Dissolved silica mobilization in the conterminous USA

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 arithropogenic and water-body influence in their catchments. The average area of the catchments is 3890 km(2). 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 km(2). The average lithology polygon size is 75 km(2). The developed multi-lithological, non-linear, lumped model for annual DSi mobilization describes 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 sedimenary 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. (C) 2009 Elsevier B.V. All rights reserved.