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Recuperated-OSL dating of quartz from Aegean (South Greece) raised Pleistocene marine sediments: current results
Quaternary Geochronology 5 (2010) 65–75
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Research Paper
Recuperated-OSL dating of quartz from Aegean (South Greece) raised Pleistocene marine sediments: current results C. Athanassas a, b, *, N. Zacharias a a b
Laboratory of Archaeometry, Institute of Materials Science, N.C.S.R. ‘Demokritos’, Aghia Paraskevi, Athens153 10, Greece Department of Geology, University of Athens, Zografou, Athens 157 84, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2008 Received in revised form 11 September 2009 Accepted 23 September 2009 Available online 2 October 2009
The purpose of this work is to provide an OSL dating framework for raised marine sequences in the South-West coast of Greece during Upper Quaternary. Paleontological investigations and Geographic Information System (GIS) analysis on elevated marine landforms have proved that a record of uplift and eustacy exists in South Greece since the Early Pleistocene (w1.6 Ma). Hereby, we test the suitability of the SAR methodology for recuperated-OSL (Re-OSL), proposed by Wang et al. (2007), on coarse-grained quartz aliquots from emerged nearshore outcrops. Protocol’s performance is examined on the basis of signal characteristics, dose response, sensitivity changes, recovery of known doses and Re-OSL bleachability to sunlight. The accuracy of the Re-OSL dates calculated for natural samples is also discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Aegean Marine terraces Nannoplankton GIS TT-OSL Recuperated-OSL Sensitivity changes Oxygen Isotope Stage
1. Introduction South Aegean Greece constitutes one of the most tectonically active regions worldwide. This activity dates back to the Oligocene (e.g. McKenzie, 1970; McClusky et al., 2000). Intense seismicity, normal faulting and crustal uplift are a few, out of several, geodynamic phenomena which prevail in the development of the region. Sequences of raised and well-developed marine terraces occupy extensive segments of the south coast, revealing a steady motif of uplift throughout the Pliocene and Pleistocene (Kelletat et al., 1976). Specifically, in the southwestern corner of the country, a set of stranded platforms have been mapped (Fig. 1), capped by shallow marine to brackish sediments. Former (Marcopoulou-Diacantoni et al., 1991; Fountoulis and Moraiti, 1994; Kourampas and Robertson, 2000) and current paleontological investigation set the beginning of sedimentation in the Early Pleistocene. Nannoplanktonic populations of Gephyrocapsa, Pseudoemiliania lacunosa, and Calcidiscusleptoporus, all correlated with the MNN-19e biozone
* Corresponding author. E-mail address: [email protected] (C. Athanassas). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.09.010
(Rio et al., 1990), bound the chronology of the Quaternary outcrops as far back as 1.6–0.95 Ma. The paucity of younger paleontological finds and absolute dates restrains the chronological discrimination between the successions. Using luminescence dating, we favor to correlate the uplifted Pleistocene facies with highstands of higher resolution, as they have been detailed in the Oxygen Isotope Stages timescale (Martinson et al., 1987). Depending on the range of the locally experienced dose rates, standard optically stimulated luminescence (OSL) measurements using the single aliquot regenerated dose protocol (SAR) by Murray and Wintle (2000) can accurately yield the palaeodose within the Last Interglacial-Glacial Cycle (e.g. Murray and Olley, 2002), this is the last w125 ka. Beyond that stage, the uncertainty increases significantly (Murray and Funder, 2003). Likewise, Late Quaternary quartz aliquots from the southwestern coast of Greece have been found to approach saturation at doses over w100 Gy (Athanassas, 2005), thus, making the possibility of exploring the age earlier geologic events using OSL dating, problematic. Wang et al. (2006a,b) have testified the capability of thermally transferred OSL (TT-OSL) signal, and particularly its recuperated component (Re-OSL), to calculate the equivalent dose in the region where the electron traps responsible for the fast OSL component in
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C. Athanassas, N. Zacharias / Quaternary Geochronology 5 (2010) 65–75 Table 1 Single aliquot regenerated dose method for Re-OSL (Wang et al., 2007). Step
Treatment
Part 1: TT-OSL measurement 1–1 Dose Di 1–2 PH (260 C, 10 sec) 1–3 OSL (125 C, 270 sec) 1–4 PH (260 C, 10 sec) 1–5 OSL (125 C, 100 sec) 1–6 Test dose TD 1–7 PH (220 C, 10 sec) 1–8 OSL (125 C, 100 sec) Part 2: BT-OSL measurement 2–1 Anneal to 300 C for 10 sec 2–2 OSL (125 C, 100 sec) 2–3 PH (260 C, 10 sec) 2–4 OSL (125 C, 100 sec) 2–5 Test dose TD 2–6 PH (220 C, 10 sec) 2–7 OSL (125 C, 100 sec)
Fig. 1. Relief image of the study area. GIS analysis of raster contour maps, 1; 5,000 in scale, revealed a sequence of four at least platforms, possibly related to Middle to Late Quaternary sea-level highstands. Elevation (colour graded scale) is given in m. Grid data are in WGS’84. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Measurement
LTT-OSL
TTT-OSL
LBT-OSL
TBT-OSL
sea-level highstands and the bedrock. Progressive uplift during lowstands resulted in this flank of abandoned landforms. Characteristic arched perimeters, parallel to the present shoreline, and higher elevations in the central-east segment, seen in Fig. 1, resulted in a dome-like topography. From this we can draw preliminary evidence for stronger uplift in the middle part of the area. Field campaigns located stacks of miscellaneous marine facies as well as fossil sand dunes. Later Quaternary offshore and nearhore clastic formations, such as silts, sands, sandstones and conglomerates have been deposited on a predominant Early Pleistocenic background. The last has been endorsed by nannoplankton examination (current research). A typical succession of the study sediments is illustrated in Fig. 2.
quartz are practically saturated. Wang et al. (2006a,b), Tsukamoto et al. (2008), Pagonis et al. (2008) provided investigations and basics on TT-OSL dating while Li and Li (2006), Pagonis et al. (2007) and Adamiec et al. (2008) put forward mechanisms on TT-OSL generation. Especially, Adamiec et al. (2008) elucidated a new mechanism according which TT-OSL derives from a trap with thermal stability similar to that of OSL, but less sensitive to the sunlight. During a preheat (260 C for 10 sec) applied immediately after an optical stimulation, a proportion of charge moves from that source trap, via the conduction band, to the main OSL trap. Subsequent optical stimulation will give rise to the TT-OSL signal, which has saturation levels significantly higher than those of the conventional OSL. In this work we test the efficiency of the SAR method for Re-OSL (Table 1), as developed by Wang et al. (2007), to extract ages for earlier Pleistocenic events in the Southwest Greece.
2. Site and sample description Although the number of terraces, appearing along the southwestern Greece coast, has been debated (Kelletat et al., 1976; Kourampas and Robertson, 2000), analysis using Geographic Information Systems (ArcGIS 9.0 software) has revealed a succession of at least four stranded platforms (Fig. 1). They have, most probably, been generated by the long-lived interaction between
Fig. 2. Typical sequence of nearshore deposits appearing in a coastal cliff at ‘MAT’ sample site (Fig. 1). From the bottom to the top, the section starts with a basal conglomerate and goes over silty sands, fine sands, silty sands, conglomerate, fine sands, conglomerate and terminates in a cross-bedded medium to coarse sandstone. The sequence stands at w12 m above modern sea-level. Samples MAT1 (offshore sand) and MAT2 (nearshore sandstone) were collected from this section.
