BLEACHING OF K-FELDSPARS IN TURBID WATER SUSPENSIONS - A COMPARISON OF PHOTOLUMINESCENCE AND THERMOLUMINESCENCE SIGNALS

In order to investigate at what levels of turbidity the luminescence signals of waterlaid sediments can be reset for dating, bleaching experiments have been performed in a test tube containing different suspensions of glacial origin. Light comparable to the solar spectrum was introduced at the surface, and grains (100-200-mu-m) of K-feldspars were exposed to subdued light through the suspensions for different periods of time. Thermoluminescence (TL) and photoluminescence (PL) signals from exposed and unexposed samples recorded on an automated Riso Reader were compared. For dilute suspensions (< 0.02 g/l) it was found that an air-generated turbulence introduced to keep material in suspension to a large extent was responsible for the observed reduction in bleaching. It is believed that a similar reduction from turbulence itself is likely to take place in natural streaming waters, when air is dragged into suspension. At the applied level of turbulence the PL signal was reduced by 95% within 20 hr whereas the TL only was reduced by 25-50%. In the more dense suspensions (> 0.05 g/l) light levels were very low (< 0.005 mW/cm2), and the light reduction was mainly caused by the suspended particles rather than by the turbulence. Only little bleaching of any of the luminescence signals was observed here. These results illustrate the potential but also some limitations of the optical technique for dating waterlaid sediments.