DEVELOPMENT OF GYPSUM OSL DATING METHODS: INVESTIGATING THE TIMELINES OF PUNCTUATED CLIMATE EVENTS IN LATEST PLEISTOCENE-TO-HOLOCENE LAKE BONNEVILLE GYPSUM DUNES

Dunes serve as repositories of climatic change and may record punctuated events marked by bounding surfaces. This study is focusing on establishing preparation and analysis methods to date such events within gypsum dunes of Knolls, Utah using OSL. The Knolls dunes are within the boundaries of former Lake Bonneville (LB) which once covered most of western Utah, USA, and was the largest of Late Pleistocene megalakes in the Great Basin. By establishing OSL methods that yield accurate dune formation ages from gypsum, we aim to refine geochronology of latest Pleistocene-to-Holocene, particularly timing of LB latest regression.

We have investigated various OSL analysis approaches from a series of samples. Dry or wet sieving, 10% HCl, 49% HF, and <2.37 g/cm³ lithium metatungstate (LMT) separation have been explored, and preparation steps examined optically and with XRD analyses. OSL measurements set-up for quartz (Risø TL/OSL DA-20 system with blue diodes; Hoya U-340 detection filters), but with a pause of 10,000 s replacing usual preheat (Clark-Balzan, 2016) in SAR protocol, was used to test luminescence characteristics, dose recovery thresholds, and determine initial equivalent dose values. Bleaching experiments were conducted to determine effect of lab lights on OSL signals.

Current preparation approaches suggest dry sieving and HCl wash is working best for gypsum, but not for samples with significant quartz. LMT may be appropriate for more quartz-rich samples, although lack of complete separation due to possible interaction between lithium and gypsum needs to be further investigated. Natural OSL signals are weak but measurable; modified SAR protocols have good recycling, low recuperation, and dose recovery ratios near unity. Results of bleaching experiments indicate subdued red light lab conditions are most appropriate.

Completion of this study will allow sixteen samples from Knolls, UT to be dated and may refine the geochronology of the late regressive stage of LB, the hydrograph of which is currently comprised of a broad range of ¹⁴C dates. This work also adds to a growing body of data suggesting gypsum as a reliable luminescence chronometer.

Clark-Balzan, L., 2016, Source and characteristics of blue, infrared (IR), and post-IR IR stimulated signals from gypsum-rich samples: Ancient TL, 34, p. 6-13.

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Session No. 53

T197. No Dates: No Rates—Utilizing Geochronometers to Quantify Rates of Geomorphic Processes or Archaeological Development over a Wide Range of Temporal and Spatial Scales

Sunday, 22 October 2017: 1:30 PM-5:30 PM

Room 3B (Washington State Convention Center)

Geological Society of America Abstracts with Programs. Vol. 49, No. 6
doi: 10.1130/abs/2017AM-306540

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