Davison V. Vivit

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

Abstract The pristine Rio Icacos watershed in the Luquillo Mountains in eastern Puerto Rico has the fastest documented weathering rate of silicate rocks on the Earth’s surface. A regolith propagation rate of 58 m Ma−1, calculated from iso-volumetric saprolite formation from quartz diorite, is comparable to the estimated denudation rate (25–50 Ma−1) but is an order of magnitude faster than the global average weathering rate (6 Ma−1). Weathering occurs in two distinct environments; plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith. These environments produce distinctly different water chemistries, with K, Mg, and Si increasing linearly with depth in saprolite porewaters and with stream 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 inorganic nutrients such as K. Long-term elemental fluxes through the regolith (e.g., Si = 1.7 × 10−8 moles m−2 s−1) are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface (200 Ma). Mass losses attributed to solute fluxes are determined using a step-wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water. Pressure heads decrease with depth in the shallow regolith (−2.03 m H2O m−1), indicating that both increasing capillary tension and graviometric potential control porewater infiltration. Interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr−1 at average field 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 × 10−8 moles m−2 s−1) are compared to watershed flux rates (e.g., Si = 2.7 × 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 several hundred thousand years have not significantly impacted weathering rates in the Luquillo Mountains of Puerto Rico. This has important ramifications for tropical environments and global climate change.

The role of disseminated calcite in the chemical weathering of granitoid rocks

Accessory calcite, present at concentrations between 300 and 3000 mg kg−1, occurs in fresh granitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and as replacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized water saturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by Ca and bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during this time and accounted for 57–98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously removed by natural weathering. The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces.

The effect of temperature on experimental and natural chemical weathering rates of granitoid rocks

The effects of climatic temperature variations (5–35°C) on chemical weathering are investigated both experimentally using flow-through columns containing fresh and weathered granitoid rocks and for natural granitoid weathering in watersheds based on annual solute discharge. Although experimental Na and Si effluent concentrations are significantly higher in the fresh relative to the weathered granitoids, the proportional increases in concentration with increasing temperature are similar. Si and Na exhibit comparable average apparent activation energies (Ea) of 56 and 61 kJ/mol, respectively, which are similar to those reported for experimental feldspar dissolution measured over larger temperature ranges. A coupled temperature–precipitation model, using an expanded database for solute discharge fluxes from a global distribution of 86 granitoid watersheds, produces an apparent activation energy for Si (51 kJ/mol), which is also comparable to those derived from the experimental study. This correlation reinforces evidence that temperature does significantly impact natural silicate weathering rates. Effluent K concentrations in the column study are elevated with respect to other cations compared to watershed discharge due to the rapid oxidation/dissolution of biotite. K concentrations are less sensitive to temperature, resulting in a lower average Ea value (27 kJ/mol) indicative of K loss from lower energy interlayer sites in biotite. At lower temperatures, initial cation release from biotite is significantly faster than cation release from plagioclase. This agrees with reported higher K/Na ratios in cold glacial watersheds relative to warmer temperate environments. Increased release of less radiogenic Sr from plagioclase relative to biotite at increasing temperature produces corresponding decreases in 87Sr/86Sr ratios in the column effluents. A simple mixing calculation using effluent K/Na ratios, Sr concentrations and 87Sr/86Sr ratios for biotite and plagioclase approximates stoichiometric cation ratios from biotite/plagioclase dissolution at warmer temperatures (35°C), but progressively overestimates the relative proportion of biotite with decreasing temperature. Ca, Mg, and Sr concentrations closely correlate, exhibit no consistent trends with temperature, and are controlled by trace amounts of calcite or exchange within weathered biotite. The inability of the watershed model to differentiate a climate signal for such species correlates with the lower temperature dependence observed in the experimental studies.

Specific-lon electrode determinations of sulfide preconcentrated from San Francisco Bay waters

Measurements of low-level dissolved-sulfide concentrations in estuarine water from San Francisco Bay have been made using the sulfide-specific electrode after preservation, separation, and preconcentration of the sulfide species. The separation and preconcentration were acheived by coprecipitation of ZnS with Zn(OH)2 followed by collection and dissolution of the precipitate, giving concentration factors up to 160-fold Preconcentration provided sulfide solutions that were adequately measurable within the practical working range of the specific-ion electrode The sulfide detection limit with the preconcentration step is 0 02 μg/l Spike recoveries in the range of 81 to 10 1% have been achieved for laboratory-prepared samples having S2− concentrations as low as 0 6 μg/l and 84 to 100% for an estuarine sample spiked in the field with 2 μg/l (S(−II) Positive correlations have been found between dissolved S(−II) concentrations and concentrations of dissolved Cd, Cu, and Ni, negative correlations have been found between bisulfide (HS−) activity and activities of Cd2+, Cu2+, and Ag+ species