SHEDDING NEW LIGHT ON THE AGE OF THE PALOUSE LOESS, SOUTHEASTERN WASHINGTON, USING OPTICALLY STIMULATED LUMINESCENCE DATING OF QUARTZ
Abstract
A single-aliquot regenerative-dose (SAR) protocol was applied to fine-silt (4-11 micron), coarse-silt (35-50 micron), and sand-sized (90-125 micron) quartz grains extracted from loess from the Palouse, southeastern Washington. Since its introduction in 2000, this optically stimulated luminescence (OSL) dating method has been applied to a wide variety of sediments, and has been shown to be robust and reliable. A clearly defined set of quality control checks has been developed to assess the suitability of samples to which this method is applied; if a sample passes these quality checks, then the ages are considered to be reliable. For the Palouse loess samples in this study, the luminescence characteristics of the various size fractions differed, and not all quartz fractions proved to be suitable for OSL dating. The coarse-grain quartz samples appear to be contaminated with feldspar, probably as inclusions within the quartz grains. OSL ages were determined using the 35-50 micron quartz fraction; the OSL signals from both this and the 4-11 micron quartz fraction passed all of the quality control checks associated with determining an accurate and precise quartz SAR OSL age. On the basis on the luminescence characteristics, therefore, the quartz OSL ages generated are believed to be robust and reliable. At some sites within the Palouse Loess region, the quartz OSL ages generated were in agreement with independent age control provided by chemical identification of tephra layers associated with known eruptions. However, at a number of sites the quartz OSL ages and the tephra evidence did not agree. The cause of this discrepancy was unknown, but given the anticipated reliability of the quartz OSL ages, attention turned to re-evaluation and re-analysis of the tephra layers which had originally been sampled and analysed more than 15 years prior to collection of the samples for quartz OSL dating. The chronology at the site was further complicated by two much earlier studies which applied IRSL and TL dating methods to polymineral fine-grains, generating ages which were acknowledged by the authors to potentially underestimate the true age of the loess units, whilst other ages appeared to agree broadly with the ages of the tephra units identified. Both the previously published luminescence ages and tephrochronologic data, and the newly generated OSL and tephra data are evaluated; the implications for stratigraphic interpretation at these and other loess sites is discussed.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMPP21B1353R
- Keywords:
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- 1105 GEOCHRONOLOGY / Quaternary geochronology;
- 1145 GEOCHRONOLOGY / Tephrochronology