Water Resources of the Caribbean
Climatic effects on chemical weathering in watersheds-- application of mass balance approaches
Arthur F. White and Alex E. Blum
U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, USA
Weathering rates of primary minerals in soils and regolith are of considerable interest in understanding the biogeochemistry of natural and perturbed watersheds. Besides the transport of some of the most abundant elements of the earth's surface, issues include the impacts of weathering on soil development, nutrient cycling and atmospheric acid deposition. Recently, interest has focused on the interrelationship between chemical weathering and global climate change (e.g. Volk, 1987; Berner, 1991, 1994; Brady, 1991). Silicate weathering is the most important long-term sink for protons in the lithosphere and can potentially buffer atmospheric concentrations of greenhouse gases such as CO2. Inorganic nutrients, such as base cations and phosphorus, are released by weathering in soils and impact fertility, and therefore productivity and the global cycling of organic carbon. Finally, soil weathering is important in regulating hydrologic processes such as evapotranspiration and runoff which effect the exchange of moisture between the atmosphere and hydrosphere.
The origin of the term "weathering" implies that chemical weathering is strongly affected by climate. The climatic controls on chemical weathering are related principally to moisture and temperature (Ollier, 1984). Moisture is influenced by the total amount, intensity and seasonality of precipitation, humidity, evapotranspiration, runoff and infiltration. Thermal effects include average air temperatures, seasonal temperature variations and thermal gradients in soils. Changes in these climatic parameters are expected to directly impact chemical weathering. Watersheds represent an important resource in discerning the interconnection between climate and chemical weathering. Watersheds are upstream inputs to larger continental and oceanic hydrochemical systems, but are less encumbered by complex and poorly constrained regional meteorologic, biological and geochemical processes. In climatic studies, watersheds can also be selected to minimize variables such as geology, geomorphology, agricultural impacts and regional atmospheric contamination, and to accentuate climatically controlled biogeochemical interactions. Watersheds can provide important information on chemical weathering under different climatic conditions existing today, as well as serving as a potential harbinger of climatic changes in the future.
The first part of this chapter will briefly review fundamental geochemical processes that both affect weathering rates and are potentially sensitive to climatic differences. Subsequent sections will review existing watershed weathering data in the context of the impacts of precipitation, evapotranspiration and temperature. These impacts can be investigated from three perspectives: (a) temporal chemical changes in a specific watershed related to historical climatic changes (b) comparison of soil weathering in different watersheds related to long-term climate differences and comparison of solute fluxes from different watersheds related to present climatic differences. The historical perspective is the most direct approach. However, as in many types of climate studies, the historical record is too short, and the magnitudes of the chemical changes are too small to directly correlate with any obvious systematic change in the climatic record of a specific watershed (Driscoll et al., 1989).
The present chapter will therefore focus on the last two approaches, i.e. correlating soil and solute chemistries in watersheds with different climates. Techniques used to make these comparisons involve chemical mass balances for soils and solutes. The chapter will utilize existing watershed data in this synthesis. Intercomparison of chemistry based on climatic differences is a relatively new approach in watershed studies and climate-specific data are often lacking. A benefit of the present review includes recognition of data needed to make more detailed climatic comparisons between watersheds.
White, A.F., and Blum, A.E., 1995, Climatic effects on chemical weathering in watersheds-- application of mass balance approaches in Trudgill, S.T., ed., Solute Modelling in Catchment Systems, John Wiley and Sons, Ltd., London, p. 102-131.