HR: 0830h
AN: H51E-1120 INVITED     [PDF]
TI: Fluvial Bedrock Incision Rates in Taroko Gorge (Taiwan) From in Situ-Produced Cosmogenic Nuclides
AU: * Schaller, M
EM: mirjam@umich.edu
AF: Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ United Kingdom
AU: Hovius, N
EM: nhovius@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ United Kingdom
AU: Willett, S
EM: swillett@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Ivy-Ochs, S
EM: ivy@particle.phys.ethz.ch
AF: Institute of Particle Physics, ETH Hoenggerberg, Zuerich, 8093 Switzerland
AU: Synal, H
EM: synal@particle.phys.ethz.ch
AF: Paul Scherrer Institut, c/o Institute of Particle Physics, Zuerich, 8093 Switzerland
AB: Climate, through its influence on erosion, plays a key role in the tectonic evolution of the continental crust. Tectonics may in turn moderate both local and global climate, for example through the construction of mountain belts. Geodynamic modeling has yielded testable hypotheses concerning the feedbacks between climate, erosion and tectonics. The models, however, are not matched by an observational data base to test their predictions. This is primarily because erosion, the key parameter linking cause and consequence, has proved difficult to measure. Cosmogenic nuclide concentrations can be used to determine present and past rates of river incision, a key erosional process, and thus establish how bedrock rivers have responded to recent change of climate. The effects of such changes are recorded in fluvially sculpted landforms in mountain valleys. The age of these fluvially sculpted surfaces can be dated by cosmogenic nuclides. The concentration of cosmogenic nuclides in rocks exposed at the Earth's surface is proportional to the total duration of their exposure. The age of the surface and their absolute altitude above the active river channel is used to constrain valley lowering rates. We applied this approach in the Taroko gorge of the Liwu Chi river, Taiwan, an ideal natural laboratory for fluvial erosion studies because of its extreme, monsoonal climate and steady tectonic forcing, absence of significant glaciation, and availability of high-quality measurements of present day erosion. Fluvial incision rates between February 2000 and May 2002 are around 5 mm/yr (Hartshorn et al., 2002). Similar to these short-term incision rates are the long-term exhumation rates estimated from apatite fission tracks. However, 2.4 kyr old terrace deposits upstream of the Taroko gorge imply higher intermediate-term incision rates of 11 mm/yr (Liew, 1988). Minimum surface exposure ages derived from the concentration of in situ-produced $^{36}$Cl in marble, and corrected for topographic shielding range between 0.3 kyr and 5.8 kyr. Ages increase from 0.3 kyr 3 m above the modern river channel to an age of 3.6 kyr at an elevation of 55 m above the channel. Incision rates for these lowermost samples were calculated using the sample altitude above the river channel divided by the exposure age of the sample. The incision rates are high for the time period between 0.3 kyr and 1.0 kyr (43 mm/yr). Rates are lower for the time period between 1 kyr and 3.6 kyr (8 mm/yr). Samples collected at elevations between 55 m and 165 m show highly variable surface exposure ages. Simple age-elevation interpretation yields a high incision rate of 28 mm/yr. Above 165 m the ages decrease again to 1.0 kyr. This pattern can be explained by multiple exposure histories (e.g. exposure, burial by landslide material and re-exposure) or may be the result of complex weathering processes (e.g. preferential leaching of $^{36}$Cl over time).
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting