Bedrock Weathering Research Articles

Mineral Magnetic Properties of Polluted Topsoils: A Case Study in Fujian Province, Southeast China

AbstractWe have conducted mineral magnetic investigations of polluted topsoil samples, which were collected in the vicinity of a steel plant and a coal‐burning power plant in Sanming City, Fujian Province, southeast China. The methods used include the temperature‐dependence of magnetic susceptibility, hysteresis loops and isothermal remanent magnetization acquisition curves. High‐temperature susceptibility measurements suggest that magnetite, partially oxidized magnetite, hematite and pyrrhotite exist in the polluted topsoil samples. The studied samples show mean magnetic grain‐sizes of large pseudo‐single domain, and multidomain‐like or multidomain. For the samples with significantly high susceptibility (x) and low frequency‐dependent susceptibility (x), x and xfd measurements can be used as an effective method for detecting the polluted topsoils. For the samples with relatively low x and high xfd, the presence of ferrimagnetic iron sulphide can serve as a potential pollution indicator in the case that an origin from bedrock weathering has been excluded. We further suggest that mineral magnetic measurements can provide a better and more complete understanding of the pollution processes forindustrially‐polluted topsoils.

Modeling of the interactions between forest vegetation, disturbances, and sediment yields

The controls of forest vegetation, wildfires, and harvest vegetation disturbances on the frequency and magnitude of sediment delivery from a small watershed (∼3.9 km2) in the Idaho batholith are investigated through numerical modeling. The model simulates soil development based on continuous bedrock weathering and the divergence of diffusive sediment transport on hillslopes. Soil removal is due to episodic gully erosion, shallow landsliding, and debris flow generation. In the model, forest vegetation provides root cohesion and surface resistance to channel initiation. Forest fires and harvests reduce the vegetation. Vegetation loss leaves the land susceptible to erosion and landsliding until the vegetation cover reestablishes in time. Simulation results compare well with field observations of event sediment yields and long‐term averages over ∼10,000 years. When vegetation is not disturbed by wildfires over thousands of years, sediment delivery is modeled to be less frequent but with larger event magnitudes. Increased values of root cohesion (representing denser forests) lead to higher event magnitudes. Wildfires appear to control the timing of sediment delivery. Compared to undisturbed forests, erosion is concentrated during the periods with low erosion thresholds, often called accelerated erosion periods, following wildfires. Our modeling suggests that drainage density is inversely proportional to root cohesion and that reduced forest cover due to wildfires increases the drainage density. We compare the sediment yields under anthropogenic (harvest) and natural (wildfire) disturbances. Disturbances due to forest harvesting appear to increase the frequency of sediment delivery; however, the sediment delivery following wildfires seems to be more severe. These modeling‐based findings have implications for engineering design and environmental management, where sediment inputs to streams and the fluctuations and episodicity of these inputs are of concern.

Historical trace metal fluxes in the Mexico City Metropolitan Zone as evidenced by a sedimentary record from the Espejo de los Lirios lake.

The accumulation of selected trace metals (Ag, Cd, Co, Cr, Cu, Hg, Ni, Pb, V and Zn) was studied in a sediment core collected at Espejo de los Lirios lake, a precipitation-dominated seepage lake in Northern Mexico City Metropolitan Zone (MCMZ). A (210)Pb-derived chronology, obtained from the same core, was used to reconstruct the historical metal fluxes at the site, allowing evaluation of the impact of environmental changes promoted by the development of the City during the last approximately 125 years. The highest levels of metal enrichment above natural concentration levels (NCL) in the sediments from Espejo de los Lirios lake were found for Ag and Pb (approximately 250%) as well as a slight enrichment for Cd (55%), Cr (84%), Co (20%), Cu (60%), Hg (47%), Ni (45%), V (59%) and Zn (66%). Fluxes of trace metals appeared to have noticeably increased from the last 45 years showing the maximum increments for Cd, Co, Cr, Ni, V and Zn during the 1980's (9 to 13 fold natural fluxes), for Ag and Cu (17 and 12 fold, respectively) during the 1990's and for Hg and Pb (2 and 13 fold) during the middle 1970's. Low levels of metal enrichment observed have evidenced that the most conspicuous consequences of the expansive growth of this area of the MCMZ, are mostly related to deforestation and erosion of the surrounding areas, rather than to trace metal pollution. Based on PCA, it can be assumed that atmospheric deposition, weathering of bedrock and soil within the watershed and authigenic production, are the most important processes that explain the trace metal distribution in the site.

