『Abstract
Mineral weathering plays a prominent role in many biogeochemical
and geomorphological processes. It supplies nutrients to soils
and streams, accelerates physical erosion by weakening bedrock
and producing easily erodible soil, and modulates Earth's long-term
climate by drawing down atmospheric carbon dioxide. We calculate
mineral-specific chemical weathering rates at two field sites
in the Rio Icacos catchment, Puerto Rico, by combining new mineral
abundance measurements from quantitative powder X-ray diffraction
(XRD) with existing measurements of (i) soil production rates
from cosmogenic nuclides, (ii) chemical alteration of the regolith
from X^-ray fluorescence (XRF), and (iii) dust deposition rates.
The central purpose of this paper is to show that combining measurements
of cosmogenic nuclides with XRF-based geochemistry and XRD-based
mineralogy can, in favorable cases, provide weathering rates of
abundant, soluble mineral phases in actively eroding terrain to
an accuracy of better than 20% of the mean, even in places with
high dust deposition rates. Mineral weathering at our two field
sites is dominated by plagioclase, at rates of 3274±575 mol ha-1
yr-1 and 3077±541 mol ha-1 yr-1,
followed by hornblende, at 187±71 mol ha-1 yr-1
and 308±93 mol ha-1 yr-1. Within the uncertainty
of our data, all weathering of these primary minerals occurs below
the saprolite-soil interface. Our measurements imply that kaolinite
production in saprolite is roughly 1.3 times faster than kaolinite
weathering in the soil. Our results are the first to show that
field measurements f cosmogenic nuclides, XRF, XRD, and dust fluxes
can be combined within the geochemical mass balance method to
quantify long-term mineral weathering rates, even in locations
with high dust deposition rates. This implies that the mass balance
method can be a valuable tool for quantifying the effects of climate,
vegetation, tectonics, and other factors on weathering rates of
individual mineral phases.
Keywords: Chemical weathering; Mineral weathering; Cosmogenic
nuclides; Rio Icacos; Puerto Rico』
1. Introduction
2. A study state framework for measuring mineral-specific chemical
weathering rates in saprolite and soil
3. Field sites: Rio Icacos, Puerto Rico
4. Methods
4.1. Soil production rates inferred from cosmogenic nuclides
4.2. Mineral abundances inferred from powder XRD patterns
4.3. Uncertainties in inferred mineral-specific chemical weathering
rates
5. Results and discussion
5.1. Mineral abundances
5.2. Mineral-specific weathering rates
5.3. Comparison to prior measurements of mineral weathering rates
5.3.1. Plagioclase weathering rates
5.3.2. Hornblende weathering rates
5.3.3. Biotite weathering rates
5.3.4. Quartz weathering rates
5.3.5. Kaolinite weathering rates
5.4. How much does dust deposition affect estimates of chemical
and physical erosion rates?
6. Conclusions
Appendix A. Supplementary data
References