~(10)Be inventories in Alpine soils and their potential for dating land surfaces

摘要

To exploit natural sedimentary archives and geomorphic landforms it is necessary to date them first. Landscape evolution of Alpine areas is often strongly related to the activities of glaciers in the Pleistocene and Holocene. At sites where no organic matter for radiocarbon dating exists and where suitable boulders for surface exposure dating (using in situ produced cosmogenic nuclides) are absent, dating of soils could give information about the timing of landscape evolution. This paper explores the applicability of soil dating using the inventory of meteoric ~(10)Be in Alpine soils. For this purpose, a set of 6 soil profiles in the Swiss and Italian Alps was investigated. The surface at these sites had already been dated (using the radiocarbon technique or the surface exposure determination using in situ produced ~(10)Be). Consequently, a direct comparison of the ages of the soils using meteoric ~(10)Be and other dating techniques was made possible. The estimation of ~(10)Be deposition rates is subject to severe limitations and strongly influences the obtained results. We tested three scenarios using a) the meteoric ~(10)Be deposition rates as a function of the annual precipitation rate, b) a constant ~(10)Be input for the Central Alps, and c) as b) but assuming a pre-exposure of the parent material. The obtained ages that are based on the ~(10)Be inventory in soils and on scenario a) for the ~(10)Be input agreed reasonably well with the age using surface exposure or radiocarbon dating. The ages obtained from soils using scenario b) produced ages that were mostly too old whereas the approach using scenario c) seemed to yield better results than scenario b). Erosion calculations can, in theory, be performed using the ~(10)Be inventory and ~(10)Be deposition rates. An erosion estimation was possible using scenario a) and c), but not using b). The calculated erosion rates using these scenarios seemed to be plausible with values in the range of 0-57 mm/ky. The dating of soils using ~(10)Be has several potential error sources. Analytical errors as well as errors from other parameters such as bulk soil density and soil skeleton content have to be taken into account. The error range was from 8 up to 21%. Furthermore, uncertainties in estimating ~(10)Be deposition rates substantially influence the calculated ages. Relative age estimates and, under optimal conditions, absolute dating can be carried out. Age determination of Alpine soils using ~(10)Be gives another possibility to date surfaces when other methods fail or are not possible at all. It is, however, not straightforward, quite laborious and may consequently have some distinct limitations.