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Stratigraphic correlations and sediment properties such as grain size and incorporated fossil fauna reflect the proximity of these deposits to sea-surface. As a result, they may be suitable for inferring relative vertical movements. Also, the abundance of quartz grains in the silty-sand grain size fraction, its extended exposure to daylight, and its assumed age, renders these sediments appropriate material for OSL dating. Sampling for the purposes of luminescence measurements involved collecting stratified sands, silts as well as coastal sand dunes in aluminum tubes from natural sections spread along the coastline and hinterland. 3. Sample preparation and measurement settings The procedure of purification of quartz was the typical followed in OSL dating including the steps: 10% HCl, 10% H2O2, first sieving, density separation using sodium heteropolytungstate with densities of 2.70 and 2.63 gcm3, 40% HF, rinsing with 10% HCl, and finally second sieving to receive the 90–125 mm fraction used in this study. Instrumentation for the signal measurements included a RISØ TL-DA-15 reader supplied with blue diodes (470D30 nm). An EMI9635Q photomultiplier and two 3 mm Hoya U-340 filters were attached on the system. The reader was equipped with a 90Sr/90Y beta source for automated irradiations, delivering 6.30 Gy min1. The measurement procedure of Table 1 was used in order to estimate the intensity of the regenerated sensitivity-corrected thermally transferred optical signals. This SAR methodology is a two-part protocol with the first applying to the calculation of the corrected TT-OSL and the second to the corrected BT-OSL. Removal of fast and medium OSL components in 1–3 (Part 1, Table 1) was achieved by stimulating the sample with blue LEDs at 125 C for 270 sec. Soon after, preheat (PH) at 260 C for 10 sec was used in 1–4 to generate thermal transfer, as it has been suggested earlier by Wang et al. (2006a,b). This treatment allows thermal transfer of maximum electron population from the source trap to that responsible for OSL (Adamiec et al., 2008), without depleting the last. These PH conditions are in agreement with the experiments of Tsukamoto et al. (2008). All OSL, TT-OSL and BT-OSL measurements were conducted at 125 C for 100 sec. Net signal intensity was calculated as the integral of the first 2 sec of the measurement, subtracted by that of the last 10 sec. In order to isolate BT-OSL from TT-OSL, annealing the sample at an elevated temperature was necessary. Experimental data from both Wang et al. (2006a,b) and Tsukamoto et al. (2008) unite that a 300 C heating for 10 sec (in step 2–1) is appropriate to completely remove Re-OSL. Therefore, the signal measured in 2–4 is the net BT-OSL. This is the heating treatment used here as well. Large doses were given in pulses of a 105 Gy interposing a 240 C preheat. The use of pulsed irradiation has been proven necessary when large doses are involved in TT-OSL dating, otherwise significant underestimations may be expected (Wang et al., 2006b). Stepped irradiation approximates better the natural trapping conditions during the absorption of the dose (Bailey, 2004). This treatment was applied to all samples and experiments (sensitivity changes, dose recoveries, measurement of natural De) when doses grater than 105 Gy were utilised. A similar dose increment have been used by Wang et al. (2006b). Finally, where bleaching prior dosing is stated in this text, it involves exposure of quartz aliquots directly to sunlight for 60 min. Signal recordings after bleaching resulted in that all three OSL components have significantly been cut down to almost background levels. The same solar exposure seems also to have
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a significant impact on the TT-OSL signal and it is able to reduce it down to very small residual values, as discussed in Section 4.5. 4. General performance of the Re-SAR method 4.1. TT-OSL characteristics Replicate runs of the protocol in Table 1, using a range of regenerated doses, results in the estimation of the sensitivity-corrected TT-OSL (LTT-OSL) and BT-OSL (LBT-OSL) signals. The difference of these numbers is the sensitivity-corrected regenerated Re-OSL (LRe-OSL) and its growth in response to the regenerated dose can be schematically described by a growth curve. One quartz aliquot of KRN6 (marine sand) was bleached in sunlight and then went through a sequence of 315, 945, 1260 and zero Gy regenerated doses using the steps shown in Table 1. Both TT-OSL and BT-OSL signals were corrected by a 94.5 Gy test dose. Fig. 3 shows the successive TT-OSL (Fig. 3a) and BT-OSL (Fig. 3b) decay curves, measured after each irradiation. The relative test dose signals (TTT-OSL and TBT-OSL) are given in Fig. 3c and d. All decay curves have been normalised to the first 0.4 sec of measurement. Normalised TT-OSL decay curves in Fig. 3a seem to be dominated by the fast component within the first 2 sec of optical stimulation. This component is distinctively more luminous than the slow that follows on. Apparent decay curves look identical, indicating that there is a constant proportion in signal growth among components as the dose increases. Similar observations stand for less intense BT-OSL in Fig. 3b as well as for OSL recorded after the test dose (Fig. 3c,d). Fig. 4 illustrates the corrected TT-OSL dose response curve, resolved in its corrected Re-OSL and BT-OSL constituents. The corrected TT-OSL and Re-OSL growth are represented by single saturating exponentials respectively, while the BT-OSL dataset is fitted by a straight line. For comparison, the growth of the typical OSL signal at the same doses is inserted in Fig. 3. Likewise, corrected OSL has been fitted by a single saturating exponential. As expected, thermally transferred signals continue to grow in the dose region where standard OSL is practically saturated. Specifically, the characteristic saturation dose (D0) was calculated for all exponential signals above using the equation (1).
D I ¼ Imax 1 eD0
(1)
D0 for OSL is 135 Gy, but this number becomes significantly higher in TT-OSL and Re-OSL growth curves; 710 and 490 Gy respectively. This indicates that Re-OSL approaches saturation at doses at least 4 times greater than standard OSL and it is very likely that this signal can be used to calculate ages older than those yielded in standard OSL-SAR dating procedures (Murray and Wintle, 2000). These D0 values are sufficient enough to investigate earlier quaternary events. It is also possible that an additional linear component may describe the Re-OSL behavior at higher doses, as hinted by the ability to recover doses larger than the above D0 value. Yet, this was not a subject of further investigation. 4.2. Sensitivity changes In order to monitor and therefore correct the sensitivity changes occurring during the Re-SAR thermal treatments, two sets of bleached aliquots from PTC2 sample (aeolian dune sand) were tested, each set containing three disks. The first set received 315 Gy and, combined with a test dose of 10.5 Gy, was recycled seven times using the procedure presented in Table 1. Similarly, the second set went through the same procedure using a 625/73.5 Gy dose/test
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Fig. 3. Normalised successive decay curves for TT-OSL (a) and BT-OSL (b) after 315, 945, 1260 Gy dosing as well as the following test dose OSL responses namely, TTT-OSL (c) and TBT(d), using a constant dose of 94.5 Gy.
OSL
dose combination. Fig. 5 demonstrates the relative sensitivity changes for TT-OSL and BT-OSL, this is the test dose response of each cycle normalised to the test dose signal of the first measurement, in a way similar to Fig. 1 in Wang et al.’s (2007) paper. Examination of the data in Fig. 5 reveals sensitivity change patterns significantly different from those presented by Wang et al. (2007) for Chinese loess quartz. Particularly, in 315/10.5 Gy and for both TT-OSL and BT-OSL signals, there do occur sensitivity changes through the cycles. A steady increase in the averaged sensitivity is observed up to cycle No 5 (No 4 for BT-OSL) in a fashion similar to that resulting from high temperature preheats (Armitage et al., 2000), and then a slight decrease. However, in 630/73.5 Gy only a minor sensitivity change is observed (Fig. 5c,d). The overall normalised TTT-OSL and TBT-OSL tend to remain constant as cycles progress. The effect of preheats inserted during pulsed irradiation may be a possible interpretation for the difference observed between the two sensitivity change motifs. In 630/73.5 the measurement of the TT-OSL signal is preceded by 6 105 Gy irradiations alternated by 5 240 C preheats, plus those in 1–2, 1–4 and 1–7 shown in Table 1. This total of 8 PHs may have sensitised the OSL signal, prior the measurement of the test dose response. Thereinafter, plots of LTT-OSL versus TTT-OSL (measured in steps 1–5 and 1–8) and LBT-OSL versus TBT-OSL (measured in 2–4 and 2–7)
were created and these are seen in Fig. 6. Each graph shows data from one representative aliquot from each dose/test dose combination. A linear relationship between the two signals is observed. Contrary to Wang et al. (2007) and Tsukamoto et al. (2008), measurement of the first cycle is collinear with the fitted data. Still, fitted lines do not pass through the origin. Wintle and Murray (2006) have claimed that ‘the OSL sensitivity measured using a test dose should be directly proportional to the sensitivity applicable to the preceding regeneration dose’ and in principle, those test doses might not be appropriate to monitor the sensitivity change for PTC2. An additional run was applied to both previously measured aliquots but giving a 0 Gy dose this time and keeping the tests doses constant. Next, plots of corrected Re-OSL versus measurement cycle were produced for the same representative aliquots (mentioned in Fig. 6), including the final zero dose cycle (8th). Fig. 7 shows that both sets of corrected Re-OSL (filled rectangles and circles) converge to a central value. In 315/10.5 Gy combination, a small increase of the corrected Re-OSL in the second cycle can be noticed, due to cumulative effect of the remnant recuperated signal (Wang et al., 2007; Tsukamoto et al., 2008). The oscillation around the central value (0.07) becomes even tighter if the zero Gy-corrected Re-OSL is subtracted from all, but the first, cycles. (open rectangles). This oscillation is very similar to
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Wang et al.’s (2007) normalised Re-OSL, after zero dose-correction, in a similar graph (Fig. 2c and f therein). Yet, data from 630 Gy are in line with the value of 0.027 (filled circles) and correction using zero Gy dose-Re-OSL (open circles) is not necessary. Nevertheless, this consistency of both data sets with a central value indicates that sensitivity correction using a test dose may be feasible for Re-OSL. This assumption is enforced by the dose recovery results discussed in the following paragraph. 4.3. Dose recovery
Fig. 4. Growth curves for corrected TT-OSL (filled rectangles), Re-OSL (empty circles) and BT-OSL (filled triangles). The OSL response to the same doses is also provided in the inset.
The efficiency of the Re-SAR method was further investigated by studying the recovery of artificially given doses. Specifically, three aliquots of PTC2 were bleached, then irradiated with 60, 315, 525 and 1050 Gy correspondingly and they were measured using the Re-SAR of Table 1. Test doses were the 10% of the administered values, since this relationship is considered to lead in best recovery (Wang et al., 2007). In Section 4.2 it was seen that correction by subtracting the zero dose normalised Re-OSL was not always necessary. Since we were unable to predict whether this subtraction was needed or not, the correction was systematically applied to all data, except that of the
Fig. 5. Plots of relative sensitivity versus seven repeated cycles thereupon the measurement of TT-OSL (a,c) and BT-OSL (b,d) signals for sample PTC2. Two regenerated dose/test dose combinations were used; 315/10.5 (a,b) and 630/73.5 Gy (c,d). Open symbols represent results from individual aliquots and filled rectangles show the mean relative sensitivity change per dose/test dose combination.
