INGERSON LECTURE: RIVERINE GEOCHEMISTRY AND CHEMICAL WEATHERING ACROSS PANAMA

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 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 during both wet and dry seasons during 2006-2009, 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. Temperature, conductivity, pH, and dissolved oxygen were measured in the field at the time of sample collection. Dissolved constituents F, Br, Cl, NO₃, SO₄, PO₄, Si, Ca, Mg, Na, K, Li, Sr, & Ba were subsequently measured in the laboratory by ion chromatography and ICP-OES spectrometry. Respective compositional variations are large, e.g. SPC = 38-2924 μS/cm, Si = 0.005-3.02 mM & Ca/Mg = 0.68-5.32 for large streams and rivers compared to SPC = 32-1736 μS/cm, Si = 0.011-1.601 mM, & Ca/Mg = 0.66-33.7 for soil seeps and small streams. 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. This feature and the large degree of compositional overlap between large rivers and small tributary streams implies that chemical weathering of silicate materials in the soil zone is the predominant process determining the geochemistry of streams and rivers in this tropical environment. 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.