Luminescence dating of feldspar in sedimentary environments: The problem of zeroing

Luminescence dating of feldspar in sedimentary environments : The problem of zeroing. Postdepositional latent luminescence of mineral grains buried in sediments increases with time, which makes the luminescence dating method particularly suitable for the dating of recent environments. The difficulty one has to demonstrate that the sediment has been fully zeroed before burial is still limiting its universal application. In the case of a poorly bleached sediment, the intensity of luminescence in the course of a paleodose measurement will be dominated by luminescence emitted by the unbleached grains and the age obtained will be overestimated. In order to solve this problem, a new method is being developed which is based on measurements of single grains. The ratio of the accrued luminescence intensity upon added dose over the natural component (R(i)=L(N+y)/L(N)) for single grains reflects the individual contribution to total luminescence from the discrete grains. Measurements carried out on three sediments of different depositional origin but of similar age suggest this parameter may eventually be used to assess the degree of zeroing for a sediment. A second application of the single grain approach allows the dating of individual minerals. This method has been applied to a sediment deposited in the lateglacial Champlain Sea (10 ka old) for which apparent luminescence ages of ca. 65 ka had been obtained. The distribution of ages obtained from the analysis of 15 grains reveals that only six of those grains have been properly bleached, with a mean apparent age of 6 ka. This underestimation of age may be caused by methodological (change of sensitivity) and/or physical (anomalous fading) factors.