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1.
The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited ∼14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m−2 yr−1—similar to rates in other ∼10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.  相似文献   

2.
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   

3.
The plagioclase of the kaolinised granite of Tirschenreuth is decomposed quantitatively and the biotite almost quantitatively, on the contrary potassium feldspar and mucovite of the parent rock remained unaffected by kaolinisation. The quantity of produced kaolinite is equivalent to the quantity of decomposed plagioclase and biotite. The rare elements Pb, Cu, Cr, Ni, P, and Ti from the kaolinised minerals are adsorbed quantitatively by the kaolin. All other elements are diminished, no element has been added to the kaolin from other sources than from parent rock material. The selective decomposition of plagioclase and biotite in the kaolin-feldspar deposit of Tirschenreuth and the association of rare elements in the kaolin are explainable by weathering processes only. Kaolins from Hirschau-Sehnaittenbach are characterised by extremely high concentrations of the rare elements Ba, Sr, Pb, Cu, and P. The association of rare elements of the kaolin has been originated from the decomposed potassium feldspar of the kaolinised arcoses. The rare elements Pb, Cu, Cr, and P are adsorbed quantitatively by the kaolin. The other elements are diminished, Ba and Sr inclusive and in spite of their high absolute concentrations. The composition of the arcoses of Hirschau-Schnaittenbach prior to kaolinisation is calculated to 56% of quartz and 44% of potassium feldspar by means of the quantities of the elements Pb and Cu analysed in the feldspar and in the kaolin. The kaolin of Hirschau-Schnaittenbach has originated by weathering of the potassium feldspar of the Triassic arcoses. The kaolinisation is of Triassic age because kaolins of the Hirschau-Schnaittenbach type were redeposited near Ehenfeld in the Cenomanian. Kaolin deposits of hydrothermal origin are characterised by a zonal structure and are associated by ores. They are different in geological behaviour and mineral association from kaolin deposits which originated by weathering of feldspathic rocks like the deposits of Tirschenreuth and Hirschau-Schnaittenbach (Upper Palatinate, Bavaria).  相似文献   

4.
The spatial and temporal changes in hydrology and pore water elemental and 87Sr/86Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying argillic horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated horizons extending to depths in excess of 15 m. Hydraulic fluxes (qh), based on Cl mass balances, vary from 0.06 to 0.22 m yr−1, resulting in fluid residence times in the terraces of 10-24 yrs.As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO3 > HCO3 > SO4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87Sr/86Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87Sr/86Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate.Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates Rsolute are estimated from linear Na and Si pore weathering gradients bsolute such that
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5.
Weathering can be used as a highly effective relative age indicator. One such application involves etching of hornblende grains in soils. Etching increases with time (duration) and decreases with depth in soils and surficial sediments. Other variables, related to intensity of weathering and soil formation, are generally held as constant as possible so as to only minimally influence the time-etching relationship. Our study focuses on one of the variables usually held constant—climate—by examining hornblende etching and quartz/feldspar ratios in soils of similar age but varying degrees of development due to climatic factors. We examined the assumption that the degree of etching varies as a function of soil development, even in soils of similar age. The Spodosols we studied form a climate-mediated development sequence on a 13,000-yr-old outwash plain in Michigan. Their pedogenic development was compared to weathering-related data from the same soils. In general, soils data paralleled weathering data. Hornblende etching was most pronounced in the A and E horizons, and decreased rapidly with depth. Quartz/feldspar ratios showed similar but more variable trends. In the two most weakly developed soils, the Q/F ratio was nearly constant with depth, implying that this ratio may not be as effective a measure as are etching data for minimally weathered soils. Our data indicate that hornblende etching should not be used as a stand-alone relative age indicator, especially in young soils and in contexts where the degree of pedogenic variability on the geomorphic surface is large.  相似文献   

6.
Biotite‐muscovite‐garnet gneisses at Einasleigh contain quartz‐feldspar veins composed of the same minerals as found in the enclosing rock. The vein‐gneiss boundaries are commonly irregular and on a microscopic scale, gradational.

