Testing a simplified SAR TT-OSL protocol for loess deposits in the midcontinental United States
thesisposted on 07.12.2012 by Nathan D. Brown
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The Mississippi and Missouri river valleys in the midcontinental United States contain extensive loess-paleosol sequences that are used to constrain glacial-interglacial transition times. Previous studies have been unsuccessful in producing finite ages for sediments older than ~200 ka due to saturation of luminescence emissions. Since the thermally transferred optically stimulated luminescence (TT-OSL) signal has been shown to grow with high (kGy) radiation doses, the TT-OSL dating technique is tested on the fine-grained (4-11 μm) quartz fraction of these loess deposits. The TT-OSL continued to increase with radiation dose > 900 Gy. Equivalent dose values are highly sensitive to preheat temperatures. Recycling ratios, zero-dose response values, and dose recovery tests all perform well for samples with burial doses > ~200 Gy. TT-OSL ages compare well with most thermoluminescence (TL) and infrared stimulated luminescence (IRSL) ages; all samples from the Peoria Loess, Roxana Silt, Teneriffe Silt, and Loveland Silt agree with both TL and IRSL ages within 1σ. For the oldest unit, the Crowley’s Ridge Silt, TT-OSL ages are younger than IRSL or TL ages by ~20%. This is interpreted as underestimation related to TT-OSL signal contamination, which can be avoided by isolating the fast component of the TT-OSL. Preliminary fast component TT-OSL ages for the Crowley’s Ridge Silt favor deposition during Marine Isotope Stage (MIS) 7 or 8, contrary to a previous inference of a MIS 12 deposition.