长江下游地区下蜀黄土堆积与成壤环境演变——以南京江北地区一典型剖面为例

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

通过对南京江北地区一个典型剖面（TZC剖面）进行野外调查、室内磁化率、粒度等替代性指标分析和光释光断代研究，探讨了该地区第一层古土壤形成时的粉尘堆积与成壤环境演变特点。结果表明：第一层古土壤（400～50cm）形成于全新世最适宜期（8500～3100aBP），是在末次冰期下蜀黄土堆积成壤基础上的再发育，与下伏黄土之间存在发生学联系。根据沉积年代和沉积厚度对沉积速率估算，该地区末次冰期沉积速率约为11．17cm／ka，而且可以推测古土壤顶部经历过强烈的水土侵蚀作用，侵蚀的厚度为98．67cm，侵蚀下来的黄土在地势低洼的地方形成次生黄土，这表明了次生黄土同为风尘成因，只是经过后期雨水的侵蚀、搬运和再堆积而已。全新世晚期3100年以来，季风转变，沙尘暴加剧，土壤退化，在南京江北地区堆积成厚约50cm左右的现代黄土层或表土层。

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.