Thufur formation in northern Iceland and its relation to holocene climate change

This gaper compares thufur formation in the northern volcanic zone of Icela nd with that of other periglacial hummock forms. The aim of this work is to further the understanding of the extent of thufur in Iceland, in terms of cryogenic mechanisms related to specific edaphic conditions. Thufur may be considered to be hummocks, of a mainly minerogenic nature and specifically developed in volcanic loesses on drained basaltic rocks or openwork glacial or fluvioglacial substratum. They are thus connected to the oceanic cold c limate as well as to the occurrence of volcanic loesses with specific physi cal properties resting on permeable substratum. Their development does not rely on either permafrost or a water table. Rather, they result from deep s easonal freezing, with exaggerated frost heave limited by snow accumulation along with the development of injection patterns along dilation cracks. Th ufur may be considered as a subtype of hummock, or of raised-centre non-sor ted polygons related to seasonal frost and snowdrift. In Iceland, the high geothermal gradient may, through exaggerated frost heave, enhance hummocky microrelief locally. Thufur expansion is related to the post-Hypsithermal ( Atlantic) climate degradation of the Holocene linked to orbital forcing. Th anks to tephro-stratigraphy, it is possible to define several main episodes of thufur growth: around 4500 (BP); after 2600 (BP); prior to colonization (1104 AD); as well as two phases during the Little Ice Age. These growth p hases are coherent with the pedoclimatic record on both sides of the North Atlantic. (C) 1998 John Wiley & Sons, Ltd.