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Chemical weathering rates of a soil chronosequence on granitic alluvium: III. Hydrochemical evolution and contemporary solute fluxes and rates
Abstract
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.
... This study links information on soil mineralogy and texture (Doetterl et al., 2018) with proxies for microbial transformation and SOM composition to derive a better understanding of long-term SOM dynamics along weathering gradients. The study area is located in the California Central Valley (Merced County, CA, USA), where granitic debris with a minor share of mafic minerals from the Sierra Nevada foothills was deposited on alluvial terraces of the Merced River during interglacial periods (Harden, 1987;White et al., 1996White et al., , 2005. The alluvial deposits cover a temporal range of 3 million years, with the youngest terrace only 100 years old (Harden, 1987;Doetterl et al., 2018). ...
... The degree of mineral weathering along the chronosequence was assessed based on successive depletion and/or accumulation of geochemical components. For instance, the Fe total : Si total ratio indicates how the mostly pedogenically formed Fe total relates with Si total , an element that abounds in the felsic parent material and accumulates in the semi-arid environment of the study area (White et al., 2005). Hence, the ratio decreases with increasing soil weathering. ...
... Older soils are increasingly weathered, and soil fertility decreases, as indicated by the lower soil pH and CEC (see Table 2). Further, the increasing dominance of Si with ongoing mineral weathering indicates increasing amounts of low reactive clay-sized silicates, i.e. kaolinite, which offer fewer binding sites to SOM (White et al., 2005). At the same time, significantly lower total C in strongly weathered soils were accompanied by an increasing trend of pedogenic iron oxides, i.e. ...
A central question in carbon research is how
stabilization mechanisms in soil change over time with soil development and
how this is reflected in qualitative changes in soil organic matter (SOM).
To address this matter, we assessed the influence of soil geochemistry on
bulk SOM composition along a soil chronosequence in California, USA, spanning
3 million years. This was done by combining data on soil mineralogy and
texture from previous studies with additional measurements on total carbon
(C), stable isotope values (δ13C and δ15N), and
spectral information derived from diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS). To assess qualitative shifts in
bulk SOM, we analysed the peak areas of simple plant-derived (S-POM),
complex plant-derived (C-POM), and predominantly microbial-derived organic matter (OM; MOM) and their changes in abundance across soils with several millennia
to millions of years of weathering and soil development. We observed that
SOM became increasingly stabilized and microbial-derived (lower C : N ratio,
increasing δ13C and δ15N) as soil weathering
progressed. Peak areas of S-POM (i.e. aliphatic root exudates) did not
change over time, while peak areas of C-POM (lignin) and MOM (components of
microbial cell walls (amides, quinones, and ketones)) increased over time
and depth and were closely related to clay content and pedogenic iron
oxides. Hence, our study suggests that with progressing soil development,
SOM composition co-varied with changes in the mineral matrix. Our study
indicates that structurally more complex OM compounds (C-POM, MOM) play an
increasingly important role in soil carbon stabilization mechanisms as the
mineral soil matrix becomes increasingly weathered.
... The study area is located in the California Central Valley (Merced County, CA, USA), where granitic debris with a minor share of mafic minerals from the Sierra Nevada foothills was deposited on alluvial terraces of the Merced River during interglacial periods (Harden, 1987;White et al., 1996;White et al., 2005). The alluvial deposits cover a temporal range of 3 90 million years, with the youngest terrace only 100 years old (Harden, 1987;Doetterl et al., 2018). ...
... The degree of mineral weathering along the chronosequence was assessed based on successive depletion and/or accumulation of geochemical components. For instance, the Fetotal:Sitotal ratio indicates how the mostly pedogenically formed Fetotal relates with Sitotal, an element that abounds in the felsic parent material and accumulates in the semi-arid environment of the study area (White et al., 2005). Hence, the ratio decreases with increasing soil weathering. ...
... Older soils are increasingly weathered and soil fertility decreases, as indicated by the lower soil pH and CEC (see Table 2 and 3). Further, the increasing dominance of Si with ongoing mineral weathering indicates increasing amounts of low reactive 230 clay-sized silicates, i.e. kaolinite, which offer less binding sites to SOM (White et al., 2005). At the same time, lower total C in strongly weathered soils were accompanied by increasing amounts of pedogenic iron oxides, i.e. ...
A central question in carbon research is how stabilization mechanisms in soil change over time with soil development and how this is reflected in qualitative changes of soil organic matter (SOM). To address this matter, we assessed the influence of soil geochemistry on bulk SOM composition along a soil chronosequence in California, USA spanning 3 million years. This was done by combining data on soil mineralogy and texture from previous studies with additional measurements on total carbon (C), stable isotope values (δ13C and δ15N), and spectral information derived from Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS). To assess qualitative shifts in bulk SOM, we analysed the peak areas of simple plant-derived (S-POM), complex plant-derived (C-POM), and predominantly microbially derived OM (MOM) and their changes in abundance across soils varying several millennia to millions of years in weathering and soil development. We observed that SOM became increasingly stabilized and microbially-derived (lower C : N ratio, increasing δ13C and δ15N) as soil weathering progresses. Peak areas of S-POM (i.e. aliphatic root exudates) did not change over time, while peak areas of C-POM (lignin) and MOM (components of microbial cell walls (amides, quinones, and ketones)) increased over time and depth and were closely related to clay content and pedogenic iron oxides. Hence, our study suggests that with progressing soil development, SOM composition co-varies with changes in the mineral matrix. Our study indicates that a discrimination in favour of structurally more complex OM compounds (C-POM, MOM) gains importance as the mineral soil matrix becomes increasingly weathered.
... White et al. (2008), and calculated based on the retention fractions of elements for the other profiles (see Appendix B). We adopted Zr as an immobile element ( Chadwick et al., 2003), quartz as an immobile quantity ( White et al., 2005), and Al and Ti as immobile elements (Appendix B) for the Hawaiian soils, the Riverbank profile, and the other profiles as well as the Neoarchean-Paleoproterozoic paleosols, respectively. b Values of temperature, observed pH and observed soil P CO2 are from the references given in the last column ...
... We compared the values of pH and P CO 2 calculated by the present method to those observed. The modern weathering profiles selected for the present study were Santa Cruz chronosequence (SCTs 1-5, White et al., 2008White et al., , 2009) and Riverbank formation ( White et al., 2005) developed on granitic rocks, and Belbex (Chesworth et al., 1981), Baynton (Nesbitt and Wilson, 1992) and Hawaii (soils E, I, J, L and M, Chadwick et al., 2003) profiles developed on basaltic rocks. For these weathering profiles, detailed information is reported, sufficient to test the present method; the information includes mineralogy, porewater compositions, temperature, concentration depth-profiles of major cations and, though only for two sites (Santa Cruz and Riverbank), "soil P CO 2 ". ...
... Although ET has a large range, we can assume <90% of ET (Holland, 1978;White and Blum, 1995) for the paleosols. Actually, ET = $80% in Riverbank with an annual rainfall of 0.30 m yr À1 ( White et al., 2005) and ET = 60-80% in Santa Cruz with an annual rainfall of 0.37-0.69 m yr À1 ( White et al., 2009). ...
The apparent activation energy of silicate weathering is a key parameter for understanding the regulation of atmospheric CO2 and surface-temperature of the Earth. Combining the atmospheric composition effects on the apparent activation energy with the compensation law for silicate-weathering flux, the relationship between the temperature dependence of atmospheric CO2 (ΔHCO2′), temperature (T) and silicate-weathering flux (FCO2) has been recently established (Kanzaki and Murakami, 2018). The present study examined the effects of atmospheric CO2 and CH4 on silicate weathering in the Precambrian based on the above T-ΔHCO2′-FCO2 relationship and the greenhouse effects of CO2, which represent ΔHCO2′ on the global scale, with and without the presence of CH4. Calculation of the ratio of the change in FCO2 to the corresponding change in the partial pressure of atmospheric CO2 (PCO2) as an indicator of the silicate-weathering feedback on CO2 revealed hitherto unknown weathering-climate interplays. The states where PCO2 < 10−0.5 atm and T > ∼30 °C are unstable due to the positive feedback, and immediately change with slight CO2 changes to either the states of PCO2 > 10−0.5 atm or those of PCO2 < 10−0.5 atm and T < ∼30 °C, both of which are stable due to the negative feedback. The latter states are especially stable against glaciations, because the feedback becomes more negative as temperature decreases, possibly explaining the stable climates in the Mesoproterozoic. When CH4 is present in atmosphere with CH4/CO2 ratio within a limited range (∼0.03–0.15), a positive feedback operates at low temperatures (<∼5 °C) and thus global glaciations can occur, which may explain the glaciation in the late Archean. The temperature and PCO2 transitions in the Precambrian were finally calculated based on the relationship between ΔHCO2′, T and FCO2 and the greenhouse effects of CO2. The calculated CO2 levels are high enough that the temperature could have been maintained at >0 °C only by CO2 through the Precambrian. The consistent PCO2 estimates from paleosols (fossil weathering profiles) in the literature support the argument. The calculated temperatures suggest that the Earth could have been cool to hot until around the end of Archean and cool to moderate afterwards.
... A chronosequence is a set of soils that differ in age of formation but have similar parent material and form under similar climatic, topographic and biological regimes. As such, soil chronosequences are ideal environments in which to simultaneously compare critical zone processes over both geologic and contemporary time scales and to determine if present-day weathering fluxes and rates are comparable to those of the past (White et al., 1996;Blum and Erel, 1997;Stewart et al., 2001;White et al., 2005). ...
... Each site was extensively instrumented to define chemistry, hydrology, climate and biology using methods previous described by White et al. (1996White et al. ( , 2005. Regolith samples characterized in the present paper were air dried and size separated into <2.0 and >2.0 mm fractions. ...
... Kaolinite abundances increase with terrace age, becoming more concentrated in argillic horizons at depths of 0.5-1.5 m (Fig. 5). Similar argillic horizons are reported for many older soils developed in the Mediterranean climate of California (Torrent and Nettleton, 1978;Hobson and Dahlgren, 1998;White et al., 2005). XRD analyses indicate that kaolinite grain size and crystallinity does not increase with age in the Santa Cruz terraces. ...
A soil chronosequence developed on marine terraces along coastal California, exhibits deeper and more intensively weathered mineral profiles with increasing age (65 to 226 kyrs). Feldspar concentrations generally increase linearly with terrace depth. The slope or weathering gradient is defined by the ratio of the weathering rate and the velocity at which the profile penetrates into the regolith.A spread sheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations are dominated by exponential changes in mineral surface to volume and at high residual feldspar concentrations by the approach to thermodynamic saturation.These parameters, in addition to the kinetic rate constant, are of secondary importance to the fluid flux qh which controls the weathering velocity and solute fluxes from the profile.
... We compared the values of pH and calculated by the present method to those observed. The modern weathering profiles selected for the present study were Santa Cruz chronosequence (SCTs 1 5, White et al., 2008; and Riverbank formation (White et al., 2005) developed on granitic rocks, and Belbex (Chesworth et al., 1981), Baynton (Nesbitt and Wilson, 1992) and Hawaii (soils E, I, J, L and M, Chadwick et al., 2003) profiles developed on basaltic rocks. For these weathering profiles, detailed information is reported, sufficient to test the present method; the information includes mineralogy, porewater compositions, temperature, concentration depth-profiles of major cations and, though only for two sites (Santa Cruz and Riverbank), "soil 2 CO P ". ...
... Although ET has a large range, we can assume < 90 % of ET (Holland, 1978;White and Blum, 1995) for the paleosols. Actually, ET = ~80 % in Riverbank with an annual rainfall of 0.30 m yr −1 (White et al., 2005) and ET = 60 80 % in Santa Cruz with an annual rainfall of 0.37 0.69 m yr −1 . The larger the ET value is, the larger the pore ε value is. ...
... Reverse triangles, circles and triangles denote the data points at maximum, geometric-mean and minimum 2 CO P levels, respectively, with the thermodynamic data by Nesbitt (1977) . We adopted Zr as an immobile element (Chadwick et al., 2003), quartz as an immobile quantity (White et al., 2005), and Al and Ti as immobile elements (Appendix B) for the Hawaiian soils, the Riverbank profile, and the other profiles as well as the Neoarchean-Paleoproterozoic paleosols, respectively. c Kmagnesite and Kankerite were calculated using thermodynamic data for magnesite and ankerite given by Holland and Powell (1998) and ...
Atmospheric CO2 levels reflect Earth’s surface temperatures directly, and have been discussed especially in association with the faint young Sun and Snowball Earth in the Precambrian. In addition, atmospheric O2 levels in the Precambrian have been estimated from paleosols, fossil weathering profiles, based on the estimates of atmospheric CO2 levels. Nevertheless, atmospheric CO2 levels in the Neoarchean and the Paleoproterozoic have remained as a debatable topic. In order to precisely estimate atmospheric CO2 levels in the Neoarchean-Paleoproterozoic, we developed a new method that calculates CO2 levels from the chemical compositions of paleosols. The new method (i) calculates the cation concentrations in porewaters at the time of weathering from those of paleosols, (ii) describes the relationships between partial pressure of atmospheric CO2 ( ), pH and cation concentrations based on the charge balance between the cations and anions including carbonate species in porewaters, and (iii) finally calculates levels at a given temperature constraining pH by thermodynamics of weathering secondary-minerals. By applying the new method to modern weathering profiles, we obtained a good agreement between the calculated and observed levels. The weathering rate deduced from the new method was proportional to with fractional dependence of 0.18 and the apparent activation energy of weathering was 40-55 kJ mol−1, which is consistent with the laboratory and field results. The application to modern weathering and the formulated characteristics of weathering strongly indicate that the new method is valid and robust.
... The study of soil chronosequences yields information about the rate and trend of pedological change, and these soils are valuable tools in soil-geomorphic research (Pillans et al., 1997;Hugget, 1998). Numerous soil chronosequences have been used to study a wide variety of geochemical and pedogenic changes (Mellor, 1986;Litcher, 1998;White et al., 2005). Reclaimed minesoils are especially suitable for soil carbon (C) sequestration-chronosequence research because of their welldefined ages (time since reclamation), which allows calculation of rates and directions of change for C storage and related timedependent processes. ...
... Other soil properties have been affected by time. Proportional declines in minesoil clay, with corresponding increases in silt or sand with time since reclamation, are likely the result of continued weathering of coarse rock materials and erosion of detached, unstable clay particles as these minesoils aged (White et al., 2005). Significantly higher CEC in the older minesoils indicated development of a more complex surface exchange system able to retain more nutrient cations, generating better chemical conditions supporting an environment favorable to biological processes. ...
... Earlier findings on changes in soil chemical properties in chronosequences as a result of time-dependent pedological processes (e.g., weathering, organic matter accumulation) support this observation (White et al., 2005). Multiple regression analysis relating measured CEC to minesoil clay and SOC contents, for each location sampled in this study, revealed that the statistical significance of the regression coefficients for both clay and SOC changed with time since reclamation. ...
