Biological dating

Two biological dating techniques have proved useful for dating glacier forelands: dendrochronology and lichenometry. In regions where glaciers descend into areas with trees, the annual pattern of tree growth may be affected by the proximity to the glacier. In recently deglaciated glacier forelands, it is important to establish the age of living trees by counting annual rings and the age of abnormal (normally reduced) growth rates both in living and dead trees (e.g. Schweingruber, 1988). The age of the oldest living tree provides a minimum age for déglaciation. This technique has been used with success, especially in western North America (e.g. Luckman, 1986). One dendrochronological technique tries to date glacier-induced growth rates from trees partly broken or tilted but not overrun or killed by the glacier (e.g. Luckman, 1986). Trees killed by the glacier and later exposed by glacier retreat may be cross-dated with living trees or dated by the radiocarbon method, an approach widely used in the European Alps (e.g. Holzhauser, 1984). In front of Briksdalsbreen, two Salix trunks, exposed during recent glacier advance, were dated to 7650 ±85 radiocarbon yr bp (8405 (8485-8340) calyrBp) and 7530 ±100 radiocarbon yr bp (8325 (8400-8165) calyrBP) (Nesje, unpublished).

Lichenometric dating, developed in the context of recently deglaciated terrain by Beschel (1950, 1957, 1961), has been widely used, in particular the yellow-green Rhizo-carpon geographicum (Innes, 1985a,b, 1986a,b). There are two main applications of the method. The 'indirect' approach is based on the assumption that there is a relationship between lichen size and terrain age. Interpolation between points of known age can be used to date other surfaces by using lichen size. The greatest limitation of the indirect lichenometric approach is the need for several surfaces of known age (control points). Commonly, the age estimates obtained by this method are given with an accuracy of ±10 per cent (Bickerton and Matthews, 1993). McCarroll (1994) developed a new approach to lichenometric dating, based on large samples of the single largest lichen on each boulder. On surfaces of uniform age, lichen sizes are close to normally distributed, and mean values can therefore be used to construct lichenometric dating curves.

Direct lichenometric dating means that a growth curve is established by direct measurements of lichen growth rates. This dating technique has, however, had little success, mainly due to slow lichen growth, great variability of lichen growth rates, and problems of linking growth curves to site age (Matthews, 1992).

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