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1.
Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO2, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients.When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.  相似文献   

2.
Geochemical fluxes from watersheds are typically defined using mass-balance methods that essentially lump all weathering processes operative in a watershed into a single flux of solute mass measured in streamflow at the watershed outlet. However, it is important that we understand how weathering processes in different hydrological zones of a watershed (i.e., surface, unsaturated, and saturated zones) contribute to the total geochemical flux from the watershed. This capability will improve understanding of how geochemical fluxes from these different zones may change in response to climate change. Here, the geochemical flux from weathering processes occurring solely in the saturated zone is investigated. This task, however, remains exceedingly difficult due to the sparsity of subsurface sampling points, especially in large, remote, and/or undeveloped watersheds. In such cases, springflow is often assumed to be a proxy for groundwater (defined as water residing in fully saturated geologic formations). However, springflow generation may integrate different sources of water including, but not limited to, groundwater. The authors’ hypothesis is that long-term estimates of geochemical fluxes from groundwater using springflow proxies will be too large due to the integrative nature of springflow generation. Two conceptual models of springflow generation are tested using endmember mixing analyses (EMMA) on observations of spring chemistries and stable isotopic compositions in a large alpine watershed in the San Juan Mountains of southwestern Colorado. In the “total springflow” conceptual model, springflow is assumed to be 100% groundwater. In the “fractional springflow” conceptual model, springflow is assumed to be an integration of different sources of water (e.g., groundwater, unsaturated flow, preferential flow in the soil, etc.) and groundwater is only a fractional component. The results indicate that groundwater contributions in springflow range from 2% to 100% overall and no springs are consistently composed of 100% groundwater; providing support for the fractional springflow conceptual model. Groundwater contributions are not strongly correlated with elevation, spring contributing area, spring discharge, or seasonality. This variability has a profound effect on long-term geochemical fluxes. The geochemical fluxes for total springflow overestimate long-term solute release by 22–48% as compared to fractional springflow. These findings illustrate that springflow generation, like streamflow generation, integrates many different sources of water reflecting solute concentrations obtained along many different geochemical weathering pathways. These data suggest that springs are not always ideal proxies for groundwater. Springs may be integrating very distinct portions of the groundwater flow field and these groundwater contributions may become mixed at the spring emergence with much younger sources of water that have never resided in the groundwater system.  相似文献   

3.
Biotite is a common constituent of silicate bedrock. Its weathering releases plant nutrients and consumes atmospheric CO2. Because of its stoichiometric relationship with its transformational weathering product and sensitivity to botanical activity, calculating biotite weathering rates using watershed mass-balance methods has proven challenging. At Coweeta Hydrologic Laboratory the coupling of biotite to its transformational weathering product is only valid if the stoichiometric relationship for the two phases is known; this relationship is unlikely layer-for-layer. Rates of biotite weathering and transformation of its secondary weathering product at the Coweeta Hydrological Laboratory are comparable with other Appalachian watersheds. The magnitude and sign of the difference between field- and laboratory-determined biotite weathering rates are similar to those of other silicate minerals. The influence of major-cation proportions in biomass on the rates of biotite weathering and transformational weathering product is greatest for watersheds with high biomass aggradation rates. The watershed with the lowest bedrock reactivity and highest flushing rate yielded the highest gibbsite formation rate of ~500 mol ha?1 year?1 and lowest kaolin-group mineral formation rates of 4–78 mol ha?1 year?1. The kaolin-group mineral formation rate increases as bedrock reactivity increases and flushing rate decreases to a maximum of ~300 mol ha?1 year?1, with a similar minimum gibbsite formation rate. The relative differences in bedrock reactivity and flux of water through Coweeta Hydrological Laboratory watersheds studied appear to be invariant over geologic timescales.  相似文献   

