1. Burial age dating
The ratio of cosmogenically formed aluminum-26 to beryllium-10 can assign numerical ages to depositional events. Both isotopes are formed – analogue to other cosmogenic nuclides like 14C, 36Cl, 3He - at the earths surface in exposed rocks by the transformation of elements (Si and O) induced by high-energy cosmic rays. If once exposed rock is buried deep and shielded from cosmic ray influence, for example due to transport into the subsurface (cave), the production of cosmogenic nuclides stops and the nuclides decay following their individual half-life and the time of sediment transport into a cave can be determined.Analogue, sediment deposition on top of once exposed rocks results in a decrease of cosmogenic nuclides production and increase of decay. The different production rates and decay rates of 26Al and 10Be in combination with the burial depth can be used to calculate the time of capping. The temporal resolution of this numerical dating method is between 100 000 years and 5 million years. Datable events are landslides, terrace deposition, sediment transport into caves (alluvial deposits) and turbidite flows. The prerequisite for a reliable age is that the formation mechanism of sediments to be dated was geologically reconstructed. Processing steps include mechanical sample preparation, chemical pre-cleaning and total digestion followed by chemical cleaning and separation with cation and anion exchange resins. The extracted Al and Be hydroxides are converted into oxides and the samples are sent to accelerator mass spectromenry for analysis.
2. Exposure dating with 10Be (and 26Al)
Alpine glaciers erode material from the bedrock, transport it within the ice towards the valley and finally deposit rock at the ice margin, creating linear sediment deposits (moraines) that delineate the ice margin. Moraines in today's glacier forefield below the recent ice margin are witnesses of stable glacier positions in the past. Dating these deposits allows insights into glacier- and climate dynamics of thousands of years in the past and thus extend the record of glacier changes into the past beyond the instrumental record. A commonly used approach to dating moraines in the high alpine region is exposure dating using 10Be. Cosmogenic 10Be forms in rocks by exposure to high-energy cosmogenic radiation. Rock material is steadily eroded at a high rate from the solid rock below the ice masses by glacier flow and transported to the edge of the glacier (tongue). This material is assumed to be free of cosmogenic nuclides at the time of deposition by ice. Once this material melts out at the ice margin, cosmogenic nuclides are produced. The accumulation of cosmogenic nuclides is a function of time and can be used as a chronometer, because increasing exposure time, following a known production rate, results in increase of cosmogenic nuclides. If rock samples are taken from moraine deposits and the cosmogenic nuclide 10Be is extracted, the corresponding concentration can be measured by accelerator mass spectrometry (AMS), and the exposure age of this rock and thus the position of a glacier in the past can be determined.