Testing the Potential of Osl, Tt-osl, Irsl and Post-ir Irsl Luminescence Dating Printer-friendly Version Interactive Discussion Climate of the past Discussions Testing the Potential of Osl, Tt-osl, Irsl and Post-ir Irsl Luminescence Dating on a Middle Pleistocene Sediment Record of Lake El'gygytgyn,

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc This discussion paper is/has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Lake El'gygytgyn is a 12 km wide crater lake located in remote Chukotka in the far East Russian Arctic about 100 km to the north of the Arctic Circle. It was formed by a meteorite impact about 3.58 Ma ago. This study tests the paleomagnetic and proxy data-based Mid-to Late-Pleistocene sediment deposition history using novel luminescence 5 dating techniques of sediment cores taken from the centre of the 175 m deep lake. For dating polymineral and quartz fine grains (4–11 µm grain size range) were extracted from nine different levels from the upper 28 m of sediment cores 5011-1A and 5011-1B. Polymineral sub-samples were analysed by infra-red stimulated luminescence (IRSL) and post-IR infra-red stimulated luminescence (post-IR IRSL) using single aliquot re-10 generative dose (SAR) sequences. SAR protocols were further applied to measure the blue light optically stimulated luminescence (OSL) and thermally-transferred OSL (TT-OSL) of fine-grained quartz supplemented by a multiple aliquot TT-OSL approach. According to an independent age model, the lowest sample from 27.8–27.9 m below lake bottom level correlates to the Brunhes-Matuyama (B/M) reversal. Finally, the SAR 15 post-IR-IRSL protocol applied to polymineral fine grains was the only luminescence technique able to provide dating results of acceptable accuracy up to ca. 700 ka. Major factors limiting precision and accuracy of the luminescence chronology are, for some samples, natural signals already approaching saturation level, and overall the uncertainty related to the sediment water content and its variations over geological times.


Introduction
Luminescence dating is long established as a reliable tool to provide absolute chronologies for Late Pleistocene sediments from numerous depositional environments.The event being dated by luminescence is the exposure of the sediments to sunlight prior to deposition and coverage.Most dating studies are based on quartz optically stimulated luminescence (OSL) applied on sediment archives of aeolian, fluvial or lacustrine Introduction

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Full origin.However, the use of quartz for OSL dating is typically limited to the last 100-150 ka because of saturation effects of the quartz luminescence signal with increasing dose.Infrared stimulated luminescence (IRSL) of potassium-rich feldspars has the potential to extend the datable age range because dose-response curves from K-rich feldspars or polymineral fine grains show a much higher saturation dose compared to quartz.But it has been known for several decades now that the IRSL signal can show anomalous fading (Wintle, 1973).This signal loss over burial time often gives rise to significant age underestimation.Fading correction procedures have been developed (Lamothe and Auclair, 1999;Huntley and Lamothe, 2001) to correct for these underestimations but they are inapplicable at samples, in which dose response curves advance the saturation level.Recent studies (Thomsen et al., 2008;Buylaert et al., 2009;Li and Li, 2011) have presented a measuring protocol, in which a high temperature IRSL signal is measured after a low temperature IRSL signal.This post-IR IRSL is less prone to fading but has the same high saturation dose limits as the low temperature IRSL.These characteristics make this a method of great importance for dating Middle and Late Pleistocene deposits and it has become the preferred protocol for feldspar dating if fading corrections are unreliable or not valid (Buylaert et al., 2012).
Only very few studies have focussed on luminescence dating of lake sediments from the Upper and Middle Pleistocene so far (e.g.Forman et al., 2007;Juschus et al., 2007;Lowick and Preusser, 2011;Lukas et al., 2012).This is probably due to the fact that such environments are afflicted with several complications.One important complication is the accurate estimation of the palaeo-water content.This variable is the most crucial, because water attenuates external radiation and causes a lowering of the dose rate received by the dosimeters, i.e. the sediment grains.The dose rate is a substantial part of the age equation and faulty dose rate calculations lead to inaccurate ages.A second problem that can affect the dose rate determination in lacustrine environments is the presence of radioactive disequilibria in the uranium decay chain (Krbetschek et al., 1994).Similar to changes in water content, this imparts a non-constant dose rate on the sampled material through time.A third question that has to be considered comprises Introduction