4.
To quantify chemical weathering and biological uptake, mass-balance calculations were performed on two small forested watersheds located in the Blue Ridge Physiographic Province in north-central Maryland, USA. Both watersheds, Bear Branch (BB) and Fishing Creek Tributary (FCT), are underlain by relatively unreactive quartzite bedrock. Such unreactive bedrock and associated low chemical-weathering rates offer the opportunity to quantify biological processes operating within the watershed. Hydrologic and stream-water chemistry data were collected from the two watersheds for the 9-year period from June 1, 1990 to May 31, 1999. Of the two watersheds, FCT exhibited both higher chemical-weathering rates and biomass nutrient uptake rates, suggesting that forest biomass aggradation was limited by the rate of chemical weathering of the bedrock. Although the chemical-weathering rate in the FCT watershed was low relative to the global average, it masked the influence of biomass base-cation uptake on stream-water chemistry. Any differences in bedrock mineralogy between the two watersheds did not exert a significant influence on the overall weathering stoichiometry. The difference in chemical-weathering rates between the two watersheds is best explained by a larger proportion of reactive phyllitic layers within the bedrock of the FCT watershed. Although the stream gradient of BB is about two-times greater than that of FCT, its influence on chemical weathering appears to be negligible. The findings of this study support the biomass nutrient uptake stoichiometry of K1.0Mg1.1Ca0.97 previously determined for the study site. Investigations of the chemical weathering of relatively unreactive quartzite bedrock may provide insight into critical zone processes.  相似文献   

5.
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   

6.
长江流域面积巨大,岩性多变,加之三峡大坝等重大水利工程的影响,干流河水的水化学成因存在较大争议。此外,以往研究中流域矿物风化过程的碳汇通量估算一般基于阳离子来源分析,但该算法通常涉及多种矿物端元的参数选取,结果具有不确定性。本次研究对长江干流水化学的时空演变进行了整体分析,并基于上游河水样品HCO3~-含量的校正与计算,提出了一种计算矿物风化过程碳汇通量的新方法。研究结果表明,蒸发盐溶解、循环盐作用、矿物风化及硫酸盐溶解是控制长江干流河水离子组成的主要水文地球化学作用,而人类活动主要影响了离海距离3 000 km以内河水NO3~-含量;长江上游干流硅酸盐风化消耗CO2速率为1.16×10~5 mol/(km~2·a),碳酸盐风化消耗CO2速率为4.75×10~5 mol/(km~2·a)。本研究有助于加深对长江干流主要水文地球化学作用的认识,丰富和完善碳循环研究理论。  相似文献   

7.
There is a fast growing interest in understanding the coupling between mineralogical and biological processes responsible for the migration of elements through continental ecosystems. This issue has fundamental impacts at the soil/plant scale because it can explain the tight links between soil and plant development and at the watershed scale because it gives a direct access to the water quality. In the present study, we performed an extended investigation of the bio-geochemical cycle of boron, which is an element known to be suitable for investigating water/rock interactions and vegetation cycling. New B data are provided along the hydro-bio-geochemical continuum in a forest ecosystem (Strengbach basin, Vosges, France), from rainwaters down to the outlet of the basin including systematic analyses of throughfalls, soil solutions, springs and brooks scattered in the watershed. At the watershed scale, we evidence a relationship between the B isotopic composition of river waters and the weathering regime outlining a predominant control of the parent rock mineralogy on the B geochemical behavior. At the soil/plant scale, it appears that the B geochemical cycle is controlled by the vegetation cycling, which is characterized by an uncommon, easy to distinguish, B isotopic composition (δ11B ranging from about +30‰ to +45‰). Each year the amount of B being involved in the vegetation cycle is about four times greater than that of B being exported out of the watershed. At 10 cm depth in soil, where the plant roots are expected to be the most active, we observe a marked seasonal oscillation of the B isotopic values, which is interpreted as resulting from the vegetation activity. A mass balance calculation based on the assumption that that 10B is preferentially accumulated in the biomass tends to indicate that the soil/plant system does not behave at steady state with respect to B.Because of the very distinct B isotopic signature of vegetation and minerals in soil, box modeling allows to quantify the part of the B fluxes involved in the vegetation cycling and the mineral reactions, respectively. This calculation reveals a clear correlation between the amount of B derived from soil weathering and the amount of B absorbed by plant roots. This result clearly supports the idea that a coupling exists between mineral weathering and plant activity, for the study of which B isotopes appear particularly suitable.  相似文献   

