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Chemical weathering rates of a soil chronosequence on granitic alluvium
... ments. In general, studies of chemical weathering processes and igneous rocks such as granite, andesite , and basalt weathering rings discuss only major element variations (Colman 1982; Nesbitt and Young 1982, 1984, 1989 Wang et al. 1991; White and Brantley 1995; White et al. 1996 White et al. , 2001 Sharma et al. 2013). There has been little study of trace element variations (White et al. 2008). ...
... Furthermore, sedimentary rocks are spread widely on Earth's surface and are generally regarded as the end products of chemical weathering. Some researchers have discussed rock outcrop profiles (Rule 1956; White et al. 1996 White et al. , 2008 Jin et al. 2010); however, studies of lake or marine environments have garnered less attention, even though they are the best delivery destination of sediments . Outcrop profiles, while used in paleoclimate research, are not an optimal choice because the chemical weathering for pre-or postdeposition cannot be accurately determined. ...
... To compare the migration ability of each element, previous studies regarded aluminum and titanium as immobile elements in weathering processes (Brimhall et al. 1991; Merritts et al. 1991; Cornu et al. 1999; Riebe et al. 2001). Quartz has long been considered an indicator of weathering intensity (Ruhe 1956 ) and has been used as a conservative component in mass balance calculations (Sverdrup and Warfvinge 1995; White et al. 1996). However, the SiO 2 content in sediment is obviously influenced by changes in provenance rocks. ...
In studies of paleoclimate change, variations in major and minor elements are often used as proxies for chemical weathering in soil or lake sediments. To prove the suitability of chemical weathering proxies and further discuss the impact of mineral dissolution on element migrations, various parent rocks in Taiwan were chosen to react with sulfuric acid for discussing mineral dissolution rankings and element leaching during chemical weathering processes. The parent rocks include granite, andesite, actinolite schist, slate, quartz sandstone, calcium sandstone, and mudstone. Experimental results show that dissolution rates are mainly controlled by mineral structures and the chemical compositions of rocks. The ranking of rock dissolution rates is as follows: mudstone 1 slate 1 actinolite schist 1 quartz sandstone > granite. Dissolution rates of andesite and calcium sandstone cannot be compared due to secondary minerals being produced in experiments. Moreover, the dissolution sequences of minerals are of the following order: calcite > chlorite > muscovite and illite > augite > hornblende ≈ feldspar > quartz. This result is consistent with the stability of silicate polymerization. Most notable among the results was that the dissolution rates of clay minerals were faster than those of feldspar. Element migration ranking in dissolution processes is as follows: Mg > Ca ≈ Rb > K > Na > Sr > Al > Zr > Fe ≈ Ti. The dissolution rates of univalent elements follow the sequence of bonding forces; however, those of divalent elements are less ordered. This is due to divalent elements being rich in different mineral phases. Therefore, the study suggests that univalent elements are more suitable as chemical weathering proxy candidates. Moreover, whether ratios of Rb/Sr and K/Rb or chemical index of alteration values in sediments are suitable proxies of paleorainfall intensity is discussed in detail.
... Chemical weathering models have been developed by many authors to investigate ecosystem sensitivity to acidic deposition (Christoffersen and Seip, 1982; April et al., 1986; Fritz et al., 1992; Probst et al., 2000 ). These models are usually based on the assumption that the most abundant minerals like plagioclase, K-feldspar, biotite and muscovite in granite bedrock are the major cation sources (Blum et al., 1994; Bullen et al., 1997; Négrel et al., 2001). Trace minerals such as calcite or apatite are often not taken into account. ...
... In spite of the restrictions discussed above, the data give some information about the weathering sequence of the Lehstenbach granite, similarly as has been found elsewhere (Bullen and Kendall, 1998; Bullen et al., 1997; Blum and Erel, 1997; Hodson et al., 1996; White et al., 2001). These results are summarized as follows. ...
... Correspondingly , some of the unweathered rock leachates and the deep groundwater samples from the Lehstenbach catchment had high factor values for this component (Fig. 8 ). This corresponds to the fact that biotite is considered to be easily weathered (Blum and Erel, 1997; Bullen et al., 1997; Hodson et al., 1996). In contrast, Drever and Clow (1995) argue that biotite weathering rates do not exceed those of feldspars due to oxide coatings attached to the minerals. ...
The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The ⁸⁷Sr/⁸⁶Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering.
... The residual mass decreases with decreasing depth in a profile while specific mineral surface area may be expected to increase, reflecting decreases in residual grain sizes and increases in surface pitting and roughness (White et al., 1996). These opposing trends may result in relatively small variations in volumetric surface areas across significant portions of a weathering profile and may be responsible for the commonly observed pseudo-linear nature of many weathering gradients (Lichtner, 1988). ...
... (15) and (16)) may reflect the failure to account for the inputs of a more radiogenic component in K-feldspar. Plagioclase weathering rates, based on any single approach , generally exhibit modest declines with increasing terrace age, an effect has been previously documented for other chronosequences (Taylor and Blum, 1995; White et al., 1996; Hodson and Langan, 1999) and for compilations of single weathering environments of different ages (White and Brantley, 2003). Due to the relatively short time scale of weathering at Santa Cruz, (65–226 kyrs), declines in weathering with age is less pronounced than in some other chronosequences such as at Merced, which has a time span of 10–3000 kyrs (White et al., 1996; White et al., 2005). ...
... Plagioclase weathering rates, based on any single approach , generally exhibit modest declines with increasing terrace age, an effect has been previously documented for other chronosequences (Taylor and Blum, 1995; White et al., 1996; Hodson and Langan, 1999) and for compilations of single weathering environments of different ages (White and Brantley, 2003). Due to the relatively short time scale of weathering at Santa Cruz, (65–226 kyrs), declines in weathering with age is less pronounced than in some other chronosequences such as at Merced, which has a time span of 10–3000 kyrs (White et al., 1996; White et al., 2005). The Santa Cruz rate data generally parallels a power law curve describing the general decline with time of average plagioclase weathering rates derived from a large number of sources (White and Brantley, 2003). ...
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.
