White et al.(2005)による〔『Chemical weathering rates of a soil chronosequence on granitic alluvium: III Hydrochemical evolution and contemporary solute fluxes and rates』(1975p)から〕

『花崗岩質沖積層における土壌クロノシーケンスの化学風化速度V.水文化学的進化およびその同時期の溶質フラックスと速度』


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.』

1. Introduction
2. Methodologies
3. Results
 3.1. Hydrology
  3.1.1. Precipitation
  3.1.2. Soil pore water
  3.1.3. Br tracer distributions
 3.2. Chemical compositions
  3.2.1. Precipitation
  3.2.2. Pore water compositions
  3.2.3. Cation exchange
  3.2.4. Biomass
  3.2.5. Soil gas
4. Discussion
 4.1. Relative importance of precipitation, evapotranspiration and contemporary weathering on solute compositions
 4.2. Hydrochemical evolution in the chronosequence
  4.2.1. Effects of weathering intensity and development of argillic horizons and duripans
  4.2.2. The effects of seasonal and yearly climate variability on pore water concentrations in younger regoliths
  4.2.3. The effects of increased weathering on fluid and solute transport in older regoliths
 4.3. Mass balances
  4.3.1. Contemporary fluxes
  4.3.2. Comparison with long-term fluxes
  4.3.3. Long-term changes in solute compositions
  4.3.4. Exchange equilibrium and solute compositions
 4.4. Geochemical controls o weathering
  4.4.1. Thermodynamic solubilities
  4.4.2. Reaction rates
5. Conclusions
Acknowledgments
References


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