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 (D _e) 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 35-50 μ m diameter which were isolated using hydrofluorosilicic acid (H ₂SiF ₆). 'Preheat plots' were constructed to select appropriate D _e values for use in subsequent age determinations. The D _e values of the younger (Peoria loess) samples varied little with preheat temperatures between 160 and 300 ^{{ring operator}} C. However, the older (Pisgah Formation) samples showed a decrease in D _e 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 D _e 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 D _e values with preheat temperature for older samples. In addition, two modified SAR procedures were tested in an attempt to generate more reproducible D _e values. The data from the three SAR methods are examined, and the question of how to discriminate between D _e values is discussed. This study demonstrates that preheat temperature can play a significant role in the determination of D _e values. The use of plots of normalised luminescence signal (L _x / T _x) versus preheat temperature is proposed to supplement dose recovery tests and preheat plateau tests.