8.
《Applied Geochemistry》1994,9(5):583-595
A steady-state geochemical model has been developed to study water-rock interactions controlling metal release from waste rock heaps at the Aitik Cu mine in northern Sweden. The Cu release in drainage waters from the site is of environmental concern. The waste rock heaps are treated as single completely mixed flow-through reactors. The geochemical model includes kinetices of sulphide and primary silicate mineral weathering, heterogeneous equilibrium with secondary mineral phases and speciation equilibrium. Field monitoring of drainage water composition provides a basis for evaluation of model performance.The relative rate of oxidative weathering of sulphides and dissolution of primary silicate minerals, using published kinetic data, are consistent with net proton and base cation fluxes at the site. The overall rate of Fe2+ oxidation within the heap is three orders of magnitude faster than that which could be explained by surface-catalysed reaction kinetics. This suggests significant activity of iron-oxidizing bacteria. The absolute weathering rates of sulphides and silicate minerals, normalized to a measured BET surface area, are approximately two orders of magnitude lower at field scale than published rates from laboratory experiments. Because of the relative absence of carbonate minerals, the weathering of biotite and plagioclase feldspar are important sources of alkalinity.  相似文献   

9.
The draw down of CO2 from the atmosphere during mineral weathering plays a major role in the global budget of this greenhouse gas. Silicate minerals remove twice the CO2 of carbonate minerals per mole of calcium in runoff during weathering. Bedrock weathering chemistry was investigated in the White River watershed of northeastern USA to investigate whether there are seasonal differences in carbonate and silicate weathering chemistry. Geographic Information Systems analyses of bedrock geology were combined with major element concentrations in river waters to gain an understanding of the consistency of mineral weathering during three seasons. The percent of carbonate mineralogy comprising the bedrock in tributaries of the White River varied from less than 5% to 45% by area. A mass balance calculation using major element concentrations in waters was applied to estimate the seasonal relationships between bedrock geology and bicarbonate flux. In all tributaries and the main stem of the White River the highest calculated percent of bicarbonate from carbonate mineral weathering was measured in the late fall. The results suggest that carbonate and silicate bedrock weathering processes are seasonally controlled. Thus single season sampling could not accurately represent an entire year's geochemical budget. In the White River, water samples obtained solely during the summer would consistently underestimate the total yearly source of bicarbonate from carbonate bedrock weathering. The same sample set would also provide data that would lead to an underestimation of the yearly atmospheric CO2 draw down by bedrock weathering in the watershed. For example at four of the seven locations studied there was an almost two-fold difference between summer and spring calculated atmospheric CO2 consumption rates.  相似文献   

10.
Watershed mass-balance methods are valuable tools for demonstrating impacts to water quality from atmospheric deposition and chemical weathering. Owen Bricker, a pioneer of the mass-balance method, began applying mass-balance modeling to small watersheds in the late 1960s and dedicated his career to expanding the literature and knowledge of complex watershed processes. We evaluated long-term trends in surface-water chemistry in the Loch Vale watershed, a 660-ha. alpine/subalpine catchment located in Rocky Mountain National Park, CO, USA. Many changes in surface-water chemistry correlated with multiple drivers, including summer or monthly temperature, snow water equivalent, and the runoff-to-precipitation ratio. Atmospheric deposition was not a significant causal agent for surface-water chemistry trends. We observed statistically significant increases in both concentrations and fluxes of weathering products including cations, SiO2, SO4 2?, and ANC, and in inorganic N, with inorganic N being primarily of atmospheric origin. These changes are evident in the individual months June, July, and August, and also in the combined June, July, and August summer season. Increasingly warm summer temperatures are melting what was once permanent ice and this may release elements entrained in the ice, stimulate chemical weathering with enhanced moisture availability, and stimulate microbial nitrification. Weathering rates may also be enhanced by sustained water availability in high snowpack years. Rapid change in the flux of weathering products and inorganic N is the direct and indirect result of a changing climate from warming temperatures and thawing cryosphere.  相似文献   

11.
Lithological maps have been proved to be a useful tool to evaluate regional and global matter fluxes from chemical weathering. However, lithological classes provide aggregated information of mineralogical, geochemical, sedimentary or diagenetic properties. Thus, using a limited number of lithological classes introduces by their definition a bias to the analysis of weathering fluxes, specifically in the case of multilithological catchments. Here it is shown that the coupling of geochemical with lithological information may help to better identify the sources of the weathering fluxes, using the example of dissolved Ca to better distinguish weathering rates of silicate and non-silicate minerals.  相似文献   

12.
The study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored.In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (<3 pmol/L, ICP-MS detection limits). Strongly insoluble secondary thorianite (ThO2) and primary zircon (ZrSiO4) crystals control their mobilization, respectively. This finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith.Chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed. Biogenic recycling significantly influences the low-Cl input fluxes. Sodium is a good tracer of chemical weathering in the watershed. The sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate. Even though low (2.8 mm/kyr), the chemical weathering rate predominates over the mechanical weathering rate (1.9 mm/kyr). Compared to the Rio Icacos watershed, the most studied tropical site, the chemical weathering fluxes of silica and sodium in the Mengong are 16 and 40 times lower, respectively. This is not only related to the protective role of the regolith, thick in both cases, but also to differences in the hydrological functioning. This is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level.  相似文献   

13.
Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm−2 ka−1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.  相似文献   

14.
Concentrations of weathering products in streams often show relatively little variation compared to changes in discharge, both at event and annual scales. In this study, several hypothesized mechanisms for this “chemostatic behavior” were evaluated, and the potential for those mechanisms to influence relations between climate, weathering fluxes, and CO2 consumption via mineral weathering was assessed. Data from Loch Vale, an alpine catchment in the Colorado Rocky Mountains, indicates that cation exchange and seasonal precipitation and dissolution of amorphous or poorly crystalline aluminosilicates are important processes that help regulate solute concentrations in the stream; however, those processes have no direct effect on CO2 consumption in catchments. Hydrograph separation analyses indicate that old water stored in the subsurface over the winter accounts for about one-quarter of annual streamflow, and almost one-half of annual fluxes of Na and SiO2 in the stream; thus, flushing of old water by new water (snowmelt) is an important component of chemostatic behavior. Hydrologic flushing of subsurface materials further induces chemostatic behavior by reducing mineral saturation indices and increasing reactive mineral surface area, which stimulate mineral weathering rates. CO2 consumption by carbonic acid mediated mineral weathering was quantified using mass-balance calculations; results indicated that silicate mineral weathering was responsible for approximately two-thirds of annual CO2 consumption, and carbonate weathering was responsible for the remaining one-third. CO2 consumption was strongly dependent on annual precipitation and temperature; these relations were captured in a simple statistical model that accounted for 71% of the annual variation in CO2 consumption via mineral weathering in Loch Vale.  相似文献   

15.
Art F. White   《Chemical Geology》2002,190(1-4):69-89
Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.

Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10−17 mol m−2 s−1 for biotite. Faster weathering rates of 1.8 to 3.6×10−16 mol m−2 s−1 are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.  相似文献   


16.
Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO2 consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO2 consumption to silicate weathering than methods utilizing Ca/Sr and 87Sr/86Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and 87Sr/86Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (PCO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca2+-Mg2+-HCO3− waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca2+ and Sr2+, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and PCO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO2. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg2+, Ca2+, and HCO3− mass balances support interpretations of preferential Ca2+ removal by calcite precipitation. On the basis of patterns of saturation state and PCO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca2+ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca2+ during riverine transport brings the Ca/Sr and 87Sr/86Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr2+ budget and suggest that the conventional application of two-component Ca/Sr and 87Sr/86Sr mixing equations has overestimated silicate-derived Sr2+ and HCO3− fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.  相似文献   

17.
The hydrochemical balance of the Yaou catchment in French Guiana was determined over a period of 1 year, combined with a detailed characterization of the primary minerals and their weathering products, in order to estimate ultramafic rock weathering rates in a rainforest environment. The time required to develop the main horizons of the laterite profile was obtained from estimations of the weathering rates and dissolution kinetics of some major parent minerals (chlorite, serpentine, biotite, calcite).