Reclaimed minesoils have well-defined ages (time since reclamation), making them suitable for studying temporal changes in terrestrial carbon sequestration. The objective of this research was to assess the effect of time since reclamation on soil organic carbon (SOC) sequestration and related soil properties such as texture, bulk density, and cation exchange capacity in three West Virginia minesoils along a chronosequence. The minesoils’ surface 750 Mg ha−1 (0–6 cm) was sampled at 1, 4, and 21 years and again at 2, 5, and 22 years postreclamation, giving a total of 6 site-years of information. Average SOC stocks (Mg C ha−1) were highest in the oldest minesoils. Soil bulk density was highest and unrelated to SOC concentration in the youngest minesoil, reflecting recent compressive reclamation techniques. The cation exchange capacity of older minesoils was influenced more by SOC than by clay, whereas the opposite was observed in younger minesoils. The relationship of SOC stock to time since reclamation was best described by a logarithmic diminishing returns model. Short-term (1 year) SOC sequestration rates (Mg C ha−1 y−1) were not appropriate to describing the change in SOC sequestration rate occurring along the chronosequence. When taken as the first derivative of the diminishing returns model, long-term SOC sequestration rates were shown to decline precipitously (80%) in the first 5 years after reclamation. The model predicts that the surface 750 Mg ha−1 of minesoil will contain about 13.3 Mg SOC ha−1 at 50 years after reclamation. About 75% of that SOC storage is predicted to be achieved in the first decade after reclamation.
... The influence of soil hydrology on chemical weathering rates has mostly been assessed through proxy variables, like soil water availability or soil moisture (Daly and Porporato, 2005;Moore et al., 2015;White et al., 2005). Soil water availability can be approached using the ratio of mean annual rainfall to potential evapotranspiration, as illustrated in studies of soil development along climatic gradients (Chadwick et al., 2003;Dixon et al., 2016;Schoonejans et al., 2016) and at the global scale (Calabrese and Porporato, 2020). ...
... The annual solute fluxes of cations and DSi (T.Cat) are taken here as a proxy for contemporary soil chemical weathering rates following Clow and Drever (1996), Guicharnaud and Paton (2006), and White et al. (2005). The annual fluxes below the TU-UP profile (15-19 g m −2 yr −1 ) are systematically higher than under CU-UR (3-11 g m −2 yr −1 ; Table 5). ...
Vegetation plays a key role in the hydrological and biogeochemical cycles. It can influence soil water fluxes and transport, which are critical for chemical weathering and soil development. In this study, we investigated soil water balance and solute fluxes in two soil profiles with different vegetation types (cushion-forming plants vs. tussock grasses) in the high Ecuadorian Andes by measuring soil water content, flux, and solute concentrations and by modeling soil hydrology. We also analyzed the role of soil water balance in soil chemical weathering. The influence of vegetation on soil water balance and solute fluxes is restricted to the A horizon. Evapotranspiration is 1.7 times higher and deep drainage 3 times lower under cushion-forming plants than under tussock grass. Likewise, cushions transmit about 2-fold less water from the A to lower horizons. This is attributed to the higher soil water retention and saturated hydraulic conductivity associated with a shallower and coarser root system. Under cushion-forming plants, dissolved organic carbon (DOC) and metals (Al, Fe) are mobilized in the A horizon. Solute fluxes that can be related to plant nutrient uptake (Mg, Ca, K) decline with depth, as expected from biocycling of plant nutrients. Dissolved silica and bicarbonate are minimally influenced by vegetation and represent the largest contributions of solute fluxes. Soil chemical weathering is higher and constant with depth below tussock grasses but lower and declining with depth under cushion-forming plants. This difference in soil weathering is attributed mainly to the water fluxes. Our findings reveal that vegetation can modify soil properties in the uppermost horizon, altering the water balance, solute fluxes, and chemical weathering throughout the soil profile.
... The influence of soil hydrology on chemical weathering rates has mostly been assessed through proxy variables, like soil water availability or soil moisture (Daly and Porporato, 2005;Moore et al., 2015;White et al., 2005). Soil water availability can be approached using the ratio of mean annual rainfall to potential evapotranspiration, as illustrated in studies of soil 45 development along climatic gradients (Chadwick et al., 2003;Dixon et al., 2016;Schoonejans et al., 2016) and even at the global scale (Calabrese and Porporato, 2020). ...
... The Na concentration in the soil solutes varies only between 3 and 7 mg L -1 , and exhibits no clear trends with depth and no consistent influence of vegetation. The annual solute fluxes of cations and DSi (T.Cat) are taken here as a proxy for contemporary soil chemical weathering rates following Clow & Drever (1996), Guicharnaud & Paton (2006) and White et al. (2005). The annual fluxes below the 450 TU-UP profile (14 -17 g m -2 y -1 ) are systematically higher than under CU-UR (5 -14 g m -2 y -1 ; Table 4). ...
Vegetation plays a key role in the hydrological and biogeochemical cycles. It can influence soil water fluxes and transport which are critical for chemical weathering and soil development. In this study, we investigated soil water balance and solute fluxes in two soil profiles with different vegetation types (cushion-forming plants vs. tussock grasses) by measuring soil water content, flux, and solute concentrations and by modeling soil hydrology. We also analyzed the role of soil water balance in soil chemical weathering. The influence of vegetation on soil water balance and solute fluxes is restricted to the A horizon. Evapotranspiration is 1.7 times higher and deep drainage is 3 times lower under cushion-forming plants than under tussock grass. Likewise, cushions transmit almost threefold less water from the A to lower horizons. This is attributed to the vertical distribution of soil properties associated with the root systems. Under cushion-forming plants, DOC and metals (Al, Fe) are mobilized in the A horizon. Solute fluxes that can be related to plant nutrient uptake (Mg, Ca, K) decline with depth as expected from bio-cycling of plant nutrients. Dissolved silica and bicarbonate are minimally influenced by vegetation and represent the largest contributions of solute fluxes. Soil chemical weathering is higher and constant with depth below tussock grasses; while lower and declining with depth under cushion-forming plants. This difference in soil weathering is attributed mainly to the water fluxes. Our findings reveal that vegetation can modify soil properties in the uppermost horizon altering the water balance, solute fluxes, and chemical weathering throughout the soil profile.
... Accounting for the evolution of porous media is not only of importance in man-made and engineered systems, but is equally relevant in natural environments, where transport and chemical properties can exhibit significant variation in space and time (Gouze and Coudrain-Ribstein 2002;White et al. 2005;Jin et al. 2013;Opolot and Finke 2015). A key example is the progress of weathering and soil formation. ...
... Crust formation has also been observed as a result of gypsum precipitation on historical monuments (Dewanckele et al. 2012). Furthermore, permeability decrease in sedimentary basins (Aharonov et al. 2004) or soils (White et al. 2005) due to mineral precipitation have been observed. The formation of secondary minerals may lead to fractures through generated internal mechanical stresses. ...
... However, Na + is in good correlation with K + (Figure 10) and the concentrations of the latter are several times higher in streams (average 0.7 mg·L −1 ) than in snow (average 0.1 mg·L −1 ). Higher concentrations of K + in the arctic stream water compared to precipitation suggest that weathering reactions involved a silicate mineral [2,7,34,52,53]. ...
... However, Na + is in good correlation with K + (Figure 10) and the concentrations of the latter are several times higher in streams (average 0.7 mg·L −1 ) than in snow (average 0.1 mg·L −1 ). Higher concentrations of K + in the arctic stream water compared to precipitation suggest that weathering reactions involved a silicate mineral [2,7,34,52,53]. Dissolution of silicates in natural waters is kinetically controlled [1,2,7,30,54] and only flow paths of the highest residence times attain high concentrations of silicate weathering products [55]. However, Dragon and Marciniak, [12]; Tranter et al. [56]; Brown, [57] and Williams et al. [58] show that arctic stream water that is characterised by a similar flow distance (less than 1 km) to that of streams in the Lake Saana catchment could already have been mineralised with similar K + and Na + contents. ...
The study focuses on chemical composition of stream and subsurface water in the catchments of two small arctic alpine lakes in the Kilpisjärvi area (northwest Finland). Differences and changes in chemical components of both water types are followed in order to detect spatial variability and impact of environmental factors. To achieve this, ion compositions of subsurface water and streams were measured at 12 sites in the catchments of Lakes Saana and Saanalampi during four years (2008-2010, and again in 2017). In the Lake Saanalampi catchment, the salinity of stream water (7.0 to 12.7 µS·cm −1) corresponded to that of snow. In the catchment of Lake Saana, however, the conductivity in stream water was much higher (40 to 220 µS·cm −1), connected mainly to the increase of SO 4 2− and less with Mg 2+ and Ca 2+ contents, especially in the western part of the Saana catchment. These results demonstrate that arctic conditions do not preclude intense chemical weathering where conditions are favourable. Although chemical composition of the soil fluid does not match the geochemical signal from the local soil, rock composition, especially the presence of pyrite, is the main controller of chemical weathering rates of the rocks on the area. This supports earlier views that the character of precipitation mostly controls water chemistry of local lakes in the Kilpisjärvi area.
... As White et al. (2005), Mavris et al. (2010) and others show, decreases in Mg/Na and Ca/Na ratios in soil solutions are related to advanced states of soil weathering. Pore waters from soils of increasing age exhibit decreasing solute concentrations and increasing proportions of Na relative to K, Mg and Ca (White et al., 2005) as well as accelerated Na depletion (Williams et al., 2010). ...
... As White et al. (2005), Mavris et al. (2010) and others show, decreases in Mg/Na and Ca/Na ratios in soil solutions are related to advanced states of soil weathering. Pore waters from soils of increasing age exhibit decreasing solute concentrations and increasing proportions of Na relative to K, Mg and Ca (White et al., 2005) as well as accelerated Na depletion (Williams et al., 2010). Vegetation change induced by climate warming would be expected to decrease Ca/Na ratiosbut not under all conditions. ...
Climate change and a related increase in temperature, particularly in alpine areas, force both flora and fauna to adapt to the new conditions. These changes should in turn affect soil formation processes. The aim of this study was to identify possible consequences for soils in a dry-alpine region with respect to weathering of primary minerals and leaching of elements under expected vegetation and climate changes. To achieve this, a field empirical approach investigating an altitudinal sequence was used in combination with laboratory weathering experiments simulating several scenarios. The study sites are located in Sinks Canyon and Stough Basin of the Wind River Range, Wyoming, USA. The following sites (from moist to dry with increasing temperature along the sequence) were investigated: 10 soil profiles (Typic Haplocryoll) in a tundra ecotone, 10 soil profiles (Ustic Haplocryoll) in a pine-fir forest and 20 soil profiles (Ustic Argicryoll) in sagebrush. All soils developed on granitoid moraines. Soil mineralogy was analysed using cathodoluminescence and X-ray diffraction. This revealed that biotite and plagioclase were both weathered to smectite while plagioclase also weathered to kaolinite. Cooler, wetter, altitude-dependent conditions promoted weathering of primary minerals. Furthermore, the soils of the tundra and forest zone exhibited a higher acidity and more organic carbon.
... While soil chemistry integrates weathering signals over thousands to millions of years, elemental chemistry of natural waters refl ects contemporary weathering rates. Furthermore, pore fl uids document weathering extent on the pedon and catena scales, and stream waters average elemental fl uxes on the watershed scale (e.g., White et al., 2005;Gaillardet et al., 1999;Quade et al., 2003;West et al., 2005;White, 2008). Most weathering reactions can be followed by investigating major element concentrations in the pore fl uids, whereas water isotopes are the best tracers to study water fl owpaths and to estimate residence times of water within a watershed (e.g., Schellekens et al., 2004;Brooks et al., 2010). ...
... For this study, the lysimeters were chosen to match the same protocol used by the USGS in similar studies in order that our data be useful in comparison with previously published USGS data. Specifi cally, chemistry data from long-term in situ soil water collection using the same types of lysimeters have been reported (e.g., White et al., 2005). For this study, we pulled the same vacuum on the lysimeters (−0.5 bar) during each sampling, and the changes in the measured water chemistry therefore refl ect spatiotemporal variation rather than diff erences in sampling protocol. ...
We investigated variations of Mg concentration, delta D and delta O in precipitation, soil water along a planar hillslope, groundwater, and first-order stream water at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Water flows vertically in the unsaturated zone of the hillslope, but hydrological saturation periodically causes lateral flow along interfaces of permeability contrast between the A-B and B-C soil horizons. Changes in soil water Mg concentration respond to hydrological changes and are ultimately controlled by the kinetics of clay mineral dissolution, but are buffered by the soil exchange capacity. Clay dissolution predominantly occurs within the A and B horizons, and Mg released from these zones of "low-flow" diffuses or flows into the "high-flow" zones at horizon interfaces. The Mg concentrations are low in these high-flow zones because fresher (younger) water flows in through macropores. The amplitude of seasonal variations in water isotopes (data from 2008-2010) decreases in the following order: precipitati on (delta D: 286 parts per thousand) >> soil water (delta D: 86 parts per thousand) > shallow groundwater (delta D: 26 parts per thousand), indicating water becomes progressively older along the flowpath. Fractures and preferential high-flow paths make the watershed hydrologically responsive: the average time water stays in the shallow subsurface is inferred to be <2 yr. The stream water chemistry is affected by inputs of old groundwater that is relatively high in Mg concentration but relatively limited in range in delta D, as well as by inputs from young soil water that is relatively low in Mg concentration with a wide range in delta D. The relative contributions of these two sources to the stream change seasonally.
... Older soils of chranokite groups have less hydraulic conductivity because they have more time to accumulate clay over the bed rock (Sawarkar 1980). Immediate changes, particularly by land use, raise hydraulic action and ensure high accumulation of clay over the bed rock (Reheis et al. 1995;White et al. 2005). The hill had two first order streams, according to topographical sheets from a 1972 survey by survey of India (SoI). ...
Morphology of the landscape is subjected to continuous change across the geological history over the Earth surface. Series of extreme weather events are the catalyst for accelerating the rate of geomorphic changes. The rising global atmospheric temperature, warming of oceans and extreme rainfall events cause hazardous impact on the landscape of Kerala in South India. This study aims to examine the causes of extreme rainfall events in Kerala during July and August 2018 and its consequent results in landslides and floods in Kerala. The analysis reveals that the pattern of tropical circulation has undergone rampant modifications in the last few decades, particularly in the nature of South-West Monsoon (SWM) over the Indian sub-continent. Since 1901, Kerala has recorded a decreasing trend of annual rainfall along with tremendous decrease in number of rainy days, while the extreme rainfall events are increasing in the past two decades. The incidence of landslips has accelerated as a result of extreme rainfall events and modifications in the land use. Further, the low-lying areas in the state are also exposed to flood as well as marine transgression at different intervals.
... Generally, fractions rich in kaolinite often showed less C contents and the smectite-rich fractions contain organic C within a wide (Manjaiah et al. 2010). The presence of kaolinite thus indicates the presence of low reactive clay-sized silicates which offer fewer binding site to SOC contrary to smectite which indicate the presence of high reactive clay-sized silicates (White et al. 2005;Manjaiah et al. 2010). However, both 2:1 and 1:1 clay layers contain pH-dependent charges corresponding to surface charge generated by protonation-deprotonation reactions of surface hydroxyl groups (Singh et al. 2017). ...