Measuring the Extent of Chemical Weathering in Subarctic Alpine Environments: Implications for Future Research

Chemical transformations associated with the weathering of bedrock to regolith, and ultimately to soils are poorly known despite the importance of these transformations in the generation of surface and groundwater solutes, the preparation of bedrock for erosion, the production of plant nutrients, and ultimately the formation of soil itself. This lack of knowledge is particularly apparent in polar and subpolar lands where traditionally chemical processes have been regarded as subordinate to mechanical weathering processes. This paper examines the nature of chemical changes associated with the transformation of bedrock to soil in three subarctic alpine locations by comparing weathering indices calculated from whole-rock chemical analyses of surface soil, regolith, rock fragments, and bedrock. The results of this largely post hoc integration of our previous reconnaissance studies suggests that as bedrock weathers to soil there is a loss of silicon, alkali earths, and alkalis relative to resistant chemical constituents, as might be expected from first principles. However, the loss is not systematic, as bedrock fragments derived from crustal rock represent the most resistant elements of a weathering/soil profile, and surface soils commonly experience eolian addition of chemical constituents. The results of this study are viewed as being “enticing” and suggest a potentially productive line of chemical weathering investigation in polar and subpolar environments that will throw light on patterns of release of chemical constituents in the initial stages of the production of residual soils.

Investigation of in situ weathering of quartz diorite bedrock in the Rio Icacos basin, Luquillo Experimental Forest, Puerto Rico

The Rio Icacos basin, in Puerto Rico, is the site of the highest measured chemical solute fluxes for a catchment on granodiorite; this is partly attributable to high annual rainfall (4300 mm), high average temperature (23 °C), and moderate relief. The bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock (0.5–1.5 m thick) underlying saprolite. These zones are characterized by systems of onion-skin “rindlets” (each 3–10 cm thick) in which porosity development is dominated by weathering of plagioclase to kaolinite. Fe-bearing aluminosilicate minerals hornblende, augite, and biotite persist in the weathered rock after plagioclase has weathered to completion, but hornblende and augite disappear between the porous rindlet systems and saprolite. A new watershed-scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock (except quartz and biotite) to kaolinite and Fe 3+ oxides. Our findings indicate that steady-state assumptions are reasonable and useful in studying weathering at this site. Estimated steady-state sediment yields (based on net solute fluxes and average bedrock composition) of 3.24×10 −9 to 3.60×10 −9 kg m −2 s −1 are smaller than measured values, indicating that sediment generation is currently accelerated with respect to steady-state; however, sediment generation has recently been closer to steady-state. Since weathering within the rindlet systems occurs on multiple fronts moving upward and downward, two new steady-state conceptual models are developed to account for (1) multiple parallel weathering fronts, and (2) differential rates of advance of weathering fronts due to porosity development. Based on these models, in situ weathering rates for plagioclase at two sample locations of 2.7×10 −15 and 5.0×10 −15 mol Na g −1 s −1 are calculated, values comparable to BET surface area normalized rates of 2.7×10 −16 to 5.0×10 −15 mol Na m −2 s −1.

Modeling unsaturated flow and transport in the saprolite of fractured sedimentary rocks: Effects of periodic wetting and drying

Whereas a number of unsaturated zone modeling studies have been conducted on dual‐porosity systems where the matrix has low permeability, few have been conducted on systems where the matrix has relatively high permeability. In humid climates, in situ weathering of bedrock can form saprolite. Saprolite usually has high matrix porosity (and variable permeability) that is a reservoir for solute storage and can have relict fractures that transport solute rapidly. On the basis of a field investigation where natural chemical tracers were monitored at high resolution during storm events, a numerical model was created that simulated variably saturated transport in the saprolite. A series of simulations were performed to explore solute transport during cycles of wetting and drying. Modeling results indicated that the advective flux of solutes from the fractures into the matrix during wetting was greater than from the matrix back into the fractures during drying, resulting in a net storage of solutes in the matrix. We hypothesize that the amount of net solute storage in the matrix may increase as the frequency of wetting/drying cycles increases, up to an optimum frequency. At frequencies higher than the optimum, the amount of solute storage in the matrix may decrease because the system behaves more like a fully saturated system where diffusion is the dominant transport process between fractures and matrix. These conclusions have significant implications for such processes as remedial strategies for contaminants in the unsaturated zone, the application of fertilizers, and quantification of mineral weathering and dissolution rates.

