CHEMICAL-WEATHERING IN A TROPICAL WATERSHED, LUQUILLO MOUNTAINS, PUERTO-RICO - I - LONG-TERM VERSUS SHORT-TERM WEATHERING FLUXES

The pristine Rio Icacos watershed in the Luquillo Mountains in eastern Puerto Rico has the fastest documented weathering rate of silicate ro cks on the Earth's surface. A regolith propagation rate of 58 m Ma(-1) , calculated from iso-volumetric saprolite formation from quartz diori te, is comparable to the estimated denudation rate (25-50 Ma(-1)) but is an order of magnitude faster than the global average weathering rat e (6 Ma(-1)). Weathering occurs in two distinct environments; plagiocl ase and hornblende react at the saprock interface and biotite and quar tz weather in the overlying thick saprolitic regolith. These environme nts produce distinctly different water chemistries, with K, Mg, and Si increasing linearly with depth in saprolite porewaters and with strea m waters dominated by Ca, Na, and Si. Such differences are atypical of less intense weathering in temperate watersheds. Porewater chemistry in the shallow regolith is controlled by closed-system recycling of in organic nutrients such as K. Long-term elemental fluxes through the re golith (e.g., Si = 1.7 x 10(-8)moles m(-2) s(-1)) are calculated from mass losses based on changes in porosity and chemistry between the reg olith and bedrock and from the age of the regolith surface (200 Ma). M ass losses attributed to solute fluxes are determined using a step-wis e infiltration model which calculates mineral inputs to the shallow an d deep saprolite porewaters and to stream water. Pressure heads decrea se with depth in the shallow regolith (-2.03 m H2O m(-1)), indicating that both increasing capillary tension and graviometric potential cont rol porewater;infiltration. Interpolation of experimental hydraulic co nductivities produces an infiltration rate of 1 m yr(-1) at average fi eld moisture saturation which is comparable with LiBr tracer tests and with base discharge from the watershed. Short term weathering fluxes calculated from solute chemistries and infiltration rates (e.g., Si = 1.4 x 10(-8) moles m(-2) s(-1)) are compared to watershed flux rates ( e.g., Si = 2.7 x 10(-8) moles m(-2) s(-1)). Consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation, temperature, and vegetation over the last s everal hundred thousand years have not significantly impacted weatheri ng rates in the Luquillo Mountains of Puerto Rico. This has important ramifications for tropical environments and global climate change. Cop yright (C) 1998 Elsevier Science Ltd.