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David A. Stonestrom - One of the best experts on this subject based on the ideXlab platform.

  • chemical weathering of a marine terrace chronosequence santa cruz california part ii solute profiles gradients and the comparisons of contemporary and long term weathering rates
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Art F. White, Marjorie S Schulz, Davison V. Vivit, David A. Stonestrom, John A Fitzpatrick, T Bullen, Kate Maher, Alex E. Blum
    Abstract:

    The spatial and temporal changes in hydrology and pore water elemental and 87 Sr/ 86 Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying Argillic Horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated Horizons extending to depths in excess of 15 m. Hydraulic fluxes (qh), based on Cl mass balances, vary from 0.06 to 0.22 m yr � 1 , resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO3 > HCO3 >S O 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87 Sr/ 86 Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87 Sr/ 86 Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates Rsolute are estimated from linear Na and Si pore weathering gradients bsolute such that Rsolute ¼ qh bsolutebSv

  • the role of reaction affinity and secondary minerals in regulating chemical weathering rates at the santa cruz soil chronosequence california
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Kate Maher, Art F. White, Carl I Steefel, David A. Stonestrom
    Abstract:

    In order to explore the reasons for the apparent discrepancy between laboratory and field weathering rates and to determine the extent to which weathering rates are controlled by the approach to thermodynamic equilibrium, secondary mineral precipitation and flow rates, a multicomponent reactive transport model (CrunchFlow) was used to interpret soil profile development and mineral precipitation and dissolution rates at the 226 ka marine terrace chronosequence near Santa Cruz, CA. Aqueous compositions, fluid chemistry, transport, and mineral abundances are well characterized (White et al., 2008, GCA) and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. When primary mineral weathering rates are calculated with either of two experimentally determined rate constants, the nonlinear, parallel rate law formulation of Hellmann and Tisser and [2006] or the aluminum inhibition model proposed by Oelkers et al. [1994], modeling results are consistent with field-scale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site. Observed maximum clay abundances in the Argillic Horizons occur at the depth and time where the reaction fronts of the primary minerals overlap. The modeling indicates that the Argillic horizon at Santa Cruz can be explained almost entirely by weathering of primary minerals and in situ clay precipitation accompanied by undersaturation of kaolinite at the top of the profile. The rate constant for kaolinite precipitation was also determined based on model simulations of mineral abundances and dissolved Al, SiO{sub 2}(aq) and pH in pore waters. Changes in the rate of kaolinite precipitation or the flow rate do not affect the gradient of the primary mineral weathering profiles, but instead control the rate of propagation of the primary mineral weathering fronts and thus total mass removed from the weathering profile. Our analysis suggests that secondary clay precipitation is as important as aqueous transport in governing the amount of dissolution that occurs within a profile because clay minerals exert a strong control over the reaction affinity of the dissolving primary minerals. The modeling also indicates that the weathering advance rate and the total mass of mineral dissolved is controlled by the thermodynamic saturation of the primary dissolving phases plagioclase and K-feldspar, as is evident from the difference in propagation rates of the reaction fronts for the two minerals despite their very similar kinetic rate laws.

  • chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles
    Geochimica et Cosmochimica Acta, 2008
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Suzanne P Anderson
    Abstract:

    Abstract The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow Argillic Horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile bs is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux qh, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (qh = 0.025–0.17 m yr−1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of qh and the ratio of solubilized to solid state feldspar (msat/Mtotal) define the weathering velocity. The weathering gradient bs reflects the kinetic rate of reaction where Sv is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates (R = 5–14 × 10−16 mol m−2 s−1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

