关键词: 冰楔假型, 末次冰期, 气候变化, 河流阶地, 黄土高原, 光释光测年

Abstract:

The ice-wedge casts were considered to be an important indicator on paleoenvironment. The climatostratigraphic information in them could implicate the geomorphic evolution. In this paper, the ice-wedge casts were described in detail, which were found in the lower terrace of Huanjiang river in the northern margin of Loess Plateau. Through the field investigation and optically stimulated luminescence (OSL)dating, the age of sandy gravel deposited in the wedges was about (30.01±3.31) ka B.P., the fluvial silty sand layer on the upper of the ice wedges was deposited about (28.64±3.13) ka B.P.. This finding shows that a cold event occurred in the phase of Marine Oxygen Isotope Stage 3 (MIS 3)in the Last Glaciation. Also, the average ground temperature then was estimated to be 17.7 ℃ lower than the temperature nowadays. Besides, the discovery of the ice-wedge casts provided evidence for the phasic variation of the permafrost boundary during the Last Glaciation. In addition, the climatostratigraphic position of ice-wedge casts in the river terrace indicated that the fluvial aggradation occurred slowly and even ended during the freezing period. When the climate was getting warmer in (28.64±3.13)ka B.P., the river started to undercut and the terrace was forming. This provided a basis for the formation mechanism of river terrace and showed the development of terrace was controlled by the changing climate.

Key words: ice-wedge cast, Last Glaciation, climate change, river terrace, Loess Plateau, OSL dating