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- 1897-1695
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- 04 Jul 2007
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- English
Loess-paleosol sequences (LPS) are terrestrial archives containing information about paleoenvironmental conditions during the Quaternary (see e.g. Antoine
Dating of the sediments is crucial to place the climatic information of a loess-paleosol record into a temporal context. The most commonly used dating technique in LPS is luminescence dating. Loess samples consisting of fine grains of quartz are thought to be the ideal material for optically stimulated luminescence (OSL) dating (Roberts, 2008), but recent studies indicate that this is not necessarily correct. Timar-Gabor
Especially for older samples, the application of infrared stimulated luminescence (IRSL) of (coarse-grained) feldspars or (fine-grained) polyminerals is an alternative technique for dating loess deposits. In particular, post-infrared-IRSL methods (pIRIR) are interesting, as these only show negligible fading rates in contrast to conventional IRSL methods (Buylaert
The major aim of this study is to construct a reliable luminescence chronology for the Stalać section in the Central Balkan region as a cornerstone for paleoenvironmental investigations (cf. Obreht
The Stalać section is located in central Serbia (43°40.64812’N, 21°25.06967’E), close to the city of Kruševac (Fig. 1). It is exposed in an active brickyard at the village of Stalać on the east side of the Južna (South) Morava River,
Figure 1
Distribution of loess and loess derivates modified after Haase et al. (2007), Lindner et al, (2017), and Vandenberghe et al. (2014) (A). Red square indicates the extent and location of map B. Map B shows a simplified geological map (Boljevac, 1968; Dolić et al., 1980;Krstić et al., 1978; and Rakić et al., 1975). The location of the Stalać section is indicated by a white circle.
The loess deposits at Stalać are formed as a plateau-like structure covering the eastern slopes of the tectonically controlled Morava-floodplain-basin. Geomorphology and present day land-use-patterns indicate that wide spread and thick Middle and Late Pleistocene loess is restricted to the eastern slopes of this basin north of the confluence of the Južna Morava and the Zapadna (West) Morava Rivers indicating prevailing north-western winds. Southeast to northwest trending 2 to 3 km long valleys are cut into the plateau. The depositional pattern of the loess-paleosol-sequence argues for their potentially constant activity throughout the time of loess deposition. Shorter maximum 1 km long valleys occur as tributaries of the main valleys and seem to be formed as gullies during single and dramatic erosional events and later filled up with eolian loess. The northwest trending depth line of such a structure is presently located just 200 m to the South of the sampled outcrop wall and might form the base-level of the observed erosional surface in the loess sequence.
The current climatic conditions are characterized with moderate continental conditions. From 1981 to 2010, the mean annual temperature (with a high interannual variability) was 11.4°C at a nearby climate station in Kruševac; the mean annual precipitation was 628 mm and exhibits precipitation maxima in June and November (RHMS of Serbia, 2016).
Fig. 1 Shows the loess distribution of Southeastern Europe and the location of the section. Moreover, it displays a simplified geological map. The geological map (Boljevac, 1968; Dolić
Kostić and Prostić (2000) undertook a first paleopedological and mineralogical investigation at the Stalać section. Based on their grain-size, clay minerology and carbonate content result, they showed that the climatic dynamics of this region gradually shifted from a dominated Mediterranean climate towards a more continental climate. This was characterized with decreasing precipitation and possible overall cooling from the Middle to Late Pleistocene. However, no numerical dating was applied, thus their interpretation is based on solely correlative stratigraphy, and will be evaluated by our geochronological investigations.
The section is exposed in a northeast to south-westward direction.
For a better overview, a composite profile was made out of the four individual profiles using the paleosol and tephra layers (Fig. 2). Stratigraphic labeling followed the scheme proposed by Marković
Figure 2
Sketch of composite profile, showing the nomenclature of paleosols and loess (after Marković et al., 2015) and the investigated luminescence samples.
