WEATHERING INTENSITY VARIATIONS IN A HOLOCENE LOESS-PALEOSOL PROFILE FROM UPPER-HANJIANG CATCHMENT IN CENTRAL CHINA

The Holocene sediment profile was surveyed in detail and sampled systematically at the Liaowadian site on the first river terrace in the upper Hanjiang valley. The stratigraphic and sedimentological characteristics,pedogenic modification characteristics and the profile structure were investigated in the field.Magnetic susceptibility,particle-size distribution,geochemical characteristics and optical simulated luminescence(OSL)age were analyzed in laboratory.Based on the stratigraphic correlation and OSL dating,the chronologic framework of the LWD profile was established as follows:Malan loess L1 was accumulated in the last glacial period;the transitional loess Ltwas accumulated in the early Holocene(11 500~8 500 aB.P.);the palaeosols(S0)was formed in the middle Holocene(8 500~3 100 aB.P.);Holocene loess L0 was accumulated in the late Holocene(3 100 aB.P.so far).A detailed study comparing the contents of major elements,content of clay,magnetic susceptibility,Rb/Sr ratio,CIA,(K2O/N2O),(CaO+K2O+N2O)/Al2O3 was conducted in this paper to explore the environmental features in the Holocene.The analytical results show that,Na,Mg and Al are relatively intensive leached,K,Fe and Ca are leached in a certain extent,and the element mobility and migration ability of the major elements are ranked in the following order:NaMgAlKFeCaSi.The distributions of content of clay,magnetic susceptibility,Rb/Sr ratio,CIA,(K2O/N2O),(CaO+K2O+N2O)/Al2O3 imply that during the palaeosols(S0)formation,chemical weathering was intense,and the climate was warmer and wetter,the leaching and migrating was intense,the stage of Malan loess L1 accumulation,weathering intensity was weakened,and the climate turned to be relatively dry,the leaching and migrating was not conspicuous.During the stage of Holocene loess L0 accumulation,the climate turned to be improved,the weathering intensity was between those during palaeosols(S0)formation and Malan loess L1 accumulation,the leaching and migrating was weaker.The CIA,(K2O/N2O)and(CaO+K2O+N2O)/Al2O3 of 205-165 cm layer during palaeosols(S0)formation occurred significant changes:the CIA and(K2O/N2O)have an obvious peak area,and the(CaO+K2O+N2O)/Al2O3 have an obvious low ebb area,which indicate the weathering and pedogenic modification was intense and there may be a secondary environmental change that the climate became relatively warm and wet.The palaeoflood slackwater deposits(SWD)was lied in the middle-upper of the profile,presenting gray-white(10 YR6/1),homogeneous texture,loose,very uniform fine slit,the stratigraphic boundary of its upper and lower adjacent showed a mutation relationship and clear boundaries.To direction of vertical bank of river tip off quickly,the palaeoflood SWD samples was OSL dated to 1 810-1 710 aB.P.(AD 200-300),which is equal to the Eastern Han dynasty to Wei Jin era(AD 25-316),indicating that a dramatic climate change event for the upper-Hangjiang catchment occurred in the end of Eastern Han dynasty during the late Holocene,which caused catastrophic floods in the upper-Hangjiang catchment.