The present work was designed to study a soil sequence in Mount Mandara, in order to identify the influence of altitude, soil characteristics, and land use on the accumulation of soil organic carbon (SOC). The study was conducted in four sites in the Far-North region of Cameroon, including Zamai, Kossohone, Sir, and Rhumsiki. Three pits were dug down to the weathering horizons in three positions (upslope, mid-slope, and footslope) along a representative toposequence in each site. Samples were taken from each pit at regular increment of 25 cm from the soil surface. The total SOC stock (T-SOCS) contents are 128.63 ± 5.25 Mg ha−1 in Arenosols at Zamai (608 m a.s.l.), 158.248 ± 10.52 Mg ha−1 in Leptosols at Kosohone (865 m a.s.l.), 158.99 ± 13.25 Mg ha−1 in Luvisols at Sir (970 m a.s.l.), and 144.79 ± 24.23 Mg ha−1 in Regosols at Rhumsiki (1050 m a.s.l.). The main secondary minerals are smectite, kaolinite, sepiolite, lepidocrocite, hematite, and calcite. Clay minerals and iron oxides are good receptacle for SOC and might constitute a major asset for the accumulation and the sequestration of SOC. Increase in elevation leads to decrease in the annual temperature which affect microbial activity, leading thus to a slow rate of soil organic matter (SOM) decomposition, which thereby affected SOCS. This is confirmed by the significant correlation between altitudinal gradient and T-SOCS (r = 0.70), with altitude contributing to the accumulation of SOC for 49.68%. Texture also plays a central role in carbon sequestration in the studied area, confirmed by the significant and positive correlation between silt fraction and SOM. Under Regosols, there is a decrease in T-SOCS value as a result of a reduction of the quantity of organic matter returned to the soil and more rapid SOM decomposition due to ploughing. This research provides a preliminary assessment for SOC stock at Mount Mandara. It suggests that altitudinal gradient, land use, and soil characteristics should be included in SOCS models and estimations at local and regional scales.
... The degree of soil development affects biogeochemical soil gradients, and when all other factors are equal, age controls the degree of soil development and the nature of depth dependent gradients. Soil chronosequences provide a useful framework for assessing the co-evolution of soil physicochemical properties and microbial communities through time (Lawrence et al., 2014;Moore et al., 2010;Stonestrom et al., 1998;White et al., 2005). ...
Although it has been shown that the interaction of climate and time shape the dynamics of soil organic matter (SOM) storage and preservation in soil, the role of soil microbial communities in this dynamic remains unclear. Microbial communities are present throughout soil profiles and likely play critical roles in SOM and nutrient cycling, however the influence of other factors such as soil development (i.e., age) and the composition SOM on microbial community variation with depth has yet to be quantified. Improving our understanding of the relationship between soil development, soil depth, and microbial communities may provide insight to the critical role they play in cycling and preservation of SOM, as well as more mechanistic predictions of the response of soil communities to change, such as landscape-scale changes in available moisture or temperature regimes. Here we compare soils spanning a soil age by climate gradient (i.e., climo-chronosequence) to better understand the mechanisms which influence soil microbial community structure and the molecular composition of SOM. While we observed little depth-dependence in metrics of microbial community structure (i.e., composition, diversity, dissimilarity) across the range of soil development under a wetter climate, we found significant depth-dependent changes in community metrics under a drier climate, which became more pronounced as soils became older. This shift in bacterial and archaeal community structure and diversity is most apparent below a clay-rich argillic horizon formed in the older, drier soils. The molecular composition of SOM as measured by high resolution mass spectrometry (i.e., FTICR MS) also exhibited similar shifts in composition with soil depth and age. Our results highlight how soil moisture shapes the interaction of soil development, SOM, and microbial community composition. Differences in the moisture regime between our two study sites drive differences in biogeochemical depth gradients and subsequent variation in soil microbes and SOM. This suggests that knowledge of not just the pedogenic trajectory of soil development, but also the spatial position relative to distinct pedogenic features, are important for explaining variations in the depth-dependencies of microbial communities and associated SOM.
... 1,33 Both chronosequences were located in a semiarid environment. 1,33 The parent material at the Merced sites was dioritic to granitic sand deposits from glacial outwash 34,35 and was microporphyritic basalt at the SAGA sites. 1 Surface soils (0−15 cm) were collected, air-dried, homogenized, ground, and passed through 100 mesh and preserved under dark and dry conditions. The same soil samples were used in previous studies, 28,32,34 and more detailed information about the study sites, soil sampling, and pretreatments can be found in previous publications. ...
Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge XANES spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO3 and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH desorbed phosphate and exchangeable Ca2+ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca2+ regardless of the presence or absence of CaCO3 whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.
... For surfaces with an extended period of formation, the timing of abandonment is more likely to coincide with events of interest such as reorganisation of a drainage network ( Bufe et al., 20 2017); changes in climate, sediment supply, or base level ( Steffen et al., 2009;Tofelde et al., 2017;Mouslopoulou et al., 2017;Brooke et al., 2018); or tectonic deformation such as faulting, uplift, or subsidence (e.g., Frankel et al., 2007Frankel et al., , 2011Ganev et al., 2010). Abandonment ages would also benefit any study that uses surface exposure dating to measure the rate of a post-depositional process, such as in situ weathering (e.g., White et al., 1996White et al., , 2005D'Arcy et al., 2015D'Arcy et al., , 2018, the topographic decay of landforms (e.g., Hanks et al., 1984;Andrews & Bucknam, 1987;Spelz et al., 2008), or channel 25 avulsion and incision (e.g., Schildgen et al., 2012;Finnegan et al., 2014;Malatesta et al., 2017). Yet the abandonment of a surface represents a cessation in the deposition of dateable material, and therefore cannot be directly dated. ...
Information about past climate, tectonics, and landscape
evolution is often obtained by dating geomorphic surfaces comprising
deposited or aggraded material, e.g. fluvial fill terraces, alluvial fans,
volcanic flows, or glacial till. Although surface ages can provide valuable
information about these landforms, they can only constrain the period of
active deposition of surface material, which may span a significant period
of time in the case of alluvial landforms. In contrast, surface abandonment
often occurs abruptly and coincides with important events like drainage
reorganization, climate change, or landscape uplift. However, abandonment
cannot be directly dated because it represents a cessation in the deposition
of dateable material. In this study, we present a new approach to inferring
when a surface was likely abandoned using exposure ages derived from in situ-produced cosmogenic nuclides. We use artificial data to measure the
discrepancy between the youngest age randomly obtained from a surface and
the true timing of surface abandonment. Our analyses simulate surface dating
scenarios with variable durations of surface formation and variable numbers
of exposure ages from sampled boulders. From our artificial data, we derive
a set of probabilistic equations and a MATLAB tool that can be applied to a
set of real sampled surface ages to estimate the probable period of time
within which abandonment is likely to have occurred. Our new approach to
constraining surface abandonment has applications for geomorphological
studies that relate surface ages to tectonic deformation, past climate, or
the rates of surface processes.
... org/10.1038/s41561-018-0168-7. 44 . Partly, this campaign resampled terraces and sites where legacy data on soil mineralogy and geochemistry was available [45][46][47][48][49][50] . Where new sites have been selected, ratios of Ti/Zr and Fe total /Fe DCB have been measured as indicators for the weathering stage of the deposits and compared with the existing mineralogical data to constrain the placement of the selected sites along the timeline of the terrace sequence. ...
Quantifying soil carbon dynamics is of utmost relevance in the context of global change because soils play an important role in land–atmosphere gas exchange. Our current understanding of both present and future carbon dynamics is limited because we fail to accurately represent soil processes across temporal and spatial scales, partly because of the paucity of data on the rela-tive importance and hierarchical relationships between microbial, geochemical and climatic controls. Here, using observations from a 3,000-kyr-old soil chronosequence preserved in alluvial terrace deposits of the Merced River, California, we show how soil carbon dynamics are driven by the relationship between short-term biotic responses and long-term mineral weathering. We link temperature sensitivity of heterotrophic respiration to biogeochemical soil properties through their relationship with microbial activity and community composition. We found that soil mineralogy, and in particular changes in mineral reactivity and resulting nutrient availability, impacts the response of heterotrophic soil respiration to warming by altering carbon inputs, carbon stabilization, microbial community composition and extracellular enzyme activity. We demonstrate that biogeochemical alteration of the soil matrix (and not short-term warming) controls the composition of microbial communities and strategies to metabolize nutrients. More specifically, weathering first increases and then reduces nutrient availability and retention, as well as the potential of soils to stabilize carbon.
... It has been a big issue in numerous studies to quantify mineralogical sources of Ca and Mg in rivers because the release of Ca and Mg from minerals during chemical weathering is closely related to biogeochemical processes and global carbon cycle (e.g., Berner et al. 1983;Likens et al. 1996;White et al. 2005). However, this is quite difficult to explain how the chemical weathering is related to various type of rocks that contain multiple Ca-and Mgbearing minerals, and therefore, most studies have indirectly constrained source apportionment using major ion balances or strontium isotope ratios ( 87 Sr/ 86 Sr) Miller et al. 1993;Négrel et al. 1993;Spence and Telmer 2005;Wu et al. 2008). ...
The sources impacting the water chemistry of the Nakdong River (NR) in South Korea were investigated in order to examine the pollution mechanism including the fate and transport of the contaminants and how much such sources may affect its main channel of water resource. Water samples were collected between 2007 and 2008 from 8 sites along a 510 km downstream transect of the NR, and chemical and isotopic compositions of the water samples were evaluated to identify natural and anthropogenic sources contributing to the water chemistry of the NR. The results showed that the major ion concentrations were mainly controlled by chemical weathering that occurred in the watershed, in which a silicate weathering is more dominant than a carbonate weathering. The ⁸⁷Sr/⁸⁶Sr ratios of the water samples were in a range from 0.71043 to 0.71520 within those of the Mesozoic volcanogenic sedimentary rocks long developed in the watershed, thereby supporting the fact that the water chemistry is governed by a chemical weathering. The δ³⁴S values varied in a narrow range of 1.8–3.1‰, regardless of spatial and seasonal variations. Mass balance calculations indicated that the contributions of chemical weathering and anthropogenic sources during summer time increased along a downstream transect, from 67.3 ± 1.3 to 73.6 ± 0.5% and from 6.0 to 15.7%, respectively. In contrast, the contribution of chemical weathering during winter time decreased from 82.7 ± 0.8 to 72.5 ± 0.3%, while anthropogenic contribution increased up to 22.2%. These results indicated that the water chemistry of the NR was mainly caused by a chemical weathering, followed by anthropogenic inputs and rainwater. This study will provide baseline information for comparing the water quality issue before and after the implementation of the Four Rivers Restoration Project of South Korea.
... Decreased soil saturation during 24 summer months and frozen soils during winter months prevented samples from being taken during several collection 25 campaigns. Samples were collected from four transects ( Figure 1) using "nested suction-cup lysimeters (48-mm 26 diameter, Soil Moisture 1900 series)" at suction of 50 kPa for a period of 1 week ( Thomas et al 2013) prior to 27 sampling following the collection protocols of White et al (2005). We note that suction cup lysimeters may 28 underestimate the degree of isotopic enrichment in the unsaturated zone by sampling only mobile and partially29 mobile soil water ( Sprenger et al. 2015;Thomas et al., 2013). ...
Naturally occurring stable water isotope tracers provide useful information for hydrologic model development and calibration. Existing models include varied approaches concerning unsaturated zone percolation mixing (preferential versus matrix flow) and evapotranspiration (ET) partitioning. We assess the impact of unsaturated zone simplifying assumptions when simulating the Shale Hills Watershed (SHW), a small (7.9 ha), temperate, forested watershed near Petersburg, Pennsylvania, USA with a relatively simple model. We found that different model structures/assumptions and parameterizations of unsaturated zone percolation had substantial impacts on the agreement between simulated and observed unsaturated-zone water isotopic signatures. We show that unsaturated zone percolation mixing primarily affects the unsaturated zone δ18O and δ2H during winter and spring and that percolation was best represented as a combination of both preferential and matrix flow. We evaluate the importance and implications related to the partitioning of ET into evaporation (E) and transpiration (T) and demonstrated that incorporation of a plant growth model for ET partitioning substantially improved reproduction of observed hydrologic isotopic patterns of the unsaturated zone during the spring season. We show that unsaturated zone percolation mixing and ET partitioning approaches do not substantially influence stream δ18O and δ2H and conclude that observed streamflow isotopic data is not always a strong predictor of model performance with respect to intra-watershed processes.
... During dry periods and droughts, soil clay minerals lose sorbed water (Velde, 1992) and aggregate to become less mobile (Fig. 9A), but during precipitation events soil aggregates disintegrate, allowing clay particles to become fluidized and erode (Fig. 9B) (Bryan, 2000). We posit that during precipitation events in the Transverse Ranges, clays are able to sorb water, becoming more mobile and erodible (Velde, 1992;Grabowski et al., 2011), and are transported downstream to be deposited in SBB, while during dry periods soil moisture decreases (White et al., 2005), and clays become progressively more immobile in the catchments (Fig. 9 inset). Thus clay mineral abundances increase in SBB sediments during intervals associated with frequent precipitation events, and decrease during droughts (Fig. 7). ...
The recurrence and magnitude of southern California hydroclimate extremes are poorly resolved due to the relatively short duration (<140 years) of modern instrumental precipitation and stream gauge records. Terrigenous detrital sediments are often used to reconstruct long-term hydroclimate changes as precipitation increases river runoff and sediment transport into nearby basins. Here we assess the potential of elemental and mineralogical sediment composition from Santa Barbara Basin (SBB, California) box core SPR0901-04BC, a ∼250 year record, as a proxy for precipitation and/or river runoff. Additionally we explore the impact of anthropogenic modification of rivers on sediment composition. Potassium and Ti concentrations and kaolinite + chlorite abundances are significantly correlated with regional precipitation. Transfer function modeling demonstrates that precipitation alone predicts Ti concentration variability well, but not clay mineral abundances. However, when dam construction within catchments draining into SBB is included, kaolinite + chlorite abundances can be modeled. We propose kaolinite + chlorite and illite sources in the upper reaches of catchments are trapped behind dams, while smectite sources in lower catchment areas are unimpeded and continue to be deposited in the basin. Linear correlations and model results suggest detrital elemental concentrations are more suitable for precipitation and river runoff reconstruction than clay mineral composition. Correlations observed between sediment composition and precipitation demonstrate the potential for marine sediment proxies to extend weather and climate records beyond the instrumental record, however anthropogenic land use modification, specifically damming, must be considered.
