SILICATE WEATHERING AND CO2 CONSUMPTION IN A MOUNTAINOUS TROPICAL SETTING: INSIGHTS FROM THE ISTHMUS OF PANAMA

Although they represent only a small percentage of the Earth’s surface, small mountainous rivers (SMRs) have been shown to deliver a disproportionate 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. The Isthmus of Panama, which comprises a variety of silicate magmatic arc lithologies, a large spatial variation in rainfall, and an array of physical erosion rates, offers an ideal location to evaluate the controls on silicate weathering rates in the tropics. A multiyear data set of stream and river chemistry from 71 watersheds across a ~450-km transect in west-central Panama, provides insight into controls on chemical weathering in tropical SMRs. Central Panama water compositions are generally dilute, with a mean TDSc value of 118±91 with bicarbonate and silica the predominant dissolved species. Solute contents and stable isotope compositions are consistent with dissolution of igneous rocks present across the Panama arc by meteoric precipitation, with geochemical signatures largely acquired in river headwater regions. Cation weathering yields (Casil+ Mgsil + Na + K as tons/km²/y) span about an order in magnitude, from 3 to 32 tons/km²/y. CO₂ consumption rates range from 166 to 1157 x10³ mol/km²/y, falling towards the upper end of the global range, and are consistent with higher CO₂ consumption rates observed for mafic to intermediate igneous terrains. Strong positive correlations of chemical weathering fluxes and CO₂ consumption are observed with precipitation, mean watershed elevation, extent of land surface forest cover, and physical erosion rate and confirm previous findings on the importance of runoff and mechanical denudation in maintaining elevated weathering rates on small mountainous rivers.