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Full the potential sediment transport processes and remobilisation processes by turbidites or landslides.Lake El'gygytgyn is located in Central Chukotka, NE Russia, ∼ 100 km north of the Arctic Cycle (Fig. 1) and even today the surface is frozen and covered with ice for about 9 months of the year (Melles et al., 2012).Fluvial and aeolian input is thus limited to the few summer months when the lake surface is open water and the light conditions allow a reasonable bleaching of the sediment grains.However, the crater is roughly 18 km in diameter and the catchment is less than three times the lake's surface area (Melles et al., 2011).About 50 small inlet streams drain into the lake (Nolan et al., 2002) and it is relevant if the distance of just a few km is sufficiently long to enable full bleaching of the transported sediment.Today, the fluvial input to the lake is very low and much of the sediment is deposited at the mouth of the inflows in shallow lagoons, which are dammed by gravel bars formed by wave and lake ice action (Melles et al., 2011).Temporary deposition in these lagoons and further transport through the clear surface waters, giving a Secchi transparency depth of 19 m in summer (Melles et al., 2012), might enable a reasonable bleaching of the suspended matters.In addition, as a result of short transport distances, sediments may have experienced insufficient cycling prior to deposition, which may lead to poor OSL-sensitivity in quartz samples (Preusser et al., 2006;Fuchs and Owen, 2008), which in turn makes extraction of a dateable signal sometimes challenging.
The samples analysed in this study originate from cores A and B of ICDP site 5011, which was drilled from February until April 2009 employing a 100 tons drilling platform on the artificially thickened ice cover in 170 m water depth in the central part of Lake El'gygytgyn (Melles et al., 2011; Fig. 1).An independent age model for the 3.6 Ma core composite is provided by magnetostratigraphy and tuning of proxy data to the regional insolation and global marine isotope stratigraphy (Melles et al., 2012;Nowaczyk et al., 2012).The objective of our study was to test different approaches of luminescence dating on the exceptionally long sediment record drilled, and provide complemental information on the core stratigraphy.

Sample preparation
The cores from ICDP site 5011 were taken in transparent plastic liners of 10 cm diameter.For luminescence dating, 10 cm thick pieces were taken from replicate core parts and wrapped with black tape to prevent further light exposure.These liner pieces were then opened under subdued red-light conditions in the luminescence sample preparation laboratory in Cologne.The sediment was pushed out of the liner and a small block of 3×3×6 cm was cut out of the inner core to eliminate any grains that were exposed to light during cutting and preparing of the liner pieces.The small block was then treated with hydrochloric acid, hydrogen peroxide and sodium oxalate to remove carbonate, organics and dissolve coagulations.Due to the lack of sufficient sand-sized minerals, the 4-11 µm fine grain fraction was prepared following Frechen et al. (1996).Quartz was separated by etching the polymineral fine grains with hexafluorosilicic acid for 7 days.
The remaining sediments from the liner were dried, homogenised and prepared for gamma-ray spectrometry.The effective water content was determined by weight loss after drying the bulk samples and is given in relation of weight water to dry mass.Comparing the results with the original water content, measured at a second correlative core soon after drilling, most of the values are in fairly good agreement (Table 1), indicating that no significant water loss has happened between coring and sample preparation.Only sample 1A1H3, taken from a turbidite layer, shows a significant discrepancy.

Dosimetry
Radionuclide analyses (uranium, thorium, potassium) were carried out by gamma-ray spectrometry in the Cologne laboratory and at the VKTA Rossendorf e.V. (D.Degering, Dresden), respectively (see Table 1).157 g of dry sample material was packed in 90 × 25 mm polystyrol containers and stored for 4 weeks to compensate for radon loss induced by sample preparation.Introduction