... Climate has a marked influence on weathering rates, and an increase in temperature and precipitation may increase weathering rates of minerals (Bolt and Bruggenwert, 1976) producing dissolved Si, P and alkalinity (Table 2). However, factors such as soil age, lithology, runoff and land cover may have a more pronounced effect on weathering rates at the regional to continental scale White et al., 2005). Tropical river basins play a major role in chemical weathering and transfer of dissolved Si and alkalinity to rivers and oceans due to their geological and climatic settings (Jennerjahn et al., 2006). ...
In river basins, soils, groundwater, riparian zones and floodplains, streams, rivers, lakes and reservoirs act as successive filters in which the hydrology, ecology and biogeochemical processing are strongly coupled and together act to retain a significant fraction of the nutrients transported. This paper compares existing river ecology concepts with current approaches to describe river biogeochemistry, and assesses the value of these concepts and approaches for understanding the impacts of interacting global change disturbances on river biogeochemistry. Through merging perspectives, concepts, and modeling techniques, we propose integrated model approaches that encompass both aquatic and terrestrial components in heterogeneous landscapes. In this model framework, existing ecological and biogeochemical concepts are extended with a balanced approach for assessing nutrient and sediment delivery, on the one hand, and nutrient in-stream retention on the other hand.
... , 2005). Moreover, the non-flood decline in DSi concentrations may be due to the weak silicate weathering due to low temperature, which significantly modifies the river DSi level (White and Blum, 1995; Gaillardet et al., 1999; White et al., 2005; Hartmann and Moosdorf, 2011). DSi concentrations are lower during non-flood season than that in flood period at equivalent water flows (Fig. S5), which could indicate the seasonal variation of silicate weathering. ...
... During water-rock interactions, primary minerals transform into secondary phases, solutes are generated, and the bedrock or parent sediments are converted to soils, which can be mobilized and transported downslope (Anderson et al., 2004;Brantley and White, 2009;Chadwick et al., 1990;Jin et al., 2010;Stallard, 1992;White et al., 2005). Thus, the mineral dissolution kinetics have important implications for the development of subsurface reaction fronts, and landscape evolution (e.g., Anderson et al., 2007;Brantley et al., 2007;Ma et al., 2013). ...
The application of multiple isotope proxies on the same location within a Critical Zone (CZ), which we term “CZ-tope”, elucidates the interactions of geochemical, geomorphological, hydrological and biological processes together with anthropogenic influences in the CZ across widely disparate timescales. We exemplify the CZ-tope approach by summarizing the emerging hypotheses developed from isotopic measurements at the Susquehanna Shale Hills CZ Observatory (SSHCZO), Pennsylvania (U.S.A).
... yr À1 and r = (0.53-1.0) Â 10 3 L m À3 , White et al., 1998White et al., , 2002White et al., , 2005White et al., , 2009, supporting the validity of the method. ...
A weathering model was developed to constrain the partial pressure of atmospheric O2 (PO2) in the Paleoproterozoic from the Fe records in paleosols. The model describes the Fe behavior in a weathering profile by dissolution/precipitation of Fe-bearing minerals, oxidation of dissolved Fe(II) to Fe(III) by oxygen and transport of dissolved Fe by water flow, in steady state. The model calculates the ratio of the precipitated Fe(III)-(oxyhydr)oxides from the dissolved Fe(II) to the dissolved Fe(II) during weathering (φ), as a function of PO2. An advanced kinetic expression for Fe(II) oxidation by O2 was introduced into the model from the literature to calculate accurate φ-PO2 relationships. The model's validity is supported by the consistency of the calculated φ-PO2 relationships with those in the literature. The model can calculate PO2 for a given paleosol, once a φ value and values of the other parameters relevant to weathering, namely, pH of porewater, partial pressure of carbon dioxide (PCO2), water flow, temperature and O2 diffusion into soil, are obtained for the paleosol.The above weathering-relevant parameters were scrutinized for individual Paleoproterozoic paleosols. The values of φ, temperature, pH and PCO2 were obtained from the literature on the Paleoproterozoic paleosols. The parameter value of water flow was constrained for each paleosol from the mass balance of Si between water and rock phases and the relationships between water saturation ratio and hydraulic conductivity. The parameter value of O2 diffusion into soil was calculated for each paleosol based on the equation for soil O2 concentration with the O2 transport parameters in the literature. Then, we conducted comprehensive PO2 calculations for individual Paleoproterozoic paleosols which reflect all uncertainties in the weathering-relevant parameters. Consequently, robust estimates of PO2 in the Paleoproterozoic were obtained: 10-7.1-10-5.4atm at ~2.46Ga, 10-5.0-10-2.5atm at ~2.15Ga, 10-5.2-10-1.7atm at ~2.08Ga and more than 10-4.6-10-2.0atm at ~1.85Ga. Comparison of the present PO2 estimates to those in the literature suggests that a drastic rise of oxygen would not have occurred at ~2.4Ga, supporting a slightly rapid rise of oxygen at ~2.4Ga and a gradual rise of oxygen in the Paleoproterozoic in long term.
... During dry periods and droughts, soil clay minerals lose sorbed water (Velde, 1992) and aggregate to become less mobile (Fig. 9A), but during precipitation events soil aggregates disintegrate, allowing clay particles to become fluidized and erode (Fig. 9B) (Bryan, 2000). We posit that during precipitation events in the Transverse Ranges, clays are able to sorb water, becoming more mobile and erodible (Velde, 1992;Grabowski et al., 2011), and are transported downstream to be deposited in SBB, while during dry periods soil moisture decreases (White et al., 2005), and clays become progressively more immobile in the catchments (Fig. 9 inset). Thus clay mineral abundances increase in SBB sediments during intervals associated with frequent precipitation events, and decrease during droughts (Fig. 7). ...
... This includes liberally interchanging geometric surface area (GSA), specifi c surface area (SSA), and reactive surface area (RSA). Mineral GSA typically refers to a surface area computed from an average grain size and assuming a particular geometric grain shape, typically perfectly smooth spheres (White et al. 2005;Alemu et al. 2011). Mineral SSA refers to the total or rough surface area per gram mineral, often measured via the Brunauer, Emmett, Teller (BET) analysis (Brunauer et al. 1938). ...
The recent profusion of microscopic characterization methods applicable to Earth Science materials, many of which are described in this volume (Anovitz and Cole 2015, this volume; Noiriel 2015, this volume), suggests that we now have an unprecedented new ability to consider geochemical processes at the pore scale. These new capabilities offer the potential for a paradigm shift in the Earth Sciences that will allow us to understand and ultimately quantify such enigmas as the apparent discrepancy between laboratory and field rates (White and Brantley 2003) and the impact of geochemical reactions on the transport properties of subsurface materials (Steefel and Lasaga 1990, 1994; Steefel and Lichtner 1994; Xie et al. 2015). It has only gradually become apparent that many geochemical investigations of Earth materials have suffered (perhaps inadvertently) from the assumption of bulk or continuum behavior, leading to volume averaging of properties and processes that really need to be considered at the individual grain or pore scale. For example, a relationship between reaction-induced porosity and permeability change can perhaps be developed based on bulk samples, but ultimately a mechanistic understanding and robust predictive capability of the associated geochemical and physical processes will require a pore-scale view.
The question still arises: Do we need pore-scale characterization and models in geochemistry and mineralogy? The laboratory–field rate discrepancy (or enigma) is a good example of where a pore-scale understanding may provide insights not easily achievable with bulk characterization and models. If the reasons for this apparent discrepancy between laboratory and field rates cannot be explained, then it appears unlikely that scientifically defensible and quantitative models for a number of important Earth Science applications ranging from chemical weathering and its effects on atmospheric CO2, to subsurface carbon sequestration, to nuclear waste storage, to contaminant remediation and transport, …
... Soil gas samplers were constructed and installed on the same ridgetop as per White et al. (2005), within ~ 3 m of the site augered for this study. Gases were sampled three times during 2008-2009 in different seasons, and O 2 measured by gas chromatography using a Carle Series 400 AGC gas chromatograph (GC) equipped with a thermal conductivity detector (USGS, Menlo Park, CA). ...
Fe oxidation is often the first chemical reaction that initiates weathering and disaggregation of intact bedrock into regolith. Here we explore the use of pyrosequencing tools to test for evidence that bacteria participate in these reactions in deep regolith. We analyze regolith developed on volcaniclastic rocks of the Fajardo formation in a ridgetop within the rainforest of the Luquillo Mountains of Puerto Rico. In the 9 meter-deep regolith profile, the primary minerals chlorite, feldspar, and pyroxene are detected near 8.3 m but weather to kaolinite and Fe oxides found at shallower depths. Over the regolith profile, both total and heterotrophic bacterial cell counts generally increase from the bedrock to the surface. Like other soil microbial studies, the dominant phyla detected are Proteobacteria, Acidobacteria, Planctomycetes, and Actinobacteria. Proteobacteria (a, b, g and d) were the most abundant at depth (6.8 - 9 m, 41 - 44%), while Acidobacteria were the most abundant at the surface (1.4 - 4.4 m, 37 - 43%). Despite the fact that Acidobacteria dominated surficial communities while Proteobacteria dominated near bedrock, the near-surface and near-bedrock communities were not statistically different in structure but were statistically different from mid-depth communities. Approximately 21% of all sequences analyzed did not match known sequences: the highest fraction of unmatched sequences was greatest at mid-depth (45% at 4.4 m).
... To improve our understanding of that control, many studies about carbonate and silicate weathering have been carried out (Nesbitt and Young 1982;Suchet and Probst 1995;Macpherson et al. 2008;Li et al. 2010;Heckman and Rasmussen 2011;Moosdorf et al. 2011;Zeng et al. 2012). Silicate weathering studies are conducted through relatively input-output calculations, from changes in solute compositions in pore water or ground water, or from the elemental differences between protolith and the weathered regolith (Kenoyer and Bowser 1992;White et al. 1996;Murphy et al. 1998;White 2002;White et al. 2005b). Chemical weathering of silicate minerals has been proposed to be a sink of atmospheric CO 2 on the geological time scale (Ridgwell and Zeebe 2005). ...
To further understand the roles of carbonate and silicate rocks in regulating the atmosphere/soil CO2 level, the flux of CO2 consumed by the chemical weathering of silicate and carbonate rocks was determined from the elemental change in soil profiles. Results showed that the chemical weathering of carbonate rocks mainly occurred at the rock-regolith interface, and that the further weathering of the residua soil on the carbonate rocks was similar to that of the granite profile. Chemical weathering of the silicate rocks occurred through the whole profiles. Therefore, CO2 consumed per volume by the silicate profiles [M sr (CO2)] and the residues on carbonate rocks [M cr (CO2)] were calculated based on the elemental weathering gradients. CO2 consumed by carbonate protolith [M cp (CO2)] was calculated from the elemental change at the rock-regolith interface. The M sr (CO2) were about tens to thousands orders of magnitude greater than M cr (CO2). Even so, this demonstrated that the residues on carbonate rocks could be a sink of CO2 on long-term scales. The M cp (CO2) was about four times larger than M sr (CO2), which demonstrated that carbonate rocks played a more important role in regulating the CO2 level than the silicate rocks did during the pedogenic process of the profiles.
... The presence of metals in water may come from two independent processes. It could come from a natural weathering process occurring in soils and rocks [2,3] with the results then transmitted in the air [4,5] and water [6]. Another factor could be anthropogenic activities [7][8][9][10]. ...
This study was conducted in a major agricultural area in the state of Chihuahua,
Mexico. Sediment samples were collected from five sites in the San Pedro River
basin with the objective of detecting levels of cadmium (Cd), arsenic (As) and
lead (Pb). Site 1 was located near the Presa las Virgenes dam (PV), site 2 near
the locations of Delicias-Rosales (RD), site 3 near the town of Meoqui (M), site
4 in the community of El Torreon (ET) and site 5 near the communities of Junta
de los Rios Conchos and San Pedro (LJ). The metals analysis was conducted
using an Inductively Couplet Plasma Atomic Emission Spectrometer (ICP-OES)
Perkin Elemer 2000. The results were compared with NOM-147-
SEMARNAT/SSA1-2004. Cd levels were not determined in any soil sample.
The results indicated that Pb levels were below the established NOM-147 and
that location M had the highest value with a mean of 9.15 mg kg-1. The highest
concentration levels of arsenic were found in the RD locale with 14.56 mg kg-1
during the December sampling. In conclusion, lead and arsenic levels in mud
from the San Pedro River basin are below the norm. Still, As levels might be
significant to human health, especially for the communities established along the
San Pedro River.
Keywords: metals, lead, arsenic, Mexico, human health.
... Here, Si infillings in subsoil horizons during wet seasons can be hardened by repeated desiccation in dry seasons. If this process continues for a longer time, duripan horizons will develop (Torrent et al., 1980;Chartres, 1985;Chadwick et al., 1987a, b;Blank and Fosberg, 1991;Moody and Graham, 1997;Monger and Kelly, 2002;Kendrick and Graham, 2004;White et al., 2005). The cementation of grains in duripans is detectable by micromorphological analysis (Brewer et al., 1972;Flach et al., 1974;Norton, 1994). ...
... Quantification and understanding of mineral weathering have important implications for many environmental problems, such as the relationship between silicate weathering and global climate over geological timescales (Berner, 1992), the availability of inorganic nutrients in soils (Federer et al., 1989;Likens et al., 1998;Huntington et al., 2000), geological carbon sequestration , global geochemical cycles (Lasaga et al., 1994), safety of radioactive waste repositories (Spycher et al., 2003), impacts of acid mines drainage and neutralization of acid precipitation in watersheds (Drever and Clow, 1995), release of toxic elements to soils and to the hydrologic cycle, and the distribution of porosity and permeability in hydrocarbon reservoir rocks (Morad et al., 2010). Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies (Schnoor, 1990;Stumm, 1992; van Grinsven and van Riemsdijk, 1992;Anbeek, 1993;Casey et al., 1993;Velbel, 1993;Blum and Stillings, 1995;Brantley, 1995, 2003;White et al., 1996White et al., , 2005White et al., , 2008Drever, 2003;Zhu et al., 2004;Zhu, 2005;Ganor et al., 2007;Moore et al., 2012). The many differences between experimental conditions in the laboratory and natural conditions in the field have been thoroughly discussed in previous studies (e.g., White and Brantley, 2003;Reeves and Rothman, 2013, and references therein), but the discrepancy was never fully reconciled. ...
Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. The differences between experimental conditions in the laboratory and natural conditions in the field have been thoroughly discussed in previous studies, however, the discrepancy was never fully resolved. It has been shown in past work that if the field conditions are fully simulated in standard laboratory experiments, it is not possible to measure the slow rates of mineral dissolution that are observed in the field using standard laboratory experiments. Therefore, a novel method that uses the change of Si isotopes ratio in spiked solutions is used in the present study to measure weathering rates of feldspar under close-to-natural conditions.
... Una cronosecuencia se refiere a una serie de suelos que difieren en su edad de formación, pero tienen un material parental similar y se forman bajo condiciones climáticas, topográficas y biológicas similares. Por lo mismo, una cronosecuencia de suelos es un ambiente ideal para comparar procesos similares a escalas de tiempo distintas, como también para observar si los procesos actuales son similares a aquellos del pasado (Bockheim, 1980;Muhs, 1982;White et al., 1996;Blum y Erel, 1997;Vitousek et al. 1997;Chadwick et al., 1999;Stewart et al., 2001;White et al., 2005). ...
