HR: 13:40h
AN: H23J-01    [Abstracts]
TI: 7Be/210Pbxs Ratio as an Indicator of Suspended Sediment Age or Fraction New Sediment in Suspension
AU: * Matisoff, G
EM: gerald.matisoff@case.edu
AF: Case Western Reserve University, Department of Geological Sciences 110-112 AW Smith Building, Cleveland, OH 44106-7216 United States
AU: Wilson, C G
EM: CWILSON@msa-oxford.ars.usda.gov
AF: USDA-ARS, National Sedimentation Lab, 598 McElroy Dr., , Oxford, MS 38655 United States
AU: Whiting, P J
EM: peter.whiting@case.edu
AF: Case Western Reserve University, Department of Geological Sciences 110-112 AW Smith Building, Cleveland, OH 44106-7216 United States
AB: We present a technique to use the 7Be/210Pbxs ratio as a measure of suspended sediment age or as an indicator of the fraction of the suspended sediment that is recently eroded from the landscape. Although both 7Be and 210Pbxs are delivered seasonally and stochastically to the landscape by precipitation, the ratio of the two radionuclides varies substantially less. The 7Be/210Pbxs ratios measured in three different watersheds decrease in the following manner: precipitation (~16) > suspended sediments in rivers (6-7) > suspended sediments in estuaries (4-6) > sediment collected in sediment traps in the estuary (~1) > surface sediment of the estuary (~0.5). Decreases in the 7Be/210Pbxs ratio in suspended sediments can be interpreted to be the result of increased age of the sediment, since 7Be decays faster than 210Pb. Alternatively, a decrease in the 7Be/210Pbxs ratio in suspended sediments can be interpreted to be the result of dilution of newly-tagged 7Be-rich sediment by 7Be-dead sediment, for example, by erosion of soil below the 7Be-enriched surface layer or by resuspension of 7Be-dead bottom sediment. We present a model which uses the 7Be/210Pbxs ratio in suspended sediments to determine the time since the particles were tagged by precipitation-derived radionuclides (i.e., the age of the suspended sediment). In addition, we present an alternative model to determine the fraction of the sediment that is `newly-tagged'. These two models are applied to three watersheds - Old Woman Creek, Ohio; Weeks Bay, Alabama; and South Slough, Oregon - and yield similar findings at all three sites. Sediment ages increase from 0 in newly tagged material to 50-80 days in rivers to about 80-100 days in the estuaries to about 200 days in the sediment traps to about 300 days on surface bottom sediments. Alternatively, the percent new sediment decreases from 100% in newly-tagged material to about 35-50% in rivers to 25-35% in the estuary to less than 10% in the sediment traps to 1-4% on the surface of the bottom sediments.
DE: 1115 Radioisotope geochronology
DE: 1130 Geomorphological geochronology
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005