Direct measurement of the combined effects of lichen, rainfall, and temperature on silicate weathering. Geochimica Cosmochimica Acta 63

Abstract-A key uncertainty in models of the global carbonate-silicate cycle and long-term climate is the way that silicates weather under different climatologic conditions, and in the presence or absence of organic os7# activity. Digital imaging of basahs in Hawaii resolves the coupling between temperature, rainfall, and weathering in the presence and absence of lichens. Activation energies for abiotic dissolution of plagioclrtse . (23, 1 t 2.5 kcal/mol) and olivine (21.3 t 2.7 kcallmol) are similar to those measured in the laboratory, and are roughly double those measured from samples taken underneath lichen. Abiotic weathering rates appear to be r3roLrortional to rainfall. Dissolution of da~ioclase and olivine underneath lichen is far more sensitive to rai~fali. Copyright 01999 Elsevier Scie~ce kd ,,,. INTRODUCTION By casting the Earth, oceans, and atmosphere as participants in a vast chemical reaction whose elementary steps can be largely understood through coordination chemistry, Werner Stumm focused the efforts of at I'easttwo generations of scientists on unraveling the inner secrets of global geochemical cycling. No geochemical cycle has received more attention in recent decades than that of carbon, primarily because of concerns about global warming. To paraphrase Werner, "Although we are clearly performing a massive titration of C02 into the atmosphere, our limited understanding of the carbon cycle makes it difficult to predict the outcome of the experiment." Presumably, a clearer understanding of the complex linkages involved would provide a more certain view of future climate shifts, as well as a means for understanding global change in the geologic past. Over geologic time spans (> 105 year), silicate weathering reactions control the movement of carbon between the atmosphere and oceans, and during the past 400 million years, biota have played an important role in the process. Weathering of Ca and Mg silicates is the primary sink for atmospheric C02 over geologic time, and the amplification of weathering caused by the appearance of biota in the Precambrian almost certainly caused a decrease in atmospheric C02 levels (Lovelock and Whitfield, 1982). Although the cause-and-effect linkages are fairly straightforward, the actual magnitude of the shift from abiotic to organically mediated weathering is unclear. Biotic enhancement has been argued to be "on the order of at least 100 to perhaps more than 1000" Qchwartzman and Volk, 1989). Drever (1994) argued for a much smaller effect. The uncertain biotic enhancement is a critical unknown in models of early climate as large-scale shifts in atmospheric C02 levels would likely result, owing to the greenhouse effect, in temperature * Author to whom correspondence should be addressed (pvbrady@ sandia.gov), This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. excursions. A minor enhancement of weathering by biota would imply that global temperature changed relatively little with the colonization of land by the biota. A high biotic weathering enhancement would point to a substantial lowering of temperature. For example, Schwartzman and Volk (1989) calculated an abiotic Earth 15°C warmer than the present if biotic weathering is 10 times faster than abiotic weathering. The abiotic Earth temperature was cdcttiated to be 30"C warmer if biotic weathering is 100 times greater than the abiotic case. The order of magnitude uncertainty in the biotic effect makes it difficult to model global habitability over geologic time. It would be useful to know to within at least an order of magnitude, to what degree soil microorganisms, lichens, and vascular plants accelerate weathering. Tlis is not an easy question to answer because the factors that control the weathering of the Earth's crust are complex, often coupled, and, as a result, understood at the field scale only in a semiquantitative sense. Soils rich in organic matter often have high C02 pressures and abundant organic acids, and are often warmer than soils that are not. Soils exposed to heavy rainfall often have high organic activity. Any, or all, could result in accelerated weathering. To separate the various effects, watershed studies have focused on denudation fluxes from multiple basins, differing primarily in the variables of interest (e.g., temperature or runoff) (Velbel, 1993; White and Blum, 1995). Weathering rates increase with temperature, ambient moisture, and organic activity, although the derived dependencies are somewhat approximate because no two basins are the same in a mineralogic, hydrologic, or biological sense. To resolve more precisely the controls on weathering, we looked directly at the silicate minerals, as opposed to the solutions in contact with them, and built a model based on measurements made at a single rnineralogically and hydratrlically similar field site. We digitally imaged weathering rates of plagioclase and olivine as a function of mean temperature, organic activity, and rainfall on a series of basalt flows in Hawaii. This allows us to gauge the role of temperature and 329