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Changes in natural OSL sensitivity during single aliquot regeneration procedure and their implications for equivalent dose determination

Singhvi A. K., Stokes S. C., Chauhan N., Nagar Y. C., Jaiswal M. K.

Geochronometria

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2011

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Vol. 38, no. 3

231-241

EN

Measurement of low temperature (90o, C-120o,C) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensi-tivity changes can occur during measurement of the natural Optically Stimulated Luminescence (OSL). During the measurement of natural signal, the luminescence sensitivity of samples can change by 40%. The sensitivity changes both during the initial preheat and the measurement of natural OSL. The currently used version of Single Aliquot Regeneration (SAR) protocol measures and corrects for the sensitivity changes after preheat and readout of natural OSL. However, it does not take into ac-count the changes in sensitivity during the readout of the natural signal. We therefore developed a correction procedure so that both the natural and the regenerated OSL in-tensities can be measured and plotted with the same sensitivity and suggest that in the absence of such a correction, a considerable fraction of the SAR based ages could have systematic errors. The correc-tion for the sensitivity is based on the use of sensitivity of 110o, C TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110o, C peak provides a reasonable measure of the changes in OSL sensitivity of quartz. A modified Natural Sensitivity Corrected–SAR (NSC-SAR) procedure, that comprises the measurement of, 1) the TL intensity of 110o, C peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110o,C peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correc-tion Factor (NCF), then provides a way to correct for sensitivity changes. Results on samples from diverse depositional environments indicated that the NSC-SAR consistently (without exception) provided improved distribution in paleodoses i.e. a lower scatter compared to the standard SAR protocol. In addition, the use of this protocol also resolved anomalous cases where the intensity of natural OSL was significantly above the saturation intensity of the regenerated OSL. Implicitly, this study implies a caution on the use of palaeodoses obtained from single grains as such a correction is not possible in the currently used automated single grain OSL measurement systems. The only way now on will be to analyze aliquots with only a grain on them.

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Palaeoflood records from Upper Kaveri River, Southern India: evidence for discrete floods during Holocene

Kale V. S., Achyuthan H., Jaiswal M. K., Sengupta S.

Geochronometria

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2010

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Vol. 37

49-55

EN

A record of six discrete middle Holocene floods has been established based on sedimentological and stratigraphical studies in the upper Kaveri catchment at Siddapur. The flood events are represented by six discrete, sharp-bounded, sand-silt couplets. Texturally and geochemically the suite of couplets is quite distinct from the overlying and underlying structureless fluvial deposits. Based on OSL ages the suite of couplets cover the Holocene from ~8 to ~2 ka. Such evidence is not present or reported from any other river originating in the Western Ghat in the Indian Peninsula. We argue that the six couplets represent short-term, high discharge events or flash floods. The initiation of this phase of flash floods broadly corresponds with the southward migration of ITCZ and a gradual decline in Indian summer monsoon precipitation starting at ~7.8 ka. Comparison of the elevation of the highest couplet with the high flood level (HFL) of the 1961 extraordinary flood on Kaveri demonstrates that the 20th century flood was higher than the mid-Holocene palaeofloods.

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Residual luminescence in quartz from slack water deposits in Kaveri Basin, South India : a single aliquot approach

Jaiswal M. K., Chen Y. G., Kale V. S., Achyuthan H.

Geochronometria

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2009

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Vol. 33

1-8

EN

Optical bleaching of quartz may be incomplete due to decreased daylight intensity and narrower wavelength spectrum in water column, leading to age overestimation in young fluvial sediments. This hypothesis was tested on a low energy slack water deposits from Kaveri Basin in south India using the SAR protocol. Various tests suggest successful application of SAR on quartz from the study area despite low sensitivity and few numbers of bright grains. Partial bleaching can be ruled out if using SAR on small aliquots. The results suggest a uniform residual luminescence and a common sedimentary condition in this area. Due to partial bleaching, the lowest values of palaeodoses should be considered in preference to the average values.

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Feasibility of the SAR technique on quartz sand of terraces of NW Himalaya: a case study from Devprayag

Jaiswal M. K., Srivastava P., Tripathi J. K.

Geochronometria

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2008

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Vol. 31

45-52

EN

Optically Stimulated Luminescence (OSL) dating technique based on the Single Aliquot Regenerative dose (SAR) protocol is being used increasingly as a means of establishing sediment burial age in the late Quaternary studies. Thermal transfer, low and changing luminescence sensitivity of quartz grains of young sedimentary belts of the New Zealand Alps and the north-east Himalaya poses problems in using SAR protocol. Records of active tectonics and signatures of palaeo-climate are preserved in the Quaternary – Holocene terrace sediments. Therefore, to unfold the history of successive tectonic and palaeo-climate events, robust chronological technique is needed. Palaeoflood deposits in NW Lesser Himalayan region receive quartz from the weathering of various rock types such as quartzite and phyllite in the Alaknanda Basin. A series of tests e.g. dose recovery, preheat plateau, thermal recuperation and change in sensitivity, were performed to check the suitability of quartz grains collected from the terrace sediment of Devprayag of the NW Himalaya, for OSL studies. Inferences were drawn regarding the source of the quartz grains on the basis of the geochemistry and luminescence intensity of the terrace sediment. The study shows that though quartz from the North West Himalaya are low in luminescence intensity but the reproducibility of De value makes the quartz sand suitable for SAR dating technique. Relation between luminescence intensity with CIA values help to predict the provenance of quartz sand. Tests show that the quartz from NW Himalaya is suitable for SAR protocol in OSL.