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Fig. 6. Plots of LTT-OSL versus TTT-OSL (a) and LBT-OSL versus TBT-OS (c) for 315/10.5 Gy of a representative aliquot (aliquot #1 from Fig. 5a), and LTT-OSL versus TTT-OSL (b) and LBT-OSL versus TBT-OSL (d) for 630/73.5 Gy (aliquot #2 from Fig. 5d).
first cycle. The resulting equivalent doses are compared with the original ones in Fig. 8 (filled rectangles). The accordance of the dataset with the y ¼ x line signs for accuracy in dose calculation. The same experiment was extended to KRN6 sample. In this case, doses of 315, 525, 630, 840, 945 and 1260 Gy were given to sets of three disks respectively. The mean recovered values are demonstrated in Fig. 8 as open circles. Only aliquots succeeded to yield recycling ratios close to unity were considered in the calculations (see Section 4.4). As earlier, recovery is again in line with the initial doses. The error in the mean recovery is little and in most cases smaller than 10%, with the only exception that at 840 Gy (14%). The imminent consequence from these observations is that the Re-SAR method is sufficient to calculate the correct value, at least up to 1260 Gy.
change, by the completion of the full SAR sequence (Wintle and Murray, 2006). Fig. 9 displays in the form of a histogram recycling ratios from 19 KRN6 aliquots which went through the Re-SAR protocol of Table 1 using 315, 945, 1260, 0 Gy regenerated doses. The 315 Gy dose cycle was repeated at the end of the sequence. Calculations generate a mean value of 0.97 0.1 which is very close to unity and the majority of recycling ratios (about 2/3 of the population) fall within the 0.90–1.10 limits (Murray and Wintle, 2000). The last implies that changes in sensitivity can be controlled at a certain amount in the sequence of measurements in Table 1. Those which have recycling ratios outside this boundary resulted in De with larger uncertainty and thus were not included in the final mean De for age calculations.
4.4. Recycling ratio
4.5. Sensitivity of Re-OSL to sunlight
Sufficient recycling ratios can validate more the success of the sensitivity change corrections (Murray and Wintle, 2000). Since sensitivity alters progressively within the Re-SAR cycles, the corrected Re-OSL responses of identical doses (one measured immediately after the ‘natural’ cycle and an one identical measured at the end of the sequence) are able to estimate the amplitude of its
In OSL procedures, stimulation of a few seconds by visible light ensures the drop of fast and medium components down to almost background noise. Consequently, exposure of quartz aliquots directly to sun rays, say for an hour, not only removes the fast and medium components completely, but also can reduce the slow component down to negligible levels. This is what is seen in our
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Fig. 7. Corrected Re-OSL versus cycle for 315/10.5 Gy (rectangles) and 630/73.5 Gy data sets (circles). Correction by subtracting the zero dose corrected recuperated signal (8th cycle) has been applied to all cycles (open rectangles and circles respectively) except the first.
experiments when measuring OSL at 125 C after so long sunlight exposures. However, complete zeroing is not guaranteed in chronologies utilising TT-OSL. Recent work by Adamiec et al. (2008) explains that the source trap responsible for Re-OSL generation is not as lightsensitive as that of OSL. Experiments by Tsukamoto et al. (2008) assert that a fractional dose can always be measured even after days of exposure in a solar simulator. Residual Re-OSL could be an extra source of uncertainty, as it might embody in and consequently affect the final De numbers. So, it is sensible to calculate an estimate of its size and see how it impacts on the final equivalent dose . Fresh PTC2 quartz, containing only the naturally absorbed dose, was prepared in sets of disks, each set including three of those. Different sets of aliquots were turned in direct sunlight on a bright day, for different periods of solar exposure; 0, ½, 1,2 and 3 h. Fig. 10 presents the resulting mean dose measured for each set using the Re-SAR after bleaching. The residual recuperated De lessens significantly within the first half hour of exposure, from w55 Gy to 7 Gy. This is about 13% of the original dose, given of course the
Fig. 8. Dose recovery results for PTC2 (filled rectangles) and KRN6 (open circles). Results from both samples are in good agreement with the administered doses, strongly supporting Re-SAR’s capacity for accuracy in dose recovery. Error bars are given only for KRN6.
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Fig. 9. Histogram of recycling ratios from aliquots used in the experiments described earlier (n ¼ 19). The recycled regenerated dose was 315 Gy using a 94.5 Gy test dose for correction. The majority lies within 10% deviation from unity, testifying Re-SAR’s suitability to correct for sensitivity changes.
experimental errors. Onwards, residual dose tails off at a very low rate. These results permit to assume than one hour of solar exposure may be an adequate time for Re-OSL to reduce to very low levels. Thus, its contribution to the mean equivalent dose will not be a problem, if large doses are assumed as happens in the experiments elaborated earlier. However, when smaller sizes of dose are involved, less than 100 Gy for example, the remnant dose should be considered. 5. Re-OSL ages The Re-SAR protocol of Table 1 was applied to ten samples from the area of Fig. 1, in order to generate De and ultimately age estimates. Initial procedures involved measuring the equivalent dose using both TT-OSL and conventional OSL for comparison. About eight different aliquots from each sample were subjected to the method of Table 1 and another similar set to the standard OSL-SAR protocol (Murray and Wintle, 2000). Preliminarily, the
Fig. 10. Relationship between time of solar exposure and magnitude of remaining dose for PTC2 calculated using Re-SAR.
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Fig. 11. This figure portrays the relationship between ages calculated by applying ReOSL and those measured using conventional OSL.
mean De was calculated by averaging the apparent equivalent doses and the uncertainty was the standard deviation of the individual estimates. Results from both methods are given in Table 2. With regard to the dose rates, Neutron Activation analysis (University of Missouri, Research Reactor Center) provided the U, Th and K concentrations. A number of samples, selected in a way to represent the textural and mineralogical variety of the studied sediment were also measured using PIPS alpha-counting (Michael et al., 2008) to check for possible disequilibrium effects. Finally, all samples were found in secular equilibrium. The conversion factors given by Liritzis and Kokoris (1992), also considering the gamma dose-correction presented at Liritzis et al. (2001), resulted the beta- and gamma-dose rates from U, Th and 40K. The resulted dose rate, corrected for water content, cosmic irradiation and grain size attenuation are listed in Table 2. It is interesting to note the low radioactivity of those sediments, owing mostly to significant carbonate content. Similar dose rates are common in the limestone-rich environments of West Greece (Zacharias et al., 2008). The last column in Table 2 shows the calculated Re-OSL ages (in ka). Also, the OSL ages are shown for comparison. Fig. 11 depicts the relationship between OSL and Re-OSL ages with respect to the y ¼ x line. Principally, OSL ages seem overestimated compared to the Re-OSL ones by some tens of thousands of years, with only a few exceptions. Particularly, in the case of FTR1, the OSL overestimation is almost double in size.
Errors for OSL dates are larger than their respective Re-OSL estimates, except LGV1. Only in PTC2, LGV1 the OSL ages fit in with those measured using Re-SAR and within the relatively large associated uncertainties on these samples. Considering the hard-to-bleach residual Re-OSL remaining after prolonged solar exposure, someone would anticipate overestimation in Re-OSL De relative to the OSL rather than vice versa. To the contrary, the small remnant dose calculated earlier might be incorporated in the errors, as long as it remains little, and practically may have a minimal effect in the mean Re-equivalent dose. Therefore, the cause of the discord between the results of the two OSL techniques should be sought elsewhere. A possible solution is that the respective OSL signals are very likely to be close to saturation at those large doses. As explained in the introduction, measurements on samples of similar age, locality and origin resulted in overestimated OSL equivalent doses because the natural signals interpolated the saturating segment of the growth curve. Fig. 12 arrays pairs of OSL and Re-OSL growth curves from representative samples, standardised by the size of their respective test dose (Roberts and Duller, 2004). There is a remarkable difference between the dose response characteristics from the two methods of measurement for two at least representative samples, namely FTR1 (Fig. 12a,b) and KNR6 (Fig. 12c,d). Here, intersection of the natural signal onto the OSL growth curve occurs near to saturation. It has been seen, as explained in the introduction, that very low growth curve slope angles lead in overestimation of the natural dose (Athanassas, 2005). Conversely, Re-OSL is qualified by greater signal capacity which allows luminescence intensity to grow up to significantly higher doses. Interpolation in case of Re-OSL is more likely to point at the actual De. This may be the reason for deviation between the two methods of measurement in Table 2 and thus, the recuperated-OSL ages could be in principal more appropriate values for age implications. The same rationale could be expanded to the majority of samples from Table 2 which exhibit similar behavior. Within the whole number of measured samples, ages calculated for PTC2 and LGV1 stand out. The Re-OSL data appear in good agreement with those evaluated using standard OSL. The argumentation described earlier does not interpretate fairly this striking coherence though. PTC2 is a younger sample. Both recuperated and conventional OSL are far from saturation and respond in a similar manner (Fig. 13e,f). In this instance, the light-insensitive fraction of dose in the TT-OSL source trap inevitably impacts upon the estimated recuperated De and consequently, the slight overestimation in the Re-OSL over the OSL age could be justified. In LGV1 (Fig. 12g,h) recuperated and conventional OSL signals exhibit common dose response for doses up to 500 Gy, resulting in almost identical equivalent doses. This growth behavior is predictable for Re-OSL, but unusual in OSL from quartz. It is
Table 2 Sample names, depths, radioelement content water concentration dose rates, OSL De and OSL ages as well as Re-OSL De and Re-OSL ages. No water content was measured for cemented samples. All samples are marine apart from PTC2 which corresponds to an aeolianite. Sample
Depth (m)
U (mg g1)
Th (mg g1)
K(wt %)
Water (wt%)
Dose rate (Gy/ka)
OSL De (Gy)
Re-OSL De (Gy)
OSL Age (ka)
Re-OSL Age (ka)
MAT1 MAT2 AGS1 AGS2 GRG1 LGV1 FTR1 FTR2 PTC2 KRN6
2.6 0.45 4.2 1.5 2 4 3 4.2 3 5
0.70 0.09 0.52 0.11 0.47 0.50 0.79 0.11 1.02 0.11 0.81 0.15 0.78 0.13 0.45 0.49 0.86 0.09 0.74 0.04
1.07 0.27 1.60 0.33 0.66 0.27 1.58 0.38 0.90 0.17 2.01 0.43 1.71 0.41 0.62 0.30 2.12 0.28 0.61 0.12
0.56 0.03 0.54 0.03 0.82 0.02 0.47 0.03 0.07 0.18 0.25 0.03 0.42 0.03 0.78 0.02 0.18 0.02 0.07 0.01
2 d 3 1 1 1 1 3 d d
0.82 0.03 0.75 0.03 1.01 0.03 0.86 0.04 0.69 0.02 0.89 0.02 0.80 0.03 0.93 0.03 0.83 0.02 0.40 0.01
94.33 18.66 83.00 18.00 174.33 21.08 98.00 27.04 124.02 12.30 225.25 7.64 147.41 19.24 167.50 41.36 54.10 10.50 176.50 38.40
73.50 10.07 57.33 8.20 145.6 13.30 90.00 8.90 100.80 8.64 231.00 12.20 84.31 9.07 105.00 14.11 56.24 7.12 122.21 17.00
115.04 23.00 110.66 24.00 172.61 20.90 113.95 31.40 179.74 17.82 253.01 8.58 184.26 24.05 180.11 44.47 65.30 12.70 438.00 96.00
91.90 12.30 76. 44 10.00 144.12 13.20 104.65 10.30 146.87 12.52 259.55 13.61 105.39 11.34 112.90 15.17 67.76 8.57 305.52 42.50
C. Athanassas, N. Zacharias / Quaternary Geochronology 5 (2010) 65–75
Fig. 12. Pairs of Re-OSL and OSL dose response curves from representative samples. Dotted lines represent the interpolation of the natural signal.