  • chemical weathering rates of a soil chronosequence on granitic alluvium iii hydrochemical evolution and contemporary solute fluxes and rates
    Geochimica et Cosmochimica Acta, 2005
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Jennifer W Harden
    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. Elemental balances in the soils are described in terms of solid state, exchange and pore water reservoirs and input/output fluxes from precipitation, ET, biomass, solute discharge and weathering. Solute mineral nutrients are strongly dependent on biomass variations as evidenced by an apparent negative K weathering flux reflecting aggradation by grassland plants. The ratios of solute Na to other base cations progressively increase with soil age. Discharge fluxes of Na and Si, when integrated over geologic time, are comparable to solid-state mass losses in the soils, implying similar past weathering conditions. Similarities in solute and sorbed Ca/Mg ratios reflect short-term equilibrium with the exchange reservoir. Long-term consistency in solute ratios, when contrasted against progressive decreases in solid-state Ca/Mg, requires an additional Ca source, probably from dry deposition. Amorphous silica precipitates from thermodynamically-saturated pore waters during periods of high evapotranspiration and result in the formation of duripans in the oldest soils. The degree of feldspar and secondary gibbsite and kaolinite saturation varies both spatially and temporally due to the seasonality of plant-respired CO2 and a decrease in organically complexed Al. In deeper pore waters, K-feldspar is in equilibrium and plagioclase is about an order of magnitude undersaturated. Hydrologic heterogeneity produces a range of weathering gradients that are constrained by solute distributions and matrix and macropore flow regimes. Plagioclase weathering rates, based on precipitation-corrected Na gradients, vary between 3 and 7 × 10−16 mol m−2 s−1. These rates are similar to previously determined solid-state rates but are several orders of magnitude slower than for experimental plagioclase dissolution indicating strong inhibitions to natural weathering, partly due to near-equilibrium weathering reactions.

Art F. White - One of the best experts on this subject based on the ideXlab platform.

  • chemical weathering of a marine terrace chronosequence santa cruz california part ii solute profiles gradients and the comparisons of contemporary and long term weathering rates
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Art F. White, Marjorie S Schulz, Davison V. Vivit, David A. Stonestrom, John A Fitzpatrick, T Bullen, Kate Maher, Alex E. Blum
    Abstract:

    The spatial and temporal changes in hydrology and pore water elemental and 87 Sr/ 86 Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying Argillic Horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated Horizons extending to depths in excess of 15 m. Hydraulic fluxes (qh), based on Cl mass balances, vary from 0.06 to 0.22 m yr � 1 , resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO3 > HCO3 >S O 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87 Sr/ 86 Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87 Sr/ 86 Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates Rsolute are estimated from linear Na and Si pore weathering gradients bsolute such that Rsolute ¼ qh bsolutebSv

  • the role of reaction affinity and secondary minerals in regulating chemical weathering rates at the santa cruz soil chronosequence california
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Kate Maher, Art F. White, Carl I Steefel, David A. Stonestrom
    Abstract:

    In order to explore the reasons for the apparent discrepancy between laboratory and field weathering rates and to determine the extent to which weathering rates are controlled by the approach to thermodynamic equilibrium, secondary mineral precipitation and flow rates, a multicomponent reactive transport model (CrunchFlow) was used to interpret soil profile development and mineral precipitation and dissolution rates at the 226 ka marine terrace chronosequence near Santa Cruz, CA. Aqueous compositions, fluid chemistry, transport, and mineral abundances are well characterized (White et al., 2008, GCA) and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. When primary mineral weathering rates are calculated with either of two experimentally determined rate constants, the nonlinear, parallel rate law formulation of Hellmann and Tisser and [2006] or the aluminum inhibition model proposed by Oelkers et al. [1994], modeling results are consistent with field-scale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site. Observed maximum clay abundances in the Argillic Horizons occur at the depth and time where the reaction fronts of the primary minerals overlap. The modeling indicates that the Argillic horizon at Santa Cruz can be explained almost entirely by weathering of primary minerals and in situ clay precipitation accompanied by undersaturation of kaolinite at the top of the profile. The rate constant for kaolinite precipitation was also determined based on model simulations of mineral abundances and dissolved Al, SiO{sub 2}(aq) and pH in pore waters. Changes in the rate of kaolinite precipitation or the flow rate do not affect the gradient of the primary mineral weathering profiles, but instead control the rate of propagation of the primary mineral weathering fronts and thus total mass removed from the weathering profile. Our analysis suggests that secondary clay precipitation is as important as aqueous transport in governing the amount of dissolution that occurs within a profile because clay minerals exert a strong control over the reaction affinity of the dissolving primary minerals. The modeling also indicates that the weathering advance rate and the total mass of mineral dissolved is controlled by the thermodynamic saturation of the primary dissolving phases plagioclase and K-feldspar, as is evident from the difference in propagation rates of the reaction fronts for the two minerals despite their very similar kinetic rate laws.

  • chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles
    Geochimica et Cosmochimica Acta, 2008
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Suzanne P Anderson
    Abstract:

    Abstract The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow Argillic Horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile bs is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux qh, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (qh = 0.025–0.17 m yr−1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of qh and the ratio of solubilized to solid state feldspar (msat/Mtotal) define the weathering velocity. The weathering gradient bs reflects the kinetic rate of reaction where Sv is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates (R = 5–14 × 10−16 mol m−2 s−1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

  • chemical weathering rates of a soil chronosequence on granitic alluvium iii hydrochemical evolution and contemporary solute fluxes and rates
    Geochimica et Cosmochimica Acta, 2005
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Jennifer W Harden
    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. Elemental balances in the soils are described in terms of solid state, exchange and pore water reservoirs and input/output fluxes from precipitation, ET, biomass, solute discharge and weathering. Solute mineral nutrients are strongly dependent on biomass variations as evidenced by an apparent negative K weathering flux reflecting aggradation by grassland plants. The ratios of solute Na to other base cations progressively increase with soil age. Discharge fluxes of Na and Si, when integrated over geologic time, are comparable to solid-state mass losses in the soils, implying similar past weathering conditions. Similarities in solute and sorbed Ca/Mg ratios reflect short-term equilibrium with the exchange reservoir. Long-term consistency in solute ratios, when contrasted against progressive decreases in solid-state Ca/Mg, requires an additional Ca source, probably from dry deposition. Amorphous silica precipitates from thermodynamically-saturated pore waters during periods of high evapotranspiration and result in the formation of duripans in the oldest soils. The degree of feldspar and secondary gibbsite and kaolinite saturation varies both spatially and temporally due to the seasonality of plant-respired CO2 and a decrease in organically complexed Al. In deeper pore waters, K-feldspar is in equilibrium and plagioclase is about an order of magnitude undersaturated. Hydrologic heterogeneity produces a range of weathering gradients that are constrained by solute distributions and matrix and macropore flow regimes. Plagioclase weathering rates, based on precipitation-corrected Na gradients, vary between 3 and 7 × 10−16 mol m−2 s−1. These rates are similar to previously determined solid-state rates but are several orders of magnitude slower than for experimental plagioclase dissolution indicating strong inhibitions to natural weathering, partly due to near-equilibrium weathering reactions.

Alex E. Blum - One of the best experts on this subject based on the ideXlab platform.

  • chemical weathering of a marine terrace chronosequence santa cruz california part ii solute profiles gradients and the comparisons of contemporary and long term weathering rates
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Art F. White, Marjorie S Schulz, Davison V. Vivit, David A. Stonestrom, John A Fitzpatrick, T Bullen, Kate Maher, Alex E. Blum
    Abstract:

    The spatial and temporal changes in hydrology and pore water elemental and 87 Sr/ 86 Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying Argillic Horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated Horizons extending to depths in excess of 15 m. Hydraulic fluxes (qh), based on Cl mass balances, vary from 0.06 to 0.22 m yr � 1 , resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO3 > HCO3 >S O 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87 Sr/ 86 Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87 Sr/ 86 Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates Rsolute are estimated from linear Na and Si pore weathering gradients bsolute such that Rsolute ¼ qh bsolutebSv

  • chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles
    Geochimica et Cosmochimica Acta, 2008
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Suzanne P Anderson
    Abstract:

    Abstract The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow Argillic Horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile bs is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux qh, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (qh = 0.025–0.17 m yr−1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of qh and the ratio of solubilized to solid state feldspar (msat/Mtotal) define the weathering velocity. The weathering gradient bs reflects the kinetic rate of reaction where Sv is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates (R = 5–14 × 10−16 mol m−2 s−1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

