LUMINESCENCE IN A DIFFERENT LIGHT: AN EXPLORATION OF THE BENEFITS AND CHALLENGES TO LUMINESCENCE DATING IN THE CRITICAL ZONE

Luminescence dating provides an age estimate for the last time sediment was exposed to light or heat. The luminescence signal is acquired in quartz and feldspar mineral grains following burial due to exposure to radioactive decay from the surrounding sediment and incoming cosmic radiation. Often, optical and infrared stimulated luminescence dating (OSL and IRSL) is applied to archaeologic materials or sands and silts from alluvial, colluvial and eolian deposits below the active soil profile, to provide ages used for calculating rates/timing of deposition. Soils and pedogenically-altered parent material are typically avoided due to mixing of sediment with various exposure histories, and changes in the dose-rate environment, both of which affect the accuracy and precision of luminescence ages. However, luminescence analysis of mineral fractions from the critical zone or soil profile can provide information about grain sourcing, mixing, and cycling that is not as readily available from other geochemical and geochronologic techniques.

The sensitivity, or brightness of the luminescence signals, provides information about grain mobility and cycling at the Earth’s surface, with luminescence sensitivity increasing with greater number of cycles of zeroing (bleaching of the luminescence signal) and dosing (burial). Single-grain analysis can provide valuable assessments on the rate and depth of soil processes by recording the last time grains were exposed to light (or heat from wildfire) at the land surface. On the other hand, luminescence exploration of saprolite may provide information from the soil-regolith interface regarding bedrock weathering. Thus, in addition to providing an age for sediment deposition or artifact manufacture, luminescence techniques provide a unique and not fully utilized method to understand processes in the critical zone. Characterization of luminescence signals from soil/regolith profiles are especially enlightening when coupled with information from geochemical analyses and measurement of short-term (e.g. Pb-210, Cs-137) and cosmogenic (Be-10, Cl-36, C-14) nuclide concentration. This presentation will discuss the benefits and provide examples of challenges to luminescence dating in the critical zone.