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Fig. 13. Comparison between the distribution of Re-OSL ages of Table 2 with the Oxygen Isotope Stages timescale. Isotopic data from Lea et al. (2002).
possible, even after careful sample preparation, that some amount of feldspar contamination might be present. It is known that feldspars have D0 values larger than those of quartz, allowing the signal to grow up to extremely high doses. Some feldspar grains scattered among LGV1 quartz grains might be responsible for the coincidence in De from either dating approach. An additional prolonged cleansing with concentrated fluorosilicic acid might help to reduce the amount of the contamination but this treatment has not been applied to LGV1 yet. TT-OSL properties of feldspar is unknown so far and do not lie within the immediate interests of this paper. Thus, any further commenting on this sample’s luminescence behavior is therefore impossible. 6. Criticism on the Re-OSL chronology In the earlier sections, the potentiality of the SAR method designed for Re-OSL was tested as a novelty in dating old marine deposits from SW Greece. Series of experiments were able to sustain protocol’s validity to a certain extent. Also, in Section 5, we attempted to explain the divergence of Re-OSL dates from those calculated by standard OSL on the basis of the recuperated signal’s capacity. In the first place and before applying any geologic justification these ages might tentatively suggest that this project is dealing with Middle-Late Pleistocene deposits. Nonetheless, the currently available data are not sufficient enough to exceed the veracity of the Re-OSL dates on firmer ground. For example, there is no independent control over the recuperated-OSL ages. Regional nanoplanktonic traces abound mainly in the Early Pleistocenic stages. Restricted precision related with the mean equivalent doses could be another drawback. In most circumstances the standard deviation associated with every Re-OSL date is larger than w15% (see Table 2). To some degree, large errors may owe to the limited number of aliquots which have been measured to obtain these preliminary ages. Consequently, efforts to associate the dated facies with distinct peaks within the global marine isotopic record (e.g. Martinson et al., 1987) would result in perplexity. Since the amount of uncertainty does not allow direct correlation with Oxygen Isotope Stage (OIS) curve peaks, Fig. 13 illustrates a looser comparison between a histogram of the measured Re-OSL ages and the secular sea-level oscillation for the last 350 ka. The majority of dates (bars on the left) seem to relate with the last temperate cycle of Quaternary known as the Last Integlacial. Foraminifera, such as Ammonia Asterigerinata, Cibicides Lobatulus, Globigerinoides Ruber and Orbulina, were observed in the dated
samples and they emphasise the warm and shallow character of the waters these samples were formed. Also, these samples come from the westernmost terrace which has been mapped in Fig. 1 (I). The last is assumed to be the youngest of all and formed during the same period of risen sea-level. This may administer some initial reason for the validity of the Re-OSL ages. Still, the causes of uncertainty described earlier make difficult any further correlation with the Late Quaternary geological and climatological framework for the time being. Geochronological associations become even more complex in the older stages as the number of samples from older terraces (Fig. 1) is very limited. The oldest of the Re-OSL dates (LGV1, KRN6) point to a period of an overall sea-level rise around 300 ka, but none of the estimated ages is able to capture the range of intermediate sea-level maxima between 150–250 ka. Again, increased uncertainty in the mean De (KRN6) and possible feldspar contamination (LGV1) are major obstacles when aiming to relate these large dates directly to the OIS timescale. Taking everything into account, calculated Re-OSL ages lack substantial support from any independent chronology so as to examine their reliability. In addition, the considerable amount of spread in the equivalent dose among single aliquots constrains the precision of the ages to a limited extent. Temporarily, we attribute this uncertainty to the small population of disks measured per sample during this preliminary investigation. Even so, the relative coincidence between Re-OSL ages from the youngest terrace in Fig. 1 and sea-level during the last temperate stage-derived from the isotopic data of Lea et al. (2002)– could be a preliminary indication for TT-OSL as a dating method which may shed light on the so far absent Quaternary chronology of the SW coast of Greece. We acknowledge thus the necessity of additional labour in order to substantiate the calculated Re-OSL ages on a more solid basis. Scrutiny for later pleistocene nannofossils will provide additional control on the Re-OSL chronology. Furthermore, measurement of larger aliquot populations and use of advanced statistical analysis may promote more secure De and therefore age estimates. When future data allow, the authors will update the criticism on the Re-OSL chronology. 7. Conclusions To sum up, application of Wang et al.’s (2007) Re-SAR method to coarse-grained quartz from Pleistocene terraces of SW Greece generated recuperated-OSL signals which dose response was sufficient enough to calculate large artificial equivalent doses and possibly appropriate to explore the chronology of earlier periods of Pleistocene. Experiments on sensitivity changes using recycled laboratory doses indicated that monitoring by means of a test dose may not be as accurate as Murray and Wintle (2000) and Wintle and Murray (2006) judge. To the contrary, corrected Re-OSL signals yielded specific values in repeated dose sequences, implying that the test dose may at least correct adequately. This hypothesis is further consolidated by the accurate results of dose recovery experiments on two samples, as well as by an acceptable number of recycling ratios. On the other hand, solar exposures of an hour or longer are able to remove an adequate amount of the residual Re-OSL. The little size of the remnant dose seems to lead only in small overestimations in the Re-OSL De, when the size of the last is of a few tens of Grays. Though, its impact is negligible at higher doses. Regarding De measurements on natural samples, Re-SAR and standard SAR results are in good accordance, as long as OSL is far from saturation and residual dose remains low. For doses above 100 Gy there is a systematic overestimation in OSL equivalent dose against Re-OSL due to saturation of the conventional optical signal.
