OSL dating of fine-grained quartz from Holocene Yangtze delta sediments

Thursday, 18 December 2014
Saiko Sugisaki1, Jan-Pieter Buylaert2,3, Andrew Sean Murray2, Ryuji Tada4, Hongbo Zheng5, Wang Ke6, Keita Saito1, Tomohisa Irino6, Luo Chao7, Li Shiyi7 and Masao Uchida8, (1)University of Tokyo, Department of Earth and Planetary Science, Tokyo, Japan, (2)Aarhus University, Risø Campus, Nordic Laboratory for Luminescence Dating, Department of Geoscience, Roskilde, Denmark, (3)Technical University of Denmark, Center for Nuclear Technologies, DTU Risø Campus, Roskilde, Denmark, (4)University of Tokyo, Department of Earth and Planetary Science, Bunkyo-ku, Japan, (5)Nanjing Normal University, College of Geographic Science, Nanjing, China, (6)Hokkaido University, Faculty of Environmental Earth Science, Sapporo, Japan, (7)Nanjing University, School of Earth Science and Engineering, Nanjing, China, (8)National Institute for Environmental Studies, Center for Environmental Measurement and Analysis, Tsukuba, Japan
Flood events in the Yangtze River are associated with variation in East Asian Summer Monsoon (EASM) precipitation. Understanding the frequency and scale of the EASM precipitation during the Holocene is a key to understanding the mechanism and cyclicity of floods and droughts. Because about 70% of the annual discharge occurs during the flood season, the Yangtze delta sediments provide a good archive of EASM precipitation. In this study, we investigate the possibility of applying OSL dating to establishing high-resolution chronologies for the Yangtze delta sediment cores YD13-1H and G3. The objectives of this study are: (1) test whether fine grained quartz in present day suspended particle matter (SPM) is fully bleached or reset before deposition, (2) where possible, test quartz fine- and coarse-grain OSL dating against radiocarbon shell ages, (3) interpret the sediment transport processes through the differential bleaching of quartz and feldspar OSL signals.

We show that the SPM collected from the surface water column of the Yangtze River during the flood season is well-bleached (offset ~60 years). Fine-grained pro-delta sediments are thus potentially a good dosimeter for OSL dating. OSL ages sediment cores indicate a pronounced change in sedimentation rate at ~6 ka and ~2ka. These events are consistent with what is known of the evolution of the Yangtze catchment and delta. The delta began to build at ~6 ka (Zhao et al., 1979), and human activities increased significantly in the catchment at ~2ka (Chen et al., 1985). It is however surprising that the entire top 9 m of sediment only records these two events. The question of whether significant deposition was limited to 2 ka and 6 ka, or whether the record has been disturbed by erosion/reworking remains. These issues are discussed in terms of the reliability of the quartz OSL ages, the degree of bleaching by comparison with polymineral OSL signals, and the relationship of the OSL ages to the sedimentary record.