The bottom of the profile starts with a 20 cm thick loess layer. On top, a reddish cambisol (11.9–12.7 m), and a 2 m thick pale yellow loess layer (L2) is exposed. The loess layer is intercalated by a deep-red vertic (11.4511.6 m, L2SS1) and two weak brown humic horizons (10.8–10.9 m, 10.43–10.55 m). At
Samples for equivalent dose (De) determination were extracted from the metal tubes under subdued red light conditions; the outer 2 cm of sediment were removed. Samples were oven dried (50°C), the water content was determined (water content over dry mass) and samples were then treated with 10% hydrochloric acid, 10% hydrogen peroxide and 0.01 N sodium oxalate to remove carbonates, organic matter, and to dissolve aggregates. Where needed, 30% hydrogen peroxide was used. After thoroughly removing the chemicals, samples were separated into the 4–11 μm fraction. One part of each fraction was etched in 34% hexafluorosilicic acid for one week to extract fine-grained quartz minerals. The other part remained as a polymineral fraction.
Continuous wave optically stimulated luminescence measurements (CW-OSL) were carried out on a Risø TL/OSL DA 20 reader. The reader is equipped with a 90Sr/90Y β source, blue light emitting diodes (LEDs), emitting at 470 nm (FWHM = 20 nm), and IR LEDs, emitting at 870 nm (FWHM = 40 nm). Signals of quartz were detected through a Hoya U-340 filter and signals of polymineral fine grains were detected through a 410 nm interference filter. A single aliquot regenerative dose (SAR) protocol (cf. Murray and Wintle, 2000, 2003) was applied for quartz. Sensitivity changes, recycling ratios and recuperation were monitored by the SAR protocol (Murray and Wintle, 2000). A preheat plateau test (e.g. Murray and Wintle, 2000) and a dose recovery test (e.g. Murray and Wintle, 2003) were performed on sample C-L3787 prior to De measurements to assess the proper measurement settings. This sample was used for testing after preliminary geochemical results were available. These showed similarities between samples C-L3780 and C-L3787, which is why the tests were done on one sample only. Preheat plateau tests (PHTs) with different preheat and cutheat temperatures between 160°C and 280°C were carried out. Cutheat temperatures were always 20°C lower than preheat temperatures. Dose recovery tests (DRTs) included optical stimulation for 100 s and laboratory irradiation to create a known laboratory dose. The chosen irradiation dose was equal to the expected De. We used an early background subtraction for equivalent dose calculation (Ballarini
For polymineral fine grain measurements the pIR50IR290 SAR protocol was used (Thiel
Samples for dose rate determination were oven dried (50°C); the water content was determined, and samples were homogenized and packed into plastic cylinders for radionuclide measurements. After a resting period of four weeks to compensate for radon emanation during pretreatment, radionuclide concentrations were measured on a high-purity germanium gamma-ray spectrometer. The dose rates and ages were calculated using DRAC, v.1.2 (Durcan
After the age calculation, ages bracketing the paleosols and the tephra were combined to one density function per layer, of which an estimate for the ages of the layers themselves including uncertainties was calculated. This was done conservatively by determining 1- and 2- sigma uncertainties of the combined densities.
First, we investigated the dependency of the equivalent dose on the preheat temperature and the ability to recover a given dose (Fig. 3). Equivalent doses are decreasing with increasing preheat temperature and no plateau was observed. Several aliquots showed sensitivity changes (recycling ratios between 1.11 and 1.13; see Fig. 4 for more detail), but recuperation was negligible (values below 0.77%). All De-values are smaller than 2×D0 and do not depend on the signal integration interval (see De(t)-plot in
Figure 3
Top shows a plot of De as a function of preheat temperature for five aliquots per temperature of the quartz fraction of sample C-L3787. The standard error is shown. Bottom shows a dose recovery test of the same sample using different preheat temperatures. Aliquots were bleached and given a dose of ~150 Gy. Area between dashed lines displays the desired ratio. Error bars show standard deviation.
Figure 4
Response to a testdose (Tx) normalized to the testdose response of the natural signal (Tn) throughout the PHT measurements of sample C-L3787 showing the sensitivity changes occurring during the SAR-cycle.
To further investigate the quartz luminescence behavior, the OSL signal components were fitted (e.g.
Figure 5
Relative photoionisation cross sections of medium (top) and slow (bottom) components with regard to the fast component at different preheat temperatures during the PHT of C-L3787. Note that only one aliquot per preheat temperature is shown and that aliquots do not behave equally. Horizontal lines show the values observed by Jain et al. (2003).