En la paleobahía de Tongoy (30°S), ubicada en Chile centro-norte, se encuentran cuatro terrazas de abrasión marina, denominadas TV, TIV, TIII y TII, las cuales se asocian a los estadios isotópicos marinos (MIS, por sus siglas en inglés) MIS 1, MIS 5e, MIS 7 y MIS 11. Sobre las tres terrazas más antiguas, se desarrollan suelos carbonáticos cementados, que se conocen en la literatura como calcretas. Las calcretas de Tongoy alcanzan un estado de desarrollo maduro en las tres terrazas donde se presentan, llegando en la terraza TII al estado VI en la secuencia de desarrollo de Machete (1985), el cual corresponde al más avanzado. La formación de estas calcretas responde a una serie de procesos pedogénicos, los cuales dan cuenta de una ciclicidad en el paleoclima de la zona.
Las calcretas estudiadas, se subdividen en calcreta masiva, calcreta laminar y calcreta brechosa, esta última presente solo en la terraza TII que corresponde a la más antigua. La calcreta masiva posee una serie de rasgos que muestran la participación de organismos en la precipitación del carbonato, presentando una fábrica de tipo “beta”. La calcreta laminar presenta una fábrica de tipo “alfa”, que corresponde a la precipitación de calcita por procesos físico-químicos. La calcreta brechosa, corresponde al fracturamiento de la calcreta laminar, y se da en estados avanzados de desarrollo de este tipo de suelos.
El desarrollo de calcretas se da en determinados climas, por lo que su presencia en las terrazas TII, TIII y TIV sugieren condiciones climáticas similares entre el MIS 11 (412 ka) y posterior al MIS 5e (125 ka). Sin embargo, existen rasgos y procesos que sugieren la existencia de fluctuaciones entre climas más áridos y más húmedos que el óptimo para la formación de calcretas. A modo de ejemplo se pueden citar la presencia de cristales de yeso y halita en las partes bajas de la calcreta masiva de los perfiles de la terraza TII, los cuales indican la presencia de climas áridos previo a la formación de la calcreta masiva; mientras que la presencia de arcillas iluviadas en el límite entre la calcreta masiva y laminar, sugiere la ocurrencia de períodos con mayor cantidad de precipitaciones, las que habrían disminuido posteriormente para generar la calcreta laminar. La ocurrencia de paleocanales en los niveles TIV y TII que erosionan la calcreta, indican períodos con presencia de precipitaciones torrenciales, posterior a los cuales no se formó calcreta de ningún tipo, ya sea por no existir las condiciones climáticas necesarias, o bien por ser fenómenos de data reciente.
Esta ciclicidad climática reflejada en los procesos geomorfológicos y pedogénicos, mediante la aplicación de métodos de datación absoluta, permitirán realizar estudios de paleoambiente en la zona, aportando con datos paleoclimáticos hasta el MIS 11, para una zona que carece de información de largo plazo. En este sentido, este estudio constituye una base en la cual se explican los procesos pedogénicos asociados a una cronosecuencia de suelos desarrollados sobre terrazas de abrasión marina, los cuales pueden ser utilizados a futuro para la reconstrucción paleoclimática del Norte Chico.
... Many natural soil chronosequences have been studied to illustrate temporal changes in soil properties and the rates and trajectories of soil forming processes (e.g. Bernasconi et al., 2011;Crews et al., 1995;He et al., 2008;Muhs, 1982;Nieuwenhuyse et al., 2000;Ryan and Huertas, 2009;White et al., 1996White et al., , 2005White et al., , 2008White et al., , 2012Zhang et al., 2007; J. Zhou et al., 2013). However, few such studies have involved anthropogenic soils. ...
Chronosequences and associated space-for-time substitutions are an important and fruitful means for investigating the rates and directions of soil and ecosystem evolution across multiple time-scales ranging from decades to millions of years. This paper reviews the use of chronosequences for studying biogeochemistry of paddy soil evolution to improve our understanding of the fundamental processes, the dynamic changes in soil properties and the associated environmental thresholds at different stages of paddy soil evolution under the intensive anthropogenic managements. Rice paddy cultivation results in accumulations of various nutrients (e.g. organic carbon, nitrogen, and phosphorus) over a much longer time period than predicted by typical long-term (< 50 years) field experiments, although it is not clear how long it takes paddy soils with different origins to reach a steady-state of these important nutrients. Extensive investigations of a 2000-year paddy soil chronosequence derived from calcareous marine sediments in the coastal region of Zhejiang Province (P.R. China) illustrate three phases of paddy soil evolution and the associated pedogenic thresholds: an initial phase during the first few decades dominated by rapid desalinization, loss of magnetic susceptibility, accumulation of topsoil organic matter and formation of a compacted plow pan due to extrinsic thresholds resulting from anthropogenic activities; the second phase lasts several centuries comprising Fe and clay enrichment in the illuvial horizon, and the loss of phosphorus and Mn coincident with the near complete removal of CaCO3 (recognized as the intrinsic threshold); in the third phase (> 700 years), (trans-)formation and redistribution of metal oxides are accompanied by clearly visible hydromorphic patterns in paddy subsoils. We also note that after 2000-years, paddy soils still lack evidence of silicate weathering and neo-formation of pedogenic clay minerals. Paddy soil management is adjusted to match landscape positions (e.g. well-drained sloping uplands, alluvial plains with groundwater fluctuation, and poorly drained bog areas with near surface water table) and this influences the trajectory and magnitude of pedogenic changes with prolonged rice cultivation. However, the parent material effects on paddy soil evolution seem to diminish with the lapse of time and vary considerably among different soil properties or processes. Given our universal dependence on paddy soils for food production, their value as an excellent opportunity for investigating anthropedogenesis, and their critical roles in global biogeochemical cycling, we put forward several open questions that must be resolved to maintain the millennial-scale sustainability of these important wetlands.
21st Century soil science must deeply grapple with the enormity of the agricultural impact on soils, and the complex ways in which this impacts soil sustainability. The first step is to remove the cloud of mysticism that sometimes surrounds soil and farming, reframe the narrative, and be clear about the enormous difficulty of creating a truly steady state and/or C-neutral soil management system. The objective of this paper is to examine the global footprint of agriculture on soils, and its impact on soil erosion, carbon, and nutrients. Stabilizing any one of these 3 mass balances globally will require complex and highly integrated collaborations between social scientists, policy experts, engineers, farmers, politicians, and natural scientists. There are no magical or simple solutions. Yet, there are untapped opportunities for research and solution-driven activities that can help to create a new framework for more effective efforts by our science to address wicked societal problems.
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.
A numerical coupling of a dynamic biogeochemical model (B-WITCH) and a model of forest balance water (BILJOU) was used to simulate the concentration of major species within soil profiles from two contrasted ecosystems: beech and spruce stands located in the Vosges Mountains and their evolution during decades. This coupled modelling allows an important vertical discretization of water and cation cycle and a direct comparison with the hydrological and geochemical data available over twenty-five years and on a seasonal basis. The processes controlling the past evolution of soil solution chemistry have been identified and tested. The biogeochemical cycles of cations estimated on the two sites shows that the biological fluxes control up to 70% (for Ca and Mg) and up to 95% (K) of the chemical composition of the soil solutions. The Ca and Mg concentrations decrease over the last decades, which can be explained by the evolution of atmospheric inputs and by the behavior of the exchange complex. This paper highlights the weak contribution of mineral dissolution and the key role of biological recycling and cation exchange processes in the soil solution signatures. The future sustainability of forest, in mountainous environment on base poor bedrock, depends strongly on the forest management and evolution of exchangeable pool nature.
The determination of the mechanisms and extent of soil mineral weathering can be challenging, and the caveats reside in 1) difficulty identifying minerals that are actually involved in weathering reactions, 2) non-stoichiometric release of cations during weathering processes due to coupled dissolution, precipitation and transformation reactions and, 3) impact of vegetation activity on elemental cycles in upper soil horizons.
To better characterize mechanisms controlling mineral weathering in soils and trace the evolution of B concentration and isotope ratios during chemical weathering, quantitative mineralogical analyses were coupled to B isotopes in a group of minerals (biotite, muscovite, K-feldspar and albite). Samples were selected along an Alocrisol (Alumic Cambisol, WRB FAO) soil profile from the bedrock (at 130 cm depth) up to 20 cm depth, developed on granitic bedrock in the Breuil-Chenue forest (France). The samples consist of residual primary minerals associated with weathering secondary phases (vermiculite, kaolinite…) in varying proportions.
The B isotopic compositions of the most pristine minerals span a very narrow range of values (around −31‰), whereas all secondary phases point to a much heavier value (around −16‰), regardless of mineralogy. Our results also show a mineral-dependent evolution of B concentration or isotopic composition as weathering progresses: no variation is observed during dissolution of K-feldspars; B behaves like a very mobile element in micas (biotite and muscovite), whereas it concentrates in weathered products derived from albite. However, rates of B concentrations and changes in isotopic compositions appear to be much faster than those inferred from mineralogy or major element concentrations determined by XRD and bulk chemical analyses, respectively. These results indicate that B is involved in very early weathering reactions and raises the question of its actual location in the structure of the various soil minerals as well as its pathway to solution.
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.
Currently, knowledge about the dissolution kinetics of silicate minerals under seawater conditions in general and about feldspar minerals specifically is lacking, even though feldspars are the most common minerals in the earth’s crust. Consequently, global geochemical cycles of elements and isotopes overlook the potential effects of silicate dissolution in seawater and its contribution to mass balances. The aim of the present study is to quantify the effect of salinity on albite dissolution kinetics under seawater conditions and the possible impact of salinity on Sr concentrations and isotopic compositions in seawater.
A series of albite dissolution experiments with synthetic NaCl solutions (0.0001–1 molal) at pH 5 and 25 °C were conducted using multi-point batch experiments (MPBEs). Above the Na concentration threshold (0.01 molal), the albite dissolution rate was found to increase with the NaCl concentration and with the ionic strength. Based on the present study and previous experimental studies, a combined model that describes the effects of pH, Na concentration and ionic strength on the albite dissolution rate was developed. The proposed model suggests that albite dissolution under natural seawater conditions is mainly controlled by the effect of ionic strength. Considering that seawater solutions are under far-from-equilibrium conditions, the dissolution rate of albite is more than order of magnitude faster in seawater than in the continental, abiotic environment. Hence, the potential of silicate dissolution in seawater to contribute elements to the oceans is significant.
In addition, a MPBE was conducted with a Bancroft, Ontario, Canada albite sample using synthetic seawater (SSW). Based on the changes in Si, Al, Sr, Ca and ⁸⁷Sr/⁸⁶Sr with time in this experiment, it is suggested that the relative dissolution rates of albite and apatite inclusions in the albite sample control the Sr concentrations and isotopic compositions. The estimated contribution of Sr release due to feldspar dissolution in seawater under far-from-equilibrium conditions is 2.67∙10⁸ mol y⁻¹. This result is significant if considering other forms of Al-Si mineral dissolution on the ocean floor.
Heavy metals contamination of groundwater has been of public health concern in recent years because of their implication in human health and environment. This study therefore assesses heavy metals contamination in groundwater from selected communities in Sagamu, Ogun State, Nigeria. Fifty groundwater samples were purposively collected for Physicochemical and heavy metals determination, and also spatial distribution maps of selected heavy metals were produced using Arc GIS software. Data were analyzed using descriptive statistics. Physicochemical results of groundwater revealed that the water was acidic with low conductivity and total dissolved solids (TDS). The levels of arsenic and chromium were found to be within the Maximum Contaminant Levels (MCL) while that of Pb and Cd were above MCL. The spatial distribution showed that there was predominantly high concentration of lead across the study area and cadmium was predominantly high especially in area close to industries. Abstract-Heavy metals contamination of groundwater has been of public health concern in recent years because of their implication in human health and environment. This study therefore assesses heavy metals contamination in groundwater from selected communities in Sagamu, Ogun State, Nigeria. Fifty groundwater samples were purposively collected for Physicochemical and heavy metals determination, and also spatial distribution maps of selected heavy metals were produced using Arc GIS software. Data were analyzed using descriptive statistics. Physicochemical results of groundwater revealed that the water was acidic with low conductivity and total dissolved solids (TDS). The levels of arsenic and chromium were found to be within the Maximum Contaminant Levels (MCL) while that of Pb and Cd were above MCL. The spatial distribution showed that there was predominantly high concentration of lead across the study area and cadmium was predominantly high especially in area close to industries. This study concludes that the population is at risk to heavy metals toxicity and acidic water, thereby making the water unsafe for drinking. Therefore, it recommends routine environmental monitoring and comprehensive treatment of groundwater source for areas situated close to industrial layout.
Geologic carbon storage (GCS) involves capture and purification of CO2 at industrial emission sources, compression into a supercritical state, and subsequent injection into geologic formations. This process reverses the flow of carbon to the atmosphere with the intention of returning the carbon to long-term geologic storage. Models suggest that most of the injected CO2 will be “trapped” in the subsurface by physical means, but the most risk-free and permanent form of carbon storage is as carbonate minerals (Ca,Mg,Fe)CO3. The transformation of CO2 to carbonate minerals requires supply of the necessary divalent cations by dissolution of silicate minerals. Available data suggest that rates of transformation are highly uncertain and difficult to predict by standard approaches. Here we show that the chemical kinetic observations and experimental results, when they can be reduced to a single cation-release time scale that describes the fractional rate at which cations are released to solution by mineral dissolution, show sufficiently systematic behavior as a function of pH, fluid flow rate, and time that the rates of mineralization can be estimated with reasonable certainty.
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.
Soils containing horizons cemented with silica occur worldwide, especially in the subhumid, Mediterranean, and semiarid regions. When the subsoil is cemented by silica into a hardpan, it is referred to as a “duripan” or in the geological literature as “silcrete” or “duricrust” (Milnes and Twidale 1983). In Soil Taxonomy (Soil Survey Staff 2010), a duripan is defined as a “silica-cemented subsurface horizon with or without auxiliary cementing agents.” Other cementing agents could be Fe oxides and CaCO3. The duripan horizon can occur concurrently with a petrocalcic horizon (Chadwick et al. 1987; Boettinger and Southard 1991; Blank and Fosberg 1991; Eghbal and Southard 1993a, b; Blank et al. 1998; Hobson and Dahlgren 1998); the duripan horizon often underlies an argillic or cambic horizon (Flach et al. 1969; Torrent et al. 1980; Chadwick et al. 1987; Blank and Fosberg 1991; Munk and Southard 1993; Blank et al. 1998; Hobson and Dahlgren 1998). A key aspect of the duripan is that it slakes in KOH or NaOH but does not readily slake in 1 M HCl (Soil Survey Staff 2010).
