Presented on Wednesday, Oct 21st at 4:20pm at the 2009 GSA Meeting in Portland, OR
Oregon Convention Center, Portland Ballroom 252
LYONS, W. Berry1, HARMON, Russell S.2,3, GOLDSMITH, Steven T.1, CAREY, Anne E. 1, WELCH, Kathleen A.1, WELCH, Susan A.1, MCELWEE, Gregg T.1, MITASOVA, Helena2, and LONG, David T.4
1. School of Earth Sciences, The Ohio State University, Columbus, OH 43210
2. ARL Army Research Office, PO Box 12211, Research Triangle Park, NC 27709 russell.harmon@us.army.mil
3. Department of Marine, Earth, & Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695
4. Department of Geological Sciences, Michigan State University, East Lansing, MI 48824
Chemical weathering of geological materials is a primary process affecting landscape development, but few denudation studies have been conducted for watersheds in the humid tropics. A hydrochemical study of 97 named rivers and numerous small streams, conducted during both wet and dry seasons during 2006-2009, has been undertaken along a ~400 km transect across Panama from the Bayano region in the east to the Costa Rica border in the west, with most rivers sampled multiple times. The generalized bedrock geology consists of magmatic rocks of Late Cretaceous-Miocene age, Mesozoic marine sediments, and Pleistocene volcanics. Dissolved constituents Cl, NO3, SO4, PO4, Si, Ca, Mg, Na, K and dissolved organic carbon (DOC) were subsequently measured. Respective compositional variations are large, e.g. SPC= 38-2924 mS/cm, Si=0.005-3.02 mM, and Ca/Mg=0.68-5.32 for large streams and rivers. Watershed lithology exerts the major control on riverine chemistry. Streams on marine sedimentary rocks have higher contents of total dissolved solids. Streams and rivers on magmatic rocks exhibit the highest silica contents and increasing trends of total cation content with increasing dissolved silica. DOC concentrations varied greatly from river to river but the highest concentrations were in rivers draining marine sedimentary rocks. DOC yields are similar to or slightly lower than those previously determined for other tropical montane systems. The highest silicate weathering rates are in watersheds with young volcanic lithologies. Basin-wide CO2 consumption yields by silicate weathering, calculated from total cation content (corrected for sea salt contribution), basin area and discharge indicate, are high on a global basis.