土楔和冰楔假形及其古气候意义

由于土楔及冰楔所处冻土的含冰状况不同,二者对冻土退化的响应不同。土楔的形状及大小乃至其中的充填物可以完整地保存下来,而冰楔在融化过程中经受强烈变形,其原形状很难保存下来,以至无法辨认。已报道的冰楔假形中,有相当部分可能是原生土脉,甚至是活动层土楔。冰楔在多年冻土环境下生长,其假形可以指示古冻土曾存在过。但它与温度之间没有简单的对应关系,因为楔形构造的形成是地-气综合因素相互作用之结果。土楔可以是原生的,也可以是次生的,它与多年冻土环境并无必然联系。

Soil Wedge and Ice-wedge Pseudomorphs and Their Paleoclimatic Implications

'Thermal contraction cracking, which is caused by the response of ground surface to the tensile stress generated by a low temperature and a rapid fall in temperature, is thought to be the mechanism by which frost fissures develop. These include active-layer ground wedges, frost-fissures of both primary and secondary infillings. Ice-wedge pseudomorphs are formed by the transformation of ice wedges when permafrost degrades. They are good indicators, therefore, of the former existence of permafrost. However, they do not have simple relationship with temperature. The following points have to be borne in mind when any paleoenvironmental reconstruction is attempted. First, soil wedge and ice wedge respond differently to the thawing of permafrost. The appearances of ice-wedge pseudomorphs vary, depending on the properties of primary infilling and the host soil types. A soil wedge may be easily preserved in both shape and size. In contrast, because of the high ice content of ice wedges and the considerable deformation on melting, structures left by the melting of ice wedges may not be recognized as ice-wedge pseudomorphs, and might be termed involutions or cryoturbations. Thus, not all ice wedges will have a recognizable pseudomorphs upon thawing. Second, the occurrence of frost fissures and their pseudomorphs is associated with a number of variables in nature. Both environmental conditions and site-specific factors should be taken into account in the paleoenvironmental reconstruction. Third, Non-cryogenic features, which resemble frost fissure pseudomorphs have been reported from regions where ice-wedge pseudomorphs were also reported. Such features have been used to reconstruct paleoenvironments. Clearly, it is necessary to reevaluate some paleoenvironmental reconstructions that have been done.