Enhancement of silicate weathering rates by vascular land plants: quantifying the effect

Abstract

Theoretical arguments and biogeochemical modeling indicate that levels of paleoatmospheric CO2 depend strongly upon the degree to which the appearance, spread, and evolution of vascular plants on land accelerated the weathering of calcium and magnesium silicates (Berner, 1991, 1993). We have undertaken a series of observations and controlled experiments relating to the relative weathering effects of higher plants and their weatheringspecific responses to elevated CO2. Work to date includes a comprehensive study of Hawaiian basalt weathering beneath plant communities, along with examination of bare-rock and lichen-covered controls (modern lichens may serve as first-order analogs for terrestrial vegetation prior to the late Silurian (Retallack, 1981; Wright, 1985 ) ); analysis of the impact of plants colonizing ash and pumice from the recent eruptions of Mt. St. Helens; and growth-chamber experiments on CO2 fertilization of Loblolly pine and American beech growth on a basalt gravel substrate. Findings from these studies are presented and their implications discussed. We sampled basalt flows with ages between a few years and several thousand years from the Kilauea and Mauna Loa volcanoes on the island of Hawaii. Regions of low, moderate and high rainfall were sampled. Apparent weathering textures (leached zones, regions of partial dissolution) were not observed to penetrate unvegetated rocks less than 200 years in age to depths of more than 10/~m, even in the most humid areas. In addition, the common fruticose lichen Stereocaulon vulcani has no greater weathering effect upon its substrate than does abiotic water-rock interaction: some slight crumbling of rock surfaces might be attributable to the expansion and contraction of lichen holdfasts upon wetting, but no leaching and only negligible dissolution of substrate minerals and glass were observed. Notwithstanding the lichen's failure to digest its substrate to any marked degree, its tendency to trap and leach fine wind-blown rock fragments may result in the chemical weathering of significant quantities of material that would otherwise be transported and buried without being chemically weathered. Schwartzman and Volk ( 1989 ) have suggested that such a process might have allowed primitive land biota to restrain atmospheric CO2 from reaching excessive values before the rise of vascular plants, and indeed it may. The scale of such lichen-mediated weathering, however, does not approach that observed adjacent to the fine roots of vascular plants on these Hawaiian flows. Younger flows beneath plants such as the common Ohia tree (Metrosideros polymorpha) exhibit widening of primary microfractures into open channels and preferential congruent dissolution of plagioclase, olivine and volcanic glass. On older flows up to