Luminescence characteristics of quartz grains from the Titel Loess Plateau

Abstract

The Titel loess plateau in the Vojvodina region of Serbia is considered to contain the most detailed terrestrial paleoclimatic records in Europe, with a thick and apparently continuous record extending through the middle and late Early Pleistocene. In the past few decades the plateau has been investigated on a fairly large scale and has provided important records of climatic and environmental changes during the Pleistocene in this part of Europe. Regardless the extensive research in the past, high detailed optically stimulated luminescence (OSL) dating of the Titel loess plateau has not yet been conducted on the site, despite having been shown in China to be essential in development of reliable chronologies and in accurate reconstruction of the last two glacial-interglacial cycles. The absence of a detailed independent chronology limits the paleoclimatic interpretations possible from this valuable archive.

In order to obtain detailed stratigraphic, climatic and chronological information from the plateau, two cores were drilled in the northern area of the plateau, near the village of Mošorin (45.296225 N 20.188648 E). The first core (TLP 1) was drilled to a depth of 22.93 m while the second one (TP 1A) was drilled to a depth of 21.42 m. The two cores were drilled side by side, covering about the same stratigraphic depths. Sampling for equivalent dose determination using the Single Aliquot Regeneration (SAR) protocol was undertaken in continuous sections at 13-15 cm resolution in both cores. Sample for dose rate via laboratorygamma spectrometry was undertaken in 6 cm sections matching the luminescence samples. In core TLP1 114 samples were obtained for both equivalent dose and dose rate measurements, while from TLP 1A 105 samples were obtained. In this study, we present the results from a detailed OSL investigation of the 63-90 µm quartz from the upper part of core TLP 1.

Aliquots for the OSL measurements were obtained from samples 168110 (9.5 cm depth) – 168138 (633 cm depth), corresponding to the L1 loess-paleosol sequence. This fraction proved to have acceptable luminescence properties which were confirmed by conducting preheat plateau, purity check and dose recovery tests.

The purity of the quartz was determined by performing purity checks on each sample before further measurements. In most cases the purity of the quartz was acceptable. There was no significant infrared stimulated luminescence (IRSL) reaction at 60°C to a large regenerative dose and the 110°C TL peaks are clearly defined. The OSL signal is bright and generally dominated by the fast component. The natural and regenerated decay curves show a very close match to each other as well as to the signal from the calibration quartz.

In order to determine an appropriate preheat temperature, a preheat plateau test was performed on 24 aliquots of the sample 168122 (309 cm depth). Temperatures ranging from 160°C to 300°C were engaged for 10 s preheat in combination ith 160°C test dose preheat. The De value did not show any significant sensitivity to preheat temperature up to approximately 260°C. The recycling ratios are close to unity and the recuperation is close to 1% in the 160-300°C interval. The dose recovery test was performed on samples 168110, 168111, 168113, 168114, 168115, 168116, 168141,168143 (L1 unit) and 168158, 168159 (S1 unit). Various preheat temperatures were used ranging from 160°C to 300°C with a fixed 220°C cut heat. The average of measured to given dose ratio for all the samples (n=10; 4 aliquots per sample) was 1.06 ±0.03 which confirmed that neither the De nor the ratio of the measured to given dose are dependent of previous preheat temperature and a known laboratory dose can be recovered. Based on these results, a preheat temperature of 260°C for 10 s and cut heat of 220°C was chosen for all following measurements.

For the determination of the De values a standard single-aliquote regenerative (SAR) protocol with blue stimulation lights was used. For each sample (168110–168138; n=29), a minimum of five replicate measurements were conducted so far, except for samples 168117 and 168129 where no sufficient quartz grains could be extracted.

The dose rates are currently in the process of calculation and no precise estimation of the actual age of the samples can be conducted at this time. Nevertheless, using typical dose rate values from Serbian loess, we were able to determine great consistency of the De values with the last glacial ages.

The calculated De values range from 20 ±1 Gy for the uppermost sample to 154 ±15 Gy for the lowest samples investigated and correspond with the stratigraphic position of the samples. The De value increases consistently to a depth of 345 cm (sample 168124) where it reaches 122 ±18 Gy and where the natural OSL signal was proved to be in saturation. By fitting a single exponential saturating dose response curve, a De value of ~120 Gy was obtained. The De values of the lower samples (168125-168138), showed significant scattering and inconsistence ranging from 92 ±10 Gy for sample 168125 up to 197 Gy for sample 168132. These results confirm our hypothesis that quartz from the Titel loess plateau cannot be reliably used to determine doses larger than ~120 Gy and K-feldspar will be more suitable for the measure of the De values of older samples.

Regardless of the limited OSL characteristics of the quartz grains from the Titel loess plateau, we can deduce that a reliable dose determination up to ~120 Gy is possible, covering roughly the last glacial cycle. We hope that these preliminary results, in addition to the results from the ongoing investigations (dose rate via laboratory gamma spectrometry, K-feldspar luminescence dating) will greatly contribute to the reconstruction of climatic and environmental changes during the Pleistocene in this part of Europe.

Resources

Bibliography

Peric, Z., Stevens, T., Buylaert, J., Markovic, S., Hambach, U., Obreht, I., Zeeden, C., Veres, D., Lehmkuhl, F., Fischer, P., Vött, A., Gavrilov, M. (2016): Luminescence characteristics of quartz grains from the Titel Loess Plateau. Loess2M - Modelling & Mapping. 26-29.8. 2016, Novi Sad, Serbia.