C. Athanassas, N. Zacharias / Quaternary Geochronology 5 (2010) 65–75
In such a case, Re-OSL data should be recognised as more appropriate. However, preliminary equivalent dose measurements on natural samples did not result in high precision, possibly due to the restricted population of apparent estimates. In addition, nanoplanktonic presence is scant in the dated samples and therefore no independent control exists for the moment. Although comparison of Re-OSL dates from the youngest terrace with sea-level data for the later Quaternary may suggest somewhat an agreement, the aforesaid factors are inevitably sources of uncertainty. It is thus sensible to be cautious about the recuperated-OSL ages given in Table 2 for the time being and until further proof is available. Acknowledgments Here we acknowledge M. Triantafyllou (Department of Geology, University of Athens) for nanoplankton analysis, E. Moraiti (Geological Survey of Greece) for SEM microscopy on microfauna, Y. Bassiakos (Archaeometry Lab, Demokritos, Athens) and I. Fountoulis (Geology Department, University of Athens) for proofreading and commenting on the geology and V. Pagonis (McDaniel College, Westminster, USA) for his critical comments on this work. Authors also acknowledge advice by G. Adamiec (Silesian University of Technology, Poland) and one anonymous reviewer for the overall improvement of this paper. C.A. is funded by the Greek Scholarships Foundation (Grant no. 1500521537.008.040). Editorial handling by: R. Roberts References Adamiec, G., Bailey, R.M., Wang, X.L., Wintle, A.G., 2008. The mechanism of thermally transferred optically stimulated luminescence in quartz. J. Phys. D: Appl. Phys. 41, 1–14. Armitage, S.J., Duller, G.A.T., Wintle, A.G., 2000. Quartz from southern Africa: sensitivity changes as a result of thermal pretreatment. Radiat. Meas. 32, 571–577. Athanassas, C., 2005. Luminescence Studies of Coastal Sediments in the South West Peloponnese, Greece. Unpublished M.Phil. thesis. University of Wales, Aberystwyth, pp. 131. Bailey, R.M., 2004. Paper I dsimulation of dose absorption in quartz over geological timescales and its implications for the precision and accuracy of optical dating. Radiat. Meas. 38, 299–310. Fountoulis, I., Moraiti, E., 1994. Sedimentation, paleogeography, and neotectonic interpretation of post-alpine deposits in the Kyparissia-Kalo Nero Basin. Bull. Geol. Soc. Greece 30, 323–336. Kelletat, D., Kowalczyc, G., Schro¨der, B., Winter, K.-P., 1976. A synoptic view on the neotectonic development of the Peloponnesian coastal regions. Z. Dt. Geol. Ges. 127, 447–465. Kourampas, N., Robertson, A.H.F., 2000. Controls on Plio-Quaternary sedimentation within an active fore-arc region: Messinia Peninsula (SW Peloponnese), S. Greece. In: Panayides, I., Xenophontos, C., Malpas, J. (Eds.), 2000, Proceedings of
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Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo
Research Paper
Recuperated-OSL dating of quartz from Aegean (South Greece) raised Pleistocene marine sediments: current results C. Athanassas a, b, *, N. Zacharias a a b
Laboratory of Archaeometry, Institute of Materials Science, N.C.S.R. ‘Demokritos’, Aghia Paraskevi, Athens153 10, Greece Department of Geology, University of Athens, Zografou, Athens 157 84, Greece
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2008 Received in revised form 11 September 2009 Accepted 23 September 2009 Available online 2 October 2009
The purpose of this work is to provide an OSL dating framework for raised marine sequences in the South-West coast of Greece during Upper Quaternary. Paleontological investigations and Geographic Information System (GIS) analysis on elevated marine landforms have proved that a record of uplift and eustacy exists in South Greece since the Early Pleistocene (w1.6 Ma). Hereby, we test the suitability of the SAR methodology for recuperated-OSL (Re-OSL), proposed by Wang et al. (2007), on coarse-grained quartz aliquots from emerged nearshore outcrops. Protocol’s performance is examined on the basis of signal characteristics, dose response, sensitivity changes, recovery of known doses and Re-OSL bleachability to sunlight. The accuracy of the Re-OSL dates calculated for natural samples is also discussed. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Aegean Marine terraces Nannoplankton GIS TT-OSL Recuperated-OSL Sensitivity changes Oxygen Isotope Stage
1. Introduction South Aegean Greece constitutes one of the most tectonically active regions worldwide. This activity dates back to the Oligocene (e.g. McKenzie, 1970; McClusky et al., 2000). Intense seismicity, normal faulting and crustal uplift are a few, out of several, geodynamic phenomena which prevail in the development of the region. Sequences of raised and well-developed marine terraces occupy extensive segments of the south coast, revealing a steady motif of uplift throughout the Pliocene and Pleistocene (Kelletat et al., 1976). Specifically, in the southwestern corner of the country, a set of stranded platforms have been mapped (Fig. 1), capped by shallow marine to brackish sediments. Former (Marcopoulou-Diacantoni et al., 1991; Fountoulis and Moraiti, 1994; Kourampas and Robertson, 2000) and current paleontological investigation set the beginning of sedimentation in the Early Pleistocene. Nannoplanktonic populations of Gephyrocapsa, Pseudoemiliania lacunosa, and Calcidiscusleptoporus, all correlated with the MNN-19e biozone
* Corresponding author. E-mail address: [email protected] (C. Athanassas). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.09.010
(Rio et al., 1990), bound the chronology of the Quaternary outcrops as far back as 1.6–0.95 Ma. The paucity of younger paleontological finds and absolute dates restrains the chronological discrimination between the successions. Using luminescence dating, we favor to correlate the uplifted Pleistocene facies with highstands of higher resolution, as they have been detailed in the Oxygen Isotope Stages timescale (Martinson et al., 1987). Depending on the range of the locally experienced dose rates, standard optically stimulated luminescence (OSL) measurements using the single aliquot regenerated dose protocol (SAR) by Murray and Wintle (2000) can accurately yield the palaeodose within the Last Interglacial-Glacial Cycle (e.g. Murray and Olley, 2002), this is the last w125 ka. Beyond that stage, the uncertainty increases significantly (Murray and Funder, 2003). Likewise, Late Quaternary quartz aliquots from the southwestern coast of Greece have been found to approach saturation at doses over w100 Gy (Athanassas, 2005), thus, making the possibility of exploring the age earlier geologic events using OSL dating, problematic. Wang et al. (2006a,b) have testified the capability of thermally transferred OSL (TT-OSL) signal, and particularly its recuperated component (Re-OSL), to calculate the equivalent dose in the region where the electron traps responsible for the fast OSL component in
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C. Athanassas, N. Zacharias / Quaternary Geochronology 5 (2010) 65–75 Table 1 Single aliquot regenerated dose method for Re-OSL (Wang et al., 2007). Step
Treatment
Part 1: TT-OSL measurement 1–1 Dose Di 1–2 PH (260 C, 10 sec) 1–3 OSL (125 C, 270 sec) 1–4 PH (260 C, 10 sec) 1–5 OSL (125 C, 100 sec) 1–6 Test dose TD 1–7 PH (220 C, 10 sec) 1–8 OSL (125 C, 100 sec) Part 2: BT-OSL measurement 2–1 Anneal to 300 C for 10 sec 2–2 OSL (125 C, 100 sec) 2–3 PH (260 C, 10 sec) 2–4 OSL (125 C, 100 sec) 2–5 Test dose TD 2–6 PH (220 C, 10 sec) 2–7 OSL (125 C, 100 sec)
Fig. 1. Relief image of the study area. GIS analysis of raster contour maps, 1; 5,000 in scale, revealed a sequence of four at least platforms, possibly related to Middle to Late Quaternary sea-level highstands. Elevation (colour graded scale) is given in m. Grid data are in WGS’84. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Measurement
LTT-OSL
TTT-OSL
LBT-OSL
TBT-OSL
sea-level highstands and the bedrock. Progressive uplift during lowstands resulted in this flank of abandoned landforms. Characteristic arched perimeters, parallel to the present shoreline, and higher elevations in the central-east segment, seen in Fig. 1, resulted in a dome-like topography. From this we can draw preliminary evidence for stronger uplift in the middle part of the area. Field campaigns located stacks of miscellaneous marine facies as well as fossil sand dunes. Later Quaternary offshore and nearhore clastic formations, such as silts, sands, sandstones and conglomerates have been deposited on a predominant Early Pleistocenic background. The last has been endorsed by nannoplankton examination (current research). A typical succession of the study sediments is illustrated in Fig. 2.
quartz are practically saturated. Wang et al. (2006a,b), Tsukamoto et al. (2008), Pagonis et al. (2008) provided investigations and basics on TT-OSL dating while Li and Li (2006), Pagonis et al. (2007) and Adamiec et al. (2008) put forward mechanisms on TT-OSL generation. Especially, Adamiec et al. (2008) elucidated a new mechanism according which TT-OSL derives from a trap with thermal stability similar to that of OSL, but less sensitive to the sunlight. During a preheat (260 C for 10 sec) applied immediately after an optical stimulation, a proportion of charge moves from that source trap, via the conduction band, to the main OSL trap. Subsequent optical stimulation will give rise to the TT-OSL signal, which has saturation levels significantly higher than those of the conventional OSL. In this work we test the efficiency of the SAR method for Re-OSL (Table 1), as developed by Wang et al. (2007), to extract ages for earlier Pleistocenic events in the Southwest Greece.
2. Site and sample description Although the number of terraces, appearing along the southwestern Greece coast, has been debated (Kelletat et al., 1976; Kourampas and Robertson, 2000), analysis using Geographic Information Systems (ArcGIS 9.0 software) has revealed a succession of at least four stranded platforms (Fig. 1). They have, most probably, been generated by the long-lived interaction between
Fig. 2. Typical sequence of nearshore deposits appearing in a coastal cliff at ‘MAT’ sample site (Fig. 1). From the bottom to the top, the section starts with a basal conglomerate and goes over silty sands, fine sands, silty sands, conglomerate, fine sands, conglomerate and terminates in a cross-bedded medium to coarse sandstone. The sequence stands at w12 m above modern sea-level. Samples MAT1 (offshore sand) and MAT2 (nearshore sandstone) were collected from this section.