... Long-term reaction rates are calculated from differences between elemental or mineral compositions in the initial protolith and the weathered regolith. While extensive data in the soil literature document such compositional differences, relatively few studies have tied these changes to quantitative mineral weathering rates (Sverdrup and Warfvinge, 1995; White et al., 1996; White et al., 2001). These rates are averages for the duration of protolith weathering, commonly tens of thousands to millions of years. ...
... The K-feldspar surface areas are also high (2.3 to 8.1 m 2 g -1 ) and may reflect interstitial 44 A. F. White et al. kaolinite. Quartz has the lowest specific surface areas in the regolith (0.11 to 0.42 m 2 g -1 ), which is consistent with weathered quartz surface areas reported elsewhere (0.10 to 0.23 m 2 g -1 ,White et al., 1996; 0.15 to 0.40 m 2 g -1 , Schulz and White, 1999). The contributions of specific minerals to the bulk regolith surface areas are calculated based on the wt % of each mineral present (Fig. 7C ). ...
... The mass loss ∆m j (moles) of an element j from the regolith is related to the rate of pore water movement through a unit surface area of regolith (m 2 ) by the expression (White et al., 1996): ...
Present-day elemental and mineral weathering rates based on solute fluxes are compared quanti-tatively to past long-term rates determined from solid-state elemental fractionation in a saprolitic granite re-golith at Panola, Georgia, USA. Saturated fluid flow across a low-permeability kaolin duripan controls the rate of steady-state unsaturated flow in the underlying saprolite. Water and Cl mass balances and experimen-tal conductivities produce a minimum fluid flux density of 8×10 -2 m yr -1 and a fluid residence time of 12 years. Solute Si flux, based on pore water concentrations and infiltration rates, is 27 mmoles yr -1 , compared to a long-term flux rate of 17 mmoles yr -1 , based on regolith Si loss and reported 36 Cl dating of the regolith surface. Similarities in short-and long-term fluxes imply that parameters influencing silicate weathering, in-cluding precipitation, temperature, and vegetative cover, while not necessarily constant, have not signifi-cantly impacted Si leaching rates during the last several hundred thousand years. Linear decreases in solid-state Mg with decreasing regolith depth permit the calculation of the long-term biotite weathering rate under isovolumetric steady-state weathering conditions. A rate constant of 3× 10 -17 moles m -2 s -1 is up to 5 orders of magnitude slower than that reported for experimental dissolution of biotite, implying very different reaction kinetics during natural weathering. Short-term biotite weathering fails to produce expected increases in solute Mg and K concentrations with increasing depth and fluid residence times in the regolith. This discrepancy indicates that ion exchange disequilibrium and open-system biologic uptake in an aggrading forest ecosystem are of sufficient magnitudes to overwhelm solute fluxes resulting from biotite weathering.
... Weathering characteristics associated with these different time scales may account for significant discrepancies in silicate weathering rates reported in the literature. Experimental weathering rates for a specific silicate mineral are commonly two to four orders of magnitude faster than field-derived rates (Schnoor, 1990; Brantley, 1992; White et al., 1996). In addition, the relative rates at which silicate minerals weather in the laboratory are sometimes different than rates observed in the field. ...
... This effect is most directly observed in soil chronosequences in which time becomes the principal independent variable. Commonly in such systems, the rates of primary minerals depletion and secondary clay and metal oxide formation progressively decrease with soil age (Taylor and Blum, 1995; White et al., 1996; Hodson et al., 1999; Stewart et al., 2001). To date, the relationship between primary silicate reaction rates and the duration of chemical weathering in both experimental and natural systems has not been systematically investigated. ...
... (6) and (7)). for the weathered granite is somewhat higher than values of approximately 1 m 2 g À 1 for plagioclase separated from granitic sand weathered over comparable time periods (White et al., 1996). Discussions involving the reaction rate for the weathered plagioclase must be tempered by this uncertainty in the surface area. ...
The correlation between decreasing reaction rates of silicate minerals and increasing duration of chemical weathering was investigated for both experimental and field conditions. Column studies, using freshly prepared Panola Granite, produced ambient plagioclase weathering rates that decreased parabolically over 6 years to a final rate of 7.0×10−14 mol m−2 s−1. In contrast, the corresponding plagioclase reaction rate for partially kaolinized Panola Granite, after reaching steady-state weathering after 2 months of reactions, was significantly less (2.1×10−15 mol m−2 s−1). Both rates were normalized to plagioclase content and BET surface area. Extrapolation of decreasing rates for the fresh plagioclase with time indicated that several thousand years of reaction would be required to replicate the rate of the naturally weathered plagioclase under identical experimental conditions. Both rates would remain orders of magnitude faster than field weathering rates previously measured for a weathering profile in the Panola Granite.
... The exchangeable cations were removed from the residual soil pellets after the water extraction and before the following acid extraction following the method of Amacher et al. (1990) and White et al. (2005). Specifically, the soil residues, resultant from the water-soluble extraction, were mixed with 25 mL of 0.1 M BaCl 2 -0.1 M NH 4 Cl, shaken for 15 min and then centrifuged at 3500 rpm for five minutes. ...
Soil salinization is a global problem affecting approximately 10% of agricultural soils, particularly in irrigated aridlands. This study quantified salt-loading by flood irrigation and soil fertilizers/amendments versus atmospheric deposition, studied controls of solute transport and salt buildup, and evaluated the effectiveness of gypsum application in improving soil sodicity in the arid southwestern United States. Study sites include one natural site and two agricultural sites in fields of dominant crops of the region, a pecan orchard and an alfalfa field near El Paso, Texas. The salt-loading rate in agricultural soils was dominated by the quantity and quality of irrigation waters rather than by dust. Salt loadings by irrigation waters were estimated ~ 306 g Na⁺ m⁻² yr⁻¹, 129 g Ca²⁺ m⁻² yr⁻¹, 361 g Cl⁻ m⁻² yr⁻¹, 419 g SO4²⁻ m⁻² yr⁻¹, and 284 g HCO3⁻ m⁻² yr⁻¹, followed by soil amendments. Whereas dust and fertilizer loadings were negligible in agricultural soils. Soil texture variability physically governs water movement and solute transport; coarser soils retained significantly less water than finer soils upon irrigation (p < 0.005) facilitating salt leaching. More salts accumulated around low-permeability layers. Some soils have approached salinity thresholds after only 90 years of cultivation. The Rio Grande river flow is projected to decrease due to reduced snowfall in Colorado, leading to more groundwater of higher salinity, to be used. If ground water were to be the sole water source, the salt loading rate would almost double. Soil amendments temporarily reduce soil sodicity induced by high Na⁺ concentrations in irrigation water. Their application is needed annually to prevent soil dispersion, to improve infiltration, and to stop even faster salt accumulation. This study highlighted the challenges that the Rio Grande valley in southwestern United States and other irrigated drylands are facing.
