RECONCILING THE LUMINESCENCE- AND TEPHRO-CHRONOLOGIES OF THE PALOUSE LOESS, WASHINGTON STATE, USA

The introduction of sensitivity-corrected single-aliquot optically stimulated luminescence (OSL) measurement protocols for quartz, have brought about a step-change in both the accuracy and precision of OSL ages generated in the last 15 years. Critically, these protocols incorporate a series of internal quality control checks which can be used to assess the likely reliability of the OSL signal used for dating.

These OSL methods were applied to loess believed to be post-Last Glacial Maximum (LGM, ~18 ka) to Holocene in age, taken from within the Columbia Plateau region, SE Washington, USA. A chronostratigraphic framework had previously been developed and published for the region on the basis of numerous tephra found within this Palouse Loess, providing an independent line of evidence against which the luminescence ages could be tested. At some sites, agreement was found between the quartz OSL age determinations and the independent age estimates provided by the tephra chronostratigraphic framework. However, at a number of other sites the quartz OSL ages and the tephra evidence did not agree, in spite of the samples passing rigorous OSL quality control tests at all of the sites studied. The cause of this discrepancy between the OSL ages and the independent dating evidence was not known, and the picture was further complicated by the presence of previously published IRSL and TL work which agreed with some of the independent tephra evidence.

This discrepancy was intriguing, given the typically reliable nature of quartz SAR OSL ages in this time range demonstrated in many other studies worldwide. In this presentation, both the previously published luminescence ages and tephrochronologic evidence, and newly generated OSL ages obtained using both the latest quartz and feldspar methods will be evaluated, in order to reconcile the disparities between the various lines of chronostratigraphic evidence. The robust and coherent new luminescence evidence collected at several Palouse Loess sites suggests that at some sites, some of the previously identified Mount St Helens tephra have potentially been mis-assigned. The importance and value of quality control tests within the latest luminescence dating techniques is discussed, and the need for equally rigorous scrutiny of other independent chronologic evidence is also considered.