Certain amphibolite layers contain quartz‐feldspar veins composed of the same minerals as found in the amphibolite. Hornblende‐rich extraction zones surround these veins, and material balance calculations show that all or nearly all of the vein‐forming matter was locally derived. Variation in the abundance of hornblende and plagioclase in the amphibolite as a function of distance from a quartz‐feldspar vein can be expressed by error‐function curves, thus suggesting that the mineral‐segregation process was diffusion‐controlled. During the mineral rearrangement, the Na and Ca contents of plagioclase have evidently remained unchanged, but the vein hornblende has become slightly richer in Fe+3, Mg, and Ca, and poorer in Si and Al relative to hornblende in the adjacent amphibolite.

A certain biotite‐plagioclase rock forms layers and boudins in the gneisses and contains pegmatite veins composed of the same minerals as found in the host rock. The plagioclase in these veins is more sodic than that in the host rock while the biotite contains slightly more Ti and Fe+2 and less Si and Mg than the biotite of the enclosing rock.

The data indicate that significant portions of the vein‐forming matter at Einasleigh were locally derived. The chemistry of some minerals has changed slightly during the segregation process, resulting possibly from different diffusion rates for the different mineral‐forming constituents.  相似文献   

7.
描述了吉林东部花岗岩风化壳的特征。研究了花岗岩风化过程中主要造岩矿物的变化特点;建立了主要造岩矿物的风化序列。研究了主量元素、微量元素及稀土元素的地球化学行为;探讨了影响风化过程中元素行为的主要因素。计算了风化速率。  相似文献   

8.
The Rico paleothermal anomaly, southwestern Colorado, records the effects of a large hydrothermal system that was active at 4 Ma. This hydrothermal system produced the deep Silver Creek stockwork Mo deposit, which formed above the anomaly's heat source, and shallower base and precious-metal vein and replacement deposits. A 65 Ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased away from the centra heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stock-work molybdenum deposit and consisted of upwelling, high-temperature fluids that produced the proximal alteration facies. Distal facies alteration was produced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na2O, large addition of CaO, and variable SiO2 exchante. Distal propylitized latite samples lost small amounts of Na2O and CaO and exchanged minor variable amounts of SiO2. The edge of the plume is marked by steep Na2O exchange gradients. Na2O exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars. Initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase, and the initial molar ratio of reactants (alkali feldspar albite component to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldspar albite component in the proximal facies reacted while the, primary plagioclase was still unreacted, but the ratio for these assemblages increased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na2O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.  相似文献   

9.
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10.
1IntroductionThecomponentsoftheenvironmentaredamagedbyagriculturalpractices.Soilslosstheirfer tilityandwaterdeterioratesduetopollution .Chemicalweatheringofmineralandsoiloccursattheinterfacesbetweentheliquidandsolidphases;consequentlythesurfaceareaandcompositionofthemineralsplayanimportantroleinthisprocess.GarrelsandMackenzie ( 1 96 7)suggestedthatincon gruentweatheringreactionswouldproducedissolvedspeciesandnewsolidsthataremorestableintheweatheringenvironmentthantheoriginalbedrockminerals.Ro…  相似文献   

11.
The Parnell Quartz Monzonite in the Pilbara Block of Western Australia is a Proterozoic (1731 ± 14 Ma) pluton characterized by high modal K‐feldspar and a greater abundance of hornblende relative to biotite, as is typical of Phanerozoic monzonitic rocks in eastern Australia. The only geochemical features reflecting its setting in an Archaean terrain are high Na2O, Ni and Cr. The pluton is zoned, with an increase in K‐feldspar, quartz and biotite and a decrease in plagioclase and hornblende from margin to core. Chemically, this zoning is reflected by systematic variation of CaO, K2O, Na2O, Sr and Rb, but ferromagnesian elements have irregular trends, implying preferential extraction of feldspars relative to mafic minerals during differentiation of the magma. The unusual geochemical trends are explained by a model involving ‘in situ’ feldspar fractionation of a K‐rich residual liquid from a mafic crystalline mush.