Diatoms as quantitative indicators of July temperature: a validation attempt at century-scale with meteorological data from northern Sweden

This study evaluates the ability of diatoms to estimate past changes in temperature by directly comparing diatom-based inferences of mean July air temperature (July T) with measured values of century-long meteorological records. The comparison includes sediment cores from three lakes in northern Sweden (Alanen Laanijärvi, 365 m above sea level (a.s.l.), Lake 850, 850 m a.s.l. and Lake Njulla, 999 m a.s.l.) dated by radioisotopic methods ( 210Pb, 137Cs) and records from meteorological stations in Kiruna and Abisko. In the sediment cores of all three study lakes, benthic species dominate the diatom assemblages with percent abundance ranging from 60 to 80%. Over the past century, the diatom-based quantitative July T inferences (weighted averaging partial least squares regression and calibration) based on a regional calibration set correspond in general closely with the meteorological records. In addition, the proportion of planktonic diatoms in the stratigraphy reflects the meteorological record as well. For example, in Alanen Laanijärvi, the proportion of planktonic diatoms (e.g. Cyclotella comensis) has increased markedly since 1985, coincident with a marked increase of measured July T (>1°C) at the meteorological station in nearby Kiruna. In contrast, July T in Abisko has remained relatively constant since 1985 and relatively little changes are observed in diatom assemblages in the two nearby lakes, Lake 850 and Lake Njulla. Interestingly, periods of relatively weak correspondence between measured and inferred July T (at Lake 850 from 1910 to 1940, at Lake Njulla from 1955 to 1990) correspond with periods when diatom-inferred lake-water pH shows distinct trends of lake-water pH change. Because lake-water pH is a stronger factor influencing diatom community composition than July T, the accuracy of diatom-inferred July T appears to be reduced during periods when lake-water pH fluctuates. Overall, we conclude that diatoms can provide reliable estimates of July T over the past century from sediment samples in northern Sweden and the reliability of the inferences further increases when lake-water pH remains constant. One palaeoecological implication based on these findings is that the early Holocene is likely a problematic period for estimating July T from diatoms, because natural processes such as soil development, catchment stabilisation and weathering of bedrock resulted in a marked natural decline of lake-water pH. Similarly, short-term stochastic events, such as sediment in-wash due to erosional events, forest clearance and forest fires, also may lead to inaccurate diatom–temperature inferences through changes in lake-water pH. In order to identify periods that may affect the accuracy of diatom-based July T inferences, we suggest that diatom analyses should be combined with other palaeoecological proxy indicators (e.g. pollen, chironomids).

Rock Outcrop Plant Communities (Glades) in the Ozarks: A Synthesis

stress. Perennial grasses are largely confined to edges of openings adjacent to surrounding forest. The distinct vegetational zones of shallow soil are dominated by annual dicots, annual grasses, and perennial dicots, including some endemic species; these are called glades in the Southeast. In the Ozarks, glade areas are typically strongly sloping, so that soil erosion frequently exceeds weathering of bedrock. Soil removal rates vary greatly over short distances, producing a mosaic of soil depths and, thus, a mosaic of plant species with different depth requirements. Plants with different soil depth requirements are intermixed or distributed in small patches, rather than in distinct zones, so fire-supporting perennial grasses are interspersed among shallow soil herbs and patches of deeper soil that support cedar trees (Juniperus). Perennial grasses support periodic fires that kill most woody plants and promote continued grass abundance. Because shallow soil plants are interspersed among deeper soil plants in the glade mosaic, studies of vegetation of glades (the total vegetational mosaic) in the Ozarks find perennial grasses and cedars dominant, with shallow-soil plants, including endemics, as minor associates.