  • chemical weathering rates of a soil chronosequence on granitic alluvium iii hydrochemical evolution and contemporary solute fluxes and rates
    Geochimica et Cosmochimica Acta, 2005
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Jennifer W Harden
    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. Elemental balances in the soils are described in terms of solid state, exchange and pore water reservoirs and input/output fluxes from precipitation, ET, biomass, solute discharge and weathering. Solute mineral nutrients are strongly dependent on biomass variations as evidenced by an apparent negative K weathering flux reflecting aggradation by grassland plants. The ratios of solute Na to other base cations progressively increase with soil age. Discharge fluxes of Na and Si, when integrated over geologic time, are comparable to solid-state mass losses in the soils, implying similar past weathering conditions. Similarities in solute and sorbed Ca/Mg ratios reflect short-term equilibrium with the exchange reservoir. Long-term consistency in solute ratios, when contrasted against progressive decreases in solid-state Ca/Mg, requires an additional Ca source, probably from dry deposition. Amorphous silica precipitates from thermodynamically-saturated pore waters during periods of high evapotranspiration and result in the formation of duripans in the oldest soils. The degree of feldspar and secondary gibbsite and kaolinite saturation varies both spatially and temporally due to the seasonality of plant-respired CO2 and a decrease in organically complexed Al. In deeper pore waters, K-feldspar is in equilibrium and plagioclase is about an order of magnitude undersaturated. Hydrologic heterogeneity produces a range of weathering gradients that are constrained by solute distributions and matrix and macropore flow regimes. Plagioclase weathering rates, based on precipitation-corrected Na gradients, vary between 3 and 7 × 10−16 mol m−2 s−1. These rates are similar to previously determined solid-state rates but are several orders of magnitude slower than for experimental plagioclase dissolution indicating strong inhibitions to natural weathering, partly due to near-equilibrium weathering reactions.

Mercier Norbert - One of the best experts on this subject based on the ideXlab platform.

  • Le site stratifié du Bois Clair à Montguyon (Charente-Maritime, France) : récurrences paléolithiques, brièveté des occupations et aires de débitage spécialisées
    'OpenEdition', 2018
    Co-Authors: Bernard-guelle Sébastien, Taylor Alexis, Rué Mathieu, Fernandes Paul, Ajas Aurélie, Coudenneau Aude, Hernandez Marion, Mercier Norbert
    Abstract:

    Le site du Bois Clair constitue un nouveau jalon pour la compréhension des occupations paléolithiques proches de la façade atlantique. Fouillé dans le cadre du projet de construction de la ligne à grande vitesse Sud Europe Atlantique, il a livré plusieurs niveaux d’occupations qui s’intercalent dans des dépôts colluviaux et éoliens au sein desquels subsistent deux Horizons de sol lessivé de rang interglaciaire, l’un actuel et l’autre fossile. L’ensemble de la séquence est cadré par six datations par luminescence. Si le contexte topographique et sédimentaire a favorisé le panachage d’industries séparées dans le temps, en particulier sur le point haut du site, le degré de conservation du site reste malgré tout étonnamment bon, particulièrement dans la partie supérieure de la séquence où l’organisation spatiale est localement bien préservée, autorisant ainsi d’intéressantes interprétations comportementales des différents groupes culturels ayant fréquenté le site. Il se singularise par une récurrence de petites occupations paléolithiques s’étalant de façon non linéaire du Moustérien jusqu’au Tardiglaciaire. Elles sont toutes matérialisées par des aires de débitage très circonscrites dans l’espace et tournées vers des productions spécialisées. Cette étude présente les principales données acquises et se focalise sur les deux niveaux les mieux conservés du site : le niveau moustérien supérieur et le niveau magdalénien. Le premier est rattaché à un technocomplexe moustérien récent (OIS3) à débitage discoïde spécifiquement tourné vers une production de pointes pseudo-Levallois via des modalités de débitage originales, particulièrement bien documentées par les remontages. Le second niveau présenté correspond à une brève occupation de la phase finale du Magdalénien orientée vers une production de grandes lames réalisée à partir de cinq blocs de silex d’origine locale.The Bois Clair site is a new benchmark for the study of the Atlantic coast area Palaeolithic settlements. Excavated during the high speed Sud Europe Atlantique train line construction project, it revealed several occupation levels, inserted in colluvial and eolian deposits displaying two Argillic Horizons. The sequence is chronologically established by six luminescence datings. Despite a topographic and sedimentary context leading to the mixing of chronologically distinct industries, especially at the highest point of the site, it’s state of preservation remains surprisingly good, particularly in the upper part of the sequence where the spatial organization is locally preserved, thus allowing interesting behavioral interpretations. The site stands out through recurrent and small palaeolithic settlements, dating from the Mousterian to the Late Upper Palaeolithic. These settlements all display small debitage areas, dedicated to specialized productions. This paper presents the acquired data and focuses on the two best preserved occupation levels : the Late Mousterian and Magdalenian levels. The first belongs to the late mousterian technological complex (OIS3), displaying a discoïde debitage specifically dedicated to pseudo-levallois points production, achieved through original debitage methods well documented by many refittings. The second level represents a brief Late Upper Magdalenian settlement, displaying the shaping and debitage of five local flint cores, dedicated to long blade production

