1. Development of the pIt-IRSL protocol for dating polymineral fine grains from Chew Bahir, Ethiopia.
- Author
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Ataee, Nina, Roberts, Helen M., and Duller, Geoff A. T.
- Subjects
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MASTICATION , *DRILL core analysis , *DETERIORATION of materials , *SIGNAL processing , *PETROLOGY - Abstract
Rift lakes are perfect places for sediment accumulation and hence potentially contain continous paleoclimate records. Located in the eastern African rift valley, in southern Ethiopia, Lake Chew Bahir preserves a record of environmental change which is pertinent to the story of hominin evolution and dispersal. Independent chronology from various dating techniques is available throughout this 293 m composite core record, from which a coherent Bayesian age-depth model was generated spanning the last 620 ka and providing one of the longest directly-dated lacustrine sediment sequences in eastern Africa [1]. However, there are regions of the core where the age depth model could be improved if further datable materials were available. For instance, beyond ~50 m depth, the quartz OSL signal appears to be saturated and there is a large gap to the first 40Ar/39Ar age. The luminescence signal from feldspars saturates at a greater De value than that from quartz and therefore offers the potential to extend the maximum age range of the luminescence-derived chronology in the core, and to fill in gaps within the quartz-derived chronology where quartz is not available. The luminescence chronology for the Lake Chew Bahir core was refined and extended using the fading corrected post-IR IRSL225 signal from polymineral fine grains. However, there are challenges and complications regarding both assessment of fading and the subsequent application of the fading rates. The duration of the delay times used to assess fading, and the given laboratory dose (two critical factors in measurement of fading rates), were the same for all samples, but because of variability in lithology in the core, it is not clear whether sample specific fading rates should be used or if the mean fading rate of all samples is appropriate. Moreover, selection of the most appropriate fading correction model is critical. To circumvent the complications of fading rate measurement and its application, the recently developed post-Isothermal (pIt) IRSL protocol [2] was therefore also explored. Application of a modified pIt-IRSL225 protocol to polymineral fine grains from this core, demonstrates that this signal is successful in providing accurate ages for this material without any fading correction. Additionally, uncertainty on the derived ages were reduced in comparison with the fading corrected post-IR IRSL225 ages. However, since the calculated De from this protocol relies on the IR50/pIRIR225 De ratio, it seems to be limited to the saturation limit of the IR50 signal. This may be the reason that the pIt-IRSL225 age for one of the older samples in the core (~300 ka) does not agree with its fading corrected post-IR IRSL225 age. Possible modifications to the pIt-IRSL225 protocol to overcome this limitation will be discussed. Furthermore, the suitability of the isothermal annealing step in this protocol to account for the athermal process of signal loss (i.e. fading) was investigated. The data suggest there is a correlation between the isothermal annealing time (for which the De in the pIt- IRSL225 protocol is reached) and the g-values determined using IR50 and pIRIR225 signals. [ABSTRACT FROM AUTHOR]
- Published
- 2023