9417325 Berner The weathering of Ca and Mg silicate minerals on the continents, followed by the precipitation of the liberated Ca and Mg in the oceans as carbonates, has exerted a major control on the level of atmospheric CO2 over geologic time. An important aspect of this process is the role played by plants as they modify the rate of mineral dissolution during weathering. Did the rise and evolution of vascular land plants over the Phanerozoic affect the rate of silicate weathering, and in turn the level of atmospheric CO2? From petrographic studies using the electron microprobe we have discovered that on the island of Hawaii the growth of vascular plants on young basalt flows results in intense etching and removal of Ca and Mg silicates in the vicinity of plant rootlets. As a result relative rates of initial weathering under higher plants were found to be much greater than under lichens or in the absence of any plant life. Similarly at Mt. St. Helens we found that a ten year old volcanic ash deposit shows evidence of chemical alteration immediately adjacent to plant rootlets but no evidence away from the rootlets. Although this petrographic study has been useful in a qualitative to semi-quantitative sense, there is a need for a better way to quantify the effects of plants on silicate weathering. This can be done through the study of the chemistry of solutions draining the rocks. Our preliminary water chemical results of a multiyear ongoing controlled experiment on the effects of young growing trees and grass on silicate weathering, conducted under natural outdoor conditions at the Hubbard Brook Experimental Forest Station in New Hampshire, indicate a minimum silicate weathering rate enhancement by the trees of a factor of 1.5-3.5. We propose to check this result by examining the chemistry of waters draining the basaltic rocks of Iceland. Iceland is ideal for our proposed study in that the rocks are rich in volcanic glass and Ca and Mg silicates , contain no CaCO3, are young enough that variations in the degree of vegetational coverage is present, anthropogenic acid rain is absent and spring and stream waters can be easily sampled. (Water sampling on the island of Hawaii was impossible due to the great permeability of those young basalt flows that exhibit variations in plant coverage.) A reconnaissance visit to Iceland shows that there are several localities where waters can be collected from vegetated and unvegetated areas that are sufficiently nearly to one another that they share the same bedrock lithology, microclimate, aspect. etc. This makes it possible to conduct an experiment on the quantitative role of the local plant life on silicate weathering. Also, there is considerable expertise already available on the chemistry of weathering solutions in Iceland, in the person of Dr. Sigurdur Gislason, and we will be assisted by him in the field. Dr. Gislason's preliminary chemical data on Icelandic spring waters suggest that the effect of plants on weathering rate should be substantial.
