PHYSICAL CONTROLS AND ENSO EVENT INFLUENCE ON CHEMICAL WEATHERING ACROSS THE PANAMA CANAL WATERSHED

Previous empirical studies using temporally-limited datasets have noted the importance of tropical small mountainous rivers on silicate weathering processes. A lack of long-term datasets has limited these studies from incorporating regional climate patterns that may affect weathering trends, such as El Niño Southern Oscillation (ENSO). In this work, we used an 18-year high-temporal frequency hydrochemical dataset collected from 1998-2015 by the Panama Canal Authority to analyze annual and decadal weathering in eight sub-catchments of the Panama Canal Watershed (PCW). Here, the largely forested, steeply-sloping sub-basins on the north side of the canal developed on igneous rocks — Río Gatún, Río Boquerón, Río Pequeni, Río Chagres, and Río Indio Este, differ markedly from their mostly deforested, gently-sloping counterparts to the south developed on marine sedimentary lithologies — Río Cano Quebrado, Río Trinidad, and Río Cirí Grande. Relationships among subcatchment physical characteristics and regional climate were analyzed to identify dominant controls on chemical weathering. We observe robust positive covariation among precipitation, forest cover, discharge, cation yields (Ca²⁺, Mg²⁺, Na⁺, K⁺), and suspended sediment yields. Covariance between cation and suspended sediment yields indicate a strong relationship between physical and chemical weathering. However, individual sub-basin trends suggest that land cover determines the strength between the physical and chemical weathering relationship. We also modeled river discharge, cation flux, and sediment flux as functions of seasonality and ENSO conditions to discern the influence of regional climate patterns on long-term weathering. Results demonstrate statistically significant relationships of discharge and cation flux, and near significance on suspended sediment flux with seasonality and ENSO conditions. Further investigation reveals significant differences among La Niña, El Niño and neutral discharge and weathering fluxes. We also observe that La Niña events had a predominant impact over seasonality, whereas El Niño events did not. Our findings demonstrate the physical controls on weathering processes and highlight the importance of regional-climate patterns on long-term weathering.