Chemical weathering and soil production 1 Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms (in press) DOI: 10.1002/esp Earth Surface Processes and Landforms Earth Surf. Process. Landforms (in press) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/esp.1443 Introduction Hillslope forms evolve over time as a result of total weathering, the sum of physical and chemical processes (e.g. Riebe et al., 2001, 2003a; Anderson et al., 2002; Stallard and Edmond, 1983). Total weathering is equivalent to erosion, the net removal of soil and other material from the hillslope, and results in landscape lowering in the absence of uplift. We consider physical weathering to be the mechanical processes that produce soil, defined here as the mobile layer of sediment being transported downslope. Chemical weathering – the dissolution and precipitation of minerals via subsurface flow – occurs along mineral grain boundaries and on mineral grains (White et al., 1999, 2001; Banfield and Eggleton, 1989), and results in a reduction of the coherence and potential strength of the bedrock (Kim and Park, 2003). The chemical weathering of the bedrock underlying a mobile soil mantle is thus critical to the soil production process (White et al., 1996; Paton et al., 1995; Mudd and Furbish, 2004) but has previously remained poorly quantified across soil-mantled landscapes. We focus here on landscapes where the mass balance of the mobile soil column depends on soil production and erosion. Such landscapes have been studied widely and can be qualitatively characterized as either weathering- or Coupling chemical weathering with soil production across soil-mantled landscapes Benjamin C. Burke, 1 , † Arjun M. Heimsath 1 * and Arthur F. White 2 1 Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, USA 2 Water Resources Division, US Geological Survey, Menlo Park, CA 94025, USA † Now at Exxon Mobil Exploration Co., 233 Benmar, Houston, TX77060, USA Abstract Soil-covered upland landscapes constitute a critical part of the habitable world. Our under- standing of how they evolve as a function of different climatic, tectonic and geological re- gimes is important across a wide range of disciplines and depends, in part, on understanding the links between chemical and physical weathering processes. Extensive previous work has shown that soil production rates decrease with increasing soil column thickness, but chemical weathering rates were not measured. Here we examine a granitic, soil-mantled hillslope at Point Reyes, California, where soil production rates were determined using in situ produced cosmogenic nuclides ( 10 Be and 26 Al), and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape. We collected saprolite samples from the base of soil pits and analysed them for abrasion pH as well as for major and trace elements by X-ray fluorescence spectroscopy, and for clay mineralogy by X-ray diffraction spectroscopy. Our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation. We also show that spatial variation in chemical weathering appears to be topographically controlled: weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale. Furthermore, to determine the best measure for the extent of saprolite weathering, we compared four different chemical weath- ering indices – the Vogt ratio, the chemical index of alteration (CIA), Parker’s index, and the silicon–aluminium ratio – with saprolite pH. Measurements of the CIA were the most closely correlated with saprolite pH, showing that weathering intensity decreases linearly with an increase in saprolite pH from 4·7 to almost 7. Data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale. Copyright © 2006 John Wiley & Sons, Ltd. Keywords: saprolite; pH; landscape evolution; weathering indices; erosion *Correspondence to: A. M. Heimsath, Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, USA. E-mail: arjun.heimsath@clartmouth.edu Received 4 January 2006; Revised 27 July 2006; Accepted 3 August 2006