HR: 0830h
AN: V51C-0308    [PDF]
TI: Soil $^{234}$U-$^{238}$U and the Potential Use of U-series Disequilibrium as a Weathering Tracer
AU: * Pett-Ridge, J
EM: jcp38@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AB: Variations in $^{234}$U/$^{238}$U activity ratios may be used to constrain weathering sources, weathering fluxes, soil development timescales, and water flow-paths in a watershed. The use of uranium-series disequilibria as a tracer and chronometer, however, hinges on a full understanding of the processes governing the behavior of uranium in the whole system, especially the soil environment. We present data on uranium concentrations and $^{234}$U/$^{238}$U activity ratios in bedrock and soil mineral samples, soil porewaters, and groundwater and streamwater samples from a previously well-characterized watershed in Puerto Rico, in order to constrain the specific chemical processes controlling the supply of $^{234}$U and $^{238}$U isotopes to the streamwater. The study site, the Rio Icacos watershed in the Luquillo Mountains, has the highest documented chemical weathering rate of granitoid rocks on the Earth's surface. Trace element ratios and $^{87}$Sr/$^{86}$Sr ratios are also used along with the U-series data to elucidate the weathering history. Sequential soil extraction data in combination with XRD and isotopic ICP-MS analyses provide information on the role of Fe-oxide and organic matter sorption on the behavior of uranium in the soil. Temporal sampling of streamwaters through varying discharge allows us to connect specific isotopic signatures to flow-paths through different layers of the weathering profile. A detailed account of the weathering mass balances in the watershed is available to substantiate interpretation of the U-series data. The project investigates the potential of U-series data to date weathering profiles, trace weathering products in surface waters, and calculate weathering mass balances.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (1625)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting