『Abstract
Silicate weathering mobilizes “fresh” dissolved silica (DSi).
The major factors governing DSi mobilization by chemical weathering
on continental or global scales have not been satisfactorily quantified.
Furthermore, influence of regional variations of proposed factors
on large scale DSi mobilization is not properly assessed. A continental-scale,
process-oriented, empirical model for DSi mobilization is developed
to assess this research gap. The model is calibrated on river
chemistry data from 142 monitoring stations from the conterminous
USA, selected for minimal anthropogenic and water-body influence
in their catchments. The average area of the catchments is 3890
km2. The average observed DSi yield of the catchments
is 2.68 t SiO2 km-2 a-1.
The model calculates DSi yield as subject to catchment attributes,
i.e. climate, lithology, land cover and morphology. As lithological
source data for the model, a new lithological map of north America
was developed. The high spatial resolution of the new map allows
assessment of lithology classes with mapped extents as small as
0.5 km2. The average lithology polygon size is 75 km2.
The developed multi-lithological, non-linear, lumped model for
annual DSi mobilization describe 89% of the observed variance
of DSi yield. It uses lithology proportion and runoff as predictors.
With runoff, DSi yield increases differently for individual lithological
classes. Basic igneous rocks show the highest DSi yields with
respect to a given runoff. Consolidated sedimentary lithological
classes yield DSi in the reversed order of their defined silicon
contents. Of all lithological classes, the least DSi per runoff
is mobilized from acid plutonic rocks.
Apart from the two major predictors, analysis of the model also
provides evidence for an influence of temperature and land cover
on DSi mobilization. Comparison with existing studies shows that
controlling factors on DSi mobilization vary regionally. Thus,
studies calibrated in different regions result in significantly
different DSi yields for comparable lithological classes. This
emphasizes the need for global DSi mobilization models to be regionally
calibrated.
Keywords: Dissolved silica; Global cycles; USA; Multi-lithological
model; Continental model; Chemical weathering』
1. Introduction
2. Methods and data
3. Results
4. Discussion
4.1. Influences on DSi yield
4.1.1. Runoff
4.1.2. Lithology
4.1.3. Temperature
4.1.4. Land cover/vegetation
4.1.5. Other factors
4.2. Comparison with previous studies
4.3. Model limitations
5. Conclusions
Acknowledgements
Appendix A. Description of the lithological map of North America
Source data
Lithological class definition
Lithological class description
Alluvial deposits (AD)
Dune sands (DS)
Loess (LO)
Unconsolidated sediments (SU)
Evaporites (EV)
Carbonate sedimentary rocks (SC)
Mixed sedimentary rocks (SM)
Siliciclastic sedimentary rocks (SS)
Metamorphic rocks (MT)
Acid plutonic rocks (PA)
Intermediate plutonic rocks (PI)
Basic plutonic rocks (PB)
Acid volcanic rocks (VA)
Intermediate volcanic rocks (VI)
Basic volcanic rocks (VB)
Pyroclastics (PY)
Water bodies (WB)
Ice and glaciers (IG)
No data (ND)
Summary
Appendix B. Supplementary data
References