  • Le site stratifié du Bois Clair à Montguyon (Charente-Maritime, France) : récurrences paléolithiques, brièveté des occupations et aires de débitage spécialisées
    'OpenEdition', 2017
    Co-Authors: Bernard-guelle Sébastien, Taylor Alexis, Rué Mathieu, Fernandes Paul, Ajas Aurélie, Coudenneau Aude, Hernandez Marion, Mercier Norbert
    Abstract:

    International audienceThe Bois Clair site is a new benchmark for the study of the Atlantic coast area Palaeolithic settlements. Excavated during the high speed Sud Europe Atlantique train line construction project, it revealed several occupation levels, inserted in colluvial and eolian deposits displaying two Argillic Horizons. The sequence is chronologically established by six luminescence datings. Despite a topographic and sedimentary context leading to the mixing of chronologically distinct industries, especially at the highest point of the site, it’s state of preservation remains surprisingly good, particularly in the upper part of the sequence where the spatial organization is locally preserved, thus allowing interesting behavioral interpretations. The site stands out through recurrent and small palaeolithic settlements, dating from the Mousterian to the Late Upper Palaeolithic. These settlements all display small debitage areas, dedicated to specialized productions. This paper presents the acquired data and focuses on the two best preserved occupation levels : the Late Mousterian and Magdalenian levels. The first belongs to the late mousterian technological complex (OIS3), displaying a discoïde debitage specifically dedicated to pseudo-levallois points production, achieved through original debitage methods well documented by many refittings. The second level represents a brief Late Upper Magdalenian settlement, displaying the shaping and debitage of five local flint cores, dedicated to long blade production.Le site du Bois Clair constitue un nouveau jalon pour la compréhension des occupations paléolithiques proches de la façade atlantique. Fouillé dans le cadre du projet de construction de la ligne à grande vitesse Sud Europe Atlantique, il a livré plusieurs niveaux d’occupations qui s’intercalent dans des dépôts colluviaux et éoliens au sein desquels subsistent deux Horizons de sol lessivé de rang interglaciaire, l’un actuel et l’autre fossile. L’ensemble de la séquence est cadré par six datations par luminescence. Si le contexte topographique et sédimentaire a favorisé le panachage d’industries séparées dans le temps, en particulier sur le point haut du site, le degré de conservation du site reste malgré tout étonnamment bon, particulièrement dans la partie supérieure de la séquence où l’organisation spatiale est localement bien préservée, autorisant ainsi d’intéressantes interprétations comportementales des différents groupes culturels ayant fréquenté le site. Il se singularise par une récurrence de petites occupations paléolithiques s’étalant de façon non linéaire du Moustérien jusqu’au Tardiglaciaire. Elles sont toutes matérialisées par des aires de débitage très circonscrites dans l’espace et tournées vers des productions spécialisées. Cette étude présente les principales données acquises et se focalise sur les deux niveaux les mieux conservés du site : le niveau moustérien supérieur et le niveau magdalénien. Le premier est rattaché à un technocomplexe moustérien récent (OIS3) à débitage discoïde spécifiquement tourné vers une production de pointes pseudo-Levallois via des modalités de débitage originales, particulièrement bien documentées par les remontages. Le second niveau présenté correspond à une brève occupation de la phase finale du Magdalénien orientée vers une production de grandes lames réalisée à partir de cinq blocs de silex d’origine locale

Marjorie S Schulz - One of the best experts on this subject based on the ideXlab platform.

  • chemical weathering of a marine terrace chronosequence santa cruz california part ii solute profiles gradients and the comparisons of contemporary and long term weathering rates
    Geochimica et Cosmochimica Acta, 2009
    Co-Authors: Art F. White, Marjorie S Schulz, Davison V. Vivit, David A. Stonestrom, John A Fitzpatrick, T Bullen, Kate Maher, Alex E. Blum
    Abstract:

