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Weathering Processes in the Icacos and Mameyes Watersheds in Eastern Puerto Rico

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Weathering Processes in the Icacos and Mameyes Watersheds in Eastern Puerto Rico Weathering Processes in the Icacos and Mameyes Watersheds in Eastern Puerto Rico By Heather L. Buss and Arthur F. White Chapter I of Water Quality and Landscape Processes of Four Watersheds in Eastern Puerto Rico Edited by Sheila F. Murphy and Robert F. Stallard Professional Paper 1789–I U.S. Department of the Interior U.S. Geological Survey Contents Abstract .......................................................................................................................................................253 Introduction.................................................................................................................................................253 Weathering Processes at the Bedrock Interface ................................................................................256 Weathering Processes in the Saprolite and Soil .................................................................................260 Summary......................................................................................................................................................260 References ..................................................................................................................................................260 Figures 1. Map showing location and geology of Icacos and Mameyes watersheds ....................255 2. Photograph showing a corestone of Río Blanco quartz diorite that is weathering spheroidally ..........................................................................................................256 3. Schematic diagram of the weathering profile in the Icacos watershed .........................257 4. Backscattered electron images of thin sections taken from a corestone-rindlet complex, Icacos watershed ....................................................................................................258 Abbreviations Used in This Report ≈ nearly equal to 30Si ratio of stable isotopes of silicon, 30Si/28Si, relative to a standard Å angstrom μmol mol–1 micromoles per mole cm centimeter km2 square kilometer m meter Ma million years old m m.y.–1 meters per million years mol m–2 s–1 moles per square meter per second ICP–OES inductively coupled plasma–atomic emission spectroscopy WEBB Water, Energy, and Biogeochemical Budgets XRD X-ray diffraction Conversion Factors Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) meter (m) 3.281 foot (ft) Area square kilometer (km2) 0.3861 square mile (mi2) Weathering rate meters per million years (m m.y.−1) 3.281 feet per million years (ft m.y.−1) Weathering Processes in the Icacos and Mameyes Watersheds in Eastern Puerto Rico By Heather L. Buss and Arthur F. White Abstract in places that contains highly fractured rock embedded in its matrix. Weathering contributions to stream chemistry at base Streams draining watersheds of the two dominant litholo- flow are predicted to be more spatially variable in the Ma- gies (quartz diorite and volcaniclastic rock) in the Luquillo meyes watershed than in the Icacos watershed owing to the Experimental Forest of eastern Puerto Rico have very high more complex subsurface weathering profile of the volcanicla- fluxes of bedrock weathering products. The Río Blanco quartz stic bedrocks of the Mameyes watershed. diorite in the Icacos watershed and the Fajardo volcaniclastic rocks in the Mameyes watershed have some of the fastest documented rates of chemical weathering of siliceous rocks Introduction in the world. Rapid weathering produces thick, highly leached saprolites in both watersheds that lie just below the soil and Biogeochemical and physical weathering processes in trop- largely isolate subsurface biogeochemical and hydrologic ical watersheds produce most of the solutes and sediments dis- processes from those in the soil. The quartz diorite bedrock charged to the oceans. These watersheds contribute 50 percent in the Icacos watershed weathers spheroidally, leaving large, of the water, 38 percent of the dissolved ions, and 65 percent of relatively unweathered corestones that are enveloped by the dissolved silica in the oceans, despite covering only about slightly weathered rock layers called rindlets. The rindlets 25 percent of the terrestrial surface (Stallard and Edmond, 1983; wrap around the corestones like an onionskin. Meybeck, 1987). Thus, relative to their area, tropical systems Within the corestones, biotite oxidation is thought to are disproportionately important in terms of weathering, ero- induce the spheroidal fracturing that leads to development sion, and global CO2 cycles. In addition to influencing stream of rindlets; plagioclase in the rindlets dissolves, creating and