being located on high, steep, actively uplifting, and formerly glaciated slopes of the Swiss Central Alps, soils in the upper Rhone Valley are depleted by up to 50% in cations relative to their parent bedrock. This depletion was determined by a mass loss balance based on Zr as a refractory element. Both Holocene weathering rates and physical erosion rates of these slopes are unexpectedly low, as measured by cosmogenic 10Be-derived denudation rates. Chemical depletion fractions, CDF, range from 0.12 to 0.48, while the average soil chemical weathering rate is 33 ± 15 t km−2 yr−1. Both the cosmogenic nuclide-derived denudation rates and model calculations suggest that these soils have reached a weathering steady-state since deglaciation 15 ky ago. The weathering signal varies with elevation and hillslope morphology. In addition, the chemical weathering rates decrease with elevation indicating that temperature may be a dominant controlling factor on weathering in these high Alpine basins. Model calculations suggest that chemical weathering rates are limited by reaction kinetics and not the supply rate of fresh material. We compare hillslope and catchment-wide weathering fluxes with modern stream cation flux, and show that high relief, bare-rock slopes exhibit much lower chemical weathering rates despite higher physical erosion rates. The low weathering fluxes from rocky, rapidly eroding slopes allow for the broader implication that mountain building, while elevating overall denudation rates, may not cause increased chemical weathering rates on hillslopes. In order for this sediment to be weathered, intermediate storage, for instance in floodplains, is required.  相似文献   

12.
《Applied Geochemistry》2001,16(7-8):931-937
Weathering in an upland catchment on granitic parent material has been studied by chemical and mineralogical analyses of soils. Long-term weathering rates for base cations, calculated from chemical analyses of the mineral horizons from soil profiles using Zr as an internal, immobile, index element, are among the smallest recorded for Scottish soils (1.7–3.1 meq m−2 a−1), indicating that these soils are susceptible to acid deposition. Sodium is the base cation lost to the greatest extent from the soils, due to weathering of plagioclase feldspar, mainly in the coarse size-fractions. Calcium is lost not only from plagioclase feldspar, but also from hornblende, grains of which show dissolution etch pits and denticulate surface features when examined by scanning electron microscopy. Weathering of hornblende, present in basic inclusions in the granite, is a significant weathering process in these soils. A range of values for 87Sr/86Sr ratios in stream-waters confirms the spatial variability of the material supplying Ca to the streams. The current weathering rate, calculated from input–output budgets to be 28.9 meq m−2 a−1, is much greater than the long-term weathering rate, but small compared to other catchments on similar parent material.  相似文献   

13.
We evaluate whether the global weathering budget is near steady state for the pre-anthropogenic modern environment by assessing the magnitude of acidity-generating volcanic exhalations. The weathering rate induced by volcanic acid fluxes, of which the CO2 flux is the most important, can be expressed as an average release rate of dissolved silica, based on a model feldspar-weathering scheme, and the ratio of carbonate-to-silicate rock weathering. The theoretically predicted flux of silica from chemical weathering is slightly smaller than the estimated global riverine silica flux. After adjustment for carbonate weathering, the riverine dissolved bicarbonate flux is larger than the volcanic carbon degassing rate by a factor of about three. There are substantial uncertainties associated with the calculated and observed flux values, but the modern system may either not be in steady state, or additional, “unknown” carbon sources may exist. The closure errors in the predicted budgets and observed riverine fluxes suggest that continental weathering rates might have had an impact on atmospheric CO2 levels at a time scale of 103-104 years, and that enhanced weathering rates during glacial periods might have been a factor in the reduced glacial atmospheric CO2 levels. Recent anthropogenic emissions of carbon and sulfur have a much larger acid-generating capacity than the natural fluxes. Estimated potential weathering budgets to neutralize these fluxes are far in excess of observed values. A theoretical scenario for a return to steady state at the current anthropogenic acidity emissions (disregarding the temporary buffering action of the ocean reservoir) requires either significantly lower pH values in continental surface waters as a result of storage of strong acids, and/or higher temperatures as a result of enhanced atmospheric CO2 levels in order to create weathering rates that can neutralize the total flux of anthropogenic and natural background acidity.  相似文献   