... Acta 72, 36-68. 905 White trace element compositions of suspended sediments (relative to un-weathered continental 89 crust), which were attributed to element mobility during continental weathering. In this study, 90 low weathering intensity rates of suspended sediment were found to correlate with high rates 91 of physical denudation. ...
Chemical weathering contributes to the regulation of the global carbon cycle and biogeochemical cycles. Accordingly, the identification of the parameters that control weathering reactions and transport of weathering signals at the catchment scale is essential. The use of boron (B) isotopes have been shown to be a useful proxy in tracing weathering reactions due to large isotope fractionation during weathering processes. However, our knowledge of how boron isotopes record the weathering regime at the catchment scale and how that weathering signal is transported from source areas to the depositional environment remains limited. Here we characterize B isotope and major element behavior during chemical weathering and transport by analyzing the B isotopic (δ¹¹B) and element compositions of riverine material (riverbank sands (<63 µm), clay fractions (<2 µm) extracted from sands, and dissolved load) along the course of the Murrumbidgee River (NSW, Australia), its upstream tributaries, and monolithologic subcatchments.
In the Murrumbidgee, two distinct weathering regimes are present, one where mineral dissolution is associated with minimal neoformation at higher elevations and another where mineral neoformation dominates at lower elevations and in granitic lithologies. Significant B isotope difference between the clay fraction and the bedrock (Δ¹¹Bclay-bedrock) is observed in most monolithological catchments at high mean elevation (excluding granites), which correlates with a large B depletion. Smaller isotope difference between the clay fraction and bedrock is observed in monolithological catchments at lower elevations as well as in granitic catchments at all elevations and is associated with limited B removal. These results suggest that lithology and catchment topography influence B mobility during weathering and the isotopic composition of weathering products. By mass balance calculation, the B isotope and chemical composition of the clay and sand fractions in the Murrumbidgee River can be explained as a mixture of the clays and sands produced throughout the catchment delivered to the main channel by the tributaries. These results indicate that there is little or no chemical and isotopic modification of the river sediment during fluvial transport and that weathering signal produced in the sediment source areas is transferred to the depositional environment without significant modification. The boron content of the clay-sized fraction (∼40 ppm) is several orders of magnitude greater than that of the dissolved load while B isotope compositions of the clay-sized fraction are isotopically much lighter (up to 40‰). Because a maximum isotopic difference of 30‰ between the dissolved and solid phases is expected during adsorption processes, the observed isotope compositions in the dissolved load and the sediment clay fraction cannot be explained by pH-driven B partitioning. These observations suggest that clays are not directly precipitating from solutions compositionally similar to surface waters; deeper soil solutions are expected to play a significant role in clay formation.
This research highlights the potential of B isotopes in river sediments to describe the present and past weathering regimes at the catchment scale, including possible paleoenvironment reconstruction as the B isotope signature of riverine material records the conditions of its formation.
... Harden et al., 1986;Markewich et al., 1987Markewich et al., , 1990Muhs, 1982Muhs, , 2001Wagner et al., 2007), fluvial terraces (e.g. Bilzi and Ciolkosz, 1977;Birkeland et al., 2003;Harris et al., 1980;Howard et al., 1993Howard et al., , 1995Kendrick and Graham, 2004;White et al., 2005), glacial outwash and moraines (e.g. Douglass and Mickelson, 2007;Haugland, 2004;Mahaney et al., 2009;Pinter et al., 1994), and alluvial fans (e.g. ...
The utility of potassium adsorption isotherm analysis (KIA) for relative dating and correlation of soil chronosequences was assessed by a comparison with some widely used mineralogical techniques. The soils studied, near Detroit, Michigan, are brown (10YR) calcareous Entisols developed in Holocene (~ 4 kA) floodplain sediments, and Inceptisols and Alfisols with redder (7.5YR–5YR) cambic and argillic B horizons formed on late Pleistocene (14.5–12.4 kA) fluvial terraces, fluvial paleochannels, and paleoshorelines (beach ridges) of proglacial lakes. The soils show a positive linear correlation between soil age and clay content, depth of carbonate leaching, and Fe-oxide accumulation. In contrast, the duration of soil formation is too short for variations in heavy mineral frequencies to discriminate soil age reliably. Scanning electron microscopy shows that chemical weathering stage is indicated by progressive etching and pitting of calcite, apatite and amphibole with increasing soil age. K-isotherms are linear or slightly curvilinear indicating that ion exchange is dominant over specific adsorption. A statistically significant linear correlation between K distribution constant and soil age, attributed to progressive weathering of mica, suggests that KIA is useful for relative dating of genetically related soils once a well dated chronosequence is established. KIA produces results comparable to, or better than, mineralogical techniques. However, soil morphology and a clay-redness index are more effective than mineralogical and chemical techniques for interchronosequence correlation.
... In an undisturbed soil profile, the residence time of soil and the extent of weathering are expected to increase with decreasing depth. White et al. (1996) reported an increase in soil median grain size with increasing soil age, suggesting the dependence of soil grain size distribution on size reduction and complete dissolution of smaller grains due to chemical weathering. The observed increase in median grain size with decreasing soil depth at Frogs Hollow signifies the increase of soil r ...
