For permission to copy, contact editing@geosociety.org  2002 Geological Society of America 1143 GSA Bulletin; September 2002; v. 114; no. 9; p. 1143–1158; 9 figures; 3 tables. Weathering profiles, mass-balance analysis, and rates of solute loss: Linkages between weathering and erosion in a small, steep catchment Suzanne Prestrud Anderson* Center for Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064-1077, USA William E. Dietrich George H Brimhall Jr. Department of Earth and Planetary Sciences, University of California, Berkeley, California 94720-4767, USA ABSTRACT In a headwater catchment in the Oregon Coast Range, we find that solid-phase mass losses due to chemical weathering are equivalent in the bedrock and the soil. However, the long-term rate of mass loss per unit volume of parent rock is greater in the soil than in the rock. We attribute this finding to the effects of biotic processes in the soil and to hydrologic conditions that maximize contact time and water flux through the mineral matrix in the soil. This result stems both from earlier work in which we demonstrated that rock and soil contribute equally to the solute flux and from arguments presented here that the ba- sin is in dynamic equilibrium with respect to erosion and uplift. The silica flux of 10.7  7.1 t·km 2 ·yr 1 from the basin is several times larger than the flux from older soils elsewhere, but comparable to the flux from sites with similar physical erosion rates. This result argues that physical denudation or uplift rates play an important role in set- ting the chemical denudation rate. Physical processes appear to influence chemical- weathering rates in several ways. First, they limit chemical evolution by removing ma- terial, thus setting the residence time within the weathered rock and the soil. Second, bioturbation mixes rock fragments into the more reactive soil and maintains high soil porosity, allowing free circulation of water. Because the weathering in the soil is more intense than in the rock, we argue that the chemical denudation rate will diminish *E-mail: spa@earthsci.ucsc.edu. where uplift rates—and, hence, physical- denudation rates—are great enough to lead to a bedrock-dominated landscape. Chem- ical denudation rates will increase with physical-denudation rates, but only as long as the landscape remains mantled by soil. Keywords: chemical erosion, denudation, physical weathering, soil dynamics, uplift, weathering. INTRODUCTION The weathered profile, consisting of layers of weathered rock topped by soil, develops in response to chemical, physical, and biological processes at the Earth’s surface. The strong interplay among these processes makes it dif- ficult to disentangle their interactions in the weathered profile. Physical weathering pro- cesses expose fresh rock and mineral surfaces to chemical weathering, whereas chemical weathering reduces the strength of rock, mak- ing it more susceptible to physical breakdown. The interactions between these processes are widely recognized as critical in understanding the effects of changes in climate and of tec- tonic uplift rates on erosion (Raymo et al., 1988), landscape evolution (Anderson and Humphrey, 1989), and geochemical cycling (Gaillardet et al., 1999a). The balance between removal of debris by transport processes and the breakdown of rock into movable material by weathering exerts a strong control on landscape evolution. G.K. Gilbert (1877) described the linkage between regolith production and erosion rates, now codified by geomorphologists who differenti- ate conceptually between weathering-limited and transport-limited landscapes (Carson and Kirkby, 1972). In the former, the rate of for- mation of erodible debris by physical and chemical processes controls the rate of land- scape lowering. Landscapes dominated by bare bedrock are universally recognized as weathering-limited. In contrast, landscapes with deep regolith mantles are transport- limited. The rate of landscape lowering is lim- ited by the efficiency of transport processes, and the weathered profile thickens through time. Stallard (1985) demonstrated that these concepts could be used to explain the evolu- tion of chemical loads of large rivers. Recent empirical advances (Heimsath et al., 1997, 1999, 2000, 2001a) support the concept that regolith-production rates decline under in- creasing soil cover. In an alternative model, regolith production reaches a maximum under a particular soil depth and is reduced under both shallower and deeper soil cover (Ahnert, 1967; Carson and Kirkby, 1972; Stallard, 1985; Anderson and Humphrey, 1989; Rosen- bloom and Anderson, 1994; Small et al., 1999). Mechanistic justification for either type of production rule is usually couched in terms of the effects of increasing soil depth on chemical and/or thermomechanical (Ander- son, 1999) processes. Understanding the interplay between phys- ical and chemical weathering processes and their relationship to erosion rates has become increasingly important in understanding geo- chemical cycles (Stallard, 1992, 1995b). The potential feedbacks between erosion and chemical weathering were brought to promi- nence by the erosion-driven climate change hypothesis (Raymo et al., 1988; Raymo and Ruddiman, 1992). Given that global silicate-