A parent magma similar to the average rock composition of the pluton is deduced because high ferromagnesian trace element abundances preclude extensive fractionation of mafic minerals. Geochemical and isotopic constraints suggest that the ultimate source was chemically similar to a shoshonitic basaltic andesite, that must have been emplaced beneath the eastern margin of the Pilbara Block in the Early Proterozoic. Subsequent partial melting of this postulated underplated source at ～ 1700 Ma to produce the Parnell Quartz Monzonite was probably associated with tectonism in the Gregory Range Complex.  相似文献   

12.
The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial87Sr/86Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.  相似文献   

13.
Contents of major and most trace elements of granitoids in three intrusions associated with the Cretaceous Independence volcanic complex, Montana, correlate well with SiO2. Major-element contents in granitoids in each intrusion are accurately modeled as mixtures of minimum melts and phenocryst assemblages (presumably restite). Restite assemblages are hypersthene+augite+plagioclase, hornblende+plagioclase, and biotite+plagioclase+quartz. Residues of melting are granulite or amphibolite. Melts in two of the bodies were LREE-enriched but unfractionated in MREE and HREE. REE patterns are consistent with residues dominated by pyroxene or amphibole and feldspar. Initial 207Pb/204Pb and 206Pb/204Pb of granitoids define a line interpreted as a secondary isochron established during crustal homogenization 3.3 Ga ago. The relatively low of source rocks (8.25) suggests that they did not spend long in U-rich environments. Source regions had variable trace element patterns; Th/Pb and U/Pb were correlated, Rb/Sr and Sm/Nd moderately well correlated, but Rb/Sr and U/Pb were decoupled. This is consistent with poor correlation of Rb, Sr and Ba with SiO2 in some granitoids and may suggest that minor phases that concentrate these elements were inhomogeneously distributed in source regions. The source probably consisted of LREE-rich, Rb-poor metamorphic rocks. Archean amphibolites, exposed in the Beartooth Mountains, are similar to the postulated source materials. They contain plagioclase, hornblende, minor quartz, biotite, and muscovite, and have low Rb/Sr and high LREE/HREE. Certain trace-element characteristics of the granitoids indicate that the deep crust in this part of Montana may be dominated by metamorphosed mafic-intermediate lavas that formed on the sea-floor. Metapelites, intercalated with amphibolites at the surface, were rare in granitoid source regions. This buried supracrustal pile was isotopically homogenized 3.3 Ga ago. Although some material melted 2.7 Ga ago to form granites that dominate the exposed basement, enough remained fertile that heating by mantle-derived magmas 85–90 Ma ago produced the granitic rocks at Independence.  相似文献   

14.
 The Middle Miocene Tobe hornfels in the Sanbagawa metamorphic belt, western Shikoku, southwest Japan, is characterized by an abnormally steep metamorphic gradient compared with other hornfelses associated with intrusive bodies. The basic hornfels, originally Sanbagawa greenschist rocks, is divided into the following three metamorphic zones: plagioclase, hornblende, and orthopyroxene. The plagioclase zone is defined by the appearance of calcic plagioclase, the hornblende zone by the assemblage of hornblende+calcic plagioclase+quartz, and the orthopyroxene zone is characterized by the assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz. Calcic amphibole compositions change from actinolite to hornblende as a result of the continuous reactions during prograde metamorphism. Petrographical and thermometric studies indicate a metamorphic temperature range of 300–475°C for the plagioclase zone, 475–680°C for the hornblende zone, and 680–730°C for the orthopyroxene zone. The temperature gradient based on petrological studies is approximately 5°C/m, which is unusually high. Geological and petrological studies demonstrate that the hornfelses were formed by the focusing of high-temperature fluids through zones of relatively high fracture permeability. The steep thermal gradient in the Tobe hornfels body is consistent with a large fluid flux, greater than 8.3 × 10–7 m3 m–2S–1, over the relatively short duration of metamorphism, approximately 100 years. Received: 10 October 1995 / Accepted: 28 May 1996  相似文献   

