The Climatic Significance Of Newly Discovered Ice Wedge Pseudotypes On The Northern Margin Of The Loess Plateau And Its Implications For The Development Of Climate Terraces

The ice-wedge casts were considered to be an important indicator on paleoenvironment.And the climatostratigraphic information in them could implicate the geomorphic evolution.In this paper,the new found ice-wedge casts and the terraces were described in detail,which were located in the lower terrace of Huan Jiang river in the northern margin of Loess Plateau.Based on the field investigation and optically stimulated luminescence(OSL)dating,also combining with the previous research results,it is found that the ice-wedge casts might be significant for studying paleoclimate and evolution of river terrace.The significant for studying paleoclimate was presented as three aspects below.Firstly,the average ground temperature then was about-7?,which was estimated to be 17.7? lower than the ground temperature nowadays.Secondly,the finding shows that a cold event occurred in the phase of Marine Oxygen Isotope Stage 3(MIS 3)at 30 ka B.P.in the Last Glaciation.Besides,the discovery of the ice-wedge casts provided evidence for the phasic variation of the permafrost boundary during the Last Glaciation.Through the empirical formula from previous research,the south boundary of the permafrost was approximately reconstructed.In addition,The climatostratigraphic position of ice-wedge casts in the river terrace indicated the process of the terrace: It was a warm and moist period in 37.16 ka B.P.the Huanjiang river discharge was greater,and undercut to form the terrace 2.The character of the sediments in terrace 1 showed that the paleoclimate turned to dry and cold and fluctuation at small amplitude during the fluvial aggradation.The finding of ice-wedge casts showed that the fluvial aggradation occurred slowly and even ended during the freezing period about 30 ka ago.When the climate was getting warmer in(28.64±3.13)ka B.P.,the river started to undercut and the terrace 1 was forming.The bed rock was cut proved that the uplifting of tectonic movement was the main reason for the terrace1.And the warm and moist climate in Holocene also effected on the river undercut.This provided a basis for the formation mechanism of river terrace and showed some relationship between the development of terrace and the changing climate.Also,the paper reminds us that it is necessary to pay more attention to the changing climate in the study of river terrace.