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Full In water-lain sediments disequilibria in the uranium decay series are quite common.The impact of changes in the sediment dose rate on age estimates can be substantial but is hardly quantifiable over large time scales.An underestimation of dose rate compared to the effective dose rate over the whole time period of burial would result in an age overestimation.The Cologne gamma-ray spectrometer has a good sensitivity in the higher energy part of the spectrum and 238 U is hence quantified by peaks of 226  formation about disequilibria in the early decay chain and can reveal uranium loss or uptake since uranium is mobile in oxidising aqueous solutions.Radium is also known to be mobile in lake sediments and can accumulate in individual horizons.The samples analysed in Dresden show only a small decrease between the early isotopes and 226 Ra but two of the three samples show a slightly more significant decrease between 226 Ra and 210 Pb (Fig. S1).This indicates a radon loss and may give evidence for the uranium series not being in the state of equilibrium.However, for all the three samples analysed here the activities of the daughters 226 Ra and 210 Pb still agree within 2-sigma errors with the activity of the mother (Fig. S1).Hence, the impact on age calculation is presumably not massive.With regard to the decay rates of the nuclides this decrease can be attributed to mobilisation processes within the last about 100 yr but it is not possible to quantify the frequency of the mobilisation processes over the last several hundred thousand years and, therefore, it is not viable to qualify the real impact on the age calculation.Dose rates (see Tables 3 and 6) and ages were calculated using the "age" software (version 1999) by R. Gr ün, Canberra, which includes the dose conversion factors published by Adamiec and Aitken (1998).Alpha efficiency was set to 0.035 ± 0.02 for quartz samples and 0.07 ± 0.02 for polymineral fine grain samples, following Rees-Jones (1995).Figures

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Attenuation factors and water content
The cosmic contribution to the dose rate is usually calculated according to the sampling depth.It can be neglected for the given samples, because the influence of cosmic radiation on the minerals is completely attenuated by overlying sediment and water column.More important is the impact of the sediment water content.Attenuation of ionising radiation is more effective in sediments with water-filled interstices and has strong effects on resulting age estimates.The "as found" water contents are hardly representative for the moisture conditions during the time span of burial, especially not for such a long period of time as considered in this study.Mechanical compaction by overlying sediments and mass movements may have reduced or increased the water content during burial times and are difficult to quantify.Full saturation of the sediment can be measured using laboratory methods (Lowick and Preusser, 2009) but a retrospective determination of water content changes through time is far from being straightforward.We therefore took the measured water content for age calculation and added three more age estimates calculated with fictive water contents in the tables to illustrate the impact of water content variation on age calculation (see Tables 3 and 6).

Luminescence measurements and results
Luminescence measurements were carried out on an automated Risø TL/OSL Reader (TL-DA 12) with a calibrated 90 Sr/ 90 Y beta source delivering 4.08 Gy min −1 .A U340 filter was used for quartz measurements and an interference 410 nm filter for IRSL measurements on polymineral samples.Several different dating techniques had to be tested in this study to evaluate the most appropriate protocol for achieving reliable luminescence age estimates for such old, i.e. > 300 ka, sediments (Table 2).This expanded dating programme evolved from the fact that the sediments analysed here are comparably old and cover a time span which is not often securely dated by luminescence techniques.Introduction

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SAR-OSL on fine grain quartz
The standard SAR protocol (Murray and Wintle, 2000) was applied on three samples, using blue stimulation for 50 s at 125 • C, a pre-heat of 240 • C and a cut heat of 220 • C. Equivalent dose (D e ) values were determined using the first 0.6 s of the OSL decay curve, and subtracting the background of the last 5 s.Another approach using the early background subtraction method (Ballarini et al., 2007) with an integral of 0-0.4 s for D e determination and a background integral of 1.0-1.4s, as described by Lowick and Preußer (2011), did not improve the dataset and was hence rejected.Dose response curves were fitted with a single exponential plus linear function.An experimental doseresponse curve measured for sample 1B2H2 with 12 dose steps up to a 1237 Gy fitted well to a single saturated plus linear function and did not show any saturation effects (Fig. 2).Validation of the protocol parameters was verified by pre-heat plateau tests and dose recovery tests at different temperatures.The validation tests both confirmed an appropriate pre-heat temperature between 220 • C and 240 • C and a perfect dose recovery at 240 • C with a measured to given dose ratio of 1.00 (Fig. 3a and b).IR tests for feldspar contamination were made standard practice.
The results obtained for the standard SAR-OSL protocol on quartz underestimate the expected age range significantly.Maximum D e -values remained below 410 Gy and none of the three samples reached the saturation level.The maximum D e was obtained for sample 1A6H1B.With regard to the age model provided by Melles et al. (2012) and Nowaczyk et al. (2012) sample 1A6H1B from 17.88-17.93m blf (below lake floor) is supposed to be deposited about 465 ka ago.Sample 1A9H2 was taken between 27.817-27.917m blf and is supposed to be 757 ka years old.The SAR-OSL results of 137 ± 20 ka and 129 ± 13 ka for these two samples are far below the expected age range and show no increase in age with depth (Table 3 and Fig. 5).Although the dose response curves show no saturation effects with increasing dose, 400 Gy appears to be the maximum D e for fine grain quartz.Thus, a quartz age beyond 200 ka is not attainable.Introduction

