Dimitri Vandenberghe

IRSL and post-IR IRSL residual doses recorded in modern dust samples from the Chinese loess plateau

Using a set of modern/young (0 to about 200 years old) dust samples collected from the Chinese Loess Plateau the bleachability of IRSL measured at 50°C (IR50) and post-IR50 elevated temperature IRSL (measured at 225°C and at 290°C) is investigated by measuring the apparent (residual) doses recorded by these signals. Doses recorded by quartz OSL are used as a reference. Allowing for differences in dose rates it seems that both IRSL and post-IR IRSL signals yield residual doses that are significantly larger than the doses measured in quartz. These residual doses can be largely explained by thermal transfer caused by preheating. Nevertheless, we advise against the use of a low temperature preheat (<200°C) with IR50 to date loess samples because, as has been reported before, the signal appears to be thermally unstable. In general, we conclude that it may not be advisable to apply post-IR IRSL dating to Chinese loess samples where residuals of up to ∼20 Gy are a significant fraction of the total dose. However, these residuals quickly become unimportant when dating older samples, and this is the age range in which post-IR IRSL dating is likely to be most useful.

A comparison of methods for the annual radiation dose determination in the luminescence dating of loess sediment.

Abstract A study was made of the performance of several methods for annual radiation dose determination in luminescence dating of loess sediment. In a homogeneous loess site at Volkegem (Belgium), NaI(Tl) gamma-ray field measurements were performed, followed by collection of samples which were then analyzed in the laboratory with the following techniques: low-background NaI(Tl) gamma-ray spectrometry; low-background extended-energy range Ge gamma-ray spectrometry; k 0 -standardized reactor neutron activation analysis; thick-source ZnS alpha counting, both in the integral and in the pair counting mode; low-background GM beta counting; and atomic absorption spectrometry. A critical comparison was made of the K, Th and U concentration data, and also of calculated and measured annual radiation doses, as obtained from the different methods investigated. From the Ge gamma-spectrometric results, conclusions could be drawn concerning the equilibrium of the Th and U series.

The age and palaeoenvironmental significance of the Kalahari Sands in western Zimbabwe: a thermoluminescence reconnaissance study.

Thermoluminescence (TL) dating, has been applied to 10 samples from the aeolian Kalahari Sands of western Zimbabwe using the total bleach (additive dose) and regeneration techniques on coarse-grain quartz separates. The results suggest that the main phases of sand accumulation occurred between 10 and 96 ka. This Upper Pleistocene age is consistent with current opinions and argues against an important period of Holocene aeolian activity. The oldest sands were found in the Victoria Falls area where they overlie a ferricrete horizon. Based on our age determinations, we were able to constrain the age of this horizon to between 96±8 and 160±23 ka, which approximately coincides with the Last Integlaciation in the high latitudes. The youngest ages were obtained for sands from the southeastern part of the Hwange dune field but our limited sampling permits assigning only a minimum age of ca 20 ka to the ferricrete horizon observed in this area. Thus, we cannot ascertain if this ferricrete horizon is isochronous with the one at Victoria Falls or if it developed during a younger humid phase. Dune building activity most probably occurred in recurrent cycles of aridity interspersed with periods during which aeolian activity was limited or non-existent. Unfortunately, besides the ferricrete horizons, no other stratigraphical evidence of climatic fluctuation has been observed in the study area. However, it is possible that palaeosols have been entirely eroded before the subsequent deposition of new sediment, resulting in an incomplete stratigraphical record. The possibilities and limitations of luminescence dating for elucidating phases of climatic fluctuation in such situations are discussed briefly. In addition, we noticed serious discrepancies between the results yielded by the different techniques that were used for determining the dose rate. Hence, further work is needed to validate the accuracy of our dose rate measurements that may cause our TL ages to be about 30% too young.

A high resolution optical dating study of the Mostiştea loess-palaeosol sequence (SE Romania) using sand-sized quartz

We report on SAR-OSL dating of closely-spaced samples from the loess-palaeosol sequence near Mostiştea lake (Danube Plain, SE Romania). We used sand-sized quartz and a SAR protocol that involved a preheat of 10s at 220°C and a test dose cutheat to 180°C. It is shown that these thermal pretreatments isolate a quartz OSL signal that is dominated by the fast component. The behaviour of this signal in the SAR protocol is then documented. The ratio of the measured to given dose tends to lie slightly but systematically above unity (∼5% on average) and the recycling ratio below unity (∼6% on average); for all samples, the recuperated signal is negligible. Within analytical uncertainty, the nineteen optical ages are internally consistent and agree with the predictions from a palaeomagnetic age-depth model. Although it may be possible to optimize the SAR measurement procedure, the optical ages already confirm the chronostratigraphic position of the uppermost welldeveloped palaeosol in that it formed during MIS 5. The established chronology allows correlating the sequence near Mostiştea lake with that near Mircea VodĂ that we investigated earlier.

Residual doses in recent alluvial sediments from the ardenne (S Belgium)

Residual Doses in Recent Alluvial Sediments From the Ardenne (S Belgium) We report on our first investigations into the potential of optical dating for determining the rate of river flood sedimentation in the Ardenne region (S Belgium). Two samples collected from a recent alluvial deposit were used to investigate the extent of resetting in different particle size fractions of quartz (4-11 μm, 63-90 μm, 90-125 μm, 125-180 μm, 180-212 μm and 212-250 μm) as well as in polymineral fine (4-11 μm) grains. Both samples show satisfactory OSL and IRSL characteristics. The IRSL signals from the polymineral fine grains yield an equivalent dose (De) of 3-4 Gy, while a De of 0.3-0.6 Gy was measured using large aliquots of quartz. Small aliquot analyses of 63-90 μm and 212-250 μm quartz grains confirm that the coarser fraction contains more grains with lower Des. Furthermore, for a modern sample (< 3 years old), ~60% of the aliquots yields a De consistent with zero, indicating that these contain only well-bleached grains. These findings suggest that it might be possible to extract the true burial dose from dose distributions measured using small aliquots of coarse-grained (e.g. 212-250 μm) quartz.

