Exposure Dating Sampling Strategies in Regions Previously Covered by Cold-Based, low-Erosive Glaciers

Cosmogenic nuclide exposure dating has proven to be useful and reliable in many geomorphological studies. However, difficulties are to be expected when using a single nuclide approach on surfaces in blockfields and/or other areas previously covered by cold-based glaciers. Because of minimal subglacial erosion, bedrock surfaces may have an inherited component of cosmogenic nuclides and the total concentration will represent a composite signal. This can be avoided by using two cosmogenic nuclides with significant different half-lives (e.g. 10Be and 26Al), where their ratio in theory should detect periods of partial erosion and/or burial. However, this requires both long exposure periods prior to glacial overriding and long duration of subsequent burial, in order to detect exposure discontinuities. To circumvent this, we combine single nuclide exposure dating (10Be) and optically stimulated luminescence (OSL) dating in sample settings that include contemporaneous erosion and deposition. By using this multiple dating method approach, we expect to obtain reliable exposure age estimates and also to be able to identify burial by cold-based ice. The field area in east-central southern Norway has numerous OSL dated sedimentary deposits ranging in age from the present to 160 ka, often with evidence of glacial overriding (e.g. erratic boulders, thin tills etc.). Such deposits are commonly connected to lateral meltwater channels, glaciofluvially washed valley slopes, overflow gaps and canyons, all which have the potential to have been sufficiently eroded to remove previously accumulated cosmogenic isotopes. Three scenarios can be envisaged when comparing exposure and OSL ages from our time-synchronous sampling settings: 1) If bedrock exposure ages are older than the age of the corresponding glaciofluvial/glaciolacustrine deposit, insufficient glaciofluvial erosion can be assumed. 2) If the ages agree, the glaciofluvial erosion can be interpreted as having been large and the exposure ages can be taken as reliable estimates. 3) If the exposure ages are younger than the corresponding OSL-dated deposits, three interpretations are plausible: a) glaciofluvially eroded surface has later been covered by cold-based, low-erosive glaciers (should leave field evidence both on bedrock and sedimentary deposits), b) bedrock can be highly weathered and hence lost cosmogenic nuclides (possible to identify in the field), and c) sediments have not been zeroed during transport/deposition (should be revealed by inverted and/or inconsistent OSL dates on the deposit). Glacial erratics deposited on glaciofluvial landforms strongly argue that non-destructive glacial burial has taken place, however, the degree of erosion and removal of pre-last exposure signal can be questionable. The first 20 10Be exposure dates from this area are expected before the end of 2004.