Chemical weathering processes in a tropical rain forest soil, Puerto Rico

¹Dept. of Geosciences, Penn State University, University Park, PA 16802, USA

²U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA

Abstract

Chemical weathering fluxes from the tropical Río Icacos watershed, Puerto Rico, have been shown to be several times greater than fluxes from temperate watersheds with similar bedrock lithology, annual rainfall, and topography. The parent material of these soils is quartz diorite, but biotite and quartz are the only primary minerals remaining in most of the saprolite because of the high intensity of weathering. Due to the absence of many primary minerals, saprolite pore waters are depleted in calcium and bicarbonate with respect to stream waters (McDowell and Asbury, 1994). Stoichiometric weathering of biotite to kaolinite is expected to release 0.16 moles of magnesium, 0.20 moles of potassium, and 0.53 moles of silica per mole of bicarbonate. Pore water chemistry and water flux data indicate production of these solutes in the relative amounts 0.2, 0.2, and 0.8 moles of magnesium, potassium, and silica per mole of bicarbonate, respectively. The observed rates of production of magnesium, potassium and bicarbonate ions can be explained by the weathering of biotite. However, the silica rate is too high to be entirely explained by this process, suggesting that silica is released from other sources. SEM images of quartz grains suggest dissolution. Other possible sources of silica include transformation of kaolinite to gibbsite, and reactions among amorphous aluminosilicates.

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