15.
Enclaves in intermediate-acid plutons from Tongling can be divided into three types: xenoliths, relics and magmatogenic enclaves. The magmatogenic enclaves consist of cumulates, micrograined dioritite mixtite and dioritic chilled border enclaves. Petrologically, relics with eyed and meta-poikilitic texture are characterized by high content of biotite (>80%) and low content of cordierite and grossular. The cumulates with accumulate texture consist of a great amount of pyroxene, hornblende and minor spinel and phlogopite. The micrograined dioritic mixtite is composed of more hornblendes and feldspar and less needle apatites and an ellipsoid basic core included in plagio-clase. The chilled border enclaves have the same mineral association, but more dark minerals than the host rocks consisting of plagioclase, quartz, alkaline feldspar, hornblende and biotite. Geochemically, the relics exhibit high REE content (455.8×10-6) and high ratio of LREE/HREE, more obvious Eu negative anomaly and are rich in Cr and  相似文献   

16.
《Applied Geochemistry》1994,9(2):119-125
Uptake of K, Ca, Mg and Na by vegetation in two upland ecosystems, one situated in andesitic parent material and the other on mica-schists, has been studied in relation to the mineralogy of the soils and the rates of weathering of base cations. Rates of weathering were calculated by two methods: (1) long-term rates were calculated from losses of the elements in soil horizons using Zr as an internal index; (2) current rates were calculated from input-output budgets using rain-water and stream chemistry over a 3 a period. Vegetation uptake can be related in a general way to mineralogy and weathering trends. Most of the K released from the soil appears to be taken up by the vegetation and the content of K in grass-dominated plant communities is related mainly to the availability of K from the weathering of mica. High levels of Ca in the streams indicate an overabundance of Ca which is being released mainly by weathering of plagioclase feldspar. Amounts of Mg in the vegetation are related to variations in the content of chlorite which is easily weathered at both sites.  相似文献   

17.
In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick zone of partially weathered rock layers (∼2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive ΔV of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction.Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, forming “altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 × 10−14 mol biotite m−2 s−1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 μm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone.Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet-saprolite interface, hornblende dissolves to completion at a rate of 6.3 × 10−13 mol hornblende m−2 s−1: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O2 at the bedrock-saprolite interface.  相似文献   

18.
Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca2+ (50%) > Si (22%) > Na+ (17%) > Mg2+ (6%) > K+ (5%). For major anions, the relative molar abundance is HCO3− (89%) > SO42− (7%) > Cl− (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca2+ and HCO3− mass-balance. Stream water Ca/Sr and 87Sr/86Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; 87Sr/86Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; 87Sr/86Sr = 0.7076 to 0.7097).Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca2+ and ∼30% more Sr2+ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr2+ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.  相似文献   

19.
The stable isotope composition of strontium (expressed as δ88/86Sr) may provide important constraints on the global exogenic strontium cycle. Here, we present δ88/86Sr values and 87Sr/86Sr ratios for granitoid rocks, a 150 yr soil chronosequence formed from these rocks, surface waters and plants in a small glaciated watershed in the central Swiss Alps. Incipient chemical weathering in this young system, whether of inorganic or biological origin, has no resolvable effect on the 87Sr/86Sr ratios and δ88/86Sr values of bulk soils, which remain indistinguishable from bedrock in terms of Sr isotopic composition. Although due in part to the chemical heterogeneity of the forefield, the lack of a resolvable difference between soil and bedrock isotopic composition indicates that these soils have thus far witnessed minimal net loss of Sr; a low degree of chemical weathering is also implied by bulk soil chemistry. The isotopic composition of Sr in streamwater is more radiogenic than median soil, reflecting the preferential weathering of biotite in the catchment; streamwater δ88/86Sr values, however, are indistinguishable from bulk soil δ88/86Sr values, implying that no resolvable fractionation of Sr isotopes takes place during release to the weathering flux in the Damma forefield. Analyses of plant tissue reveal that plants (Rhododendron and Vaccinium) preferentially assimilate the lighter isotopes of Sr such that their δ88/86Sr values are significantly lower than those of the soils in which they grow. Additionally, δ88/86Sr values of foliar and floral tissues are lower than those of roots, contrary to observations for Ca, for which Sr is often used as an analogue in weathering studies. We suggest that processes that discriminate against Sr in favour of Ca, due to the different nutritional requirement of plants for these two elements, are responsible for the observed contrast.  相似文献   

20.
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   

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