Radioluminescence Dating: A New Tool for Quaternary Geology and Archaeology

 Natural minerals, such as widespread quartz and feldspars, have physical properties which enable them to be used as radiation dosimeters. The underlying luminescence phenomena have made it possible in recent decades to determine the age of a variety of materials important for quaternary geochronometry and archaeochronometry. We present a new luminescence dating method based on radioluminescence measurements of potassium feldspar. For the first time we have been able to investigate the light-emitting transition of electrons from the conduction band to an optically sensitive electron trap. Many advantages can be derived from such direct measurements of the metastable electron density in this particular trap, which produces the age-dependent signal. The method can be used to date the last light exposure of feldspar grains within a range of a few hundred to more than 200,000 years. Examples are presented of age determination of various waterlaid quaternary sands. The results of a basic study of feldspar radioluminescence also shed light on effects not sufficiently understood in conventional dating by luminescence techniques, especially in infrared optically stimulated luminescence dating of feldspar. Received: 6 August 1998 / Accepted in revised form: 14 April 1999     

Zur Frage der Bedeutung hormoneller Beziehungen bei der Insektenmetamorphose

The Science of Nature -     

Optical properties of selected plants from a tropical mountain ecosystem - Traits for plant functional types to parametrize a land surface model

The optical properties (reflectance and transmittance) of selected leaves from a tropical mountain rainforest in southern Ecuador are determined to parametrize optical traits of plant functional types (PFT) of a state of the art land model (Community Land Model, CLM). 46 spatially dominating species are selected from 4 different forest types, the subpáramo and a succession stage of pasture areas representing ecologically predefined functional types within the study area. Measurements are conducted under a standardized experimental setup with a field spectrometer covering the radiation between 305 and 1305 nm. The results of the optical properties of all species are checked for similarity by cluster analysis and are compared to the composition of species of the predefined PFTs. Furthermore the results are compared to other studies, the default values for the globally defined PFT of tropical evergreen trees in the CLM and another forest growth model operated in the same study area. The results show that the clusters aggregated by the reflectance, transmittance or combined properties do not represent the predefined PFTs. The values of the other studies suggest a reassessment of the experimental setup for the transmittance measurements. Nevertheless, new reflectance values for the regionalized PFTs can be determined. The optical values differ from the CLM-PFT of tropical evergreen trees, and new values for the reflectance are recommended.     

Effects of zooplankton diel vertical migration on a phytoplankton community: A scenario analysis of the underlying mechanisms

A mechanistic model was applied to study the influence of diurnal vertical migration (DVM) of planktonic crustaceans on the succession and composition of the phytoplankton community. While zooplankton was restricted to only one functional group, the phytoplankton community was divided into two functional groups which are distinguished by their maximum growth rates and vulnerability to zooplankton grazing. DVM causes a pulsed grazing regime and may also entail a corresponding reduction of the cumulative daily rates of ingestion and losses of zooplankton. To study the relative importance of these two mechanisms of DVM to phytoplankton we performed a scenario analysis consisting of 5 different scenarios. The results show that DVM has a strong influence on the phytoplankton community. Well edible algae benefit during the first 3–4 weeks of summer stratification by reduced daily grazing. The typical shift from small, well edible algae to larger, poorly or non-edible phytoplankton is distinctly delayed. Under the assumption of unchanged daily grazing, however, a pulsed grazing regime has nearly no influence on the resulting phytoplankton composition. As similar effects are also found for completely non-edible phytoplankton, indirect effects via phosphorus availability must be assumed. Thus, the scenario analysis reveals that the observed effects of DVM on phytoplankton can be explained by a combination of two mechanisms: (1) reduction of the daily zooplankton grazing, and (2) changed assimilation and remineralisation of phosphorus. Surprisingly and in contradiction to earlier reports there is almost no DVM effect on phytoplankton due to the sole action of a pulsed grazing regime.