Deducing mineral weathering reactions from solute profiles in highly leached regolith, Luquillo Mountains, Puerto Rico
Abstract
Porewater solute fluxes are measured on a smaller spatial scale than watershed fluxes and can thus provide more information on the mechanisms of weathering and nutrient cycling processes as well as a more accurate estimate of mineral weathering rates. Mineral weathering reactions continue influence mineral nutrient cycles even in thick, highly leached regolith. As part of the USGS Water Energy and Biogeochemical Budgets (WEBB) program and the Critical Zone Exploration Network, we are investigating mineral nutrient distributions and fluxes in depth profiles in the Bisley 1 catchment in the Luquillo Mountains of Puerto Rico for comparison to the nearby Rio Icacos catchment. Both watersheds are steep, rugged, humid, and tropical, have similar vegetation and land-use, and contain thick, well drained saprolites developed on igneous bedrock: a quartz diorite intrusion in the Rio Icacos, and a marine bedded, andesitic volcaniclastic in the Bisley. Depth distributions of physical parameters (density, porosity, water saturation) and chemistry of solids and solutes were determined. Vertical water infiltration rates in the Bisley watershed average about 4 m y-1, while infiltration rates in the Rio Icacos are about 1 m y-1 (White et al., 1998), and fluid residence times average 8 years in Rio Icacos versus 2 years in Bisley. Porosity in the Bisley regolith is slightly higher at most depths than in the Rio Icacos. The Bisley regolith is highly weathered and is depleted in primary minerals (except quartz) with respect to bedrock, while the Rio Icacos regolith contains residual biotite, which produces a Mg solute gradient with depth (White et al., 1998). The Bisley regolith also exhibits Mg solute gradients among others (notably Si); however, to date, no Mg-containing minerals have been identified in the regolith. Mg and Sr isotope ratios were measured in the pore waters at the Institut de Physique du Globe de Paris. The Mg isotope ratios increase with increasing depth from δ26Mg = -0.772 at the surface to -0.267 at 9.3 m depth, relative to the DSM3 standard. Sr isotope ratios vary from 0.71044 near the surface to 0.70692 87Sr/86Sr at 9.3 m depth. These solute gradients in Mg and Sr isotope ratios with depth suggest mixing lines between an atmospheric component at the surface and a weathering component at depth. The isotopic data indicate that even in this thick, highly weathered profile with high water infiltration rates, weathering reactions are occurring and affecting mineral nutrient cycles. White A.F. et al. (1998) Geochim. Cosmochim. Acta 62, 209-226.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H33H0975B
- Keywords:
-
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1615 GLOBAL CHANGE / Biogeochemical cycles;
- processes;
- and modeling;
- 1875 HYDROLOGY / Vadose zone;
- 1886 HYDROLOGY / Weathering