The sustainability of soil resources is determined by the balance
between the rates of production and removal of soils. Samples from four
weathering profiles at Frogs Hollow in the upper catchment area of the
Murrumbidgee River (southeastern Australia) were analyzed for their
uranium-series (U-series) isotopic composition to estimate soil
production rates. Sequential leaching was conducted on sample aliquots
to assess how U-series nuclides are distributed between primary and
secondary minerals. Soil is increasingly weathered from bottom to top
which is evident from the decrease in (234U/238U)
ratios and increase in relative quartz content with decreasing soil
depth. One soil profile shows little variation in mineralogy and
U-series geochemistry with depth, explained by the occurrence of already
extensively weathered saprolite, so that further weathering has minimal
effect on mineralogy and geochemistry. Al2O3 is
mobilized from these soils, and hence a silicon-based weathering index
treating Al2O3 as mobile is introduced, which
increases with decreasing soil depth, in all profiles. Leached and
unleached aliquots show similar mineralogy with slight variation in
relative concentrations, whereas the elemental and isotopic composition
of uranium and thorium show notable differences between leached and
unleached samples. Unleached samples show systematic variations in
uranium-series isotopic compositions with depth compared to leached
samples. This is most likely explained by the mobilization of U and Th
from the samples during leaching. Soil residence times are calculated by
modeling U-series activity ratios for each profile separately. Inferred
timescales vary up to 30 kyr for unleached aliquots from profile F1 to
up to 12 kyr for both leached and unleached aliquots from profile F2.
Muscovite content shows a linear relationship with U-series derived soil
residence times. This relationship provides an alternative method to
estimate residence timescales for profiles with significant U-series
data scatter. Using this alternative approach, inferred soil residence
times up to 33 kyr for leached samples of profile F1 and up to 34 kyr
for leached samples of profile F3 were determined. A linear relationship
between soil residence times and WIS (Si-based Weathering Index) exists
and is used to estimate soil residence times for profile F3 (up to 28
kyr) and F4 (up to 37 kyr). The linear relationship between soil depth
and calculated residence time allows determination of soil production
rates, which range from 10 to 24 mm/kyr and are comparable to the rates
determined previously using cosmogenic isotopes at the same site
(Heimsath et al., 2001b). This implies that at this site, on the
highland plateau of southeastern Australia, soil thickness has reached
steady-state, possibly as a result of stable tectonic conditions but
despite variable climatic conditions over the timescale of soil
development. Soil-mantled landscapes are the geomorphic expression of
this balance between soil production and denudation, and our results
show that in tectonically quiescent regions, this landscape can be
achieved in less than 30 kyr.
... The surface area of soil grains impacts on both availability of surfaces for dissolution and element release and availability of surfaces for sorption (c.f. Oelkers et al., 2009; Gherbi et al., 2010). Understanding the variability and evolution of mineral surface area in soils is therefore crucial for modelling the evolution of soils. White et al (1996) and Hodson (2002) showed that the specific surface area of soil minerals increases with decreasing grain size. For a constant mineralogy and even distribution of reactive sites this implies that as minerals break down into smaller grains, a soil's surface area and thus reactivity should increase. However a soil's mineralogy can vary with gr ...
The contribution of individual grain size fractions (2000-500, 500-250, 250-63, 63-2 and < 2 mu m) to bulk soil surface area and reactivity is discussed with reference to mineralogical and oxalate and dithionite extractions data. The 63-2 mu m fraction contributed up to 56% and 67% of bulk soil volume and BET surface area, respectively. Consideration of these observations and the mineralogy of this fraction suggest that the 63-2 mu m fraction may be the most influential for the release of elements via mineral dissolution in the bulk soil.
... Essential features of weathering in the field include those of seasonal wetting and drying, a generally high solid: solution ratio, and long residence times for water (Inskeep et al., 1993). The net effect is a weathering rate, determined for plagioclases, that is several orders of magnitude slower than the experimental dissolution rate (White et al., 1996(White et al., , 2005White and Brantley, 2003). Maher et al. (2009) have shown that other factors besides mineral dissolution rate and the rate of aqueous transport of solutes, notably the rate of secondary mineral precipitation, ensure a much slower rate of weathering of minerals, including feldspars, in the field than in the laboratory. ...
... Pore water thermodynamic saturation with amorphous Si (K sat % 1.5 mM) is also invoked in promoting phytolith stability (Alexandre et al., 1997). While experimental phytolith dissolution rates are determined for solutions far from amorphous silica saturation (Fraysse et al., 2006 ), natural pore waters, particularly in relatively dry climates, approach or exceed silica saturation (White et al., 1996White et al., , 2005 ). At Santa Cruz, measured pore water Si is typically undersaturated with amorphous silica (S pw = 0.1–0.5 mM,Table 4 andFig. ...
Biogenic and pedogenic processes control silica cycling in grasslands growing on a soil chronosequence and dominated by strong seasonal variabilities of a Mediterranean climate. Shallow pore water Si, in spite of significant annual uptake and release by plant growth and dieback, exhibits only moderate seasonal fluctuations reflecting strong buffering from labile biogenic Si, dominated by phytoliths and by secondary pedogenic silicates. Long phytolith residence times (340-900. yrs) reflect the seasonally dry climate and high solute Si concentrations. Water-extractable Si is closely associated with Al, indicating seasonal precipitation and dissolution of a highly labile 1:1 hydroxyaluminosilicate (HAS), probably allophane, which transforms in deeper soil into fine grained, poorly crystalline kaolinite. Shallow plant roots extract greater proportions of biogenic Si and deeper plant roots larger amounts pedogenic Si. High pore water Ge/Si in late winter and spring reflects the reinforcing effects of plant fractionation and concurrent dissolution of Ge-enriched HAS. The same processes produce pore waters with depleted 30Si/ 28Si. In the summer and fall, Ge/Si declines and 30Si/ 28Si increases, reflecting the cessation of plant uptake, continued dissolution of soil phytoliths and re-precipitation of less soluble HAS. Si inputs from weathering (2-90mmolm -2yr -1) and losses from pore water discharge (18-68mMm -2yr -1) are comparable for individual soils, decline with soil age and are significantly less than amounts of Si annual cycled through the vegetation (42-171mMm -2yr -1). Mobile Si is generally balanced in the soils with upward bio-pumping by the shallow-rooted grasses efficiently competing against downward leaching and pore water discharge. Small net annual increases in Si in the present day soils could not have been maintained over the time scale represented by the chronosequence (65-225yrs), implying past changes in environmental conditions.
