Optical dating of coarse-silt sized quartz from loess: Evaluation of equivalent dose determinations and SAR procedural checks

This study discusses the values of equivalent dose (De)(De) determined for samples taken for optical dating from a loess section at Loveland, Iowa, North America. A modified single-aliquot regenerative-dose (SAR) procedure was applied to quartz grains of 35–50μm diameter which were isolated using hydrofluorosilicic acid (H2SiF6). ‘Preheat plots’ were constructed to select appropriate DeDe values for use in subsequent age determinations. The DeDe values of the younger (Peoria loess) samples varied little with preheat temperatures between 160 and 300∘C. However, the older (Pisgah Formation) samples showed a decrease in DeDe values with increasing preheat temperature, by as much as a factor of two. Interestingly, both younger and older samples could recover a known laboratory administered radiation dose for a range of preheat temperatures. These apparently contradictory findings present a dilemma regarding which DeDe values are the most appropriate to use for dating the samples. The behaviour of the samples is examined to investigate the cause of the variations in DeDe values with preheat temperature for older samples. In addition, two modified SAR procedures were tested in an attempt to generate more reproducible DeDe values. The data from the three SAR methods are examined, and the question of how to discriminate between DeDe values is discussed. This study demonstrates that preheat temperature can play a significant role in the determination of DeDe values. The use of plots of normalised luminescence signal (Lx/Tx)(Lx/Tx) versus preheat temperature is proposed to supplement dose recovery tests and preheat plateau tests.