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Stratigraphic correlations and sediment properties such as grain size and incorporated fossil fauna reflect the proximity of these deposits to sea-surface. As a result, they may be suitable for inferring relative vertical movements. Also, the abundance of quartz grains in the silty-sand grain size fraction, its extended exposure to daylight, and its assumed age, renders these sediments appropriate material for OSL dating. Sampling for the purposes of luminescence measurements involved collecting stratified sands, silts as well as coastal sand dunes in aluminum tubes from natural sections spread along the coastline and hinterland. 3. Sample preparation and measurement settings The procedure of purification of quartz was the typical followed in OSL dating including the steps: 10% HCl, 10% H2O2, first sieving, density separation using sodium heteropolytungstate with densities of 2.70 and 2.63 gcm3, 40% HF, rinsing with 10% HCl, and finally second sieving to receive the 90–125 mm fraction used in this study. Instrumentation for the signal measurements included a RISØ TL-DA-15 reader supplied with blue diodes (470D30 nm). An EMI9635Q photomultiplier and two 3 mm Hoya U-340 filters were attached on the system. The reader was equipped with a 90Sr/90Y beta source for automated irradiations, delivering 6.30 Gy min1. The measurement procedure of Table 1 was used in order to estimate the intensity of the regenerated sensitivity-corrected thermally transferred optical signals. This SAR methodology is a two-part protocol with the first applying to the calculation of the corrected TT-OSL and the second to the corrected BT-OSL. Removal of fast and medium OSL components in 1–3 (Part 1, Table 1) was achieved by stimulating the sample with blue LEDs at 125 C for 270 sec. Soon after, preheat (PH) at 260 C for 10 sec was used in 1–4 to generate thermal transfer, as it has been suggested earlier by Wang et al. (2006a,b). This treatment allows thermal transfer of maximum electron population from the source trap to that responsible for OSL (Adamiec et al., 2008), without depleting the last. These PH conditions are in agreement with the experiments of Tsukamoto et al. (2008). All OSL, TT-OSL and BT-OSL measurements were conducted at 125 C for 100 sec. Net signal intensity was calculated as the integral of the first 2 sec of the measurement, subtracted by that of the last 10 sec. In order to isolate BT-OSL from TT-OSL, annealing the sample at an elevated temperature was necessary. Experimental data from both Wang et al. (2006a,b) and Tsukamoto et al. (2008) unite that a 300 C heating for 10 sec (in step 2–1) is appropriate to completely remove Re-OSL. Therefore, the signal measured in 2–4 is the net BT-OSL. This is the heating treatment used here as well. Large doses were given in pulses of a 105 Gy interposing a 240 C preheat. The use of pulsed irradiation has been proven necessary when large doses are involved in TT-OSL dating, otherwise significant underestimations may be expected (Wang et al., 2006b). Stepped irradiation approximates better the natural trapping conditions during the absorption of the dose (Bailey, 2004). This treatment was applied to all samples and experiments (sensitivity changes, dose recoveries, measurement of natural De) when doses grater than 105 Gy were utilised. A similar dose increment have been used by Wang et al. (2006b). Finally, where bleaching prior dosing is stated in this text, it involves exposure of quartz aliquots directly to sunlight for 60 min. Signal recordings after bleaching resulted in that all three OSL components have significantly been cut down to almost background levels. The same solar exposure seems also to have
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a significant impact on the TT-OSL signal and it is able to reduce it down to very small residual values, as discussed in Section 4.5. 4. General performance of the Re-SAR method 4.1. TT-OSL characteristics Replicate runs of the protocol in Table 1, using a range of regenerated doses, results in the estimation of the sensitivity-corrected TT-OSL (LTT-OSL) and BT-OSL (LBT-OSL) signals. The difference of these numbers is the sensitivity-corrected regenerated Re-OSL (LRe-OSL) and its growth in response to the regenerated dose can be schematically described by a growth curve. One quartz aliquot of KRN6 (marine sand) was bleached in sunlight and then went through a sequence of 315, 945, 1260 and zero Gy regenerated doses using the steps shown in Table 1. Both TT-OSL and BT-OSL signals were corrected by a 94.5 Gy test dose. Fig. 3 shows the successive TT-OSL (Fig. 3a) and BT-OSL (Fig. 3b) decay curves, measured after each irradiation. The relative test dose signals (TTT-OSL and TBT-OSL) are given in Fig. 3c and d. All decay curves have been normalised to the first 0.4 sec of measurement. Normalised TT-OSL decay curves in Fig. 3a seem to be dominated by the fast component within the first 2 sec of optical stimulation. This component is distinctively more luminous than the slow that follows on. Apparent decay curves look identical, indicating that there is a constant proportion in signal growth among components as the dose increases. Similar observations stand for less intense BT-OSL in Fig. 3b as well as for OSL recorded after the test dose (Fig. 3c,d). Fig. 4 illustrates the corrected TT-OSL dose response curve, resolved in its corrected Re-OSL and BT-OSL constituents. The corrected TT-OSL and Re-OSL growth are represented by single saturating exponentials respectively, while the BT-OSL dataset is fitted by a straight line. For comparison, the growth of the typical OSL signal at the same doses is inserted in Fig. 3. Likewise, corrected OSL has been fitted by a single saturating exponential. As expected, thermally transferred signals continue to grow in the dose region where standard OSL is practically saturated. Specifically, the characteristic saturation dose (D0) was calculated for all exponential signals above using the equation (1).
D I ¼ Imax 1 eD0
(1)
D0 for OSL is 135 Gy, but this number becomes significantly higher in TT-OSL and Re-OSL growth curves; 710 and 490 Gy respectively. This indicates that Re-OSL approaches saturation at doses at least 4 times greater than standard OSL and it is very likely that this signal can be used to calculate ages older than those yielded in standard OSL-SAR dating procedures (Murray and Wintle, 2000). These D0 values are sufficient enough to investigate earlier quaternary events. It is also possible that an additional linear component may describe the Re-OSL behavior at higher doses, as hinted by the ability to recover doses larger than the above D0 value. Yet, this was not a subject of further investigation. 4.2. Sensitivity changes In order to monitor and therefore correct the sensitivity changes occurring during the Re-SAR thermal treatments, two sets of bleached aliquots from PTC2 sample (aeolian dune sand) were tested, each set containing three disks. The first set received 315 Gy and, combined with a test dose of 10.5 Gy, was recycled seven times using the procedure presented in Table 1. Similarly, the second set went through the same procedure using a 625/73.5 Gy dose/test
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Fig. 3. Normalised successive decay curves for TT-OSL (a) and BT-OSL (b) after 315, 945, 1260 Gy dosing as well as the following test dose OSL responses namely, TTT-OSL (c) and TBT(d), using a constant dose of 94.5 Gy.
OSL
dose combination. Fig. 5 demonstrates the relative sensitivity changes for TT-OSL and BT-OSL, this is the test dose response of each cycle normalised to the test dose signal of the first measurement, in a way similar to Fig. 1 in Wang et al.’s (2007) paper. Examination of the data in Fig. 5 reveals sensitivity change patterns significantly different from those presented by Wang et al. (2007) for Chinese loess quartz. Particularly, in 315/10.5 Gy and for both TT-OSL and BT-OSL signals, there do occur sensitivity changes through the cycles. A steady increase in the averaged sensitivity is observed up to cycle No 5 (No 4 for BT-OSL) in a fashion similar to that resulting from high temperature preheats (Armitage et al., 2000), and then a slight decrease. However, in 630/73.5 Gy only a minor sensitivity change is observed (Fig. 5c,d). The overall normalised TTT-OSL and TBT-OSL tend to remain constant as cycles progress. The effect of preheats inserted during pulsed irradiation may be a possible interpretation for the difference observed between the two sensitivity change motifs. In 630/73.5 the measurement of the TT-OSL signal is preceded by 6 105 Gy irradiations alternated by 5 240 C preheats, plus those in 1–2, 1–4 and 1–7 shown in Table 1. This total of 8 PHs may have sensitised the OSL signal, prior the measurement of the test dose response. Thereinafter, plots of LTT-OSL versus TTT-OSL (measured in steps 1–5 and 1–8) and LBT-OSL versus TBT-OSL (measured in 2–4 and 2–7)
were created and these are seen in Fig. 6. Each graph shows data from one representative aliquot from each dose/test dose combination. A linear relationship between the two signals is observed. Contrary to Wang et al. (2007) and Tsukamoto et al. (2008), measurement of the first cycle is collinear with the fitted data. Still, fitted lines do not pass through the origin. Wintle and Murray (2006) have claimed that ‘the OSL sensitivity measured using a test dose should be directly proportional to the sensitivity applicable to the preceding regeneration dose’ and in principle, those test doses might not be appropriate to monitor the sensitivity change for PTC2. An additional run was applied to both previously measured aliquots but giving a 0 Gy dose this time and keeping the tests doses constant. Next, plots of corrected Re-OSL versus measurement cycle were produced for the same representative aliquots (mentioned in Fig. 6), including the final zero dose cycle (8th). Fig. 7 shows that both sets of corrected Re-OSL (filled rectangles and circles) converge to a central value. In 315/10.5 Gy combination, a small increase of the corrected Re-OSL in the second cycle can be noticed, due to cumulative effect of the remnant recuperated signal (Wang et al., 2007; Tsukamoto et al., 2008). The oscillation around the central value (0.07) becomes even tighter if the zero Gy-corrected Re-OSL is subtracted from all, but the first, cycles. (open rectangles). This oscillation is very similar to
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Wang et al.’s (2007) normalised Re-OSL, after zero dose-correction, in a similar graph (Fig. 2c and f therein). Yet, data from 630 Gy are in line with the value of 0.027 (filled circles) and correction using zero Gy dose-Re-OSL (open circles) is not necessary. Nevertheless, this consistency of both data sets with a central value indicates that sensitivity correction using a test dose may be feasible for Re-OSL. This assumption is enforced by the dose recovery results discussed in the following paragraph. 4.3. Dose recovery
Fig. 4. Growth curves for corrected TT-OSL (filled rectangles), Re-OSL (empty circles) and BT-OSL (filled triangles). The OSL response to the same doses is also provided in the inset.
The efficiency of the Re-SAR method was further investigated by studying the recovery of artificially given doses. Specifically, three aliquots of PTC2 were bleached, then irradiated with 60, 315, 525 and 1050 Gy correspondingly and they were measured using the Re-SAR of Table 1. Test doses were the 10% of the administered values, since this relationship is considered to lead in best recovery (Wang et al., 2007). In Section 4.2 it was seen that correction by subtracting the zero dose normalised Re-OSL was not always necessary. Since we were unable to predict whether this subtraction was needed or not, the correction was systematically applied to all data, except that of the
Fig. 5. Plots of relative sensitivity versus seven repeated cycles thereupon the measurement of TT-OSL (a,c) and BT-OSL (b,d) signals for sample PTC2. Two regenerated dose/test dose combinations were used; 315/10.5 (a,b) and 630/73.5 Gy (c,d). Open symbols represent results from individual aliquots and filled rectangles show the mean relative sensitivity change per dose/test dose combination.
