Abstract:

This paper presents data from QFC loesspalaeosol profile in the Hanjiang River Valleytry to illustrate weathering intensity and its environmental significance. To evaluate the degree of weathering using major elements content and related parameters, using Al as the standard rate of change of parameter calculation. Experimental results showed that: (1) magnetic susceptibility, clay composition, and CIA value of the paleo sol S0 is higher than those of typical loess L1, while Na/K value is lower than that of the typical loess L1. It indicates that it was warm and humid climate, with stronger weathering and leaching during the palaeosol S0 formation period, reflecting the strong pedogenesis; while it was  relatively dry and cold, and dust storms occur frequently during loess accumulation period, reflecting weak pedogenes．(2)The dramatic changes of carbonate content is an very important factor influencing the element distribution of loess.Elements content such as Mn and Fe increased in palaesol is mainly caused by the leaching of the carbonate.(3) In the QFC profile the major elements such as Na, Ca, Mg, K, Si and Ti in paleosol S0 are the main migration elements whose migration order is  Na＞Ca＞Mg＞K＞Si＞Ti;  Mn and Fe expressed relative slightly enriched, other major elements in the profile are expressed as relative migration. This indicates loess in upper reaches of Hanjiang valley has completed primary off the Ca and Na phase, and should go into the early off K, with moderate weathering stages．(4) Difference is clear in the value of climatic proxies in paleosol and loess. Susceptibility, clay content, clay/coarse silt ratio and the CIA are higher values in paleosol S0 while they are relatively low in loess. It indicates that the water and heat conditions was the most appropriate during paleosol S0 formation period, with strong pedogenesis; while the climate was cold and dry during the loess accumulation period, with weak pedogenesis.Weathering intensity order is paleosol S0＞ Holocene loess L0＞transition Lt＞ Malan loess L1．(5) It was cold and dry, and dust storms occur frequently, forming Malan loess L1 in the late Pleistocene (11 500 aBP). Climate shift from coldwet to warmhumid during the early Holocene (between 11 500-8 500 aBP) and dust storm activity gradually decreased to form a transitional loess layer Lt．It was warm and humid, and dust storm sediments were strongly modified to form paleosol S0 during the midHolocene (between 8 500-3 000 aBP).It tended to be cooler and drier during the late Holocene (since 3 000 aBP), and dust storms were obviously intensified to form loess L0.