Trace element differentiation in ferruginous accumulation soil patterns under tropical rainforest of southern Cameroon, the role of climatic change

Regions under tropical rainforest cover, such as central Africa and Brazil are characterised by degradation and dismantling of old ferricrete structures. In southern Cameroon, these processes are relayed by present-day ferruginous accumulation soil facies, situated on the middle and the lower part of hill slopes. These facies become progressively harder towards the surface, containing from bottom to top, mainly kaolinite, kaolinite–goethite and Al-rich goethite–hematite, and are discontinuous to the relictic hematite-dominated ferricrete that exist in the upper part of the hill slope. These features were investigated in terms of geochemical differentiation of trace elements. It appears that, in contrast to the old ferricrete facies, the current ferruginous accumulations are enriched in transitional trace elements (V, Cr, Co, Y, Sc) and Pb, while alkali-earth elements are less differentiated. This recent chemical accumulation is controlled both by intense weathering of the granodiorite bedrock and by mobilisation of elements previously accumulated in the old ferricrete. The observed processes are clearly linked to the present-day humid climate with rising groundwater tables. They slowly replace the old ferricretes formed during Cretaceous time under more seasonal climatic conditions, representing an instructive case of continuos global change.

Seasonal changes in groundwater input to a well‐mixed estuary estimated using radium isotopes and implications for coastal nutrient budgets

226Ra (half‐life 1,600 yr) and 228Ra (half‐life 5.75 yr) were used as tracers to determine seasonal changes in groundwater input to the Pettaquamscutt estuary from June 1999 to June 2000. Ra isotopes were observed to be in excess of activities due to input from streams, Rhode Island Sound water, and particle desorption. The source of excess 226Ra and 228Ra within the estuary is attributed to groundwater transport of Ra derived from the weathering of bedrock within the Pettaquamscutt watershed. Short‐lived 224Ra (half‐life 3.66 d) was used to calculate an average residence time of the estuary of 8 ± 4 d. Box models used to constrain seasonal changes in groundwater input to the estuary indicate highest groundwater input in the summer (6.4–20 L m−2 d−1) and lowest values in the winter (2.1–6.9 L m−2 d−1). These Ra‐derived groundwater fluxes are similar to rates of aquifer recharge over the annual cycle. Using these Ra‐derived groundwater fluxes, the input of inorganic nitrogen and phosphorus from groundwater to the Pettaquamscutt was estimated at 61–180 mmol m−2 yr−1 and 4.4‐13 mmol m−2 yr−1, respectively.

Chemical mobilizations in laterites: evidence from trace elements and 238U-234U-230Th disequilibria

Geochemical and mineralogical investigations, including measurements of major and trace elements, Sr isotope ratios, and 238U-234U-230Th activity ratios, were made on an old African laterite to reconstruct its formation steps and assess recent chemical mobilization. The present data support a scenario of discontinuous formation for the laterite, with different bedrock weathering conditions during the formation of each unit, rather than a scenario of continuous formation. Absolute accumulation of Fe, U, and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile. Present chemical distributions of lanthanides, as well as of Rb, K, Ba, and Sr, within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases. Complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons. These redistribution processes may be related to the chemical instability of the ferruginous cap. The 238U-234U-230Th disequilibria indicate that recent U mobilization occurs in the whole profile and that, as for lanthanides, there is a vertical redistribution of U from the uppermost ferruginous unit to the underlying horizons. Moreover, these data show that both U losses and gains exist at each level of the profile. A simple modeling of this double U mobilization process is proposed to interpret the 238U-234U-230Th data. Differences in the mobilization and fractionation intensities of the U input and removal processes can account for the two evolution trends, which distinguish the ferruginous unit from the underlying ones. Furthermore, on the basis of this modeling, the profile appears to be in a transient state because of recent changes in the U mobilization conditions, which could correspond to major Pleistocene climatic variations.

Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

This paper presents and reviews the processes responsible for the distribution and formation of regolith and associated landscapes of the Yilgarn Craton and highlights their implications for mineral exploration. It provides an analysis of regolith geology investigations that were conducted in many districts of contrasting contemporary geomorphic and climatic conditions.The Yilgarn Craton is composed of Archaean rocks, predominantly granitoids, that are crossed by north‐northwest‐trending belts of greenstones. It has an arid to humid climate at present. The gently undulating landsurface forms a partial etchplain and the topography is largely related to bedrock lithologies and a complex history of valley development and aggradation. Deep weathering has affected most lithologies and geological provinces across the craton. The depth of the weathered mantle may be as much as 150 m, but it varies considerably and rock outcrop may occur in any part of the landscape. The main factors influencing extent of weather...

Using 10Be and 26Al to determine sediment generation rates and identify sediment source areas in an arid region drainage basin

We measured 10Be and 26Al in 64 sediment and bedrock samples collected throughout the arid, 187 km 2 Yuma Wash drainage basin, southwestern Arizona. From the measurements, we determine long-term, time-integrated rates of upland sediment generation (81±5 g m −2 year −1) and bedrock equivalent lowering (30±2 m Ma −1) consistent with other estimates for regions of similar climate, lithology, and topography. In a small (∼8 km 2), upland sub-basin, differences in nuclide concentrations between bedrock outcrops and hillslope colluvium suggest weathering of bedrock beneath a colluvial cover is a more significant source of sediment (40×10 4 kg year −1) than weathering of exposed bedrock surfaces (10×10 4 kg year −1). Mixing models constructed from nuclide concentrations of sediment reservoirs identify important sediment source areas. Hillslope colluvium is the dominant sediment source to the upper reaches of the sub-basin channel; channel cutting of alluvial terraces is the dominant source in the lower reaches. Similarities in nuclide concentrations of various sediment reservoirs indicate short sediment storage times (<10 3 years). Nuclide concentrations, measured in channel sediment from tributaries of Yuma Wash and in samples collected along the length of the Wash, were used to construct mixing models and determine sediment sources to the main stem channel. We find an exponential decrease in the channel nuclide concentrations with distance downstream, suggesting that as much as 40% of sediment discharged from Yuma Wash has been recycled from storage within basin fill alluvium. Sediment generation and denudation rates determined from the main stem are greater (25%) than rates determined from upland sub-basins suggesting that, currently, sediment may be exported from the basin more quickly than it is being generated in the uplands. Independence of nuclide concentration and sediment grain size indicates that channels transport sediment in discrete pulses before rapidly depositing poorly sorted material, suggesting that differences in transport times for different size materials are minimal.

Calcite saturation in the River Dee, NE Scotland.

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SIcalcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO2 values indicate strong seasonal patterns, with the highest pH and lowest EpCO2 values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme 'end-member' case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SIcalcite-pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SIcalcite against pH gives a straight line with a gradient in the range 1.6-2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SIcalcite-pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column.

Major and trace element geochemistry in the upper Niger river (Mali): physical and chemical weathering rates and CO 2 consumption

This study describes the chemistry of the upper Niger river system upstream of the inland delta in Mali. It constitutes the first geochemical investigation of suspended and dissolved loads of this region of the world. The Niger, on its way between Banankoro and Ké-Macina and its tributary, the Bani river at Douna, were sampled during eight hydrological years (from 1991/1992 to 1997/1998). Major elements were determined for all 8 years and trace elements during 2 years (1996/1997 and 1997/1998). The total period of investigation is representative of the hydrological fluctuations over the last 25 years. Dissolved concentrations show that in comparison with other rivers, the upper Niger rivers are among the least mineralised and are strongly influenced by silicate rocks, which underlie almost the totality of the basin. The contribution of atmospheric precipitation to the rivers' chemistry (using the technique of marine contribution) appears very low. Normalised to the upper crust, the chemical composition of the different phases shows that in each tributary, suspended sediment and dissolved load are chemically complementary for the most soluble elements (Ca, Na, Sr, K, Ba, Rb and U). The depletion of these elements in the suspended phase (and the corresponding enrichment in the dissolved phase) is related to their high chemical mobility during silicate weathering. The variability over the hydrological year of trace elements in suspended matter has been attributed to the mixing between inputs of clay minerals and quartz. The distribution of the dissolved particulate ratio in the upper Niger basin leads to the conclusion that for the majority of elements, the sediment content controls the quantity of elements in the dissolved phase. As carbonate outcrops are insignificant in the upper Niger basin, it is possible to calculate chemical denudation rates by correcting water quality for atmospheric inputs. For the studied period, the denudation rates found range from 0.2 to 1.9×10 6 tons/year, corresponding to 26×10 3 to 100×10 3 mol CO 2/km 2/year. Physical denudation rates were also estimated and range from 3.9 to 8.2 tons/km 2/year. It appears that the low values of these chemical and physical denudation rates are related to a number of factors such as the lithology (silicate rocks), the lack of tectonism (permitting thick soil formation that limits bedrock weathering) and low precipitation (limiting runoff). Finally, a simple steady state model has been applied to the chemical composition of dissolved and solid products of silicate weathering. This model shows a good agreement between the modelled and the measured values of the suspended sediment concentrations, thus confirming the existence of steady state conditions in this tropical area.