... The weathering of quartz in soils is a function of both climate (maximum under tropical conditions) and time; cases have been described of soils under prolonged weathering in a temperate climate (White, 1981; Howard et al., 1995). Quartz has been considered to be almost unaltered chemically in soils from a Mediterranean climate and has thus been used in this environment as inalterable mineral for quantitative studies of mass balances (Bornand, 1978; Parra et al., 1982; Baize, 1983; Delgado et al., 1990; Martín-García, 1994; White et al., 1996). Due to its ubiquitous nature and abundance, quartz has been the subject of numerous soil and sediment studies. ...
... Using this approach and analyzing 234 U– 238 U disequilibria in sediment and their pore fluids, Maher et al. (2004) have developed a method to estimate the dissolution rate (R d ) of continentally derived sediments, which can be expressed as: where the subscripts f and s represent the fluid and the solid, respectively, and f a is the fraction of recoiled 234 U (estimated theoretically using the mean grain size of the sediment). The results obtained on marine sediments were found to be comparable with dissolution rates determined in soil chronosequences (e.g., White et al., 1996; White and Brantley, 2003). Similarily, DePaolo et al. (2006) calculated the ''communition'' age of late Pleistocene deep-sea sediments, that is ''the time elapsed between the generation of the small (o50 mm) sediment grains in the source areas by communition of bedrock and deposition on the seafloor'' and proposed that the transport time of the Site 984A sediments can vary from less than 10 kyr to approximately 300–400 kyr. ...
This chapter summarizes the recent applications of U-series nuclides to study weathering and river transport. The chapter focuses on the determination of time-scales of continental weathering and erosion, based on U-series analysis in soils, rivers, as well as in lakes. Analysis of U-series nuclides in soils and alterites can provide information on the behavior of radionuclides during weathering in the natural environment and also on the nature and time-scale of weathering processes. The chapter also indicates that determination of the time-scales of weathering processes from U-series data in soils requires a full understanding of the processes that control U–Th–Ra fractionation in regoliths. In parallel with the development of U-series disequilibrium studies in soils and weathering profiles U-series disequilibria in rivers have been used to establish weathering mass balances and/or to determine weathering time-scales at the scale of a watershed. Studies of U- and Th-series nuclides in lake waters provide information on their geochemical behavior and mixing characteristics of lakes. Some of these studies complement the investigations in river waters. Lake sediments, on the other hand, hold records of recent environmental changes. Measurements of U–Th nuclides in these sediments and associated mineral precipitates can help chronologically decipher the records in them. It has been shown that U-series disequilibria in lake water and sediment could also provide a wealth of information about the physico-chemical processes operating in lakes, such as the source and fate of elements in sediment and water, the residence time of particles or radionuclides in lakes, mixing or turnover time-scales of lakes. The chapter briefly presents some of the results obtained in these areas of research. These findings are certainly one of the major advances in U- and Th-series nuclide applications in the earth sciences and have laid the foundation for further developments in this field.
... Productivity changes are driven by N-limitation (young) to co-limitation by both N and P (middle-aged), and then P-limitation in the oldest soils (Vitousek et al., 1997). Previous studies of long-term chronosequences have focused on the aboveground ecosystem (e.g., Crews et al., 1995; Vitousek et al., 1997; Parfit et al., 2005) or on changes in soil chemistry with soil age (e.g., Walker and Syers, 1976; White et al., 1996; Vitousek et al., 1997). Given the focus on the above ground, little work has been done on root development with soil age across long-term chronosequences although one study found that roots were increasingly concentrated in surface (FH) horizons with soil age (Parfit et al., 2005 ). ...
Changes in the biomass and structure of soil microbial communities have the potential to impact ecosystems via interactions with plants and weathering minerals. Previous studies of forested long-term (1000s – 100,000s of years) chronosequences suggest that surface microbial communities change with soil age. However, significant gaps remain in our understanding of long-term soil microbial community dynamics, especially for non-forested ecosystems and in subsurface soil horizons. We investigated soil chemistry, aboveground plant productivity, and soil microbial communities across a grassland chronosequence (65,000–226,000 yrs old) located near Santa Cruz, CA. Aboveground net primary productivity (ANPP) initially increased to a maximum and then decreased for the older soils. We used polar lipid fatty acids (PLFA) to investigate microbial communities including both surface (<0.1 m) and subsurface (≥0.2 m) soil horizons. PLFAs characteristic of Gram-positive bacteria and actinobacteria increased as a fraction of the microbial community with depth while the fungal fraction decreased relative to the surface. Differences among microbial communities from each chronosequence soil were found primarily in the subsurface where older subsurface soils had smaller microbial community biomass, a higher proportion of fungi, and a different community structure than the younger subsurface soil. Subsurface microbial community shifts in biomass and community structure correlated with, and were likely driven by, decreasing soil P availability and Ca concentrations, respectively. Trends in soil chemistry as a function of soil age led to the separation of the biological (≤1 m depth) and geochemical (>1 m) cycles in the old, slowly eroding landscape we investigated, indicating that this separation, commonly observed in tropical and subtropical ecosystems, can also occur in temperate climates. This study is the first to investigate subsurface microbial communities in a long-term chronosequence. Our results highlight connections between soil chemistry and both the aboveground and belowground parts of an ecosystem.
... The weathering of quartz in soils is a function of both climate (maximum under tropical conditions) and time; cases have been described of soils under prolonged weathering in a temperate climate (White, 1981; Howard et al., 1995). Quartz has been considered to be almost unaltered chemically in soils from a Mediterranean climate and has thus been used in this environment as inalterable mineral for quantitative studies of mass balances (Bornand, 1978; Parra et al., 1982; Baize, 1983; Delgado et al., 1990; Martín-García, 1994; White et al., 1996). Due to its ubiquitous nature and abundance, quartz has been the subject of numerous soil and sediment studies. ...
