In most exposure dating studies a purely exponential decrease of nuclide production in rock is used for depth scaling. Calculations by Masarik and Reedy [1] show however that the profile in the first few centimeters below the rock surface might actually be quite flat and only decrease exponentially from about 10-14 g/cm2 onward. As most samples are taken right from the rock surface and are usually only a few centimeters thick, the accuracy of exposure ages is clearly influenced by whatever shape of the depth profile is used in the age calculations. Furthermore, the influence of any kind of surface cover (e.g. snow, forest, sediment) on the exposure age depends on the depth profile used.
We made an attempt to measure the shape of the depth profile over the first 20 cm below the rock surface. Three short drill cores were taken from a near vertical wall at Kleiner Arbersee (917 m asl), a small lake below the Grosser Arber summit, which is the highest mountain (1456 m asl) of the central-European mountain range, Bavarian Forest, in Southern Germany. The surfaces of the cores were glacially polished. Two of the cores were up to 23 cm long, while the third was only 8 cm long, and all were of almost pure quartz. A diamond saw was used to precisely cut the three cores into 1 and 1.2 cm slices. Not all slices could be measured successfully, but we obtained a set of 21 10Be concentrations to construct a measured depth profile.
The generally used apparent attenuation length for depth scaling (e.g. 140-160 g/cm2) is valid for the specific situation of a flat surface and open sky environment. It is a good approximation for the specific settings of the data set from which it was constructed. From the apparent attenuation length one can determine the physical exponential attenuation length for a collimated unidirectional beam [2]. This physical attenuation length together with the zenith angle dependence for our individual sample locations in the cores allows the calculation of theoretical depth profiles, both based on either a purely exponential shape or on a Masarik + Reedy type profile, which then can be compared to our measured data set. Both measured data and the depth profile calculations will be discussed.
[1] J. Masarik and R.C. Reedy, EPSL 136 (1995) 381-395
[2] J. Dunne et al., Geomorphology 27 (1999) 3-11
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