Abstract

Regolith, the mantle of physically, chemically, and biologically altered material overlying parent material, covers much of Earth’s continents but the rates and mechanisms of regolith formation are not well known. In an effort to quantify the influence of climate on shale weathering rates, a transect of study sites has been established on Silurian shales along a climatic gradient in the Northern Hemisphere as part of the Susquehanna Shale Hills Critical Zone Observatory, PA, USA. The climate gradient is bounded by a cold/wet end member in Wales and a warm/wet end member in Puerto Rico; in between, temperature and rainfall increase to the south through New York, Pennsylvania, Virginia, Tennessee and Alabama. Soil depth and geochemisty were measured both on ridgetop positions (where water flow is largely vertical through the soil profile) and on slope positions (where water moves both vertically and laterally downslope). The depth of ridgetop soils increases from north to south along the climate gradient, with shallow soils (~ 30 cm) in Wales and Pennsylvania and increasingly deep soils to the south (394 cm in Tennessee). Similarly, the extent of chemical depletion in ridgetop surface soils increases from <20% elemental depletion in the north to 100% depletion of some elements in the south. Soil depths on slopes, however, do not vary much along the transect and range from 52–86 cm across the Appalachian sites. In these slope soils the extent of weathering is similar across the Appalachian study sites; Mg, K and Al, for example, are ~40% depleted at the soil surface at all of the study sites. One working hypothesis based on limited quantitiative assessments of regolith age in the Appalachians suggests that ridgetop and slope soils may be more or less the same age. In this model, the increasing ridgetop soil thickness along the transect is a function of warmer and wetter climate to the south. In slope positions, however, where downslope water flow leads to decreased fluid residence time, thinner and less weathered soils are observed. Alternately, southern soils are much older than northern soils due to impacts of the Last Glacial Maximum and similar erosion rates across the study region have left an eroded mantle of uniform thickness on top of bedrock slopes.

Citation

A. L. Dere, T. S. White, L. E. Leidel AND S. L. Brantley (2013): Differences in shale weathering on ridgetops and slopes along a latitudinal climosequence. Mineralogical Magazine 77:976.

This Paper/Book acknowledges NSF CZO grant support.