Two Orthents from Sierra Nevada National Park (Spain), classified as loamy-skeletal mesic Typic Xerorthent and loamy-skeletal frigid Lithic Cryorthent were studied. They were situated at 1460 and 2000 m, derived from micaschists and quartzites under Mediterranean mountain climate conditions with mesic and cryic temperature regimes and xeric moisture regime.These Orthents are weakly developed, as shown by their reduced thickness, their lack of diagnostics subsurface horizons, sandy loam textures, skeletal, and lithochromic colors. Quartz is present in significant amounts, particularly in the fine light sand fraction.Analysis of the quartz grains of the fine light sand by scanning electron microscope (SEM) and backscattered electron images (BSE) revealed the presence of significant physical and chemical alteration features such as fractures, corrosion gulfs, fields of etch pits and dissolution of intergranular surfaces. This process is confirmed by electron microprobe analysis (EMPA) since the edges of the quartz grains have a higher proportion of cations other than Si, compared to their centre. Further evidence indicates the authigenic nature of the process. Starting from quartz etching an assessment of soil weathering has been made (W index from Marcelino et al. (1999).The alteration of the quartz, together with the development of a ultramicrofabric with several fabric units (investigated with SEM), and other characteristics (mineralogical, chemical, etc.) are considered indicative of some pedogenic evolution. These facts are especially interesting in the case of weakly developed Mediterranean mountain soils from Sierra Nevada, where processes of chemical weathering have traditionally not been considered very active.
Abstract Boulders are an important material in debris flow and their source is coupled with spheroidal weathering profiles that produce corestones. The goal of this work was to establish the geochemical transformations that produced corestones and distinguished them from the surrounding grus in two tropical granite weathering profiles (P1 and P2). Sampling was not performed in a vertical profile; instead, we gathered 13 (P1) to 16 (P2) samples displaying different weathering degrees (corestone and saprolite) and spatial positions in the profiles. We conducted the geochemistry (EDXRF/EDX and INAA) and mineralogy (petrography and XRD) of the samples. The CIA values ranged from 46 (corestones) to 93 (saprolite). Granite spheroidal weathering under a tropical mountainous slope develops mostly due to feldspar weathering (foremost plagioclase) in the following sequence: porosity growth, kaolinite, and gibbsite crystallization. Zircon weathering stability and its probable mobility as grain along the weathering profile play an important role in REE concentration. Spheroidal weathering is mainly a lixiviation process, yet specific locations (below the individualized corestones) presented REE enrichment due to translocation. They are hosted mainly by clay minerals and, to some extent, by amorphous Fe oxyhydroxide. The evolution of spheroidal weathering results in a vertical patchy weathering profile.
Physical and biogeochemical heterogeneity dramatically impacts fluid flow and reactive solute transport behaviors in geological formations across scales. From micro pores to regional reservoirs, upscaling has been proven to be a valid approach to estimate large-scale parameters by using data measured at small scales. Upscaling has considerable practical importance in oil and gas production, energy storage, carbon geologic sequestration, contamination remediation, and nuclear waste disposal. This review covers, in a comprehensive manner, the upscaling approaches available in the literature and their applications on various processes, such as advection, dispersion, matrix diffusion, sorption, and chemical reactions. We enclose newly developed approaches and distinguish two main categories of upscaling methodologies, deterministic and stochastic. Volume averaging, one of the deterministic methods, has the advantage of upscaling different kinds of parameters and wide applications by requiring only a few assumptions with improved formulations. Stochastic analytical methods have been extensively developed but have limited impacts in practice due to their requirement for global statistical assumptions. With rapid improvements in computing power, numerical solutions have become more popular for upscaling. In order to tackle complex fluid flow and transport problems, the working principles and limitations of these methods are emphasized. Still, a large gap exists between the approach algorithms and real-world applications. To bridge the gap, an integrated upscaling framework is needed to incorporate in the current upscaling algorithms, uncertainty quantification techniques, data sciences, and artificial intelligence to acquire laboratory and field-scale measurements and validate the upscaled models and parameters with multi-scale observations in future geo-energy research.
There is growing urgency for CO2 removal strategies to slow the increase of, and potentially lower, atmospheric CO2 concentrations. Enhanced weathering, whereby the natural reactions between CO2 and silicate minerals that produce dissolved bicarbonate ions are accelerated, has the potential to remove substantial CO2 on decadal to centennial timescales. The global mining industry produces huge volumes of fine wastes that could be utilised as feedstock for enhanced weathering. We have compiled a global database of the enhanced weathering potential of mined metal and diamond commodity tailings from silicate-hosted deposits. Our data indicate that all deposit types, notably mafic and ultramafic rock-hosted operations and high tonnage Cu-hosting deposits, have the potential to capture ∼1.1-4.5 Gt CO2 annually, between 31 and 125% of the industry's primary emissions. However, current knowledge suggests that dissolution rates of many minerals are relatively slow, such that only a fraction (∼3-21%) of this potential may be realised on timescales of <50 years. Field trials in mine settings are urgently needed and, if this prediction is confirmed, then methodologies for accelerating weathering reactions will need to be developed.
Landslides can behave as dynamic processes, which emerge from the complex interplay of tectonics, erosion, weathering and gravitational influences, triggered by various hydrological, mineralogical, biological and geotechnical factors. Integral studies to assess the mechanisms underlying landslide initiation and progression are mainly focussed on specific cases with high geohazard potential. The landslide near Stadtschlaining (Austria) represents a key study site to elucidate the impacts of pelitic sediment composition, weathering regime, alteration patterns and hydrochemistry on recurrent damage progression in the local infrastructure. Based on field work, soil-mechanical logging (Atterberg limits, undrained strength, friction angles), water chemistry (ICP-OES, IC, hydrochemical modeling), solid-phase characterization (XRD, XRF, SEM) and sorption experiments we establish a conceptual model for initiating and progressing of landslides: Infiltration of low mineralized meteoric water (EC: <200 μS/cm) in permeable limonitic gravels triggers chemical weathering of greenschist-derived detritus and promotes its transformation into kaolinite and smectite. The clayey strata (>50 wt% of clay minerals) create zones of mechanical and chemical weakness in the underground (~4–6 m below ground level), which are characterized by particle disintegration/delamination, slip bedding and deformations, and development of porous layers depicting water flow paths. Subsequent Na⁺ exchange for bivalent ions in the smectite interlayer delivered by percolating, highly mineralized water (EC: 1600–5100 μS/cm) is caused by de-icing salt and fertilizer applications during winter and late summer, and yield in i) decohesion and physical breakdown of the particle aggregates and ii) swelling of the clay matrix in early spring and autumn. These processes reduce the shear strength of the pelitic sediments, resulting in failure and initiation of landslides (deformation: ~500 mm within a month) and subsequent steady creeping motion (deformation: ~100 mm in 6 months). Customized engineered solutions to prevent landslides in this area are presented, which can be conveyed to analogous landslide-affected areas worldwide.
We are preparing a nationwide distribution map of strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) in Japan, using stream sediment to obtain basic ⁸⁷Sr/⁸⁶Sr data for the provenance analysis of food production and archaeological substances. To clarify the effect of particle size on ⁸⁷Sr/⁸⁶Sr in stream sediments, we analyzed stream sediment from the Shigenobu River system in Matsuyama, Ehime Prefecture, Japan, where a variety of silicate-dominated lithologies, that is, several bedrocks of andesitic, granitic, and siliciclastic sedimentary rocks, are distributed. This paper reports elemental concentrations and ⁸⁷Sr/⁸⁶Sr in stream sediments for six particle-size fractions (1000–500, 500–300, 300–180, 180–125, 125–75, and <75 μm). The results from stream sediments were compared with results from bedrock units and stream water over the catchment. The elemental concentrations in stream sediment tended to increase with decreasing particle size in all lithologies; however, Sr concentrations varied less than other elements across particle sizes. For most of the samples, ⁸⁷Sr/⁸⁶Sr varied by less than 0.001 among the six particle-size fractions, which was less than the variation among the different lithologies. Therefore, ⁸⁷Sr/⁸⁶Sr in the <180 μm particle-size fraction, which is normally used in Japanese nationwide geochemical mapping, should be a reliable proxy for bedrock ⁸⁷Sr/⁸⁶Sr. The ⁸⁷Sr/⁸⁶Sr values in water samples from the Shigenobu River system were lower and less variable than ⁸⁷Sr/⁸⁶Sr in the stream sediments, and they did not faithfully correspond to the watershed geology. The inconsistency may reflect selective dissolution of Sr from plagioclase. Interestingly, ⁸⁷Sr/⁸⁶Sr values of the exchangeable fraction of stream sediment in the <180 μm fraction were strongly correlated with ⁸⁷Sr/⁸⁶Sr of stream water samples. Because ⁸⁷Sr/⁸⁶Sr in plant and animal bodies reflects that of their water sources, the exchangeable fraction of stream sediment may be a useful proxy for geochemical provenance in Japan instead of stream water.
Four pairs of fresh and partly-weathered granitoids, obtained from well-characterized watersheds—Merced River, CA, USA; Panola, GA, USA; Loch Vale, CO, USA, and Rio Icacos, Puerto Rico—were reacted in columns under ambient laboratory conditions for 13.8 yrs, the longest running experimental weathering study to date. Low total column mass losses (<1 wt. %), correlated with the absence of pitting or surface roughening of primary silicate grains. BET surface area (SBET) increased, primarily due to Fe-oxyhydroxide precipitation. Surface areas returned to within factors of 2 to 3 of their original values after dithionite extraction. Miscible displacement experiments indicated homogeneous plug flow with negligible immobile water, commonly cited for column experiments. Fresh granitoid effluent solute concentrations initially declined rapidly, followed by much slower decreases over the next decade. Weathered granitoid effluent concentrations increased modestly over the same time period, indicating losses of natural Fe-oxide and/or clay coatings and the increased exposure of primary mineral surfaces. Corresponding (fresh and weathered) elemental effluent concentrations trended toward convergence during the last decade of reaction. NETPATH/PHREEQC code simulations indicated non-stoichiometric dissolution involving Ca release from disseminated calcite and excess K release from interlayer biotite. Effluent ⁸⁷Sr/⁸⁵Sr ratios reflected a progressive weathering sequence beginning and ending with ⁸⁷Sr/⁸⁵Sr values of plagioclase with an additional calcite input and a radiogenic biotite excursion proportional to the granitoid ages.
The rates of many heterogeneous reactions are dependent upon the mineral-water interfacial area. Examples include release of nutrients from primary minerals, rate of growth of authigenic minerals, adsorption and desorption of metal and organic contaminants on soil and sediment grains, neutralization of acid deposition by weathering reactions, oxidation and reduction of mmetal-containing phases and solutes, clumping of colloids or bacteria by electrostatic attraction, and photocatalytic degradation of organic pollutants at metal oxide surfaces (Davis et al., 1993; White and Brantley, 1995). Several workers have also shown that the surface area is a key parameter in predicting weathering rates using geochemical models (e.g. PROFILE) and soil chemistry under the influence of acid rain (Jönsson et al., 1995; Hodson et al., 1996, 1997a). Along with the permeability, the surface area is one of the most difficult physical parameters to quantify in extrapolating from the laboratory to the soil plot to the watershed (White and Peterson, 1990). Most models of solute transport in aquifers and in soils simply ignore the mineral-water surface area term by combining it with the kinetic rate constant into one fitting parameter, despite the fact that the specific surface area may vary over several orders of magnitude — 102–106 cm2/g as functions of grain size, mineralogy, oxide coating, weathering history, or biological effects, or as a combination of these factors. Despite the importance of the mineral surface area in many areas of geochemistry, little systematic effort has been expended to understand or predict this term for primary silicates (more work has been completed on the surface area of clays and simple oxides).
Geochemical systems of the Earth’s surface and interior are often studied by means of conceptual models that represent them as geochemical or biogeochemical cycles of chemical elements. Such models usually address the various geological, geochemical, geophysical, and biological processes within the cycle or system, and they focus on the model’s ability to evaluate the system changes at different time scales, often extending from the remote past into the future. The time dimension of changes taking place in the different parts of the Earth System makes it necessary to understand the mechanisms and rates of the numerous processes that control the element interactions in geochemical systems of different physical structures and degrees of complexity.
Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite ≈ hypersthene > orthoclase ⪢ apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, rr, as a function of time, t(rr = ), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (rr = kappt). In these rate laws, κ is a diffusion parameter and kapp is an apparent rate constant. The rate of advance is best fit by the interface model.
Merouane Chott ephemeral lake is an ideal natural system for studying mineral dissolution and precipitation rates because (1) it undergoes annual cycles of filling and complete evaporation, and (2) it has a simple, well-defined hydrology. The major element concentrations of Merouane Chott lake waters have been measured weekly from January to June 2003. These concentrations are used together with estimates of the chott lake volume to calculate the temporal evolution of the total mass of these major elements. Dividing the first derivative of these masses with respect to time by the chott surface area yields precipitation rate estimates for halite, calcite, gypsum, and K–Mg salts during the complete evaporation of the lake. These rates are compared with the saturation indexes of these minerals to deduce the degree to which they are consistent with laboratory measured rates available in the literature.
The Merouane Chott, located in southeastern Algeria, is an ideal natural system for studying mineral dissolution and precipitation rates because it (1) undergoes annual cycles of filling and complete evaporation, and (2) has a simple, well defined hydrology. The major element concentrations of Merouane Chott lake waters were measured weekly from January to June 2003. These concentrations are used together with estimates of the Chott lake volume and infiltration rates to calculate the temporal evolution of the total mass of these major elements. Element precipitation rates are generated by dividing the first derivative of total mass of each element with respect to time by the Chott surface area. Mass balance considerations yield precipitation rate estimates for halite, calcite, and gypsum during the complete evaporation of the lake. These rates are compared with the saturation indexes of these minerals to deduce the degree to which they are consistent with laboratory measured rates available in the literature.
Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the ²³⁴U/²³⁸U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11–15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).
Although long-term changes in solid-state compositions of soil chronosequences have been extensively investigated, this study presents the first detailed description of the concurrent hydrochemical evolution and contemporary weathering rates in such sequences. The most direct linkage between weathering and hydrology over 3 million years of soil development in the Merced chronosequence in Central California relates decreasing permeability and increasing hydrologic heterogeneity to the development of secondary argillic horizons and silica duripans. In a highly permeable, younger soil (40 kyr old), pore water solutes reflect seasonal to decadal-scale variations in rainfall and evapotranspiration (ET). This climate signal is strongly damped in less permeable older soils (250 to 600 kyr old) where solutes increasingly reflect weathering inputs modified by heterogeneous flow.
Two and three-dimensional field scale reservoir models of CO2 mineral sequestration in basalts were developed and calibrated against a large set of field data. Resulting principal hydrological properties are lateral and vertical intrinsic permeabilities of 300 and 1700 × 10−15 m2 , respectively, effective matrix porosity of 8.5% and a 25 m/year estimate for regional groundwater flow velocity.
Reactive chemistry was coupled to calibrated models and predictive mass transport and reactive trans- port simulations carried out for both a 1200-tonnes pilot CO2 injection and a full-scale 400,000-tonnes CO2 injection scenario. Reactive transport simulations of the pilot injection predict 100% CO2 mineral capture within 10 years and cumulative fixation per unit surface area of 5000 tonnes/km2 . Correspond- ing values for the full-scale scenario are 80% CO2 mineral capture after 100 years and cumulative fixation of 35,000 tonnes/km2 . CO2 sequestration rate is predicted to range between 1200 and 22,000 tonnes/year in both scenarios.
The predictive value of mass transport simulations was found to be considerably lower than that of reactive transport simulations. Results from three-dimensional simulations were also in significantly better agreement with field observations than equivalent two-dimensional results.
Despite only being indicative, it is concluded from this study that fresh basalts may comprise ideal geological CO2 storage formations.
SiO2 fluxes associated with contemporary solute transport in three deeply weathered granitoid profiles are compared to bulk SiO2 losses that have occurred during regolith development. Climates at the three profiles range from Mediterranean to humid to tropical. Due to shallow impeding alluvial layers at two of the profiles, and seasonally uniform rainfall at the third, temporal variations in hydraulic and chemical state variables are largely attenuated below depths of 1–2 m. This allows current SiO2 fluxes below the zone of seasonal variations to be estimated from pore-water concentrations and average hydraulic flux densities. Mean-annual SiO2 concentrations were 0.1–1.5 mM. Hydraulic conductivities for the investigated range of soil-moisture saturations ranged from <10−9 to >10−6 m s−1. Estimated hydraulic flux densities for quasi-steady portions of the profiles varied from 6×10−9 to 14×10−9 m s−1 based on Darcy's law and field measurements of moisture saturations and pressure heads. Corresponding fluid-residence times in the profiles ranged from 10 to 44 years. Total SiO2 losses, based on chemical and volumetric changes in the respective profiles, ranged from 19 to 110 kmoles SiO2 m−2 of land surface as a result of 0.2–0.4 Ma of chemical weathering. Extrapolation of contemporary solute fluxes to comparable time periods reproduced these SiO2 losses to about an order of magnitude. Despite the large range and non-linearity of measured hydraulic conductivities, solute transport rates in weathering regoliths can be estimated from characterization of hydrologic conditions at sufficiently large depths. The agreement suggests that current weathering rates are representative of long-term average weathering rates in the regoliths.
The temporal evolution of natural illite du Puy dissolution rates was measured from Si release rates in single-pass flow-through experiments lasting at least 100 days at 25°C and pH ranging from 2 to 12. Si release rates decreased by a factor of five and three at pH 12 and 2, respectively, during the experiments. These observations are interpreted to stem from changes in illite du Puy reactive surface area during these experiments. As the edges of clay minerals dissolve faster than the basal planes, dissolution tends to change clay mineral morphology decreasing the percentage of reactive edge sites. This continuously changing morphology prevents illite dissolution rates from attaining steady state during laboratory experiments lasting 100 to 200 days. A similar temporal decrease in dissolution rates is evident for many different sets of clay mineral dissolution rate data available in the literature. It seems reasonable, therefore, to expect that clay mineral dissolution does not attain steady state in nature, but rather their dissolution rates decrease continuously during their dissolution.
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