This difference becomes also evident in the NR(t) plots in
Furthermore, the observed difference between natural and regenerated signals is also visible in Fig. 6, which presents the normalized luminescence signal (Lx/Tx) plotted against the preheat temperature (cf. Roberts, 2006): while the natural signal (Ln/Tn) remains constant, the regenerated signal (Lx/Tx) is increasing with higher preheat temperatures (with regard to the 280°C dose point). A steeper dose response curve results in lower De values, which were evident in the preheat plateau test (Fig. 3).
Figure 6
Normalized luminescence signal (Lx/Tx) of the natural signal (N) and the regeneration dose points (indicated in Gy) during the different preheat temperature measurements. All points are normalized to the 280°C data point.
The variation within the contribution of different signal components and the sensitivity changes are not very large (<30%), and the SAR-protocol was able to correct for this at least partially (recycling ratios within 0.9–1.1 for most aliquots). Part of the preheat temperature dependency of the equivalent dose can be explained by the fact that the fast component contributes to the signal with only 70–80 %, but it remains unclear if this is the reason for the quartz behavior. Similar observations with decreasing preheat plateaus were also observed by Roberts (2006) and interpreted as thermal transfer of charge from a trap between cutheat and preheat temperatures into the OSL trap. Moreover, their study showed that falling preheat plateaus are particularly problematic for older loess samples, although the signal is not saturated. We observed the same behavior for the Stalać sample, which has equivalent doses between 140–220 Gy depending on the preheat temperature used. This might also hint towards another process within quartz OSL measurements, which is not yet understood. Especially considering the differences between grains sizes (Timar-Gabor
First, the dependency of the temperature of prior IR stimulation on the equivalent dose was investigated: the De is independent from prior IR stimulation temperatures for sample C-L3780 between 50 and 110°C, for sample C-L3784 between 50 and 140°C, and for sample C-L3787 between 50 and 170°C (Fig. 7 top). Accordingly, we continued the measurements with a first IR stimulation temperature of 50°C, which exhibit higher signal intensities. We applied test doses of ~ 30% of the equivalent dose. DRTs gave recovered/given dose ratios of 0.97 ± 0.004 (C-L3780), 1.07 ± 0.02 (C-L3784), and 0.96 ± 0.02 (C-L3787) as depicted in Fig. 7. For these ratios the residual doses were subtracted.
Figure 7
Top shows prior IR stimulation temperature plotted against measured dose. Dose of C-L3787 can be viewed at the right hand axis, while dose of C-L3780 can be viewed at the left hand axis. Standard errors are shown. Bottom shows dose recovery tests of C-L3780, C-L3784 and C-L3787. Recovered to given doses are shown as mean and standard error of at least three aliquots per sample. Given doses were 194 Gy for C-L3787, 617 Gy for C-L3784 and 645 Gy for C-L3780. Residual doses, after bleaching for 24h in a solar simulator, were subtracted.
De measurements were carried out on at least ten aliquots per sample and all aliquots passed the SAR quality checks. No residual doses were subtracted for De calculations, as suggested by Kars
Summary of the De, dose rate and age data. Water content is the one obtained in the laboratory. For alpha efficiency a 10% error was applied when standard deviation was smaller than 10%, otherwise standard deviation was used. Dose rates are calculated as described in Section 5 - Dose Rate and Age Calculation. Standard errors are indicated. Ages are expressed with a 1-sigma error range.Sample Lab Grain Depth Water U Th K Cosmic dose Alpha Total dose No. of De Age St 1 C-L3778 4-11 7 14.5 ± 7.3 33.78 ± 1.58 44.44 ± 2.26 1.69 ± 0.03 0.101 ± 0.01 0.134 ± 0.013 3.77 ± 0.2 10 630 ± 32 139 ± 11 St 3 C-L3780 4-11 6 11.8 ± 5.9 34.23 ± 1.62 45.86 ± 2.35 1.83 ± 0.03 0.111 ± 0.01 0.128 ± 0.013 4.07 ± 0.2 13 650 ± 36 136 ± 10 St 7 C-L3784 4-11 4.5 8.6 ± 4.3 33.49 ± 1.60 42.96 ± 2.21 1.76 ± 0.03 0.130 ± 0.01 0.149 ± 0.015 4.06 ± 0.2 13 622 ± 32 123 ± 8 St 9 C-L3786 4-11 6.3 8.0 ± 4.0 30.88 ± 1.63 42.65 ± 2.49 1.48 ± 0.02 0.108 ± 0.01 0.102 ± 0.013 3.71 ± 0.2 10 245 ± 12 59.9 ± 4.0 St 10 C-L3787 4-11 7 10.7 ± 5.4 30.42 ± 1.73 48.07 ± 2.9 1.75 ± 0.02 0.101 ± 0.01 0.108 ± 0.026 3.97 ± 0.2 14 191 ± 10 43.0 ± 3.6 St 11 C-L3788 4-11 5 7.8 ± 3.9 36.06 ± 1.68 45.82 ± 2.34 1.77 ± 0.03 0.123 ± 0.01 0.143 ± 0.030 4.26 ± 0.2 10 149 ± 8 28.5 ± 2.3
In fact, the measurements show optimal performance: all SAR criteria were met, dose recovery ratios were within 10% of unity and prior IR stimulation temperature plateaus were present for all samples tested. Therefore, we used the equivalent doses of the pIR50IR290 measurements of the polymineral samples for the final age assessment.