The specific transport of solute matter in the catchment is 205 kg/ha/a. The Cl and NO3 net outputs shows that the system is in dynamic equilibrium, notably with respect to the biomass reservoir. The output flux of Mg in solution is mainly supplied from the weathering of primary minerals, whereas that of Ca comes both from atmospheric contributions and from the dissolution of carbonates. The fluxes of K and, more particularly, Na derive essentially from the atmosphere. Knowing the weathering rate of primary minerals, the total CO2 consumption rate by silicate weathering is estimated at 1430 mol/ha/a.

The weathering rates of chlorite, serpentine and biotite range from 18 to 65 mol/ha/a, and those of talc and calcite are, respectively 51 and 153 mol/ha/a. Weathering rates normalized to mineral modal proportions give a decreasing order of stability resistance to weathering (calcitebiotitechlorite>serpentine>talc) that agrees with their vertical distribution in the weathering profile. The dissolution kinetics of chlorite, biotite and serpentine, expressed in relation to the Si release rate, were calculated using estimations of the mineral exchange surface by (a) optical microscope image processing of crystal outlines, and (b) BET specific surface measurements. The release rate of Si, computed for biotite, chlorite and serpentine, ranges around 10−13 and 10−14 mol/m2/s using microscope images on particle sizes. The estimated dissolution kinetics is two orders of magnitude lower when using the BET measured exchange surface, which is 100 times larger.

The saprolitization rate, calculated from the weathering rates of calcite, chlorite and biotite, is on average 7.5 m/Ma. The rate of latosol development, calculated from the weathering rate of serpentine at the saprolite–latosol interface, is estimated at 4.5 m/Ma. That means that the profile is chemically thickening at a rate of 3 m/Ma.  相似文献   


18.
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19.
黔北白云岩红色风化壳元素地球化学特征   总被引：5，自引：3，他引：2  
李锐  高杰  张莉  李今今  季宏兵 《中国岩溶》2014,33(4):396-404
选取贵州高原喀斯特地区典型白云岩风化剖面作为研究对象,探讨了红土剖面元素迁移特征及其影响因素,为进一步认识红色风化壳元素地球化学特征提供依据。结果表明:（1）红土剖面中土壤样品（XPS-4~XPS-25）Fe、K、Mn常量元素及Co、Pb、Zn微量元素的分布曲线相对于其他元素靠近 UCC,元素Mg、Ca、Na、Ti和 Li、Cr、Ba、V 的数据点则显著偏离了上部陆壳的平均组成。（2）元素对协变分析得出Ti与Fe、Ba和V的积累特征一致,呈增加趋势;Ti与Mg呈反向变化特征。（3）以Ti为参比元素的剖面迁移特征表明,绝大部分元素的地球化学行为表现为迁移淋失,仅 Fe 、Cr和V 富集。K、Ba总体上呈迁出趋势,个别层段有轻微富集;Na和Co在风化原岩处有一个富集端点。元素在风化过程中的迁移聚集与土壤剖面的pH值和剖面结构变化、粘土矿物和有机质等的吸附有关。   相似文献   

20.
《Chemical Geology》2007,236(3-4):247-265
A knowledge of the processes involved in streamflow generation are critical to an understanding of solute transport and weathering rates in upland catchments. The determination of specific flow pathways and the formulation of process-based models have proved difficult in such terrains, largely due to the heterogeneous nature of catchments and the necessary limitations of bulked parameter models. Natural geochemical tracers have proved invaluable in developing conceptual models of catchment functioning and for constraining weathering processes and geochemical cycling. Strontium isotopes have been used as a natural tracer to calculate weathering rates for Sr and Ca, and to constrain the dominant flow pathways in two upland forested sub-catchments (Afon Hafren and Afon Hore) of the River Severn at Plynlimon in Central Wales. The dominant source of Sr in the catchments is considered to be from the weathering of silicate minerals. Weathering rates for Sr and Ca in the Afon Hafren, calculated using Sr isotopes, were similar to those derived from mass balance studies. The rates for the Afon Hore were similar for Ca, but significantly different for Sr. The reason for the difference is unclear, but may be due to additional sources (calcite) in the catchment. Strontium isotope ratios for different input sources and compartments within the catchment were characterised and helped to identify potential flow pathways. The data suggest an important role for groundwater inputs in contrast to previous models which indicated a dominant role for soil waters.  相似文献   

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