『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 id 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 = [qh・(msol/Mtotal)]・[1/(Sv・bs)]・
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.』
1. Introduction
2. Site characterization
2.1. Geology
2.2. Terrace ages
2.3. Sampling and analyses
3. Results
3.1 Elemental distributions
3.2. Primary mineral distributions
3.3. Clay and Fe oxide distributions
4. Discussion
4.1. Elemental mobilities
4.1.1. Protolith compositions
4.1.2. Mobility of Na, Ca, Sr
4.1.3. Mobility of K, Rb and Ba
4.1.4. Effect of eolian deposition on weathering profiles
4.1.5. Mobility of Mg, Al, and Fe
4.1.6. Mass changes
4.1.7. Mass fluxes
4.2. Mineral weathering rates
4.2.1. A weathering profile calculator
4.3. Controls on mineral weathering
4.3.1. Role of fluid flow
4.3.2. Role of the reaction rate constant
4.3.3. Role of surface area
4.3.4. The role of differing feldspar solubilities
4.3.5. Role of thermodynamic saturation
4.4. Comparing terrace weathering profiles
4.4.1. Steady-state versus non-steady weathering profiles
4.4.2. Simple methods for estimating weathering velocities and
rates
5. Conclusions
Acknowledgments
Appendix I
Appendix II
References