RIVERINE GEOCHEMISTRY OF PANAMA

The humid tropics are an important contributor to global chemical erosion, but few denudation studies have been conducted for tropical watersheds. Panama comprises a terrane of Upper Cretaceous–Late Tertiary marine sedimentary rocks and volcanic/plutonic igneous rocks. Panama’s climate is typical of the low-latitude humid tropics, characterized by near-uniform average temperature, abundant rainfall, and a distinct seasonality of precipitation. Over 58-months from Feb. 2005 to Nov. 2009, 837 samples from 29 of Panama’s 52 hydrographic basins were collected across an ~500 km transect between 7.2°-9.6° N and 77.2°- 83.0° W, from Lago Bayano to the Costa Rican border. T, SPC, TDS, and pH were measured in the field for 114 named rivers, 96 small rivers and streams, and 17 soil seeps, with individual sites sampled between 1-11 times. Subsequently, major and minor element compositions (Si, Na, K, Ca, Mg, Li, Sr, Ba, F, Br, Cl, SO₄, NO₃, PO₄) were measured in the laboratory at OSU. Respective ranges, mean values, and coefficients of variation for T, TDS, and pH are: 12.7-34.6 (25.1±2.6, 10.4%); 6.0-1901 (144±156, 108%); and 5.05-8.72 (7.46­±0.59, 7.9%), with the greatest variation observed in small streams draining sedimentary terranes. Observed average dissolved contents (in mgL^-1) are: Si = 15.8±6.3, Ca²⁺ = 11.2±8.6, Mg²⁺ = 4.7±3.9, Na⁺ = 9.7±15.9, K⁺ = 1.2±2.9, Cl^- = 8.2±23.4, NO₃^- = 0.38±1.35, & SO₄^2- = 5.9±12.3. Overall, dissolved solute contents of rivers and streams across Panama are low suggesting a short residence time in the weathering zone for infiltrating precipitation. Compositional variability reflects both the nature of the lithology from which drainage is occurring and time since the last strong precipitation event. Respective long-term silicate weathering and CO₂ consumption rates range from 1.5 t/km²/y and 133x10³ mol/km²/y to nearly 50 t/km²/y and 3000 mol/km²/y, are toward the high end of the ranges reported in the literature.