HR: 14:25h
AN: H23J-04    [Abstracts]
TI: Origin of Pb-210 in Florida Bay, A Constant Non-atmospheric Source.
AU: * Holmes, C W
EM: cholmes@usgs.gov
AF: U.S.geological Survey, 600 Fourth Street South, St. Petersburg, FL 33701 United States
AB: Establishing the timing of surface processes with 210Pb has been commonly used over the last 4 decades. Radon diffuses from the earth's crust and decays through a series of very short lived isotopes to Pb-210. Because the production of Pb-210 depends on the expose of continental material, the atmospheric concentration at any one location is related to the proximity of continental masses and prevailing air flow. This process produces excess Pb-210 in the atmosphere and subsequently the hydrosphere. Pb-210 inventories in Florida Bay sediments were found to exceed that which could be accounted for by atmospheric deposition. Since these sediments are formed in situ, transportation of Pb-210 enriched material can not account for this disequilibrium. Detailed analysis of the entire sediment column showed that significant Pb-210 concentrations were present at the sediment rock interface. Previous isotopic measurements showed that the strata underlying Florida Bay contained high concentrations of Rn-222 diffusing into the overlying waters. Sediment dynamic monitoring studies have demonstrated that most sediment transported in Florida Bay are deposited during the winter months. It is in these months during which the water is cold which allows radon concentrations build up in the waters. With the three day half life, this radon produces Pb-210 that is available to be adsorbed on the moving sediment. This mechanism creates a constant supply of Pb-210 and produces the ideal condition 210Pb dating Florida Bay sediment. There is evidence that such a condition is common to other carbonate environments.
DE: 1022 Composition of the hydrosphere
DE: 1030 Geochemical cycles (0330)
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
SC: Hydrology [H]
MN: Fall Meeting 2005