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Fig. 6. Plots of LTT-OSL versus TTT-OSL (a) and LBT-OSL versus TBT-OS (c) for 315/10.5 Gy of a representative aliquot (aliquot #1 from Fig. 5a), and LTT-OSL versus TTT-OSL (b) and LBT-OSL versus TBT-OSL (d) for 630/73.5 Gy (aliquot #2 from Fig. 5d).
first cycle. The resulting equivalent doses are compared with the original ones in Fig. 8 (filled rectangles). The accordance of the dataset with the y ¼ x line signs for accuracy in dose calculation. The same experiment was extended to KRN6 sample. In this case, doses of 315, 525, 630, 840, 945 and 1260 Gy were given to sets of three disks respectively. The mean recovered values are demonstrated in Fig. 8 as open circles. Only aliquots succeeded to yield recycling ratios close to unity were considered in the calculations (see Section 4.4). As earlier, recovery is again in line with the initial doses. The error in the mean recovery is little and in most cases smaller than 10%, with the only exception that at 840 Gy (14%). The imminent consequence from these observations is that the Re-SAR method is sufficient to calculate the correct value, at least up to 1260 Gy.
change, by the completion of the full SAR sequence (Wintle and Murray, 2006). Fig. 9 displays in the form of a histogram recycling ratios from 19 KRN6 aliquots which went through the Re-SAR protocol of Table 1 using 315, 945, 1260, 0 Gy regenerated doses. The 315 Gy dose cycle was repeated at the end of the sequence. Calculations generate a mean value of 0.97 0.1 which is very close to unity and the majority of recycling ratios (about 2/3 of the population) fall within the 0.90–1.10 limits (Murray and Wintle, 2000). The last implies that changes in sensitivity can be controlled at a certain amount in the sequence of measurements in Table 1. Those which have recycling ratios outside this boundary resulted in De with larger uncertainty and thus were not included in the final mean De for age calculations.
4.4. Recycling ratio
4.5. Sensitivity of Re-OSL to sunlight
Sufficient recycling ratios can validate more the success of the sensitivity change corrections (Murray and Wintle, 2000). Since sensitivity alters progressively within the Re-SAR cycles, the corrected Re-OSL responses of identical doses (one measured immediately after the ‘natural’ cycle and an one identical measured at the end of the sequence) are able to estimate the amplitude of its
In OSL procedures, stimulation of a few seconds by visible light ensures the drop of fast and medium components down to almost background noise. Consequently, exposure of quartz aliquots directly to sun rays, say for an hour, not only removes the fast and medium components completely, but also can reduce the slow component down to negligible levels. This is what is seen in our
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Fig. 7. Corrected Re-OSL versus cycle for 315/10.5 Gy (rectangles) and 630/73.5 Gy data sets (circles). Correction by subtracting the zero dose corrected recuperated signal (8th cycle) has been applied to all cycles (open rectangles and circles respectively) except the first.
experiments when measuring OSL at 125 C after so long sunlight exposures. However, complete zeroing is not guaranteed in chronologies utilising TT-OSL. Recent work by Adamiec et al. (2008) explains that the source trap responsible for Re-OSL generation is not as lightsensitive as that of OSL. Experiments by Tsukamoto et al. (2008) assert that a fractional dose can always be measured even after days of exposure in a solar simulator. Residual Re-OSL could be an extra source of uncertainty, as it might embody in and consequently affect the final De numbers. So, it is sensible to calculate an estimate of its size and see how it impacts on the final equivalent dose . Fresh PTC2 quartz, containing only the naturally absorbed dose, was prepared in sets of disks, each set including three of those. Different sets of aliquots were turned in direct sunlight on a bright day, for different periods of solar exposure; 0, ½, 1,2 and 3 h. Fig. 10 presents the resulting mean dose measured for each set using the Re-SAR after bleaching. The residual recuperated De lessens significantly within the first half hour of exposure, from w55 Gy to 7 Gy. This is about 13% of the original dose, given of course the
Fig. 8. Dose recovery results for PTC2 (filled rectangles) and KRN6 (open circles). Results from both samples are in good agreement with the administered doses, strongly supporting Re-SAR’s capacity for accuracy in dose recovery. Error bars are given only for KRN6.
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Fig. 9. Histogram of recycling ratios from aliquots used in the experiments described earlier (n ¼ 19). The recycled regenerated dose was 315 Gy using a 94.5 Gy test dose for correction. The majority lies within 10% deviation from unity, testifying Re-SAR’s suitability to correct for sensitivity changes.
experimental errors. Onwards, residual dose tails off at a very low rate. These results permit to assume than one hour of solar exposure may be an adequate time for Re-OSL to reduce to very low levels. Thus, its contribution to the mean equivalent dose will not be a problem, if large doses are assumed as happens in the experiments elaborated earlier. However, when smaller sizes of dose are involved, less than 100 Gy for example, the remnant dose should be considered. 5. Re-OSL ages The Re-SAR protocol of Table 1 was applied to ten samples from the area of Fig. 1, in order to generate De and ultimately age estimates. Initial procedures involved measuring the equivalent dose using both TT-OSL and conventional OSL for comparison. About eight different aliquots from each sample were subjected to the method of Table 1 and another similar set to the standard OSL-SAR protocol (Murray and Wintle, 2000). Preliminarily, the
Fig. 10. Relationship between time of solar exposure and magnitude of remaining dose for PTC2 calculated using Re-SAR.
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Fig. 11. This figure portrays the relationship between ages calculated by applying ReOSL and those measured using conventional OSL.
mean De was calculated by averaging the apparent equivalent doses and the uncertainty was the standard deviation of the individual estimates. Results from both methods are given in Table 2. With regard to the dose rates, Neutron Activation analysis (University of Missouri, Research Reactor Center) provided the U, Th and K concentrations. A number of samples, selected in a way to represent the textural and mineralogical variety of the studied sediment were also measured using PIPS alpha-counting (Michael et al., 2008) to check for possible disequilibrium effects. Finally, all samples were found in secular equilibrium. The conversion factors given by Liritzis and Kokoris (1992), also considering the gamma dose-correction presented at Liritzis et al. (2001), resulted the beta- and gamma-dose rates from U, Th and 40K. The resulted dose rate, corrected for water content, cosmic irradiation and grain size attenuation are listed in Table 2. It is interesting to note the low radioactivity of those sediments, owing mostly to significant carbonate content. Similar dose rates are common in the limestone-rich environments of West Greece (Zacharias et al., 2008). The last column in Table 2 shows the calculated Re-OSL ages (in ka). Also, the OSL ages are shown for comparison. Fig. 11 depicts the relationship between OSL and Re-OSL ages with respect to the y ¼ x line. Principally, OSL ages seem overestimated compared to the Re-OSL ones by some tens of thousands of years, with only a few exceptions. Particularly, in the case of FTR1, the OSL overestimation is almost double in size.
Errors for OSL dates are larger than their respective Re-OSL estimates, except LGV1. Only in PTC2, LGV1 the OSL ages fit in with those measured using Re-SAR and within the relatively large associated uncertainties on these samples. Considering the hard-to-bleach residual Re-OSL remaining after prolonged solar exposure, someone would anticipate overestimation in Re-OSL De relative to the OSL rather than vice versa. To the contrary, the small remnant dose calculated earlier might be incorporated in the errors, as long as it remains little, and practically may have a minimal effect in the mean Re-equivalent dose. Therefore, the cause of the discord between the results of the two OSL techniques should be sought elsewhere. A possible solution is that the respective OSL signals are very likely to be close to saturation at those large doses. As explained in the introduction, measurements on samples of similar age, locality and origin resulted in overestimated OSL equivalent doses because the natural signals interpolated the saturating segment of the growth curve. Fig. 12 arrays pairs of OSL and Re-OSL growth curves from representative samples, standardised by the size of their respective test dose (Roberts and Duller, 2004). There is a remarkable difference between the dose response characteristics from the two methods of measurement for two at least representative samples, namely FTR1 (Fig. 12a,b) and KNR6 (Fig. 12c,d). Here, intersection of the natural signal onto the OSL growth curve occurs near to saturation. It has been seen, as explained in the introduction, that very low growth curve slope angles lead in overestimation of the natural dose (Athanassas, 2005). Conversely, Re-OSL is qualified by greater signal capacity which allows luminescence intensity to grow up to significantly higher doses. Interpolation in case of Re-OSL is more likely to point at the actual De. This may be the reason for deviation between the two methods of measurement in Table 2 and thus, the recuperated-OSL ages could be in principal more appropriate values for age implications. The same rationale could be expanded to the majority of samples from Table 2 which exhibit similar behavior. Within the whole number of measured samples, ages calculated for PTC2 and LGV1 stand out. The Re-OSL data appear in good agreement with those evaluated using standard OSL. The argumentation described earlier does not interpretate fairly this striking coherence though. PTC2 is a younger sample. Both recuperated and conventional OSL are far from saturation and respond in a similar manner (Fig. 13e,f). In this instance, the light-insensitive fraction of dose in the TT-OSL source trap inevitably impacts upon the estimated recuperated De and consequently, the slight overestimation in the Re-OSL over the OSL age could be justified. In LGV1 (Fig. 12g,h) recuperated and conventional OSL signals exhibit common dose response for doses up to 500 Gy, resulting in almost identical equivalent doses. This growth behavior is predictable for Re-OSL, but unusual in OSL from quartz. It is
Table 2 Sample names, depths, radioelement content water concentration dose rates, OSL De and OSL ages as well as Re-OSL De and Re-OSL ages. No water content was measured for cemented samples. All samples are marine apart from PTC2 which corresponds to an aeolianite. Sample
Depth (m)
U (mg g1)
Th (mg g1)
K(wt %)
Water (wt%)
Dose rate (Gy/ka)
OSL De (Gy)
Re-OSL De (Gy)
OSL Age (ka)
Re-OSL Age (ka)
MAT1 MAT2 AGS1 AGS2 GRG1 LGV1 FTR1 FTR2 PTC2 KRN6
2.6 0.45 4.2 1.5 2 4 3 4.2 3 5
0.70 0.09 0.52 0.11 0.47 0.50 0.79 0.11 1.02 0.11 0.81 0.15 0.78 0.13 0.45 0.49 0.86 0.09 0.74 0.04
1.07 0.27 1.60 0.33 0.66 0.27 1.58 0.38 0.90 0.17 2.01 0.43 1.71 0.41 0.62 0.30 2.12 0.28 0.61 0.12
0.56 0.03 0.54 0.03 0.82 0.02 0.47 0.03 0.07 0.18 0.25 0.03 0.42 0.03 0.78 0.02 0.18 0.02 0.07 0.01
2 d 3 1 1 1 1 3 d d
0.82 0.03 0.75 0.03 1.01 0.03 0.86 0.04 0.69 0.02 0.89 0.02 0.80 0.03 0.93 0.03 0.83 0.02 0.40 0.01
94.33 18.66 83.00 18.00 174.33 21.08 98.00 27.04 124.02 12.30 225.25 7.64 147.41 19.24 167.50 41.36 54.10 10.50 176.50 38.40
73.50 10.07 57.33 8.20 145.6 13.30 90.00 8.90 100.80 8.64 231.00 12.20 84.31 9.07 105.00 14.11 56.24 7.12 122.21 17.00
115.04 23.00 110.66 24.00 172.61 20.90 113.95 31.40 179.74 17.82 253.01 8.58 184.26 24.05 180.11 44.47 65.30 12.70 438.00 96.00
91.90 12.30 76. 44 10.00 144.12 13.20 104.65 10.30 146.87 12.52 259.55 13.61 105.39 11.34 112.90 15.17 67.76 8.57 305.52 42.50
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Fig. 12. Pairs of Re-OSL and OSL dose response curves from representative samples. Dotted lines represent the interpolation of the natural signal.