Table 1 shows a summary of the dose rate measurements, equivalent doses and age estimates of the studied section. We used the measured water content to calculate the total (wet) dose rate. Ages are based on the polymineral samples using the pIR50IR290 protocol. The α-efficiency was determined giving values between 0.102 ± 0.013 (C-L3786) and 0.149 ± 0.015 (C-L3784). Ages are all in stratigraphic order, ranging from 28.5 ± 2.3 ka to 139 ± 11 ka. These ages might be considered as falling in Marine Isotope Stages (MIS) 2, 3, 4 and 6. The ages of samples C-L3778 (139 ± 11 ka), C-L3780 (136 ± 10 ka), and C-L3784 (123 ± 8 ka) highlight a presumably high accumulation rate in this time range (although the slope position should not be disregarded).
Probability density functions of the ages of over- and underlying samples were combined to assess the ages of paleosol and tephra layers. This results in an age of 127 and 148 ka at a 1σ confidence level for L2SS1 (116— 159 ka, 2σ); and an age of 118–141 ka at 1σ (109-152 ka, 2σ) for the tephra. Although the samples bracketing the tephra are not in close stratigraphic distance, the difference in age is small. We restrain from this calculation for S1 because of the age differences between samples C-L3784 and C-L3786. Combining the probability density functions for the L1 paleosols gives ages of 41–62 ka 1σ (37–66 ka, 2σ) for L1SS1SSS2 and 27–45 ka 1σ (25–49 ka, 2σ) for L1SS1SSS1.
Generally, LPS have been investigated intensively in Southeastern Europe, but only some were OSL-dated in high-resolution (Constantin
The previous investigation at the Stalać section has established a correlative age model (Kostić and Protić, 2000). They correlated the cambisol (S2) to the Riss/Würm interglacial (MIS 5), while three weaker kastanozem-like paleosols above were correlated to the last glacial soil developments (S1, L1SS1SSS2, L1SS1SSS1). However, our investigation demonstrates that such a correlation is challenging and in this case very likely incorrect. Based on the luminescence data (Table 1), we show that the two younger kastanozem-like paleosols developed during MIS 3 (C-L3787 and C-L3780) and the oldest kastanozem-like paleosol (S1, see Fig. 2) formed during MIS 5, while the cambisol (S2) probably developed during the penultimate interglacial (C-L3778, C-L3784, C-L3786). This fundamentally changes the conclusion from the Kostić and Protić (2000) study. It furthermore reveals that the environmental conditions during the last two interglacials were remarkably different. Adapting the data by Kostić and Protić (2000) with respect to the new age-model suggests stronger weathering during the formation of the S2 paleosol and an upward trend of decreasing precipitation. A trend of gradual aridization was also discussed by Buggle
Geochemical analyses of glass shards found in the L1SS1LLL1 layer indicate the existence of cryptotephra layer (Obreht
Another tephra was dated to 109–152 ka (118–141 ka, 1σ) by means of samples C-L3780 and C-L3784. Though samples bracketing the L2 tephra were not closely spaced in the profile, their ages overlap within 1-σ errors. Wacha and Frechen (2011) dated a tephra in Croatian loess to 139 ± 9 ka (above) and 145 ± 12 ka (below), which is consistent with the L2 tephra found at Stalać. Moreover, a tephra in Lake Ohrid with an 40Ar/39Ar-age of 133.5 ± 2 ka (Leicher
Finally, the lowermost sample (C-L3778) places the beginning of the L2 loess accumulation phase at 139 ± 11 ka. However, the top of the S2 soil in Eurasian loess is usually placed at ~190 ka (e.g. Basarin
Additionally, we dated a pedogenic horizon (L2SS1) at 11.45–11.6 m within the L2 loess to 126–148 ka (1σ, C-L3778 and C-L3780). Also several other Balkan LPS reveal a weakly developed pedogenic layer within the L2 (Marković
In contrast to earlier views, fine quartz grains with high equivalent dose values are not always the ideal material for OSL dating, as was also shown in this study. The importance of a thorough investigation was demonstrated, underlining the necessity of a preheat plateau test and the usefulness of a plot of normalized luminescence signal versus preheat temperature. High-dosed quartz with falling preheat plateaus remains unsuitable for OSL dating until the underlying processes that cause these problems are better understood.
