The Geochemical Carbon Cycle: A Vadose-Hydrologic Study of the Effect of Plants on Chemical Weathering

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9628296 Keller Carbon dioxide (CO2) is a greenhouse gas. One avenue to understanding the relationship of atmospheric CO2 concentration to climate, is to study that relationship over geologic time. This requires that we understand the processes affecting atmospheric CO2 on long time scales. The principal pathway for long-term transfer of CO2 from Earth's atmosphere to its crust is (a) dissolution or "chemical weathering" of common minerals in soils, subsoils, and groundwater systems, followed by (b) transport of the dissolved material to the oceans in continental drainage, where (c) precipitation and deposition onto the ocean floor occur. This sequence effects transfer of CO2 because mineral dissolution consumes CO2 -containing acids and entrains the product bicarbonate in drainage waters, which is later deposited in the oceans as carbonate minerals. In the geochemical carbon cycle, this process is countervailed by the return of CO2 from subducted and buried carbonate minerals to the atmosphere by volcanism and metamorphism. The relative rates of these processes determine the masses of CO2 in the atmosphere and lithosphere at any one time and, in the process, affect Earth's climate. It is therefore important to understand the factors controlling the rates of various steps in the cycle. One such factor is the effect of plant communities, and their evolution over geologic time, on the rate of chemical weathering. Plants could enhance chemical weathering--relative to unvegetated conditions--by raising subsurface concentrations of acids, facilitating the exposure of fresh mineral surfaces, and binding mineral grains into water-retaining structures characteristic of soils. The actual extent of weathering enhancement by these mechanisms is, however, poorly understood because of the difficulty of factoring other variables out of comparative field studies. This study takes advantage of ongoing long-term, field-scale experiments in plant nutrient cycling. The experimental plots were designed for monitoring of drainage water, and a ten-year database now exists. An NSF Small Grant for Exploratory Research has been awarded to fund the (just-completed) installation of an extensive network of vadose pore-water samplers to observe how solution chemistry (the chemical weathering "load") evolves with depth and time in soils and subsoils beneath the various plots. This proposal is application for funds to continue the work, i.e., to do the geochemical and hydrologic research which these facilities can now support. The results will facilitate testing of hypotheses regarding the relative importance of weathering-enhancement mechanisms. ??