Paper No. 285-7
Presentation Time: 9:00 AM-6:30 PM
TIMING AND PROVENANCE OF LOESS IN JIUZHAIGOU, CHINA
MCGUIRE, Casey, Dept. of Geology, Oberlin College, 52 W. Lorain St., Oberlin, OH 44074, SCHMIDT, Amanda H., Geology, Oberlin College, Geology Department, Rm. 403, 52 W. Lorain St, Oberlin, OH 44074, KEEN-ZEBERT, Amanda, Division of Earth and Ecosystem Sciences, Desert Research Institute, 2215 Raggio Pkwy, Reno, NV 89512, COLLINS, Brian D., Earth and Space Sciences and Quaternary Research Center, University of Washington, Box 351310, Seattle, WA 98195 and D'ALPOIM GUEDES, Jade, Department of Anthropology, UC San Diego, 9500 Gilman Dr., La Jolla, CA 92093, cmcguir2@oberlin.edu
Loess is widely distributed in southwest China; however, the timing of loess deposition and source are unknown. To better understand this, we conducted a study of the loess in Jiuzhaigou National Park, Sichuan Province, China. This site is on the boundary of two loess regions—the Chinese Loess Plateau and the Chengdu basin—that have different sources: inland basins in northwestern China and the eastern Tibetan Plateau. Numerous hillslope terraces provide a window into the widely distributed loess of the region. Typical terrace stratigraphy from bottom to top is loess, paleosol, and “stony loess” (loess containing cm to m scale angular clasts). Samples were taken from different stratigraphic and geographic points on the terrace faces to constrain the physical properties and age of the loess. Possible loess sources include the Tibetan Plateau to the west, the Chinese Loess Plateau to the north, and the Chengdu Basin to the south.
Optically stimulated luminescence (OSL) and grain size were used to characterize the loess. Six OSL samples were analyzed: one above the paleosol and four below. The paleosol is a useful marker in dating because it is evidence of a hiatus in loess deposition. We used post-infrared optically stimulated luminescence (post-IR-OSL) on 4 mm aliquots of fine grained (63-90 µm) polymineral sediment to constrain the timing of loess deposition. The oldest OSL age from below the paleosol is 10.73 ± 1.04 ka, and the paleosol dates to 8.93 ± 0.44 ka. OSL ages indicate that the loess accumulated over ~2 ka during the Holocene. Two OSL samples (90 and 300 cm below the paleosol) have statistically identical dates, suggesting that the loess accumulated relatively quickly during the Holocene.
Loess from Jiuzhaigou is coarser (mean = 43 μm) than loess from both the Chinese Loess Plateau (mean =15 μm) and the Chengdu Basin (mean = 22 μm). This suggests a potential origin on the Tibetan Plateau to the west implying that westerlies and winter monsoons are likely the means of sediment transport. Additionally, a source of loess from the Tibetan Plateau, the closest possible source region to Jiuzhaigou, is consistent with the relatively coarse grain size.
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