Abstract

A transect of study sites has been established in the Appalachian Mountains to investigate the rates of shale weathering as a function of climate. The transect includes sites in New York, Pennsylvania, Virginia, Tennessee and Alabama. All sites are underlain by Silurian-age, iron-rich, organic-poor Rose Hill Shale (or local equivalent), providing a similar lithology from which soils are forming. Climate varies along the transect, with a mean annual temperature (MAT) of 8.3 °C in New York increasing south to 16.7 °C in Alabama. Mean annual precipitation (MAP) ranges from 100 cm in New York to 133 cm in Alabama. Previous studies at these sites have focused on characterizing ridgetop locations, or 1-D sites, where water enters the soil surface and proceeds vertically to bedrock. 2-D sites, or soils forming on slopes, represent the part of the landscape where water moves both vertically and laterally down the slope. A recent effort to characterize 2-D sites at each transect site was undertaken by students participating in a National Science Foundation Research Experiences for Undergraduates (REU) program. The students represented seven universities affiliated with sites along the transect.

To characterize 2-D soils, sampling sites were first identified on a convex slope located below the 1-D ridge top sampling sites. Soil pits were dug as deep as possible and sampled by depth and horizon. Additionally, ground penetrating radar (GPR) surveys were conducted on transects between the ridgetop and 2-D pits. In New York, the parent material consists of locally derived shale till and the augerable depth of the 2-D soil pit is 230 cm. A 2-D pit was not sampled in Virginia because the Rose Hill Shale Formation in this location is very thin; the narrow ridge is bounded on both sides by sandstone beds. In Tennessee, shallow soil pits (approximately 75 cm deep) were observed on the slope compared to the 398 cm depth measured at the ridgetop site. In Alabama, augerable soil depth on the slope is 175 cm deep compared to 220 cm deep at the ridgetop. All 2-D soil profiles have rock fragments throughout, indicating colluvial transport of material from farther upslope. Soils are brown and lack distinct horizons in
New York and are increasingly redder in Virginia and Alabama. Tennessee soils are yellow-brown, suggesting the presence of goethite. Redoximorphic features are present in all 2-D soil pits and are most strongly expressed in the Tennessee soil profile.

The 2-D soil profile in NY is 20% depleted of K at the soil surface and Na is 5% depleted, suggesting minimal weathering at this site. In Tennessee, Mg, Na and K are all approximately 50% depleted at the surface, indicative of a more highly weathered soil. Alabama geochemical profiles show depletion-addition trends for several elements, including Al, Fe, K, and Mg, suggesting these elements have been weathered from the surface and reprecipitated at depth. In general, 2-D soils are shallower and less weathered than 1-D soil profiles, and 2-D sites are more weathered moving from north to south along the transect, as evidenced by geochemical profiles. This study of 2-D weathering will help understand the rate at which shale weathers on slopes in various climates, information that could be used to predict the impact of climate change on soil production.

Citation

Leidel, L, Dere, A.L., White, T. (2011): Shale weathering on slopes across a latitudinal climosequence. AGU Annual Fall Conference Proceedings.

This Paper/Book acknowledges NSF CZO grant support.