HR: 0800h
AN: H31D-1327    [Abstracts]
TI: Fallout Radionuclides as Tracers in Southern Alps Sediment Studies
AU: * Carey, A E
EM: carey@geology.ohio-state.edu
AF: Department of Geological Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210-1398 United States
AU: Karanovic, Z
EM: karanovic.1@osu.edu
AF: Department of Geological Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210-1398 United States
AU: Dibb, J E
EM: jack.dibb@unh.edu
AF: Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Morse Hall, 39 College Road, Durham, NH 03824-3525 United States
AB: The primary geologic processes shaping the landscape are physical and chemical weathering and the transport of solids by erosion. As part of our studies on the coupling between physical erosion and chemical weathering, we have determined depositional and erosional processes in New Zealand's tectonically active, rapidly uplifting Southern Alps, specifically focusing on the Hokitika River watershed. The South Island watersheds we are studying are subject to extreme orographic precipitation (as high as 7-12 m annually) and high landslide frequency, but have modest topography due to the rapid erosion. In concert with our studies of chemical weathering and physical erosion, we have used the atmospherically-delivered radionuclides of ⁷Be, ¹³⁷Cs and ²¹⁰Pb_excess to determine the relative magnitude of particle residence time in the high elevation Cropp and Whitcombe subwatersheds and the rates of sedimentation. One- and two-box modeling with ⁷Be and ²¹⁰Pb_excess was used to determine soil and sediment residence times. Residence time of fine suspended particles is short and particles can travel the length of the river during a single storm, probably due to the short duration, high-intensity rainfalls which produce rapidly moving, steep flood waves. The readily detected peak of ¹³⁷Cs activity in Cropp terrace and Hokitika gorge soils yielded sedimentation rates of 0.06-0.12 cm yr^-1. At the Cropp terrace, inventory models of ²¹⁰Pb_excess yield soil accumulation rates significantly less than those determined using the ¹³⁷Cs activity peak. We attribute the differences to overestimation of ²¹⁰Pb_excess in surface soils and to contrasting fallout fluxes, geochemical behavior and radionuclide contents of sedimenting materials. Total inventories of ²¹⁰Pb_excess in soils greatly exceed the expected direct atmospheric deposition, suggesting that lateral transport of this nuclide occurs within the watershed. At the Hokitika gorge, all nuclides studied yielded similar sedimentation rates, confirming the potential of ²¹⁰Pb_excess for determining sedimentation rates in New Zealand watersheds with very low ¹³⁷Cs inventories.
DE: 1815 Erosion
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
DE: 1865 Soils (0486)
DE: 1879 Watershed
SC: Hydrology [H]
MN: Fall Meeting 2005