Water quality in North Alabama is becoming a critical issue and in order to understand the quality of water in the Flint Creek and Flint Rivers, concentration levels of physicochemical parameters and trace heavy metals were measured in six strategically selected sites within the streams to obtain baseline data. A total of 192 surface water samples were collected and analyzed for total phosphorous and five heavy metals of concern. Physical and chemical parameters were determined for dissolved oxygen (DO), turbidity, pH, temperature, water conductivity and total phosphorous. The concentration of the DO was above guidelines in three sites at both rivers. All metals were statistically significant (P ≤ 0.05) at 95% confidence interval by watershed. Aluminum and lead exceeded their maximum permissible levels. The results for heavy metals in this study show location and seasonal variability. There was significantly higher heavy metal content during winter>spring>summer> fall. The values for the Hazard Quotientand Hazard Index for both watershed sites were mostly more than 1, indicating some levels of concern. Additionally, surrounding rocks and other anthropogenic sources appeared to be causing the elevated chemical content of the rivers and may have resulted in significant adverse impacts on the study areas.
The objective of this thesis is to estimate the ability of boron and its isotopes to evaluate mineral weatheringbudgets and mechanisms in soils. In this goal, 3 different approaches were conducted:1) an experimental approach during which a test-mineral (biotite) undergoes the action of differentweathering agents, representative of those found in soils.2) an in-situ approach in an acid soil profile led on different minerals (biotite, muscovite, K-feldspar andalbite) handpicked at various depths and weathering state (Breuil-Chenue forest experimental site, France).The objective of this study is to determine the sensitivity of boron to soil formation processes.3) an second in-situ approach conducted on the same study site but this time on sub-surface horizons(horizon A and Al-oxi-hydroxides accumulation horizon), which aim is to evaluate the sensitivity of boron andits isotopes to mechanisms in close vicinity to vegetation (seasonal variations, vicinity of the roots).B and its isotopes able, thanks to its high reactivity during weathering mechanisms in comparison to majorelements that constitute the crystallographic network (observed in experimental as well as in in-situapproaches), to trace with a great sensitivity primary minerals dissolution or transformation mechanisms, and opens new insights for the comprehension of soils.
This study reports U-series activity ratios from river waters and six soil profiles across a soil chronosequence formed since the last glacial retreat, in Glen Feshie, Scotland. The overall aim is to examine the geochemical behaviour of the U-series nuclides in a boreal climate setting. The U-series data show that U is being both added to and leached out of the soils, to varying degrees. The U addition elevates the (U-234/U-238) of the bulk soils (up to 1.25), which is most pronounced in the youngest and the upper organic-rich soil horizons. The U addition appears to be linked to U adsorption, controlled by the degree of flooding by the Feshie River. The Feshie River has a high (U-234/U-238) ratio (similar to 1.7), a feature shared with most high-latitude Northern Hemisphere rivers. For the soil profiles with no significant U addition, U-series nuclide modelling suggests U leaching rates on the order of 0.5-2 x 10-5 y-1, a similar range to other Northern Hemisphere highlatitude areas affected by the last glaciation, e. g. Mackenzie Basin, Canada. These two observations suggest a link between weathering rates and riverine (U-234/U-238) for areas that have been glaciated recently. A global compilation of major rivers shows that high-latitude Northern Hemisphere rivers comprise a significant U flux to the ocean with high (U-234/U-238). Thus, past changes in this Northern Hemisphere high-latitude U flux may have played a major role in oceanic (U-234/U-238) variation over glacial-interglacial cycles.
For the past two decades, the spread of angiosperm plants in the Cretaceous and Paleogene has been thought to have enhanced silicate weathering fluxes of Ca and Mg to the oceans, thereby drawing down atmospheric CO2 and ultimately sequestering it in marine carbonate sediments. However, the rise of angiosperm trees in the Cretaceous was coincident with the evolution of ectomycorrhizal fungal associations in angiosperm and gymnosperm trees that have increasingly supplanted trees with the ancestral arbuscular-mycorrhizal associations. This represents the most profound alteration in root functioning to occur in plant evolutionary history, with far-reaching implications for weathering and soil biogeochemistry because the fine roots are enveloped with a fungal sheath. Ectomycorrhizal fungi provide the main nutrient and water-absorbing interface with soil, and the pathway through which organic acids and protons are actively secreted at the scale of individual mineral grains. Here, we test the hypothesis that the rise of ectomycorrhizal trees was a major contributor to the drawdown of atmospheric CO2 over the past 120 Ma through enhanced silicate weathering. We developed a process-based soil chemistry model incorporating the effects of plants with ancestral arbuscular mycorrhizas, and more recently evolved ectomycorrhizas on soil chemistry via its effects on the biological proton cycle, and integrated it into a leading model of the long-term carbon cycle (GEOCARBSULF). Our mechanistic, process-based modeling reveals that the rise of ectomycorrhizal trees can explain the CO 2 drawdown previously attributed empirically to the spread of angiosperms. We suggest, therefore, that the evolutionary rise of ectomycorrhizas represents an important driving force of the long-term carbon cycle by enhancing chemical weathering and draw-down of atmospheric CO 2 into marine carbonates.
Exchange isotherms were prepared for Na** plus in Na** plus -Ca**2** plus and Na** plus -Mg**2** plus exchange reactions, and for Mg**2** plus in Ca**2** plus -Mg**2** plus exchange reactions, at 298 K on Wyoming bentonite suspended in a 0. 05M perchlorate background. These isotherms were essentially congruent with the appropriate thermodynamic nonpreference exchange isotherms. It was concluded from this fact that there is essentially no difference in the affinity of montmorillonite clay for Ca**2** plus vs. Mg**2** plus .
Forest soil Ca depletion through leaching and vegetation uptake may threaten long-term sustainability of forest productivity in the southeastern USA. This study was conducted to assess Ca pools and fluxes in a representative southern Piedmont forest to determine the soil Ca depletion rate. Soil Ca storage, Ca inputs in atmospheric deposition, and outputs in soil leaching and vegetation uptake were investigated at the Panola Mountain Research Watershed (PMRW) near Atlanta, GA. Average annual outputs of 12.3 kg ha-1 yr-1 in uptake into merchantable wood and 2.71 kg ha-1 yr-1 soil leaching exceeded inputs in atmospheric deposition of 2.24 kg ha-1 yr-1. The annual rate of Ca uptake into merchantable wood exceeds soil leaching losses by a factor of more than five. The potential for primary mineral weathering to provide a substantial amount of Ca inputs is low. Estimates of Ca replenishment through mineral weathering in the surface 1 m of soil and saprolite was estimated to be 0.12 kg ha-1 yr-1. The weathering rate in saprolite and partially weathered bedrock below the surface 1 m is similarly quite low because mineral Ca is largely depleted. The soil Ca depletion rate at PMRW is estimated to be 12.7 kg ha-1 yr-1. At PMRW and similar hardwood-dominated forests in the Piedmont physiographic province, Ca depletion will probably reduce soil reserves to less than the requirement for a merchantable forest stand in ≃80 yr. This assessment and comparable analyses at other southeastern USA forest sites suggests that there is a strong potential for a regional problem in forest nutrition in the long term.
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.
A synthesis of the biogeochemistry of K was conducted during 1963–1992 in the reference and human-manipulated watershed-ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study (1963–65), which coincided with a drought period, the reference watershed was a net sink for atmospheric inputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term declines in volume-weighted concentration and flux of K at the HBEF; however, this pattern appears to be controlled by the relatively large inputs during the initial drought years. Net ecosystem loss (atmospheric deposition minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern of net K loss coincides with trends in the drainage efflux of SO4
2– and NO3
–, indicating that concentrations of strong acid anions may be important controls of dissolved K loss from the site. There were no long-term trends in streamwater concentration or flux of K. A distinct pattern in pools and fluxes of K was evident based on biotic controls in the upper ecosystem strata (canopy, boles, forest floor) and abiotic controls in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations and fluxes of K in vegetation, aboveground litter, throughfall and forest floor pools and soil water in the northern hardwood vegetation within the lower reaches of the watershedecosystem, when compared with patterns in the high-elevation spruce-fir zone. Abiotic control mechanisms were evident through longitudinal variations in soil cation exchange capacity (related to soil organic matter) and soil/till depth, and temporal and disturbance-related variations in inputs of strong-acid anions. Marked differences in the K cycle were evident at the HBEF for the periods 1964–69 and 1987–92. These changes included decreases in biomass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ecosystem response to decreasing rates of biomass accretion during the study. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur from release from slash, decomposition of soil organic matter and displacement from cation exchange sites. Elevated concentrations of K persist in stream water for many years after clearcutting. Of the major elements, K shows the slowest recovery from clearcutting disturbance.
The Laguna, Turlock Lake, Riverbank, and Modesto Formations are lithologically similar but may be distinguished and subdivided on the basis of soil profile development, topographic position and expression, local lithologic differences, and unconformities associated with buried soils. Post-Modesto deposits are subdivided into 4 stratigraphic units, informally designated post-Modesto I, II, III, and IV. Most of the stratigraphic units discussed are believed to represent separate alluvial episodes, recorded by fill terraces opening westward onto alluvial fans. Substantial time intervals between periods of aggradation are represented by buried paleosols. -from Authors
An experimental study was made of several cation-exchange reactions on a Yolo loam
soil by means of a leaching procedure. Adsorption isotherms for the K-Na, K-Mg, K-Ca,
and Mg-Ca ion pairs were obtained. An application of thermodynamic equations was used
to find the equilibrium constants and free-energy changes for the reactions. Activity
coefficients of the adsorbed ions were also calculated. A comparison of various selectivity
coefficients was included.
Steady-state rates were measured using a flow-through reaction vessel. Congruent dissolution was observed only in solutions that were greatly undersaturated with respect to kaolinite and quartz and calculated to be undersaturated with respect to aluminous phases. The calculated ion activity products for the solutions in which the slowest dissolution and precipitation rates were measured were used to determine an internally consistent thermodynamic equilibrium solubility product. -from Authors
Saprolite production is controlled by solution flux and reaction kinetics. Compared with the residence time of the ground water, dissolution of plagioclase and precipitation of kaolinite are relatively rapid. It is claculated that 1m of saprolite is produced in about 250 000 yr. The interaction between chemical processes at the weathering front, stream incision, and the rate of ground water recharge is discussed in terms of the geomorphology of the catchment. -K.Clayton
Reports mean annual fluxes for a 7 year observation period and fluxes for the hydrological year 1976. Forested basins are compared with an agricultural basin, and rates of erosion are calculated. Denudation is 9-14mm/1000 yr in the forested basins, annd 32mm/1000 yr in the agricultural basin. Chemical weathering is responsible for about half the work done in the forested basins. -K.Clayton
The RbSr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to ∼300 kyr were studied and revealed that the ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at aaproximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only ∼20% of that of plagioclase in the oldest soil. ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than ∼20 kyr in age which contain biotite in the soil parent material, weathering-derived values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes.
Flow pathways of water and solutes in soils form distinct patterns,
which are not a priori predictable. Macropore structure is a prime
cause, but other factors, such as differing initial or boundary
conditions, may also predispose a soil to produce bypassing of
infiltrating water. This study was conducted to assess the flow pathways
of water in different soils and to investigate the effect of initial
water content on the flow pattern. Dye-tracing experiments were carried
out at 14 different field sites. The sites represent a good portion of
soils used for agricultural crop production in Switzerland. Each site
consisted of two 1.4 by 1.4 m plots, one of which had been covered with
a plastic roof for two months before the experiment to achieve different
initial water contents. Forty millimeters of water containing the dye
Brilliant Blue FCF (C.I. Food Blue 2) were applied within 8 hours onto
the plots with a sprinkling apparatus. One day after irrigation the
plots were excavated, and the stained pattern was examined on a vertical
1 by l m soil profile. The spatial structure of flow patterns showed
remarkable differences. In most soils, water bypassed the soil matrix.
In some soils, dye penetrated beyond l m depth, whereas in others it
remained in the top 50 cm. Structured soils were more prone to produce
bypass flow, deep dye penetration, and pulse splitting than
nonstructured soils. The initial water content had a less pronounced
effect in some soils and no effect in others.
In the Piedmont and Mountain Provinces of North Carolina average annual precipitation exceeds evapotranspiration. Soil solum thickness is usually 1 to 2 m. Saprolite thickness is variable but related to rock type and geomorphic position. Slow hydraulic conductivity in the zone between the soil and the saprolite appears to restrict vertical percolation of water, thus slowing saprolite weathering on convex slopes. In saprolite zones immediately above granitic gneiss, often about 5 m below the surface, halloysite, gibbsite and X-ray amorphous aluminosilicate clay are the initial secondary minerals formed. Halloysite contents increase upward in the saprolite, apparently from resilication of gibbsite and the amorphous aluminosilicates. In the upper zones of the saprolite the halloysite is recrystallized into kaolinite which is the predominant clay-sized mineral in the soil.
Previously unpublished water quality data are used to explore the potassium chemistry of a small upland stream following the 1976 drought in England. The behaviour of potassium is a complex response to several factors: hydrological pathways operating during periods of storm runoff; sediment inputs; and the chemical properties of the transporting water. Analyses of hysteresis loops for a series of storms show that the relationship between suspended sediment and potassium concentrations is not simple; spatial and temporal variations in surface and subsurface stormflow add complexity. In addition to the specific discussion of potassium, data are presented to show the recovery of stream discharge, and of sediment and solute concentrations during the immediate post-drought period.
Predictive modeling of cation transport in soils requires knowledge of the distribution of exchangeable species between solution and adsorbed phases. Such distribution of exchangeable species may be described with an exchange selectivity coefficient. Typically, the selectivity coefficient for any cation-exchange pair varies among soils. If the exchange complex of a soil is dominated by one type of exchanger, however, then the selectivity coefficient for that soil may be similar in magnitude to that for the principal mineral (or organic) constituent. Therefore, reasonably accurate predictions of cation transport in that soil might be obtained using exchange data for the dominant exchange material. This study was undertaken to determine whether exchange data for kaolinite could be successfully used in predicting Ca, Mg, and Na retention during movement in a Mahan series soil (clayey, kaolinitic, thermic Typic Hapludult). Binary-exchange isotherms for Ca-Mg, Ca-Na, and Mg-Na were generated for Ga-1 kaolinite (well crystallized). Selectivity coefficients derived from these data were used to describe cation exchange during miscible displacement through columns of (i) kaolinite admixed with acid-washed sand and (ii) Mahan soil. Use of the selectivity coefficients for kaolinite provided good predictions of cation transport in the kaolinite/sand mixture. Predictions for the kaolinitic Mahan soil were not as good. The presence of small amounts of 2:1 minerals may have influenced the overall exchange behavior of the Mahan soil. When the uncertainty in values for selectivity coefficients and other exchange or transport parameters was considered, however, the exchange selectivity data for kaolinite could adequately describe cation transport in the Mahan soil.
This study was conducted to determine if a single unbuffered salt extractant for exchangeable bases and Al could be used in place of a buffered salt extractant for exchangeable bases and an unbuffered salt extractant for exchangeable Al. Barium chloride and a mixed NH 4 Cl‐BaCl 2 extractant were compared with NH 4 OAc for determining exchangeable bases and with KCl for determining exchangeable Al in soils. In general, BaCl 2 extracted less exchangeable K than either NH 4 OAc or NH 4 CL‐BaCl 2 . The NH 4 CL‐BaCl 2 extractant was equivalent overall to NH 4 OAc for extracting exchangeable bases and to KCl for extracting exchangeable Al, although less K was extracted by NH 4 Cl‐BaCl 2 at low levels of K for some Coastal Plain soils. The NH 4 Cl‐BaCl 2 extractant can be used by soil‐test or soil‐characterization labs as a simple one‐step extractant in place of NH 4 OAc and KCl for the determination of exchangeable cations and effective cation‐exchange capacity of soils.
Many alluvial Argids in central Nevada are cemented by illuvial silica and CaCO//3. In loamy soils, microsite deposition of these authigenic components tends to be mutually exclusive, with silica being finely distributed throughout the plasma phase and calcite plugging packing voids formed by skeleton grains and root channels. The following model is proposed to explain the differences in depositional locations of silica and CaCO//3. As soils dry, calcite precipitates rapidly in an ionic, diffusion-controlled reaction while monosolicic acid, which requires greater activation energy for Si-O bond breakage prior to precipitation, is concentrated in the solution phase. Monosilicic acid left bracket Si(OH)//4 right bracket can diffuse away from the evaporation front into smaller pores where, in contact with higher surface areas, it is absorbed onto clay, sesquioxide, and weathered primary mineral surfaces.
A continuous vertical soil‐saprolite‐granitic gneiss profile was studied to determine its morphology, micromorphology, and physical properties. A clayey, kaolinitic, thermic Typic Hapludult formed in 4.5 m of saprolite derived from a granitic gneiss was sampled in 14 different levels from the surface to the consolidated rock to determine changes with depth. Morphologically, the Piedmont soil has a gravelly sandy loam surface over a red, clayey, moderately structured subsoil. The saprolite has a 1.15 m transitional sandy clay loam horizon over 3.35 m of a sandy loam saprolitic material with rock controlled structure overlying consolidated rock. Micromorphologically, content of skeletal grains increases with increasing depth in the subsoil and saprolite. Plasma and compound void volume decrease with increasing depth. Bulk density measurements of 0.95 Mg m ⁻³ are found in the soil surface, 2.46 Mg m ⁻³ in the rock, and about 1.3 Mg m ⁻³ in the subsoil and saprolite. Pore size distribution as indicated by soil moisture release measurements shows 40% more macroporosity in the subsoil than the saprolite. Saturated hydraulic conductivity in the subsoil is 2.2 µm s ⁻¹ —over twice the 1.1 µm s ⁻¹ found in the upper saprolitic horizon. Theoretically derived unsaturated hydraulic conductivities show little difference between the subsoil and saprolite. The significant physical changes accompanying the weathering of this saprolite to soil include: a weathering of sand‐sized weatherable minerals to clay‐sized minerals, an increase in structural strength of the Bt horizon, an accompanying increase in macroporosity in the Bt horizon, and higher saturated hydraulic conductivity in the Bt horizon than in the saprolite.
The Al chemistry of seeps, soil solutions and soils was evaluated at the Hubbard Brook Experimental Forest in New Hampshire. Concentrations of both organic and inorganic forms of Al were highest at the higher elevations. The extent of spodosol development was consistent with these observations, suggesting the rate of spodosolization is greatest at high elevations in the watershed. The historical rate of organic Al deposition within the mineral soil far exceeds the current rate of organic Al precipitation from soil solutions. Processes such as vegetation turnover and windthrow may be important to the Al cycle at Hubbard Brook. Removal of modest amounts of dissolved inorganic Al from the Bhs1 horizon, and subsequent export from the soil, occurs during winter and early spring. This process may be due to atmospheric inputs of acidic substances. -from Authors
Data for three kaolinites show that Cs was preferentially sorbed by kaolinites in NaCl and CaCl 2 solutions. Cesium desorption behavior of kaolinites revealed that a variable proportion, 0–3.5% was strongly held. The Cs desorption behavior as well as the amounts of Cs fixed by kaolinites correlated well with the nature of impurities in them.
Rates of chemical weathering in two forested Adirondack watersheds were determined from mineral and elemental depletion trends in soil profiles and from input/output budgets based on precipitation and surface-water chemistry. Long-term rates of weathering have averaged about 500 to 600 eq/ha.yr for both watersheds since the glaciers retreated from the region about 14 000 yr ago. Present-day denudation rates average 1679 eq/ha.yr in the Panther Lake watershed and only 198 eq/ha.yr in the Woods Lake watershed. -from Authors
Long-term weathering rates (RLT) were determined from base cation depletion in soil profiles developed on six granitic glacial moraines varying in age from 0.4 to >=297 ka in the Wind River Mountains, Wyoming. Weathering rates were found to decrease with soil age (t) according to the power-law equation RLT = 15 c t (-0.71) where RLT is in meq c m -2 c yr -1, and t is in ka. Comparable data from the literature are consistent with this function, and it is similar to power-law equations describing changes in laboratory weathering rates with time. On the basis of our study, we predict that for a catchment unaffected by anthropogenic inputs and in a steady state with respect to the organic and cation exchange pools, RLT should be higher than present-day weathering rates derived from stream fluxes (RPD) by a factor of about 3.4.
The most important weathering reaction that neutralizes incoming atmospheric acidity in the South Cascade Lake basin is weathering of calcite, which occurs in trace amounts in veins, on joint surfaces, and as a subglacial surficial deposit. Although the basin is underlain by igneous and high-grade metamorphic rocks, weathering of plagioclase is quantitatively negligible; the principal silicate weathering reaction is alteration of biotite to vermiculite. These conclusions are based on mass-balance calculations involving runoff compositions and on mineralogical observations. For predictive modeling of the effects of increased acid deposition, it is essential to identify the relevant weathering reactions. Feldspar weathering is commonly not an important source of solutes in alpine basins underlain by granitic rocks. *Present address: Bureau of Reclamation, Pecos River Projects Office, P.O. Box 1356, Carlsbad, New Mexico 88220
Recent research suggested that plow pan development in Palouse soils of Northern Idaho may be enhanced by deposition of Si compounds. However, the mechanisms responsible for Si sorption in the plow pan zone have not been determined. We undertook this study to: (1) evaluate the differences in Si sorption between profiles with and without well-developed plow pans, and (2) evaluate possible mechanisms of Si sorption and associated reaction rates. Soil samples were collected from Palouse silt loam soils (fine-silty, mixed, mesic, Pachic Ultic Haploxerolls) both with and without well-developed plow pans. Samples were allowed to equilibrate with solutions containing Si concentrations of 40, 100, and 140 μg/ml before extraction using immiscible displacement. Silica sorption was controlled by a second-order reaction with variation of reaction constants with depth in the profile. Silica sorption in well-developed plow pans was significantly less than found above and below, probably due to the previous occupation by Si of the most accessible sorption sites. Although the sorption process was found to depend on soil pH, data suggested that the equilibrium Si concentrations found in the plow pan zones were controlled by amorphous Si deposited at the surfaces of soil particles.
Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10−17 mol m−2 s−1 for biotite. Faster weathering rates of 1.8 to 3.6×10−16 mol m−2 s−1 are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.
Feedback can be defined as the returning of a part of the effects of a given process to its beginning or to a preceding stage so as to reinforce or modify that process. Feedback processes are self-accelerating because of continuous reinforcement of the causes starting them. Up to a certain moment, reinforcement is proportional to the effects (or output) of the process. Later, de-acceleration and termination come as one of several factors (or reactants) in the process become limiting, e.g., depletion of easily weatherable minerals, hence cessation of clay formation. In soils there are many feedback, self-terminating processes such as accumulation of organic matter in Mollisols, clay accumulation in Argids, and development of windows in duripans and dayas in soils with petrocalcic horizons. The occurrence and constancy of some morphological features in many soil profiles and soilscapes can be explained by this concept.
Geochemical mole-balance models are sets of chemical reactions that
quantitatively account for changes in the chemical and isotopic
composition of water along a flow path. A revised mole-balance
formulation that includes an uncertainty term for each chemical and
isotopic datum is derived. The revised formulation is comprised of
mole-balance equations for each element or element redox state,
alkalinity, electrons, solvent water, and each isotope; a charge-balance
equation and an equation that relates the uncertainty terms for pH,
alkalinity, and total dissolved inorganic carbon for each aqueous
solution; inequality constraints on the size of the uncertainty terms;
and inequality constraints on the sign of the mole transfer of
reactants. The equations and inequality constraints are solved by a
modification of the simplex algorithm combined with an exhaustive search
for unique combinations of aqueous solutions and reactants for which the
equations and inequality constraints can be solved and the uncertainty
terms minimized. Additional algorithms find only the simplest
mole-balance models and determine the ranges of mixing fractions for
each solution and mole transfers for each reactant that are consistent
with specified limits on the uncertainty terms. The revised formulation
produces simpler and more robust mole-balance models and allows the
significance of mixing fractions and mole transfers to be evaluated. In
an example from the central Oklahoma aquifer, inclusion of up to 5%
uncertainty in the chemical data can reduce the number of reactants in
mole-balance models from seven or more to as few as three, these being
cation exchange, dolomite dissolution, and silica precipitation. In
another example from the Madison aquifer, inclusion of the
charge-balance constraint requires significant increases in the mole
transfers of calcite, dolomite, and organic matter, which reduce the
estimated maximum carbon 14 age of the sample by about 10,000 years,
from 22,700 years to 12,600 years.
Chronosequential analysis of soil properties has proven to be a valuable approach for estimating ages of geomorphic surfaces where no independent age control exists. In this study we examined pedogenic silica as an indicator of relative ages of soils and geomorphic surfaces, and assessed potential sources of the silica. Pedogenic opaline silica was quantified by tiron (4,5-dihydroxy-1,3-benzene-disulfonic acid [disodium salt], C6H 4Na2O8S2) extraction for pedons in two different chromosequences in southern California, one in the San Timoteo Badlands and one in Cajon Pass. The soils of hoth of these chronosequences are developed in arkosic sediments and span 11.5 to 500 ka. The amount of pedogenic silica increases with increasing duration of pedogenesis, and the depth of the maximum silica accumulation generally coincides with the maximum expression of the argillic horizon. Pedogenic silica has accumulated in all of the soils, ranging from 1.2% tiron-extractable Si (Sitn) in the youngest soil to 4.6% in the oldest. Primary Si decreases with increasing duration of weathering, particularly in the upper horizons, where weathering conditions are most intense. The loss of Si coincides with the loss of Na and K, implicating the weathering of feld-spars as the likely source of Si loss. The quantity of Si lost in the upper horizons is adequate to account for the pedogenic silica accumulation in the subsoil. Pedogenic silica was equally effective as pedogenic Fe oxides as an indicator of relative soil age in these soils.
Do glaciers enhance or inhibit chemical weathering rates relative to
other environments? The importance of glaciers in the global carbon
cycle and climate change hinges on the answer. We show that catchments
occupied by active alpine glaciers yield cation denudation rates greater
than the global mean rate but do not exceed rates in nonglacial
catchments with similar water discharge. Silica denudation rates are
distinctly lower in glacier-covered catchments than in their nonglacial
counterparts. Because sediment yields are high from glaciers, this
suggests that water flux, rather than physical erosion, exerts the
primary control on chemical erosion by glaciers. Potassium and calcium
concentrations are high relative to other cations in glacial water,
probably due to dissolution of soluble trace phases, such as carbonates,
exposed by comminution, and cation leaching from biotite. Preferential
weathering of biotite may result in higher
87Sr/86Sr in glacial runoff than expected from
whole-rock compositions. Thus, although glaciers do not influence total
chemical denudation rates at a given runoff, they may yield
compositionally distinctive chemical fluxes to the oceans. Disruption of
mineral lattices by grinding increases dissolution rates; this and high
surface area should make glacial sediments exceptionally weatherable.
Weathering of glacial erosion products in environments beyond the
glacier margin deserves attention because it may figure prominently in
global chemical cycles.
A modification of the steady-state centrifuge method for unsaturated hydraulic conductivity (K) measurement improves the range and adjustability of this method. The modified apparatus allows mechanical adjustment to vary the measured K by a factor of 360. In addition, the use of different flow-regulation ceramic materials can give a total K range covering about six orders of magnitude. The range extension afforded has led to the lowest steady-state K measurement to date, for a sandy soil of the Delhi series (Typic Xeropsamment). -from Authors
Hard-setting is an important but poorly understood phenomenon in seasonally wetting and drying soils. This study was conducted to determine the role, if any, of chemical cementing agents in a hard-setting soil. Undisturbed cores from the E horizon of an Aeric Albaqualf were leached in the laboratory with distilled water, 0.15 M ammonium oxalate (pH 3.0 in darkness) and 2 M NaOH/ethylene glycol. The leachates were analyzed for Si, Al, and Fe and the residual materials by infrared spectroscopy. After drying at 40°C, tensile, shear, and compressive strengths of the cores were determined. All three reagents caused a total loss of tensile strength and decreased shear strength, but had little effect on compressive strength. Amorphous silica is probably the major cementing agent, but the results also indicate that an imogolite-like aluminosilicate, a feldspathoid mineral, and possibly silica-Fe complexes may play some part in cementation.
Biotite samples (10-20 mu m lepidomelane) were mixed with solutions tha t contained various oxidants (O//2, NaOCl, H//2O//2, or Br//2) and salts (NaCl, KCl, NaCl-NaPBh//4, or none) to compare the effectiveness with which these solutions oxidize structural Fe**2** plus at 25 degree C when customary procedures and mica samples at different stages of expansion are used. In general, the treatments that minimized interlayer K exchange did not oxidize detectable amounts of Fe**2** plus , whereas those causing mica expansion oxidized equivalent fractions of the Fe**2** plus if they were given enough time. The rates of Fe**2** plus oxidation by the O//2 and NaOCl-O//2 solutions were very slow in even the expanded mica and were similar enough to suggest the OCl** minus ions did not contribute to the process under these conditions. The Br//2 and H//2O//2 treatments, on the other hand, oxidized 97 and 85% of the structural Fe**2** plus in a fully expanded mica, respectively, in a 24-h period.
Although pedogenesis and landscape evolution have occurred on the eastern North Dakota till plain since the inception of the Holocene, soils with argillic horizon are recognized on a very limited basis. Argillic soils are not mapped in well-drained positions on the youngest drift prairie landscapes. Roughly half of 47 pedons examined at a precision farming research site in Barnes County, North Dakota show evidence of genetic argillic horizons based on soil structure, hand texturing, and cutan presence. This study applies physical and micromorphological evidence to verify that diagnostic argillic horizons have formed in well-drained settings on this eastern North Dakota landscape. Total clay distributions and fine/total clay ratios show marked increases in genetic argillic horizons compared with surface horizons and parent materials. The argillic horizons average about 28 cm in thickness; only two pedons were thinner than 7.5 cm. Consequently, the increase in total clay and fine day distributions, and the thickness criteria required to meet diagnostic horizon status as established by Soil Taxonomy are satisfied. The upper argillic horizon boundary of seven profiles was 50 cm or more below the surface, possibly indicating relict argillic horizons. Micromorphologic evidence shows the presence of illuvial day in the three soils sampled for microscopy. Porosity variations caused by the argillic horizons likely affect subsurface flow and may be responsible for variable N levels measured for these soils. These soils formed in well-drained landscape settings, where stratified drift may predispose the soils to illuviation.
