Luminescence dating of a fluvial sequence using different grain size fractions and implications on Holocene flooding activities in Weihe Basin, central

Palaeoflood reconstruction is of great importance to extend the flooding records and improve the understanding of their relationships with climatic change and geomorphologic evolution. As a large active rift basin in central China, Weihe Basin subsided and infilled in response to the uplift of the Tibetan Plateau and Asian monsoon variations. Many rivers drain out the erodible Chinese Loess Plateau with tremendous detritus and then build up considerable fluvial sequences. Scarcity of organic material and potential poor bleaching make the fluvial deposits difficult to date with radiocarbon and optically stimulated luminescence (OSL) technique, and as a result hinder the palaeoflood reconstruction. This study attempts to compare OSL ages resulted from different grain size fractions of quartz, and then cross-checks with independent radiocarbon ages, aiming to choose proper method to reconstruct the chronology of a fluvial sequence from the valley bank of Shichuan River, a tributary of Weihe River. The results show that the coarse quartz grains experienced better bleaching than fine quartz grains, and the comparison with AMS C-14 ages further confirms the efficient bleaching of the coarse quartz grains before burial. The sufficient bleaching of the coarse fraction could be attributed to sediment source from the upstream mountainous areas and long-distance transportation. Finally, five palaeoflood events were identified at the times of approximately 2.3-2.6 ka, 3.7-3.9 ka, 4.1-4.7 ka, 6.2-6.8 ka, and 8.9-9.2 ka, corresponding to the periods with intensified rainstorm and vegetation degradation associated with climatic deterioration. This study provides a regional evidence to improve our understanding of the response of river system evolution to climate change.