    The spatial and temporal changes in hydrology and pore water elemental and 87 Sr/ 86 Sr compositions are used to determine contemporary weathering rates in a 65- to 226-kyr-old soil chronosequence formed from granitic sediments deposited on marine terraces along coastal California. Soil moisture, tension and saturation exhibit large seasonal variations in shallow soils in response to a Mediterranean climate. These climate effects are dampened in underlying Argillic Horizons that progressively developed in older soils, and reached steady-state conditions in unsaturated Horizons extending to depths in excess of 15 m. Hydraulic fluxes (qh), based on Cl mass balances, vary from 0.06 to 0.22 m yr � 1 , resulting in fluid residence times in the terraces of 10–24 yrs. As expected for a coastal environment, the order of cation abundances in soil pore waters is comparable to sea water, i.e., Na > Mg > Ca > K > Sr, while the anion sequence Cl > NO3 > HCO3 >S O 4 reflects modifying effects of nutrient cycling in the grassland vegetation. Net Cl-corrected solute Na, K and Si increase with depth, denoting inputs from feldspar weathering. Solute 87 Sr/ 86 Sr ratios exhibit progressive mixing of sea water-dominated precipitation with inputs from less radiogenic plagioclase. While net Sr and Ca concentrations are anomalously high in shallow soils due to biological cycling, they decline with depth to low and/or negative net concentrations. Ca/Mg, Sr/Mg and 87 Sr/ 86 Sr solute and exchange ratios are similar in all the terraces, denoting active exchange equilibration with selectivities close to unity for both detrital smectite and secondary kaolinite. Large differences in the magnitudes of the pore waters and exchange reservoirs result in short-term buffering of the solute Ca, Sr, and Mg. Such buffering over geologic time scales can not be sustained due to declining inputs from residual plagioclase and smectite, implying periodic resetting of the exchange reservoir such as by past vegetational changes and/or climate. Pore waters approach thermodynamic saturation with respect to albite at depth in the younger terraces, indicating that weathering rates ultimately become transport-limited and dependent on hydrologic flux. Contemporary rates Rsolute are estimated from linear Na and Si pore weathering gradients bsolute such that Rsolute ¼ qh bsolutebSv

  • chemical weathering of a marine terrace chronosequence santa cruz california i interpreting rates and controls based on soil concentration depth profiles
    Geochimica et Cosmochimica Acta, 2008
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Suzanne P Anderson
    Abstract:

    Abstract The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow Argillic Horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile bs is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux qh, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (qh = 0.025–0.17 m yr−1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of qh and the ratio of solubilized to solid state feldspar (msat/Mtotal) define the weathering velocity. The weathering gradient bs reflects the kinetic rate of reaction where Sv is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates (R = 5–14 × 10−16 mol m−2 s−1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

  • chemical weathering rates of a soil chronosequence on granitic alluvium iii hydrochemical evolution and contemporary solute fluxes and rates
    Geochimica et Cosmochimica Acta, 2005
    Co-Authors: Art F. White, Marjorie S Schulz, Alex E. Blum, Davison V. Vivit, David A. Stonestrom, Jennifer W Harden
    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. Elemental balances in the soils are described in terms of solid state, exchange and pore water reservoirs and input/output fluxes from precipitation, ET, biomass, solute discharge and weathering. Solute mineral nutrients are strongly dependent on biomass variations as evidenced by an apparent negative K weathering flux reflecting aggradation by grassland plants. The ratios of solute Na to other base cations progressively increase with soil age. Discharge fluxes of Na and Si, when integrated over geologic time, are comparable to solid-state mass losses in the soils, implying similar past weathering conditions. Similarities in solute and sorbed Ca/Mg ratios reflect short-term equilibrium with the exchange reservoir. Long-term consistency in solute ratios, when contrasted against progressive decreases in solid-state Ca/Mg, requires an additional Ca source, probably from dry deposition. Amorphous silica precipitates from thermodynamically-saturated pore waters during periods of high evapotranspiration and result in the formation of duripans in the oldest soils. The degree of feldspar and secondary gibbsite and kaolinite saturation varies both spatially and temporally due to the seasonality of plant-respired CO2 and a decrease in organically complexed Al. In deeper pore waters, K-feldspar is in equilibrium and plagioclase is about an order of magnitude undersaturated. Hydrologic heterogeneity produces a range of weathering gradients that are constrained by solute distributions and matrix and macropore flow regimes. Plagioclase weathering rates, based on precipitation-corrected Na gradients, vary between 3 and 7 × 10−16 mol m−2 s−1. These rates are similar to previously determined solid-state rates but are several orders of magnitude slower than for experimental plagioclase dissolution indicating strong inhibitions to natural weathering, partly due to near-equilibrium weathering reactions.