Unsaturated flow and transport under a natural hydraulic gradient in a Mediterranean climate were investigated with a field tracer experiment combined with laboratory analyses and numerical modeling. Bromide was applied to the surface of a sandy soil during the dry season. During the subsequent rainy season, repeated sediment sampling tracked the movement of bromide through the profile. Analysis of data on moisture content, matric pressure, unsaturated hydraulic conductivity, bulk density, and soil texture and structure provides insights into parameterization and use of the advective-dispersive modeling approach. Capturing the gross features of tracer and moisture movement with model simulations required an order-of-magnitude increase in laboratory-measured hydraulic conductivity. Wetting curve characteristics better represented field results, calling into question the routine estimation of hydraulic characteristics based only on drying conditions. Measured increases in profile moisture exceeded cumulative precipitation in early winter, indicating that gains from dew drip can exceed losses from evapotranspiration during periods of heavy (“Tule”) fog. A single-continuum advective-dispersive modeling approach could not reproduce a peak of bromide that was retained near the soil surface for over 3 years. Modeling of this feature required slow exchange of solute at a transfer rate of 0.5–1 × 10−4 d−1 with an immobile volume approaching the residual moisture content.
The earliest reports on the composition of deep-sea sediments resulted
from the Challenger Expedition (1873-1876) (e.g., Tizzard et al., 1885;
Murray and Renard, 1891). Many review papers on marine sediment
composition have subsequently been published, including the ones by
Revelle (1944), El Wakeel and Riley (1961), Arrhenius (1963), Goldberg
(1963), Chester and Aston (1976), Glasby (1977), Bischoff and Piper
(1979), Baturin (1982, 1988), Notholt and Jarvis (1990), Nicholson et
al. (1997), Glenn et al. (2000), and Li (2000). The constituents of a
marine sediment are often classified according to their origin ( Table
1; after Goldberg, 1963). The detrital component is made up of
cosmogenous and lithogenous materials. Cosmic spherules contain
particles of FeNi that are formed by ablation of iron meteorites as they
pass through Earth's atmosphere, as well as fragments of silicate
chondrules ( Arrhenius, 1963). Lithogenous constituents of marine
sediments are the minerals derived from weathering of rock on land or on
the seafloor, or from the volcanic eruptions ( Goldberg, 1963; see
review in Windom (1976)). The biogenous component is made up of the
tests of planktic and benthic organisms, as well as biogenic apatite
(see review in Berger (1976)). The hydrogenous fraction of marine
sediment encompasses phases formed by inorganic precipitation from
seawater. Elderfield (1976) and Piper and Heath (1989) provide
comprehensive reviews of hydrogenous material in marine sediments.
Silicates constitute more than 90% of the rocks exposed at Earth's land
surface (Garrels and Mackenzie, 1971). Most primary minerals comprising
these rocks are thermodynamically unstable at surface
pressure/temperature conditions and are therefore susceptible to
chemical weathering. Such weathering has long been of interest in the
natural sciences. Hartt (1853) correctly attributed chemical weathering
to "the efficacy of water containing carbonic acid in promoting the
decomposition of igneous rocks." Antecedent to the recent interest in
the role of vegetation on chemical weathering, Belt (1874) observed that
the most intense weathering of rocks in tropical Nicaragua was confined
to forested regions. He attributed this effect to "the percolation
through rocks of rain water charged with a little acid from decomposing
vegetation." Chamberlin (1899) proposed that the enhanced rates of
chemical weathering associated with major mountain building episodes in
Earth's history resulted in a drawdown of atmospheric CO2 that led to
periods of global cooling. Many of the major characteristics of chemical
weathering had been described when Merrill (1906) published the
groundbreaking volume Rocks, Rock Weathering, and Soils.The major
advances since that time, particularly during the last several decades,
have centered on understanding the fundamental chemical, hydrologic, and
biologic processes that control weathering and in establishing
quantitative weathering rates. This research has been driven by the
importance of chemical weathering to a number environmentally and
economically important issues. Undoubtedly, the most significant aspect
of chemical weathering is the breakdown of rocks to form soils, a
process that makes life possible on the surface of the Earth. The
availability of many soil macronutrients such as magnesium, calcium,
potassium, and PO4 is directly related to the rate at which primary
minerals weather. Often such nutrient balances are upset by
anthropogenic activities. For example, Huntington et al. (2000) show
that extensive timber harvesting in the southeastern forests of the
United States, which are underlain by intensely weathered saprolites,
produces net calcium exports that exceed inputs from weathering, thus
creating a long-term regional problem in forest management.The role of
chemical weathering has long been recognized in economic geology.
Tropical bauxites, which account for most of world's aluminum ores, are
typical examples of residual concentration of silicate rocks by chemical
weathering over long time periods (Samma, 1986). Weathering of
ultramafic silicates such as peridotites forms residual lateritic
deposits that contain significant deposits of nickel and cobalt. Ores
generated by chemical mobilization include uranium deposits that are
produced by weathering of granitic rocks under oxic conditions and
subsequent concentration by sorption and precipitation ( Misra,
2000).Over the last several decades, estimating rates of silicate
weathering has become important in addressing new environmental issues.
Acidification of soils, rivers, and lakes has become a major concern in
many parts of North America and Europe. Areas at particular risk are
uplands where silicate bedrock, resistant to chemical weathering, is
overlain by thin organic-rich soils (Driscoll et al., 1989). Although
atmospheric deposition is the most important factor in watershed
acidification, land use practices, such as conifer reforestation, also
create acidification problems ( Farley and Werritty, 1989). In such
environments, silicate hydrolysis reactions are the principal buffer
against acidification. As pointed out by Drever and Clow (1995), a
reasonable environmental objective is to decrease the inputs of acidity
such that they are equal to or less than the rate of neutralization by
weathering in sensitive watersheds.The intensive interest in past and
present global climate change has renewed efforts to understand
quantitatively feedback mechanisms between climate and chemical
weathering. On timescales longer than a million years, atmospheric CO2
levels have been primarily controlled by the balance between the rate of
volcanic inputs from the Earth's interior and the rate of uptake through
chemical weathering of silicates at the Earth's surface (Ruddiman,
1997). Weathering is proposed as the principal moderator in controlling
large increases and decreases in global temperature and precipitation
through the greenhouse effects of CO2 over geologic time (R. A. Berner
and E. K. Berner, 1997). Weathering processes observed in paleosols,
discussed elsewhere in this volume (see Chapter 5.18), have also been
proposed as indicating changes in Archean atmospheric CO2 and O2 levels
(Ohmoto, 1996; Rye and Holland, 1998).
Chemical and mineralogical characteristics have been determined for a chronosequence of six soil profiles ranging in age from 80–13,000 years BP developed on river terraces in the western Cairngorms of Scotland. The C horizons are similar chemically and mineralogically, and the soils have similar pedogenetic histories. Exchangeable Ca and Mg decrease with time and base saturation decreases exponentially from 24.6% in the Ah horizon of the youngest profile to 2.8% in the comparable horizon of the 10,000 year old profile according to the chronofunction y=3.372+22.612 exp(−0.0007365t). Long-term weathering rates of base cations, calculated from the loss of these cations relative to Zr, appear to decrease exponentially with time but this may be due to the method of calculation. The magnitude of loss of base cations decreases in the sequence Na>K>Mg>Ca but when the relative mobilities of these elements are considered, the loss is in the order Mg>Na>Ca>K; this reflects the dissolution of chlorite and loss of Mg, and the more rapid weathering of plagioclase feldspar and loss of Na and Ca (particularly in the coarse sand fraction) than K-feldspar. The clay fractions, although <2% of all horizons, also show distinct patterns with age in that chlorite and mica are less abundant in older soils and vermiculite is more abundant, the latter phase often having hydroxyaluminium polymers in the interlayer region. The chemical and mineralogical trends in the soil sequence are closely associated and are induced by pedogenic weathering.
Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall dissolution and Sr release became stoichiometric. Under steady state conditions that were far from being in equilibrium with the labradorite, we measured the log of the overall labradorite dissolution rate (mol mineral/m²/s) to be −10.6 ± 0.1 while the Sr release rate was −13.2 ± 0.1 (mol Sr/m²/s). The isotopic ratio of the output solutions did not vary with time as both the early ⁸⁷Sr/⁸⁶Sr ratios and the later, steady state ratios were all essentially the same as that of the bulk labradorite (0.704671).
Two methods based on a chloride concentration profile were applied to evaluate the annual groundwater recharge in a subhumid area cropped with maize where chloride anthropogenic inputs were greater than the natural ones. The site is located in the alluvial Po plain (NW Italy). The two methods were a steady-state model and an approximate diffusive movement equation. They were applied to the Cl− content of retention water extracted from porous cups on 24 sampling dates through one year. The sampling depth ranged from 0.2 to 2.6 m, and the concentration was steady in time below 1.6 m. Considering all the approximations introduced (homogeneous and non-dispersive medium, constant diffusion coefficient with depth, all the liquid phase in movement, no macroporosity, steady state conditions), the results were consistent with those obtained with a long-term mass-balance method. The mean annual recharge assessed using the steady-state chloride profile method was 205 mm yr−1, using the approximate diffusive movement equation it was 216 mm yr−1, while using the mass-balance method the calculated mean recharge was 174 mm yr−1. The estimate showed a moderate dependence with the sampling date. Chloride inputs were wet and dry atmospheric deposition, irrigation water and fertilizers. The incorporation of chloride fertilizers to the soil is one of the more unusual aspects of the chloride approach in this study.
The use of strontium isotopes to evaluate mineral weathering and identify sources of base cations in catchment waters requires an understanding of the behavior of Sr in the soil environment as a function of time. Our approach is to model the temporal evolution of 87Sr/86Sr of the cation exchange pool in a soil chronosequence developed on alluvium derived from central Sierra Nevada granitoids during the past 3 Ma. With increasing soil age, 87Sr/86Sr of ammonium-acetate extractable Sr initially decreases from values typical of K-feldspar to those of plagioclase and hornblende and then remains constant, even though plagioclase and hornblende are absent from the soils after approximately 1 Ma of weathering. The temporal variation of 87Sr/86Sr of exchangeable Sr is modeled by progressively equilibrating Sr derived from mineral weathering and atmospheric deposition with Sr on exchange sites as waters infiltrate a soil column. Observed decreases in quartz-normalized modal abundances of plagioclase, hornblende, and K-feldspar with time, and the distinct87Sr/86Sr values of these minerals can be used to calculate Sr flux from weathering reactions. Hydrobiotites in the soils have nearly constant modal abundances, chemistry, and 87Sr/86Sr over the chronosequence and provide negligible Sr input to weathering solutions. The model requires time and soil horizon-dependent changes in the amount of exchangeable Sr and the efficiency of Sr exchange, as well as a biologic cycling term. The model predicts that exchangeable Sr initially has 87Sr/86Sr identical to that of K-feldspar, and thus could be dominated by Sr leached from K-feldspar following deposition of the alluvium. The maximum value of 87Sr/86Sr observed in dilute stream waters associated with granitoids of the Yosemite region is likewise similar to that of the K-feldspars, suggesting that K-feldspar and not biotite may be the dominant source of radiogenic Sr in the streams. This study reveals that, when attempting to use Strontium isotopes to identify sources of base cations in catchment waters and biomass, both preferential leaching of Sr from minerals during incipient soil development and changing Sr exchange efficiency must be considered along with chemical contributions due to mineral dissolution.
Climatic effects on chemical weathering are evaluated by correlating variations On solute concentrations and fluxes with temperature, precipitation, runoff, and evapotranspiration (ET) for a worldwide distribution of sixty-eight watersheds underlain by granitoid rock types. Stream solute concentrations are strongly correlated with proportional ET loss, and evaporative concentration makes stream solute concentrations an inappropriate surrogate for chemical weathering. Chemical fluxes are unaffected by ET, and SiO2 and Na weathering fluxes exhibit systematic increases with precipitation, runoff, and temperature. However, warm and wet watersheds produce anomalously rapid weathering rates. A proposed model that provides an improved prediction of weathering rates over climatic extremes Os the product of linear precipitation and Arrhenius temperature functions. The resulting apparent activation energies based on SiO2 and Na fluxes are 59.4 and 62.5 kJ · mol-1, respectively. The coupling between temperature and precipitation emphasizes the importance of tropical regions On global silicate weathering fluxes, and suggests it is not representative to use continental averages for temperature and precipitation On the weathering rate functions of global carbon cycling and climatic change models.
We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr ≈ 0.20 μmol/nmol, 87Sr/86Sr ≈ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr ≈ 0.98 to 5.3 μmol/nmol, 87Sr/86Sr ≈ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr ≈ 1.0 to 2.8 μmol/nmol, 87Sr/86Sr ≈ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ∼0.5 to ∼55 kyr revealed that carbonate dissolution provides more than ∼90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ∼1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87Sr/86Sr ratios released by carbonate weathering decrease from ∼3.6 to ∼0.20 μmol/nmol and ∼0.84 to ∼0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.
Thirty-two chronosequences from 27 areas were selected from the literature for constructing chronofunctions and for correlating rates of change in soil properties with variables representing climate and parent material. The chronosequences originate from areas situated between 66°N and 78°S latitude and represent seven climatic regions, ranging from tropical rainy to cold desert, and seven types of parent materials, including till, aeolian sand, alluvium, mine spoil, volcanic ash, raised beach deposits, and mudflows. Fourteen of the chronosequences contain soils which range in age from 0 to 500 yr; seven span a 12,000-yr period, three a 100,000-yr period, and eight a period of greater than one million yr. Three linear and non-linear models were tested on 15 soil properties. The single-logarithmic (Y = a + b log X) model yielded the best correlation coefficients, when soil property (Y) was correlated with time (X), using linear regression techniques. The dates and equations allow for the following conclusions: 1.(1) The rates of decrease in pH and in base saturation are similar, regardless of the nature of the parent material or climate.2.(2) The rates of increase in clay content of the B horizon and solum thickness are positively correlated with clay content of the parent material.3.(3) The rates of increase in solum thickness, oxidation depth, soluble salt content of the salt-enriched horizon, and clay content of the B horizon are positively correlated with mean annual temperature whereas the rate of increase in total N in the surface mineral soil is negatively correlated with present-day mean annual temperature.4.(4) The increase in bulk density of the surface soil is positively correlated with present mean annual precipitation.5.(5) The rate of change in C:N is not correlated with variables representing climate and parent material.