HR: 0800h
AN: H41E-0341    [Abstracts]
TI: Temporal Shifts in Radiocarbon in Spring Waters: Implications for Decadal Controls on Element Cycling in a Mountain Catchment
AU: * Blumhagen, E D
EM: erik_blumhagen@umail.ucsb.edu
AF: Department of Geological Sciences, UC Santa Barbara-Building 526, Santa Barbara, CA 93106-9630 United States
AU: Clark, J F
EM: jfclark@geol.ucsb.edu
AF: Department of Geological Sciences, UC Santa Barbara-Building 526, Santa Barbara, CA 93106-9630 United States
AU: Chadwick, O A
EM: oac@geog.ucsb.edu
AF: Geography Department, University of California, Santa Barbara, Santa Barbara, CA 93106-4060 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: IGERT Program in Dept. of Earth and Atmospheric Sciences Environmental Biocomplexity, Cornell University, Ithaca, NY 14853 United States
AB: Groundwater is a good archive of chemical signals inherited from soil processes. How good it is depends on the residence time of water in aquifers, the amount of mixing of radically different flowpaths, and how well the chemical signals are preserved. Here, we present data collected from nine springs in Sagehen basin, northern Sierra Nevada, California with a focus on the variable influence the soil zone has in regulating the chemistry of recharging waters before they eventually enter the groundwater system. CFC and tritium/3He age-dating methods were used to determine the apparent geochemical ages (or mean residence times) of groundwater emerging from these springs. The apparent ages range between 15 and 43 years and correlate positively with concentrations of rock-derived cations (Ca2+, Na+), conductivity, temperature, and pH, demonstrating the geochemical evolution of a shallow groundwater system. In contrast with the major cations, delta C-13 shows little change with age (values range between -17 and -19 permil), indicating that the carbon chemistry is not evolving. Thus, carbon isotopes record soil processes in this groundwater system. Radiocarbon contents range between 87 and 110 pmC and correlate very well with age, whereby the youngest groundwater has the highest radiocarbon values. This temporal variation reflects the movement of radiocarbon, derived from atmospheric nuclear bomb tests, through the biota and soil zone. Unlike the decadal response time of the atmospheric carbon reservoir, attenuation and lag of the radiocarbon spike recorded in our groundwater data suggest that the turnover time of carbon in the soil zone is relatively long, perhaps 100s to 1000s of years. Additionally, we are investigating other tracers (Ge/Si ratios and silicon isotopes) to elucidate dynamics of element cycling and fractionation in the soil zone, that are in turn recorded in the groundwater chemistry.
DE: 1803 Anthropogenic effects
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting