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Identifying the degree of luminescence signal bleaching in fluvial sediments from the Inner Mongolian reaches of the Yellow River

Li F., Pan B., Lai Z., Gao H., Ou X.

Geochronometria

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2018

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Vol. 45, no. 1

82--96

EN

The partial bleaching of the luminescence signal prior to deposition results in age overestimation, and can be a problem in delineating fluvial evolution within an OSL chronological framework. The Inner Mongolian reaches of the Yellow River are characterised by a high sediment load and complex sources of sediments. To test the incomplete bleaching occurring in this type of environment, the residual doses and the luminescence signal characteristics of different particle size fractions from 14 modern fluvial sediment samples were investigated. Furthermore, 26 OSL ages derived from drilling cores were compared with 11 radiocarbon ages. Our results show that the residual equivalent doses principally range between 0.16 and 0.49 Gy for silt grains, and between 0.35 and 3.72 Gy for sand grains of modern samples. This suggests that medium-grained quartz has been well bleached prior to deposition, and is preferable to coarse-grained quartz when dating fluvial sediments in this region. The results also show that the De values of coarse-grained fractions display a stronger correlation with distance downstream. In addition, a comparison of OSL and radiocarbon ages from drilling cores establishes further confidence that any initial bleaching of these sediments was sufficient. As a result, we believe that the studied fluvial samples were well bleached prior to deposition.

2

Employing Minimum age model (MAM) and Finite mixture modeling (FMM) for OSL age determination of two important samples from Ira Trench of North Tehran Fault

Fattahi M., Heidary M., Ghasemi M.

Geochronometria

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2016

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Vol. 43, no. 1

38--47

EN

Ira trench site is in a point where, the surface trace of North Tehran Fault (NTF) joins the Mosha Fault (MF) in the north-eastern margin of Tehran and can provide important paleosismological information for Tehran. The Ira trench, were divided into 6 packages (I to VI), described, according to their composition, relative and absolute ages. Package I consists of units 23, 25, 26, 27, 28, 29, 30 and 31. The whole package I mainly belongs to Holocene, and provides essential constraints for the recent paleo-earthquake activity of the EMF and NTF zone. Therefore, finding accurate ages for the units of this package is very important. Three colluvial wedges (units 23, 26, 28) are present between 20 and 36.5 m north in package I, which are assigned to 3 episodes of activity on Fault 13. Central age model (CAM) provided OSL ages of 35.0 ± 6.1, 7.3 ± 1.3, 6.4 ± 0.9 and 56 ± 6.5 ka for units 23, 26, 28 and 29, respectively. The conflicting ages of 56 ± 6.5 and 35.0 ± 6.1 ka (for units 23 and 29, respectively) as compared to the underlying younger units suggest that these ages are overestimated. MAM provided OSL ages of 13.1 ± 4.3 and 3.5 ± 0.4 ka for units 23 and 29, respectively. The contribution of the new statistical age model of sample IRA4 to the paleoseismic data is discussed.

3

Performance of different luminescence approaches for the dating of known-age glaciofluvial deposits from northern Switzerland

Gaar D., Lowick S. E., Preusser F.

Geochronometria

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2014

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Vol. 41, no. 1

65--80

EN

Luminescence properties of two samples taken from sand lenses in proglacial outwash de-posits of a piedmont glacier that reached the Swiss midlands during the Last Glacial Maximum are investigated in detail. Deconvolution of CW-OSL decay curves shows that the fast component domi-nates the OSL signal of quartz. The chemistry of single feldspar grains, in particular the K content in different grains, is determined using wavelength dispersive spectrometry (electron microprobe), re-vealing an average 12.9 wt.% K of the grains contributing to the IRSL signal. De distributions are in-vestigated in order to gain insights into partial bleaching, and agreement is found for quartz OSL and feldspar IR50 and pIRIR225 ages for small aliquots and single grains when applying the Minimum Age Model. These ages are also consistent with independent age control. For one sample, ages determined using the Central Age Model result in highly overestimated ages for both feldspar and quartz.

4

Expectations of scatter in equivalent-dose distributions when using multi-grain aliquots for OSL dating

Cunningham A. C., Wallinga J., Minderhoud P. S. J.

Geochronometria

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2011

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Vol. 38, no. 4

423-431

EN

In the OSL dating of sediment, the scatter in equivalent dose (De) between grains is almost always larger than would be expected due to counting statistics alone. Some scatter may be caused by insufficient (partial) bleaching of some of the grains prior to deposition. In order to date partially bleached sediment, it is essential to estimate the amount of scatter caused by other processes (e.g. grain-to-grain variability in the natural dose rate). Measurements of such scatter are performed at the single-grain level; by contrast, most OSL dating is performed on multi-grain subsamples, for which grain-to-grain scatter is reduced through averaging. Here we provide a model for estimating the expected scatter (i.e. excluding that caused by partial bleaching) for multi-grain aliquots. The model requires as input the single-grain sensitivity distribution, the number of grains in the sub-samples, and the expected scatter at the single-grain level, all of which can be estimated to an adequate degree. The model compares well with measured values of scatter in De, determined using aliquots of various sizes, and can be used to help produce a minimum-age De from multi-grain subsamples that is consistent with single-grain data.