Chemical Weathering of Granitic Bedrock in Southeastern Puerto Rico: Influence of Precipitation

Weathering of bedrock shapes the Earths surface, produces soil, contributes dissolved ions to the hydrologic cycle and even influences the global climate. Chemical weathering is primarily controlled by temperature and moisture. Eastern Puerto Rico is home to a rainforest, and has been documented as having one of the highest rates of chemical weathering. As you travel west, however, precipitation, the source of moisture for weathering decreases from about 200 inches per year to only 50 inches per year. This creates a natural laboratory evaluate the influence of moisture variations on chemical weathering. To complete the study, historic data collected by the U.S. Geological Survey are used to determine weathering rates across the natural precipitation gradient. Results show stream water chemistry to be similar across the ten watersheds studied, plotting within the CaHCO3 hydrogeochemical facies area on Piper diagrams. Evaluation of bedrock weathering using major ion plots (Gailardet et al. 1999) shows a generally linear trend between dominantly silicate and dominantly carbonate end members. A few samples deviate from the trend toward dominantly evaporite weathering, which is likely the influence of the strongly NaCl dominated rain water chemistry. Loading rates are generally consistent across the region with a few exceptions. SiO2 loading, however, was found to range from 13.21 to 111.79 MT y-1 km-2. Silicate loading does appear to vary spatially with total precipitation, but some exceptions exist. Other major elements show less variability.