ocean chemistry and sediment loads, weathering processes additional pore spaces. Near the rindlet-saprolite interface, exert control over chemical transport through soils and regolith the remaining plagioclase dissolves, hornblende dissolves to (such as saprolite), soil and saprolite formation rates, mineral completion, and precipitation of kaolinite, gibbsite, and goe- nutrient availability, and microbial growth rates. Weathering thite becomes pervasive. In the saprolite, biotite weathers to processes are complex phenomena comprising coupled chemi- kaolinite and quartz begins to dissolve. In the soil layer, both cal, physical, and microbial processes, driven by water and quartz and kaolinite dissolve. The volcaniclastic bedrock of oxygen fluxes. These coupled processes also control the shape, the Mameyes watershed weathers even faster than the quartz position, stability, and rate of retreat of the bedrock weathering diorite bedrock of the Icacos watershed, leaving thicker sapro- interface, which in turn affect topography and erosion. lites that are devoid of all primary minerals except quartz. The To date (2012), the majority of research into weathering quartz content of volcaniclastic bedrock may help to control processes in the four U.S. Geological Survey Water, Energy, watershed geomorphology; high-quartz rocks form thick sap- and Biogeochemical Budgets program (WEBB) watersheds rolites that blanket ridges. in eastern Puerto Rico has focused on the Icacos water- Hydrologic flow paths within the weathering profiles shed. This watershed is an ideal site for studying tropical vary with total fluid flux, and they influence the chemistry of weathering processes, because it is relatively pristine, small streams. Under low-flow conditions, the Río Icacos and its (3.3 square kilometers (km2)), and lithologically homoge- tributaries are fed by rainfall and by groundwater from the neous (99 percent Río Blanco quartz diorite; Murphy and fracture zones; during storm events, intense rainfall rapidly others, 2012). Thick saprolite profiles on ridge tops allow raises stream levels and water is flushed through the soil as analysis of deep weathering and the cycling of chemical shallow flow. As a result, weathering constituents that shed elements—processes that are not directly coupled with the into streamwaters are dominated by rindlet-zone weathering surficial environment. The homogeneity of the quartz diorite processes during base flow and by soil weathering processes (relative to the volcaniclastic units that dominate the Mam- during stormflow. The upper reaches of the Mameyes water- eyes watershed) also considerably simplifies interpretation shed are characterized by regolith more than 35 meters thick of the weathering profiles. Currently, a subwatershed of the 254 Water Quality and Landscape Processes of Four Watersheds in Eastern Puerto Rico Mameyes watershed, Bisley 1 (fig. 1), is being investigated elements such as Ca, Na, Mg, and K are lost from the system, and compared with the Icacos watershed in order to better quartz makes up a larger proportion of the remaining mass understand the influence of lithology on weathering pro- than it did originally. Similarly, elements that are relatively cesses, weathering fluxes, and mineral nutrient availability. immobile during chemical weathering, such as Ti and Zr, are Weathering processes within the Bisley 1 watershed are also more abundant in saprolite relative to parent rock in both spatially more variable than in the Icacos watershed owing the Bisley watershed (bulk chemistry measured by inductively to the more varied lithology of the parent rock. The Bisley 1 coupled plasma–atomic emission spectroscopy (ICP–OES) watershed is underlain by the Fajardo Formation, a marine- after lithium metaborate fusion) and Icacos watershed (White bedded volcaniclastic bedrock that is mapped as having and others, 1998; Buss and others, 2005, 2008). several units: an upper thin-bedded tuffaceous siltstone and The Fajardo Formation, which underlies the Bisley water- sandstone, an upper thick-bedded tuff, a lower thin-bedded shed, is older than the granitic intrusion underlying the Icacos tuffaceous sandstone and siltstone, a lower thick-bedded watershed, ≈100 million years old (Ma) as compared with tuff, and an undivided unit (Briggs and Aguilar-Cortés, 47 Ma, respectively (Jolly and others, 1998; Smith and others, 1980). Bed thicknesses differ, and coarse tuff, breccias, and 1998). Regolith in the Bisley 1 watershed is also thicker than cherty or calcareous siltstone beds are found in some units. in the Icacos watershed: about 10–20 m and 5–9 m on ridge Although the few bedrock exposures in the Bisley watershed tops in the Bisley and Icacos watersheds, respectively (White appear homogeneous, the watershed is underlain by the upper and others, 1998; Schellekens and others, 2004; Buss and oth-
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