『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