HR: 0800h
AN: H31D-1330    [Abstracts]
TI: Tracing Solute Sources in a Small Tropical Granitoid Watershed
AU: * Pett-Ridge, J
EM: jcp38@cornell.edu
AF: Department of Earth and Atmospheric Sciences, Cornell University, 2122 Snee Hall, Ithaca, NY 14853 United States
AU: Kurtz, A C
EM: kurtz@bu.edu
AF: Department of Earth Sciences, Boston University, 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: Department of Earth and Atmospheric Sciences, Cornell University, 2122 Snee Hall, Ithaca, NY 14853 United States
AU: Troester, J
EM: jtroest@usgs.gov
AF: USGS, Water Resources of the Carribean, GSA Center 651 Federal Drive, Suite 400-15, Guaynabo, PR 00965-5703 Puerto Rico
AB: We studied strontium isotope systematics in the Rio Icacos watershed in Puerto Rico in order to trace the relative contributions of individual minerals to weathering fluxes. This study compliments work done with ²³⁴U/²³⁸U disequilibria and Ge/Si ratios in the same system, providing critical information about the mechanisms of biotite weathering in particular, and allowing us to trace changing sources of Sr to streamwater during storm events. We analyzed ⁸⁷Sr/⁸⁶Sr ratios in primary plagioclase, biotite, and hornblende from the Tertiary quartz diorite bedrock, as well as secondary kaolinite and altered biotite mineral separates from the soil and saprolite profile. We also measured ⁸⁷Sr/⁸⁶Sr ratios of 10 porewater samples taken from tension lysimeters at a range of depths, from 853 cm near the bedrock-saprolite interface to 15 cm near the soil surface. Streamwater samples from the catchment were also analyzed, under both baseflow and stormflow conditions. Streamwaters and the deepest porewaters have low ⁸⁷Sr/⁸⁶Sr ratios (0.7052 to 0.7063) and high [Sr], primarily reflecting plagioclase (0.7042, 560 ppm Sr) and hornblende (0.7058, 80 ppm Sr) weathering at the bedrock-saprolite interface. Soil porewater ⁸⁷Sr/⁸⁶Sr ratios steadily become increasingly radiogenic (higher value) with decreasing depth, reaching a value of 0.712 near the surface. This reflects a combination of the increasing contribution of biotite weathering, and the contribution of sea salt aerosol Sr, which is assumed to have the seawater ⁸⁷Sr/⁸⁶Sr ratio of 0.7092. The possible impact of atmospheric dust inputs on the Sr budget is investigated using Nd isotopes as a tracer of African dust. Primary bedrock biotite has a highly radiogenic ⁸⁷Sr/⁸⁶Sr ratio of 0.7827, altered biotite from the saprolite (505 cm) has an intermediate value 0.7307, and the altered biotite present in the soil (66 cm) has a ⁸⁷Sr/⁸⁶Sr ratio of 0.7055, although substantial heterogeneity was observed among altered biotite samples. We suggest that the radiogenic interlayer-site Sr in the altered biotite is gradually replaced with unradiogenic local porewater Sr during weathering. The ⁸⁷Sr/⁸⁶Sr ratios of kaolinite separates from throughout the profile indicate two distinct generations of kaolinite, one forming at the saprolite-bedrock interface, and a second forming in the soil rooting zone. Sequences measured through stormflow events showed increasing and then decreasing ⁸⁷Sr/⁸⁶Sr ratios, reflecting an increase in shallower soil and saprolite-derived Sr during peak flow.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0486 Soils/pedology (1865)
DE: 0489 Trace element cycling (4875)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005