14.
The paucity of weathering rates for quartz in the natural environment stems both from the slow rate at which quartz dissolves and the difficulty in differentiating solute Si contributed by quartz from that derived from other silicate minerals. This study, a first effort in quantifying natural rates of quartz dissolution, takes advantage of extremely rapid tropical weathering, simple regolith mineralogy, and detailed information on hydrologic and chemical transport. Quartz abundances and grain sizes are relatively constant with depth in a thick saprolite. Limited quartz dissolution is indicated by solution rounding of primary angularity and by the formation of etch pits. A low correlation of surface area (0.14 and 0.42 m2 g−1) with grain size indicates that internal microfractures and pitting are the principal contributors to total surface area.Pore water silica concentration increases linearly with depth. On a molar basis, between one and three quarters of pore water silica is derived from quartz with the remainder contributed from biotite weathering. Average solute Si remains thermodynamically undersaturated with respect to recently revised estimates of quartz solubility (<180 μM) but exceeds estimated critical saturation concentrations controlling the initiation of etch pit formation (>17–81 μM). Etch pitting is more abundant on grains in the upper saprolite and is associated with pore waters lower in dissolved silica. Rate constants describing quartz dissolution increase with decreasing depth (from 10−14.5–10−15.1 mol m−2 s−1), which correlate with both greater thermodynamic undersaturation and increasing etch pit densities. Unlike for many aluminosilicates, the calculated natural weathering rates of quartz fall slightly below the rate constants previously reported for experimental studies (10−12.4–10−14.2 mol m−2 s−1). This agreement reflects the structural simplicity of quartz, dilute solutes, and near-hydrologic saturation.  相似文献   

15.
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   

16.
《Chemical Geology》2007,236(3-4):199-216
The chemical characteristics of freshwaters draining the silicate rocks in the northern part of Okinawa Island were studied to understand solute generation processes, and to determine rates of chemical weathering and CO2 consumption. It was observed that the water chemistry is highly influenced by marine aerosols, contributing more than 60% of total solute. Significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration. It was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized. Carbonic acid was found to be the major chemical weathering agent, releasing greater than 80% of weathering-derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks. The flux of basic cations, weathering-derived silica and CO2 consumption were relatively high due to favourable climatic condition, topography and high rate of mechanical erosion. Silicate weathering rates for basic cations were estimated to be 6.7–9.7 ton km− 2 y− 1. Carbon dioxide consumed by silicate weathering was 334–471 kmol km− 2 y− 1 which was slightly higher than that consumed by carbonate weathering. In general, divalent cations (Mg and Ca) and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering. As a consequence, the evolution of chemical species in the freshwaters of northern area of Okinawa Island to a large extent could be explained by mixing of two components, characterized by waters with Na+ and Cl− as predominant species and waters enriched with Ca2+ and HCO3−.  相似文献   

17.
We present here the first available estimations of chemical weathering and associated atmospheric CO2 consumption rates as well as mechanical erosion rate for the Lesser Antilles. The chemical weathering (100–120 t/km2/year) and CO2 consumption (1.1–1.4 × 106 mol/km2/year) rates are calculated after subtraction of the atmospheric and hydrothermal inputs in the chemical composition of the river dissolved loads. These rates thus reflect only the low-temperature basalt weathering. Mechanical erosion rates (approx. 800–4000 t/km2/year) are estimated by a geochemical mass balance between the dissolved and solid loads and mean unaltered rock. The calculated chemical weathering rates and associated atmospheric CO2 consumption rates are among the highest values worldwide but are still lower than those of other tropical volcanic islands and do not fit with the HCO3− concentration vs. 1/T correlation proposed by Dessert et al. (2001). The thick soils and explosive volcanism context of the Lesser Antilles are the two possible keys to this different weathering behaviour; the development of thick soils limits the chemical weathering and the presence of very porous pyroclastic flows allows an important water infiltration and thus subsurface weathering mechanisms, which are less effective for atmospheric CO2 consumption.  相似文献   