Palaeohydrology and Bedrock Weathering in the Arid Tracts of Jaisalmer, Rajasthan

Palaeohydrology and Bedrock Weathering in the Arid Tracts of Jaisalmer, Rajasthan

Natural and human-induced landsliding in the Garhwal Himalaya of northern India

After the March 28, 1999, Garhwal earthquake, 338 active landslides, including 56 earthquake-induced landslides, were mapped in a 226-km 2-study area in the Garhwal Himalaya, northern India. These landslides mainly comprised shallow failures in regolith and highly weathered bedrock involving avalanches, slides, and flows. The total volume of active landslide debris in the region was estimated to be ∼1.3 million m 3 including 0.02 million m 3 (<2% of the total volume) moved during and within a few days of the earthquake. The denudation produced by the active landsliding within the study area is equivalent to a maximum landscape lowering of ∼5.7 mm. If active landsliding persists for a duration of between ∼1 and 10 years, then denudation due to landsliding is in the order of ∼0.6–6 mm a −1. Approximately, two-thirds of the landslides in this region were initiated or accelerated by human activity, mostly by the removal of slope toes at road cuts, suggesting that human activity is accelerating denudation in this region. Three ancient catastrophic landslides, each involving >1 million m 3 of debris, were identified and two were dated to the early–middle Holocene using cosmogenic radionuclide 10Be and 26Al. Cosmogenic radionuclide 10Be and 26Al were also used to date strath terraces along the Alaknanda River in lower Garhwal Himalaya to provide an estimate of ∼4 mm a −1 for the rate of regional denudation throughout the Holocene. Natural landsliding, therefore, contributes ∼5–50% of the overall denudation in this region and is important as a formative process in shaping the landscape.

Large-scale erosion rates from in situ-produced cosmogenic nuclides in European river sediments

We have calculated long-term erosion rates of 20–100 mm/kyr from quartz-contained 10Be in the bedload of middle European rivers for catchments ranging from 10 2 to 10 5 km 2. These rates average over 10–40 kyr and agree broadly with rock uplift, incision and exhumation rates, historic soil erosion rates, and erosion rates calculated from the measured sediment loads of the same rivers. Moreover, our new erosion rate estimates correlate well with lithology and relief. However, in the Regen, Neckar, Loire, and Meuse catchments, cosmogenic nuclide-derived erosion rates are consistently 1.5–4 times greater than the equivalent rates derived from measured river loads. This may be due to the systematic under-representation of high-magnitude, low-frequency transport events in the gauging records which cover less than a century. Alternatively the discrepancy may derive from spatially non-uniform erosion and preferential tapping of deeper sections of the irradiation profile. A third explanation relates the high cosmogenic nuclide-derived erosion rates to inheritance of an elevated Pleistocene erosion signal. Uncertainties associated with the cosmogenic nuclide-derived erosion rate estimates are not greater than the potential errors in conventional estimates. Therefore, the cosmogenic nuclide approach is an effective tool for rapid, catchment-wide assessment of time-integrated rates of bedrock weathering and erosion, and we anticipate its fruitful application to the Quaternary sedimentary record.

Elevated lead and zinc contents in remote alpine soils of the Swiss National Park.

