T165 Critical Zone Evolution: Climate and Exhumation

Monday, 28 October 2013
1:00 PM - 5:00 PM
Mile High Ballroom 2C

MINERAL WEATHERING, ELEMENTAL TRANSPORT, AND PRODUCTION OF MINERAL SURFACE AREA IN A SCHIST BEDROCK WEATHERING PROFILE, PIEDMONT PENNSYLVANIA

Paper number: T165-4
Program: Technical Sessions
Time: 1:50 PM – 2:05 PM

Authors

Abstract

We assess a deep chemical weathering profile in the Laurels Schist, a late proterozoic greenschist formation in the Christina River Basin Critical Zone Observatory located in the Piedmont region in southeastern Pennsylvania. Samples were taken from two 21-meter rotosonic drill cores at the ridge top and footslope positions in a first-order, forested watershed. The top meter was sampled at high-resolution in a soil pit adjacent to each drill core. We assess weathering processes in soil and saprolite by observing changes in mineralogy, including the emergence of secondary phyllosilicate and oxide minerals; measuring specific surface area of bulk soil and saprolite; and by quantifying elemental mass changes of major and minor rock-forming elements. Specific surface area (SSA) values decrease with increasing depth to a critical depth around 2 meters, where values of untreated (carbon-loaded) and muffled (carbon removed by heating) grains converge to baseline SSA values below 10 m2g-1. Immobile element concentrations decrease with increasing depth up to 3 meters, indicating the preferential removal of mobile elements. Variability of immobile elements in the deep weathering profile reveal variations that could be from extreme weathering in fractures but are more likely inherited from pre-metamorphosed sedimentary rock composition and particle size.