18.
The geochemistry of dissolved and suspended loads in river catchments of two low mountain ranges in Central Europe allows comparison of pertinent chemical weathering rates. Distinct differences in lithology, i.e. granites prevailing in the Black Forest compared to Palaeozoic sediments in the Rhenish Massif, provide the possibility to examine the influence of lithology on weathering. Here we determine the origin of river water using the stable isotope ratio δ18OH2O and we quantify the geogenic proportions of sulphate from stable isotope ratios δ34SSO4 and δ18OSO4. Particularly in catchments with abundant pyrite, determination of the geogenic amount of sulphate is important, since oxidation of pyrite leads to acidity, which increases weathering. Our results show that spatially averaged silicate weathering rates are higher for the river catchments Acher and Gutach in the Black Forest (10–12 t/km2/yr) compared to the river catchments of the Möhne dam and the Aabach dam in the Rhenish Massif (2–6 t/km2/yr). Correspondingly, the CO2 consumption by silicate weathering in the Black Forest (334–395 × 103 mol/km2/yr) is more than twice as high as in the Rhenish Massif (28–151 × 103 mol/km2/yr). These higher rates for watersheds of the Black Forest are likely due to steeper slopes leading to higher mechanical erosion with respective higher amounts of fresh unweathered rock particulates and due to the fact that the sediments in the Rhenish Massif have already passed through at least one erosion cycle. Carbonate weathering rates vary between 12 and 38 t/km2/yr in the catchments of the Rhenish Massif. The contribution of sulphuric acid to the silicate weathering is higher in the catchments of the Rhenish Massif (9–16%) than in the catchments of the Black Forest (5–7%) due to abundant pyrite in the sediments of the Rhenish Massif. Three times higher long-term erosion rates derived from cosmogenic nuclides compared to short-term erosion rates derived from river loads in Central Europe point to three times higher CO2 consumption during the past 103 to 104 years.  相似文献   

19.
Over time periods of 106 years and longer, atmospheric carbon dioxide content is largely controlled by a balance between silicate rock weathering and CO2 sources (degassing from the Earth plus net organic carbon oxidation). Vegetation cover can affect silicate rock weathering rates by increasing soil CO2 content, stabilizing soil cover, and producing organic acids. Forests absorb more solar radiation than most other ecosystems; this tends to warm Earth's climate, especially outside of the tropics; this warmth would tend to increase silicate rock weathering rates. Here, we develop preliminary parameterizations of this effect that could be incorporated into carbonate–silicate cycle models, based on the results of general circulation model simulations.  相似文献   

20.
《Applied Geochemistry》2006,21(4):580-613
To quantify and explain the contributions by pollution and chemical weathering to their composition, we studied the chemistries of springs and surface waters in the mountainous part of the Vouga River basin. Water samples were collected during a number of consecutive summer campaigns. Recharge rates were derived from monitored discharge rates within the basin. Very large contributions by meteoric, agricultural and domestic sources to the water chemistries were found, identified by the chloride, sulfate and nitrate concentrations: on average only 1/4 to 1/3 of the solutes could be attributed to chemical weathering. Two petrologic units characterize the river basin: granites and metasediments. The waters collected within metasediment units are distinct from those in granite terrain by a higher magnesium concentration. On that basis, it could be estimated that the Rio Vouga, when leaving the mountainous part of the basin, has for some 2/5 a signature determined by chemical weathering in the metasediments. The dominant primary minerals subject to chemical weathering are plagioclase (Pl) and biotite (in granite) or Pl and chlorite (in metasediment). Kaolinite, gibbsite and vermiculite are the major weathering products where annual precipitation (P) > 1000 mm y−1, and kaolinite, vermiculite and smectite where P was lower. Using an algorithm based on the ratio of dissolved silica to bicarbonate, the contributions of chemical weathering of primary minerals could be unraveled. The results show that in granite the export rate (as mol ha−1 y−1 wt%mineral−1) of oligoclase (Pl with An10–30) was 5.0 ± 2.6 and of biotite 3.2 ± 2.6, while in metasediment these rates for albite (Pl with An0–10) are 16.5 ± 8.9 and for chlorite are 0.5 ± 0.5. The observed decrease of dissolved silica in surface waters relative to springs was ascribed to (summer) uptake by aquatic biota.  相似文献   

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