Applications and limitations of thermoluminescence to date quaternary sediments

Abstract

The thermoluminescence (TL) method dates sediment by quantifying time-dependent dosimetric properties of mineral grains. Sunlight exposure of the sediment during transportation and deposition or heating resets the TL clock to time ‘zero’. After burial of the mineral grains, ionizing radiation from the decay of radioisotopes imparts a TL signal. This accumulated TL signal is proportional to the equivalent dose (ED), which is a measure of radiation exposure during the burial period. A TL age estimate is determined from the ratio of the ED to the environmental radiation (dose rate). The calculation of an ED is influenced by the susceptibility of sediments to optical bleaching, the emission spectra of minerals, and possible long and short term fading. There are three basic methods for determining an ED: regeneration, total and partial bleach. The regeneration technique may yield under-estimates because of sensitivity changes with laboratory bleaching. The latter two techniques are most useful for well and partially light bleached sediments, respectively. A number of sediments can be dated by the TL method. Eolian and a variety of terrestrial sediments yield apparently accurate TL age estimates, though there are possible complications associated with pedogenesis. It appears that some fluvial sediment can be accurately dated by TL, but little is known about the efficiency of optical bleaching for fluvial depositional systems. Till and glacier-proximal deposits are inadequately light bleached to be dated by TL. Sediments deposited distal from the glacier margin, which are at least partially bleached, are preferred for dating. Future research is needed to understand better the relationship between sedimentary environments and the extent of light bleaching. Also, studies should concentrate on dating feldspar extracts from well-dated sequences to more fully ascertain the temporal limitations of the method. Only by understanding the interplay of sedimentology, TL dosimetry and environmental radiation can the TL method be effectively applied to the myriad of depositional environments spanning the late to middle Pleistocene.