IN SITU MEASUREMENTS TO QUANTIFY PRESENT DAY SHALE WEATHERING RATES

Direct measurement of weathering rates is a difficult task though the data achieved from such measurements could be invaluable as we strive to understand soil production and erosion rates in the Critical Zone. To study weathering rates, a latitudinal transect of six sites was selected in the Appalachian Mountain region (New York, Virginia, Tennessee and Alabama), with end member sites in Puerto Rico and Wales. All of these locations share a similar shale bedrock parent material while the temperature and average rainfall at each site vary. Small shale chips measuring approximately two centimeters by two centimeters were cleaned and weighed, sewn into mesh bags and inserted at different depths into the wall of soil pits. After two years, the mesh bags were removed from the ground and the shale chips were reweighed to determine two-year mass loss. Subsequently, binocular microscopy was used to compare the before and after characteristics of the shale chips through comparison to photographs taken of each chip prior to burial. Based on observations of lost fragments and excessive mass loss values, 35% of the original buried chips were deemed unusable for our evaluation. The shale chips buried in Wales showed the lowest weathering rates around 10 mg per two years, which is similar to the 2 year mass loss value of 8.1 mg published by earlier researchers. Samples from Puerto Rico have at least two times more mass loss than those from Wales. The Puerto Rico values are also lower than those found for Malaysia by previous researchers, but our microscopic observations indicated soil particles remain bound to the shale chip edges after washing, thus our mass loss values for Puerto Rico are damped and could in fact be higher. The Appalachian Mountains sites display weathering rates that lie between our end member rates from Wales and Puerto Rico. Admittedly, variability in this data set is high. However, taken on the whole, an average weathering rate of 9.9 m Ma^-1 can be calculated from this data set, a number that is comparable to erosion rates calculated using cosmogenic radionuclide assays.