Modeling of Silicate Mineral Weathering Reactions in an Alpine Basin of the Southern Sierra Nevada, California

Abstract

Mineral weathering reactions from a small, Sierran alpine watershed were modeled with a stoichiometric mole-balance method, using a multi-year record of streamflow and snowpack chemical analyses, and site-specific mineral compositions. Reaction modeling was intended to determine the dependence of mineral weathering stoichiometry on (1) time, (2) lithology /mineralogy, and (3) space. Thin sections were made from 13 Emerald Lake Watershed rock samples; mild hypogene alteration was identified in most samples, indicated by quartzsericite replacement of primary minerals and the presence of epidote and minor calcite. Electron microprobe analyses of thin sections provided basin-specific chemical compositions of major mineral species for reaction modeling. Time series arrays of input (snowpack) chemistry, and output (stream) chemistry were used to calculate molar balances of component ions in the watershed. Single weathering reactions were modeled first, to determine the most important species, then reactions were combined, using mineral compositions from different lithologies, to determine dependence on geology. Weathering reaction models were evaluated primarily by linear regression of modeled versus observed differences between snowmelt and streamflow chemistry. As modeled, relative molar quantities of silicate minerals dissolved through weathering did not change appreciably over the two years considered. Also, hornblende exerted influence on streamflow chemistry disproportionate to its volumetric presence; of all mineral species considered singly, it produced the lowest final error and the best fit of modeled versus observed ion concentrations. Weathering reactions using mineral compositions from the Emerald Lake Granodiorite, a mafic-rich unit in the lower elevations of the basin, best explained the chemical differences between snowpack and streamflow. Finally, different regions within the watershed, as delineated by the four main inflows, exhibited different weathering stoichiometries due to variations in their stream chemistry. A summary of quantitative chemical analyses for Emerald Lake Watershed minerals is appended, along with a listing and explanation of the computer program used to perform the mole-balance calculations.