AN ANALYSIS OF POTASSIUM TO RUBIDIUM RATIOS IN SHORELINE SEDIMENTS: IMPLICATIONS FOR OSL DATING

Optically Stimulated Luminescence (OSL) is a rapidly growing dating technique for the study of late-Quaternary geology, but the luminescence measurements themselves only yield a sample’s absorbed dose of natural radiation, called the equivalent dose. The other value needed to produce an approximation is the environmental dose rate, which is calculated using the sample’s known concentrations of four radioactive elements: potassium (K), rubidium (Rb), thorium (Th), and uranium (U). Because some elemental analysis techniques, such as gamma counting, do not provide a direct measurement of Rb concentration, the ratio of potassium to rubidium in a given sample is often used to approximate this value. For this purpose, a constant K:Rb ratio of 200:1 has been used. The goal of this study was to evaluate this ratio for a large set of shoreline samples. Data was compiled from 200 samples that had been dated using OSL as part of previous projects. These samples varied in origin, with a total of 84 samples collected from Lake Agassiz, 63 from Lake Huron, and 53 from Lake Superior. The data was plotted on a scatterplot such that the slope of the line-of-best-fit represented the average K:Rb ratio for the data sets. The results were far more dispersed than anticipated. Overall, the samples had an average K:Rb ratio of 284:1 with data variance reported as an r-squared value of 0.83. These results, however, were largely influenced by the data from Lake Superior, which exhibited a 291:1 K:Rb ratio and a strong correlation of 0.93. The Lake Agassiz and Lake Huron samples bore much different ratios and weaker correlations, with K:Rb ratios of 180:1 and 198:1, respectively, as well as r-squared values of 0.38 and 0.45, respectively. Samples from Lake Agassiz’s beach ridges were analyzed as independent groups. Data from most of its shoreline sets were consistent with Lake Agassiz’s 180:1 mean; however, ratios from the earliest beaches, Norcross and Herman, diverged considerably. Our results indicate that a large number of K:Rb ratios from shoreline sand deposits deviate significantly from the 200:1 ratio commonly used for approximating the Rb concentration and provide data-based confirmation that independent measurement of Rb concentrations would provide more accurate OSL dose rate analyses for sediments of this type.