Chemical weathering of silicate minerals in a mountainous, tropical rain forest, Puerto Rico

¹U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA

²U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Dr ive, Guaynabo, Puerto Rico, 00965-5703, USA

Abstract

Soils are developed on granitic bedrock and range in depth from 3 to 8 meters. The upper meter of soil is unstructured, but is saprolitic below 1 meter with well preserved igneous structures and a porosity of 50 percent. Gravimetric water contents and bulk density data indicate that soils remain ~80% saturated with matric potentials <25 centibars. Mineralogy is predominantly kaolinite and quartz, with minor biotite below several meters. The concentrations of Na, Sr, Ca, Cl and S0₄ in soil water throughout the saprolite profile are much more dilute than observed in steam flow and are dominated by precipitation. This is supported by ⁸⁷Sr ⁸⁶Sr ratios of 0.710 that are nearly ide ntical to sea water and more radiogenic than ratios of 0.705 for both unreacted granitic and surface water. Si increases with depth from .05 mmol/L at 1 m to 2 mmol/L at 8 m along with corresponding increases in K and Mg. These relatively high concentrations are indicative of dissolution of kaolinite in the upper soil zone and alteration of biotite in the deeper saprolite. Weathering of feldspars occur s at depth within a narrow interface between the fresh granite and the overlying saprolite. Mass balance calculations of soil chemical and volume changes based upon Zr, Ti and Nb indicate <10% volume decrease, consistent with the saprolite textures. Na, Ca and Sr are nearly completely depleted in the soil, 50 percent of the Si has been lost, and 30 percent of the Al and Fe. These results Sr, Ca, Cl and S0₄ in soil water throughout the saprolite profile are much more dilute than observed in steam flow and are dominated by precipitation. This is supported by ⁸⁷Sr ⁸⁶Sr ratios of 0.710 that are nearly ide ntical to sea water and more radiogenic than ratios of 0.705 for both unreacted granitic and surface water. Si increases with depth from .05 mmol/L at 1 m to 2 mmol/L at 8 m along with corresponding increases in K and Mg. These relatively high concentrations are indicative of dissolution of kaolinite in the upper soil zone and alteration of biotite in the deeper saprolite. Weathering of feldspars occur s at depth within a narrow interface between the fresh granite and the overlying saprolite. Mass balance calculations of soil chemical and volume changes based upon Zr, Ti and Nb indicate <10% volume decrease, consistent with the saprolite textures. Na, Ca and Sr are nearly completely depleted in the soil, 50 percent of the Si has been lost, and 30 percent of the Al and Fe. These results demonstrate a much more intensive weathering regime than associated with temperate zone saprolites and have important ramifications for climate change scenarios.

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