Physicochemical techniques

Several physicochemical dating techniques based on isotopes or radiological processes, rock surface weathering, tephrochronology, varves (annual laminae in sediments) and palaeomagnetism have been applied to glacier forelands. Radiocarbon dates for wood, peat, soil or lacustrine sediments obtained from glacial forelands can either be of the actual event (contemporary), bracketing or limiting (Porter, 1981b). Most commonly, samples provide either maximum or minimum ages of the actual event. Dates from below and above may bracket the age of the deposited layer.

Radiocarbon dating of different soil fractions from palaeosols has provided information on glacier advance and retreat. This approach has been used with great success in New Zealand, the European Alps and Scandinavia. Dating of thicker soils developed over a considerable time span is somewhat problematic, because of problems in isolating organic layers/fractions of different ages due to greater mean residence time (Matthews, 1985). In southern Norway, Matthews and Dresser (1983) and Matthews and Caseldine (1987) reported steep age/depth gradients in soils beneath Little Ice Age moraines, ranging from about 4000 yr bp at the bottom to several hundred years at the top. The radiocarbon method itself poses a problem with precision. Age limits given with two standard deviations (95 per cent certainty) normally give an age uncertainty in the order of ±100 years. When calibrating radiocarbon dates into calendar ages, dates younger than about 400 years give equivocal dates. As an example, a radiocarbon age of 220 ±50 radiocarbon yr bp (1 sigma) is equivalent to calendar age ranges of 150-210, 280-320 and 410-420 calibrated yr bp (Porter, 1981b).

Tephra layers are stratigraphical marker horizons which may indicate the relative age of the overlying or underlying deposits. Tephra layers have been used to date moraines in areas subject to volcanic eruptions, particularly in Iceland (e.g. Dugmore, 1989) and North America (Porter, 1981b).

Laminae/varves in proglacial lakes may indicate upstream glacier fluctuations (e.g. Karlen, 1976, 1981; Nesje et al, 1991; Karlen and Matthews, 1992; Matthews and Karlen, 1992; Dahl and Nesje, 1994, 1996). (For further details, see Section 3.8.)

Rock surface colour (Mahaney, 1987), rock disintegration (Innes, 1984), rock surface hardness and roughness (Matthews and Shakesby, 1984; McCarroll, 1989; McCarroll and Nesje, 1993) and weathering-rind thickness have been used to obtain relative ages. Theoretically, weathering rates or degree of rock surface weathering can be calibrated with other dating techniques, for example the radiocarbon method, to get absolute dates (e.g. Colman, 1981).

In conclusion, direct observation and measurements, historical documents, dendrochronology, lichenometry, radiocarbon dating and tephrochronology are considered the most accurate dating approaches in glacier forelands. If possible, multi-parameter methods or several dating techniques should be used to obtain the most accurate terrain age in glacier forelands.

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