Deposits and soil-formation process of Xiashu Loess in lower reaches of the Yangtze River:a case study of TZC profile in Nanjing

Abstract

Field investigation,magnetic susceptibility,grain-size analysis and optically stimulated luminescence(OSL) dating in a loess profile of Holocene on north bank of the Yangtze River,Nanjing,have revealed the characteristics of aeolian dust deposition and pedogenic environmental change during the formation of the first paleosol.The results show that a layer of paleosol(yellow-brown soil) was formed during the Holocene climate optimum(8500-3100 aBP) on north bank of the Yangtze River,and the climate became warmer and wetter,the proxy index such as magnetic susceptibility is generally higher than other layers,and frequency graphs of grain-size distribution have double peak values,the peak values of grain size(1 um) have been formed during pedogenic processes.but only a peak value in loess layer,so the pedogenic rate was much faster than aeolian dust deposition rate.The optically stimulated luminescence(OSL age) dating indicated the loess formed in the last glacial became the parent matters that the paleosol re-developed.On the grounds of the sediments chronology and the sediment thickness,deposition rate was 12cm/ka in the last glacial on the north bank of the Yangtze River,which can deduce that serious soil erosion happened in top paleosol during Holocene climate optimum(8500-3100 aBP) and the thickness of soil erosion was at least 1 meter.The matters eroded redeposited on the lower place and formed the secondary loess.This means that the source of the secondary loess and Xiashu loess were the same,but the secondary loess redeveloped due to heavy precipitation.There was a transformation period at 3100 aBP,representing a climate change from the period dominated by the southern monsoon to the period dominated by the northern monsoon.A layer of modern loess(about 50cm) have been accumulated since 3100 aBP,this means that the period of the last 3100 years was a one with intensified dust accumulation.