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Fig. 13. Comparison between the distribution of Re-OSL ages of Table 2 with the Oxygen Isotope Stages timescale. Isotopic data from Lea et al. (2002).
possible, even after careful sample preparation, that some amount of feldspar contamination might be present. It is known that feldspars have D0 values larger than those of quartz, allowing the signal to grow up to extremely high doses. Some feldspar grains scattered among LGV1 quartz grains might be responsible for the coincidence in De from either dating approach. An additional prolonged cleansing with concentrated fluorosilicic acid might help to reduce the amount of the contamination but this treatment has not been applied to LGV1 yet. TT-OSL properties of feldspar is unknown so far and do not lie within the immediate interests of this paper. Thus, any further commenting on this sample’s luminescence behavior is therefore impossible. 6. Criticism on the Re-OSL chronology In the earlier sections, the potentiality of the SAR method designed for Re-OSL was tested as a novelty in dating old marine deposits from SW Greece. Series of experiments were able to sustain protocol’s validity to a certain extent. Also, in Section 5, we attempted to explain the divergence of Re-OSL dates from those calculated by standard OSL on the basis of the recuperated signal’s capacity. In the first place and before applying any geologic justification these ages might tentatively suggest that this project is dealing with Middle-Late Pleistocene deposits. Nonetheless, the currently available data are not sufficient enough to exceed the veracity of the Re-OSL dates on firmer ground. For example, there is no independent control over the recuperated-OSL ages. Regional nanoplanktonic traces abound mainly in the Early Pleistocenic stages. Restricted precision related with the mean equivalent doses could be another drawback. In most circumstances the standard deviation associated with every Re-OSL date is larger than w15% (see Table 2). To some degree, large errors may owe to the limited number of aliquots which have been measured to obtain these preliminary ages. Consequently, efforts to associate the dated facies with distinct peaks within the global marine isotopic record (e.g. Martinson et al., 1987) would result in perplexity. Since the amount of uncertainty does not allow direct correlation with Oxygen Isotope Stage (OIS) curve peaks, Fig. 13 illustrates a looser comparison between a histogram of the measured Re-OSL ages and the secular sea-level oscillation for the last 350 ka. The majority of dates (bars on the left) seem to relate with the last temperate cycle of Quaternary known as the Last Integlacial. Foraminifera, such as Ammonia Asterigerinata, Cibicides Lobatulus, Globigerinoides Ruber and Orbulina, were observed in the dated
samples and they emphasise the warm and shallow character of the waters these samples were formed. Also, these samples come from the westernmost terrace which has been mapped in Fig. 1 (I). The last is assumed to be the youngest of all and formed during the same period of risen sea-level. This may administer some initial reason for the validity of the Re-OSL ages. Still, the causes of uncertainty described earlier make difficult any further correlation with the Late Quaternary geological and climatological framework for the time being. Geochronological associations become even more complex in the older stages as the number of samples from older terraces (Fig. 1) is very limited. The oldest of the Re-OSL dates (LGV1, KRN6) point to a period of an overall sea-level rise around 300 ka, but none of the estimated ages is able to capture the range of intermediate sea-level maxima between 150–250 ka. Again, increased uncertainty in the mean De (KRN6) and possible feldspar contamination (LGV1) are major obstacles when aiming to relate these large dates directly to the OIS timescale. Taking everything into account, calculated Re-OSL ages lack substantial support from any independent chronology so as to examine their reliability. In addition, the considerable amount of spread in the equivalent dose among single aliquots constrains the precision of the ages to a limited extent. Temporarily, we attribute this uncertainty to the small population of disks measured per sample during this preliminary investigation. Even so, the relative coincidence between Re-OSL ages from the youngest terrace in Fig. 1 and sea-level during the last temperate stage-derived from the isotopic data of Lea et al. (2002)– could be a preliminary indication for TT-OSL as a dating method which may shed light on the so far absent Quaternary chronology of the SW coast of Greece. We acknowledge thus the necessity of additional labour in order to substantiate the calculated Re-OSL ages on a more solid basis. Scrutiny for later pleistocene nannofossils will provide additional control on the Re-OSL chronology. Furthermore, measurement of larger aliquot populations and use of advanced statistical analysis may promote more secure De and therefore age estimates. When future data allow, the authors will update the criticism on the Re-OSL chronology. 7. Conclusions To sum up, application of Wang et al.’s (2007) Re-SAR method to coarse-grained quartz from Pleistocene terraces of SW Greece generated recuperated-OSL signals which dose response was sufficient enough to calculate large artificial equivalent doses and possibly appropriate to explore the chronology of earlier periods of Pleistocene. Experiments on sensitivity changes using recycled laboratory doses indicated that monitoring by means of a test dose may not be as accurate as Murray and Wintle (2000) and Wintle and Murray (2006) judge. To the contrary, corrected Re-OSL signals yielded specific values in repeated dose sequences, implying that the test dose may at least correct adequately. This hypothesis is further consolidated by the accurate results of dose recovery experiments on two samples, as well as by an acceptable number of recycling ratios. On the other hand, solar exposures of an hour or longer are able to remove an adequate amount of the residual Re-OSL. The little size of the remnant dose seems to lead only in small overestimations in the Re-OSL De, when the size of the last is of a few tens of Grays. Though, its impact is negligible at higher doses. Regarding De measurements on natural samples, Re-SAR and standard SAR results are in good accordance, as long as OSL is far from saturation and residual dose remains low. For doses above 100 Gy there is a systematic overestimation in OSL equivalent dose against Re-OSL due to saturation of the conventional optical signal.
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In such a case, Re-OSL data should be recognised as more appropriate. However, preliminary equivalent dose measurements on natural samples did not result in high precision, possibly due to the restricted population of apparent estimates. In addition, nanoplanktonic presence is scant in the dated samples and therefore no independent control exists for the moment. Although comparison of Re-OSL dates from the youngest terrace with sea-level data for the later Quaternary may suggest somewhat an agreement, the aforesaid factors are inevitably sources of uncertainty. It is thus sensible to be cautious about the recuperated-OSL ages given in Table 2 for the time being and until further proof is available. Acknowledgments Here we acknowledge M. Triantafyllou (Department of Geology, University of Athens) for nanoplankton analysis, E. Moraiti (Geological Survey of Greece) for SEM microscopy on microfauna, Y. Bassiakos (Archaeometry Lab, Demokritos, Athens) and I. Fountoulis (Geology Department, University of Athens) for proofreading and commenting on the geology and V. Pagonis (McDaniel College, Westminster, USA) for his critical comments on this work. Authors also acknowledge advice by G. Adamiec (Silesian University of Technology, Poland) and one anonymous reviewer for the overall improvement of this paper. C.A. is funded by the Greek Scholarships Foundation (Grant no. 1500521537.008.040). Editorial handling by: R. Roberts References Adamiec, G., Bailey, R.M., Wang, X.L., Wintle, A.G., 2008. The mechanism of thermally transferred optically stimulated luminescence in quartz. J. Phys. D: Appl. Phys. 41, 1–14. Armitage, S.J., Duller, G.A.T., Wintle, A.G., 2000. Quartz from southern Africa: sensitivity changes as a result of thermal pretreatment. Radiat. Meas. 32, 571–577. Athanassas, C., 2005. Luminescence Studies of Coastal Sediments in the South West Peloponnese, Greece. Unpublished M.Phil. thesis. University of Wales, Aberystwyth, pp. 131. Bailey, R.M., 2004. Paper I dsimulation of dose absorption in quartz over geological timescales and its implications for the precision and accuracy of optical dating. Radiat. Meas. 38, 299–310. Fountoulis, I., Moraiti, E., 1994. Sedimentation, paleogeography, and neotectonic interpretation of post-alpine deposits in the Kyparissia-Kalo Nero Basin. Bull. Geol. Soc. Greece 30, 323–336. Kelletat, D., Kowalczyc, G., Schro¨der, B., Winter, K.-P., 1976. A synoptic view on the neotectonic development of the Peloponnesian coastal regions. Z. Dt. Geol. Ges. 127, 447–465. Kourampas, N., Robertson, A.H.F., 2000. Controls on Plio-Quaternary sedimentation within an active fore-arc region: Messinia Peninsula (SW Peloponnese), S. Greece. In: Panayides, I., Xenophontos, C., Malpas, J. (Eds.), 2000, Proceedings of
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