The Last Interglacial palaeosol in the Belgian loess belt: TL age record

Abstract In the Belgian loess plateau, the Rocourt soil is a well-known luvisol horizon that is regarded as representing the Last Interglacial. The name of the soil comes from the locality of Rocourt in the east of the country, where it has been observed for the first time. Later, exposures of a similar soil were found in other localities. Most typically, a sequence of three horizons is observed: a red-brown illuvial Bt horizon, overlain by a bleached horizon and a compound dark horizon. In this paper, we present the results of TL age determinations obtained on sediments directly bracketing the soil at the type locality of Rocourt and at the locality of Momalle more to the west, together with some supplementary data for the exposures at Kesselt to check a controversial result that was obtained earlier (J. Quaternary Sci. 13 (5) (1998) 487). The TL analyses involved both the (total bleach) additive dose and regeneration methods. Our TL ages confirm the assumed chronostratigraphic position (oxygen isotope stage 5) of the soil exposed at the type locality. The ages obtained at Momalle and Kesselt indicate that we are dealing with the same soil here but the palaeodose data are less consistent. This is probably due to the fact that the loess has been disturbed by post-depositional processes such as solifluction, surface run-off, cryo- and bioturbation. This seems to be a major problem with the sediments in the eastern part of the Belgian loess belt.

First evidence of Pleistocene rock art in North Africa: securing the age of the Qurta petroglyphs (Egypt) through OSL dating

Long doubted, the existence of Pleistocene rock art in North Africa is here proven through the dating of petroglyph panels displaying aurochs and other animals at Qurta in the Upper Egyptian Nile Valley. The method used was optically stimulated luminescence (OSL) applied to deposits of wind-blown sediment covering the images. This gave a minimum age of ~15 000 calendar years making the rock engravings at Qurta the oldest so far found in North Africa.

The timing of aeolian events near archaeological settlements around Heidebos (Moervaart area, N Belgium)

At the locality of Heidebos (Moervaart area, N Flanders, Belgium), a sedimentary core was taken in the Maldegem-Stekene coversand ridge and dated using optically stimulated luminescence (OSL). The study aimed at contributing to an improved understanding of the evolution of the physical landscape around archaeological settlements in this area. The core comprised a 7 m thick series of laminated and massive aeolian sands, in which several organic layers were intercalated. From this sequence, 11 samples were collected for quartz-based SAR-OSL dating; an internally consistent dataset was obtained. The ages of the lowermost 1 m of the sedimentary sequence (15.5±1.1 ka and 17.3±1.3 ka) imply that these sediments may represent the time-equivalent deposit of a deflation phase that occurred during the Late Pleniglacial and led to the formation of a widespread desert pavement, regionally known as the Beuningen Gravel Bed. However, a significant part of the sediments (at least 4 m) was deposited later, i.e. during the Allerod and/or the Late Dryas. As such, the results allow establishing the genesis of the coversand ridge at the Heidebos locality on the basis of direct age information. The relatively high sedimentation rate and the absence of extensive soil formation in the record reflect periods of pronounced aeolian activity and landscape instability during the Late Glacial, which provides part of the environmental framework for human occupation in the area.

Luminescence dating of neolithic ceramics from Lumea Nouă, Romania

Luminescence Dating of Neolithic Ceramics from Lumea Nouă, Romania Luminescence dating was applied to four Neolithic pottery fragments excavated at Lumea Nouă (Alba Iulia, Romania), with the purpose of improving the chronology for the archaeological finds, and hence, the occupational history of the site. The single-aliquot regenerative-dose (SAR) protocol was applied to both blue and infrared stimulated luminescence signals from coarse quartz and polymineral fine grains, respectively. A more conventional approach which uses a multiple-aliquot additive-dose protocol and thermoluminescence signals from polymineral fine grains was applied as well. The characteristics of the OSL and IRSL signals were investigated in terms of behaviour in the SAR protocol, dose response and dose recovery. Additionally, anomalous fading measurements of the 410 nm IRSL emission and of the blue TL emission from polymineral fine-grains were performed. Both signals were observed to be affected by anomalous fading. OSL dating of quartz using the SAR protocol is considered to be the technique of choice; it yields an average age of 6.0±0.4 ka (n = 4), which is in agreement with archaeological expectations. The dating results allow refining the typological interpretation of the objects and improve the chronological framework for the site.

OSL dating of an inland dune along the lower River Scheldt near Schellebelle (East Flanders, Belgium)

The chronostratigraphic position of aeolian dunes in East Flanders (Belgium) has been under debate for decades. Until now, the only available age information consisted of a limited number of radiocarbon dates, which provided indirect sediment deposition chronologies. This paper reports on the first direct determination, by quartz-based single-aliquot optically stimulated luminescence dating, of the time that dune sands were deposited along the Lower River Scheldt in Belgium. The sediments are dated at 12.0 +/- 0.9 ka (n = 5), which confirms that the time of inland dune formation in East Flanders dates from the Younger Dryas period and should not be constrained to the Holocene.