SURFACE WATER GEOCHEMISTRY, CHEMICAL WEATHERING, AND CO2 CONSUMPTION ACROSS PANAMA

Although they represent only a small percentage of the Earth’s surface, small mountainous rivers (SMRs) have been shown to deliver a significant amount of dissolved solutes to the global ocean. However, we are only beginning to understand the various factors that both contribute to and sustain these yields and there is little information available for the tropics. The Isthmus of Panama, which comprises a variety of silicate lithologies ranging from Late Cretaceous to Holocene age, a large spatial variation in rainfall, and an array of physical erosion rates is an ideal location to evaluate the controls on silicate weathering rates in the tropics. Here we utilize a multi-year dataset of large spatial extent to evaluate first order controls on chemical weathering in tropical SMRs. Watershed lithology was found to exert the main control on riverine chemistry with streams on marine sedimentary rocks having higher dissolved solids loads than those on igneous rocks, with the latter exhibiting the highest silica contents and increasing trends of total cations with increasing dissolved silica. Cation weathering yields (Ca_sil+Mg_sil+Na+K; corrected for sea salt contribution) range over more than an order in magnitude from 2.6 to 22.1 tons/km²/y, whilst silicate weathering yields (Ca_sil+Mg_sil+Na+K+Si) range from 6.9 to 56 tons/km²/y. Basin-wide CO₂ consumption yields by silicate weathering, calculated from total cation content, basin area and discharge, are high on a global basis. Despite regional differences in relative age and chemistry of volcanic lithology, statistical differences in yield were not observed. Finally, comparisons with showed positive strong positive correlations with mean annual runoff and weaker positive correlations with long-term physical erosion rates, annual mean/maximum precipitation, and percent forest /grassland land cover.