Novel Approaches to Improve Robustness of K-feldspar IRSL Age Estimates of Sediment from Active Tectonic Contexts

Recent developments in luminescence dating, in particular using the post-IR IRSL (InfraRed Stimulated Luminescence) at 225 degrees C for single grains of potassium feldspar, extend the range of applicable contexts for dating fault-offset geomorphic features and seismically deformed sediment. Coarse gravels recording high-energy fluvial conditions appear to provide consistent age estimates, as do sediments in desert contexts, where radiocarbon is resricted. At first order, single-grain K-feldspar post-IR IRSL-225 works extremely well.

Limitations can arise under certain specific conditions. These include situations in which a deposit composed of grains that were well-exposed to light prior to burial is reworked under low-light conditions (such as at night or during a storm) and grains are redeposited into a new sediment body that is subsequently the target for dating. The degree that these two different events may be distinguished depends on the luminescence characteristics of grains, the age difference between the two depositional events, and the proportions of grains each provides to the target deposit. This effect may be referred to as "shadowing".

The authors are currently developing new luminescence measurement protocols that help identify grains that were well-bleached (i.e., exposed to daylight sufficiently to reduce their relevant constituent trapped charge populations to a low level prior to burial). One approach uses luminescence photochronometry, that is, the assessment of past light exposure duration, for each grain as part of the dating protocol. This is achieved using multiple elevated temperature (MET) IRSL, measuring signals with different sensitivities to light. In this manner, age assessment can be based only on responses from those grains that were sufficiently well-bleached for the dating requirement; this approach reduces reliance on the assumption that shared apparent age is sufficient to identify depositional age populations, and may improve resolution between different events closely spaced in time that have the potential to lead to shadowing. It may also provide useful information regarding the timing of past events and the environmental conditions pertaining prior to deposition.