Solute flux and mineral mass balance approaches to the quantification of plant effects on silicate weathering

Abstract

The weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric CO 2 over geologic time, and vascular plants may have played an important role in this process. We have examined the role of plants in weathering in western Iceland by two methods: a solute flux approach and a mineral mass balance approach, using the chemistry of weathering products and of waters draining adjacent areas of basaltic rocks that are either barren (with a partial cover of mosses and lichens) or populated by trees. The study area was chosen to maximize vegetational differences and to minimize differences in microclimate, slope, and lithology, while avoiding hydrothermal waters and anthropogenic acid rain. Solute flux results, including data on cation uptake by growing trees, indicate that the rate of weathering release of Ca and Mg to streams is about 4 times higher in vegetated areas than in bare areas. Mineral mass balance results show that trees increase plagioclase weathering by a factor of 2 and pyroxene weathering by a factor of 10, possibly as a result of preferential weathering of pyroxene by the vegetation. Biomass-normalized weathering fluxes from the birch trees are greater than those from the conifers suggesting that angiosperms enhance weathering to a greater degree than gymnosperms. Results indicate a perpetual short-term sink for atmospheric C as tree-induced leakage of HCO (super -) 3 from the forest ecosystem to rivers and streams, in addition to the finite sink of C in forest biomass. The findings also suggest a major role for vascular plants, as they invaded upland portions of the continents between 410 and 360 my ago, in accelerating weathering and thereby lowering atmospheric CO 2 .