A reliable chronology of the Stalać LPS covering the last two glacial cycles was established by dating the polymineral fine-grain fraction. The good behavior of the polymineral samples made them ideal for measurements with the pIR50IR290 protocol and support the validity of the obtained ages. The wide time range enhances our understanding on the timing of LPS south of the typical loess distribution. Moreover, the previous chronology by Kostić and Protić (2000) was corrected by means of the new ages, highlighting the importance of luminescence dating to contribute to chronological questions in LPS. This revealed a quite different paleoenvironmental evolution of the Carpathian Basin and the Central Balkan region during the last interstadial, which shows stronger pedogenesis at Stalać. Finally, the tephra found in L2 was dated to 118–142 ka (1σ), contributing to the establishment of a reliable loess chronology of the penultimate glaciation in Southeastern Europe.
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Figure 7
Summary of the De, dose rate and age data. Water content is the one obtained in the laboratory. For alpha efficiency a 10% error was applied when standard deviation was smaller than 10%, otherwise standard deviation was used. Dose rates are calculated as described in Section 5 - Dose Rate and Age Calculation. Standard errors are indicated. Ages are expressed with a 1-sigma error range.
| Sample | Lab | Grain | Depth | Water | U | Th | K | Cosmic dose | Alpha | Total dose | No. of | De | Age |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| St 1 | C-L3778 | 4-11 | 7 | 14.5 ± 7.3 | 33.78 ± 1.58 | 44.44 ± 2.26 | 1.69 ± 0.03 | 0.101 ± 0.01 | 0.134 ± 0.013 | 3.77 ± 0.2 | 10 | 630 ± 32 | 139 ± 11 |
| St 3 | C-L3780 | 4-11 | 6 | 11.8 ± 5.9 | 34.23 ± 1.62 | 45.86 ± 2.35 | 1.83 ± 0.03 | 0.111 ± 0.01 | 0.128 ± 0.013 | 4.07 ± 0.2 | 13 | 650 ± 36 | 136 ± 10 |
| St 7 | C-L3784 | 4-11 | 4.5 | 8.6 ± 4.3 | 33.49 ± 1.60 | 42.96 ± 2.21 | 1.76 ± 0.03 | 0.130 ± 0.01 | 0.149 ± 0.015 | 4.06 ± 0.2 | 13 | 622 ± 32 | 123 ± 8 |
| St 9 | C-L3786 | 4-11 | 6.3 | 8.0 ± 4.0 | 30.88 ± 1.63 | 42.65 ± 2.49 | 1.48 ± 0.02 | 0.108 ± 0.01 | 0.102 ± 0.013 | 3.71 ± 0.2 | 10 | 245 ± 12 | 59.9 ± 4.0 |
| St 10 | C-L3787 | 4-11 | 7 | 10.7 ± 5.4 | 30.42 ± 1.73 | 48.07 ± 2.9 | 1.75 ± 0.02 | 0.101 ± 0.01 | 0.108 ± 0.026 | 3.97 ± 0.2 | 14 | 191 ± 10 | 43.0 ± 3.6 |
| St 11 | C-L3788 | 4-11 | 5 | 7.8 ± 3.9 | 36.06 ± 1.68 | 45.82 ± 2.34 | 1.77 ± 0.03 | 0.123 ± 0.01 | 0.143 ± 0.030 | 4.26 ± 0.2 | 10 | 149 ± 8 | 28.5 ± 2.3 |