『Abstract
What controls the chemical weathering of soils in tectonically
active landscapes? Recent field and modeling studies suggest that
tectonic forcing and associated increases in erosion rates may
either promote or hinder soil chemical weathering. These competing
trajectories are dependent on two primary controls: the availability
of fresh minerals and their residence time on the landsurface.
Here, we explore rates and extents of soil weathering in the San
Gabriel Mountains of California, where previous work has measured
clear tectonic fingerprints on rates of long term exhumation,
hillslope erosion and landscape morphology. We quantify chemical
weathering across this landscape by elemental analysis of soils,
saprolites and bedrock on six sites that bracket the low-gradient
hillslopes of the relict upland plateau and the high-gradient
hillslopes at the margins of the tectonically-driven incising
landscape. Average chemical depletion fractions, which measure
weathering losses from soil relative to unweathered parent material,
decrease with increasing elevation and decreasing temperature,
reflecting a combination of climate influence and potential dust
inputs from the Mojave Desert. Weathering fluxes from non-dust-affected
sites with similar elevations, climates and lithology correlate
with both erosion rates and hillslope gradient. On low-gradient
hillslopes (<25゜), weathering rates increase with increasing
erosion rates, reflecting the influence of mineral supply. However,
on high-gradient hillslopes (>25゜), weathering intensities
and rates both decrease as erosion rates increase and soils thin.
At the highest denudation rates (>300 t km-2 y-1),
saprolite production is outpaced, and soils are produced directly
from fractured rock. These patterns are consistent with those
predicted by a previously published model for denudation-weathering
relationships based on mineral weathering kinetics. Variable weathering
extents in soils indicate that weathering in the SGM is largely
kinetically limited. This study is the first to quantify decreases
in both rates and extents of soil chemical weathering with increasing
erosion rates, and suggests tectonic uplift in rapidly eroding
and incising landscapes may not stimulate increased silicate weathering.
Keywords: erosion; chemical weathering; soil production; kinetic
limitation; tectonics』
1. Introduction
2. field site and methods
2.1. Setting
2.2. Field and laboratory methods
2.3. Calculations of chemical losses
2.4. Model of kinetic and supply limited weathering
3. Results
3.1. Parent material chemistry
3.2. Soil weathering
4. Discussion
4.1. Downslope patterns of chemical weathering extents
4.2. Climate, acidity and dust
4.3. Relationships between chemical weathering and erosion
4.4. Kinetic limitation and the weathering response to uplift
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References