『Abstract
Weathering of silicate minerals impacts many geological and ecological
processes. For example, the weathering of basalt contributes significantly
to consumption of atmospheric carbon dioxide (CO2)
and must be included in global calculations of such consumption
over geological timeframes. Here we compare weathering advance
rates for basalt (wDβ), where
D and β indicate the scale at which the rate id determined and
surface area measured, respectively, from the laboratory to the
watershed scales. Data collected at the laboratory, weathering
rind, soil profiles and watershed scales show that weathering
advance rate of basalt is a fractal property that can be described
by a fractal dimension (dr≒2.3). By combining
the fractal description of rates with an Arrhenius relationship
for basalt weathering, we derive the following equation to predict
weathering advance rates at any spatial scale from weathering
advance rates measured at the BET scale:
wDβ = k0
(β/a)dr-2 e-Ea/RT.
Here, k0 is the pre-exponential factor (1.29×107
mm3 mm-2 yr-1). Ea
is the activation energy (70 kj mol-1), and a is a
spatial constant related to the scale of measurement of BET surface
area (10-7 mm) The term, (β/a)dr-2,
is the roughness. The roughness fractal dimension can be conceptualized
as a factor related to both the thickness of the reaction front
and the specific surface area within the reaction front. However,
the above equation can also be written in terms of a surface fractal
dimension and the hypothetical average grain radius. These fractal
dimensions provide insight into reaction front geometry and should
vary with lithology. Once the surface area discrepancy has been
accounted for using this method, we find a one to two order of
magnitude range in weathering advance rates measured at any scale
or temperature that can be attributed to factors such as changes
in erosional regime, parent lithology, mechanism, climate, composition
of reacting fluid, and biological activity. Our scaled equation,
when used to predict global basalt CO2 consumption
based upon global lithologic maps, yields an uptake flux (1.75×1013
mol CO2 yr-1) within the predicted
error of fluxes estimated based upon riverine measurements.
Keywords: basalt; weathering; fractal dimension; fractals; surface
area; cation denudation rates; weathering advance rates; scaling』
1. Introduction
2. Weathering advance rates
2.1. Laboratory scale
2.2. Weathering rind and soil profile studies
2.3. Watershed studies
3. Calculation of weathering advance rates
3.1. Advance rates from water chemistry
3.2. Advance rates calculated from regolith thickness
4. Results and discussion
4.1. BET surface area-normalized rates
4.2. BET surface area-normalized rates from fractal dimensions
4.3. Predicting weathering advance rates across scales
5. Fractal dimensions and reaction front geometry
6. Conclusions
Acknowledgements
References