Weathering of bedrock and pedogenic processes can result in elevated heavy metal concentrations in the soil. Small-scale variations in bedrock composition can therefore cause local variations in the metal content of the soil. Such a case was found in the remote alpine area of the Swiss National Park. Soil profiles were sampled at an altitude of about 2,400 m, representing soils developed above different bedrocks. The concentration of lead in the profiles was found to be strongly dependent on the metal content in the bedrock underlying the soil and was strongly enriched in the top 10 cm. The dolomitic bedrock in the study area contains elevated lead concentrations compared with other dolomites. Dissolution of dolomite and accumulation of weathering residues during soil formation resulted in high lead concentrations throughout the soil profile. The enrichment of lead in the topsoil, however, is largely attributed to atmospheric input. The isotopic signature of the lead clearly indicates that it is mainly of natural origin and that atmospheric deposition of anthropogenic lead contributed to about 20 to 40% to the lead concentration in the topsoil on the bedrock with elevated lead concentrations. In the soils on bedrock with normal lead concentrations, the anthropogenic contribution is estimated to be about 75%. Also, zinc was very strongly enriched in the topsoil. This enrichment was closely correlated with the organic matter distribution in the profiles, suggesting that recycling through plant uptake and litter deposition was a dominant process in the long-term retention of this metal in the soil.

Differential rates of feldspar weathering in granitic regoliths

Differential rates of plagioclase and K-feldspar weathering commonly observed in bedrock and soil environments are examined in terms of chemical kinetic and solubility controls and hydrologic permeability. For the Panola regolith, in the Georgia Piedmont Province of southeastern United States, petrographic observations, coupled with elemental balances and 87Sr/ 86Sr ratios, indicate that plagioclase is being converted to kaolinite at depths > 6 m in the granitic bedrock. K-feldspar remains pristine in the bedrock but subsequently weathers to kaolinite at the overlying saprolite. In contrast, both plagioclase and K-feldspar remain stable in granitic bedrocks elsewhere in Piedmont Province, such as Davis Run, Virginia, where feldspars weather concurrently in an overlying thick saprolite sequence. Kinetic rate constants, mineral surface areas, and secondary hydraulic conductivities are fitted to feldspar losses with depth in the Panola and Davis Run regoliths using a time-depth computer spreadsheet model. The primary hydraulic conductivities, describing the rates of meteoric water penetration into the pristine granites, are assumed to be equal to the propagation rates of weathering fronts, which, based on cosmogenic isotope dating, are 7 m/10 6 yr for the Panola regolith and 4 m/10 6 yr for the Davis Run regolith. Best fits in the calculations indicate that the kinetic rate constants for plagioclase in both regoliths are factors of two to three times faster than K-feldspar, which is in agreement with experimental findings. However, the range for plagioclase and K-feldspar rates (k r = 1.5 × 10 −17 to 2.8 × 10 −16 mol m −2 s −1) is three to four orders of magnitude lower than for that for experimental feldspar dissolution rates and are among the slowest yet recorded for natural feldspar weathering. Such slow rates are attributed to the relatively old geomorphic ages of the Panola and Davis Run regoliths, implying that mineral surface reactivity decreases significantly with time. Differential feldspar weathering in the low-permeability Panola bedrock environment is more dependent on relative feldspar solubilities than on differences in kinetic reaction rates. Such weathering is very sensitive to primary and secondary hydraulic conductivities (q p and q s), which control both the fluid volumes passing through the regolith and the thermodynamic saturation of the feldspars. Bedrock permeability is primarily intragranular and is created by internal weathering of networks of interconnected plagioclase phenocrysts. Saprolite permeability is principally intergranular and is the result of dissolution of silicate phases during isovolumetric weathering. A secondary to primary hydraulic conductivity ratio of q s/q p = 150 in the Panola bedrock results in kinetically controlled plagioclase dissolution but thermodynamically inhibited K-feldspar reaction. This result is in accord with calculated chemical saturation states for groundwater sampled in the Panola Granite. In contrast, greater secondary conductivities in the Davis Run saprolite, q s/q p = 800, produces both kinetically controlled plagioclase and K-feldspar dissolution. Faster plagioclase reaction, leading to bedrock weathering in the Panola Granite but not at Davis Run, is attributed to a higher anorthite component of the plagioclase and a wetter and warmer climate. In addition, the Panola Granite has an abnormally high content of disseminated calcite, the dissolution of which precedes the plagioclase weathering front, thus creating additional secondary permeability.