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Full Although the SAR-OSL protocol is the most accepted protocol and has proved very successful when dating quartz, there are several studies reporting age underestimations for quartz beyond the Eemian or even as young as 70 ka (Murray et al., 2008).Similar observations were made by Lowick and Preusser (2011), Lowick et al. (2010), Lai (2010), Timar et al. (2010), who all reported on underestimations with fine grain quartz.The majority of these studies show that the samples meet the standard validation criteria for the SAR protocol, which usually allows the assumption to obtain reliable OSL ages.They all observed well-shaped dose response curves, which are best fitted to a single exponential plus linear function.While the characteristics of the dose response curve suggested that determination of D e -values up to 400 Gy should be possible, Lai (2010) showed that age determination was only reliable up to 230 Gy.Lowick and Preusser (2011) and Lowick et al. (2010) reported on sedimentary quartz from Northeastern Italy.Their OSL ages agree well with biochronological constraints up to 140 Gy (70 ka) but increasingly underestimate the age beyond this point.They assume the underestimations are caused by the presence of different components of luminescence signals with different luminescence characteristics but have no real explanation for them.They also report on the slow growing dose response curve beyond 400 Gy, which is represented by a linear component.
A decomposition of the L x /T x quotient from our study revealed an obvious answer to the question, why the dose response curve shows a linear response to high doses.
Plotting the background-subtracted regenerated signals (L x ) and the backgroundsubtracted test dose signals (T x ) against the administered beta dose shows a saturation of the L x -curve above 400-500 Gy.The corresponding test dose signal (T x ) shows a small rise up to the 500 Gy regenerative dose and a decrease beyond, although the test dose was always kept constant (Fig. 2).The ratio then seems to indicate a rising L x /T x with dose but the uncorrected OSL signal is in saturation.Hence, signal growth of the sensitivity-corrected OSL signals is an artefact of the response to the test dose that is commonly well below the saturation level.Here, sensitivity correction as Introduction

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Full conducted as part of the SAR protocol results in erroneous equivalent dose estimates in dose ranges beyond the saturation level of the uncorrected regenerated OSL.

Thermally transferred OSL (TT-OSL)
Thermally transferred OSL (TT-OSL) was proposed by Wang et al. (2006) to extend the age range of quartz and to provide quartz OSL dates for Middle Pleistocene sediments.The TT-OSL signal is measured after the depletion of the conventional OSL signal and a subsequent pre-heat, which is applied to induce the thermal transfer of charge.The TT-OSL signal has a saturation limit at least an order of magnitude greater than the fast component of the conventional OSL signal (Wang et al., 2007) but, in contrast to initial suggestions by Wang et al. (2006), it is considerably less light sensitive than the fast bleaching OSL component (Tsukamoto et al., 2008;Jacobs et al., 2011).
The simplified SAR protocol for TT-OSL developed by Porat et al. (2009) (Table 2) was tested on 3 different samples but results showed that it was not suited for the quartz from Lake El'gygytgyn.Significant sensitivity changes and a non-linear dose response prevented the fitting of a sensible dose response curve to the data.A dose recovery test carried out on 1A3H1 failed to meet the validation criteria; the average measured dose overestimated the given dose by more than 25 % and resulted in a ratio of 1.32.
The recycling ratio was poor (more than the acceptable 10 % deviation) and the recuperation of 10-20 % was significantly exceeding the acceptance level of 5 % (Murray and Wintle, 2000).An additional hot bleach (OSL shine down at 300 • C for 100 s), as proposed by Stevens et al. (2009), was then added after the TT-OSL measurement to remove the residual charge carried over from the regenerated dose measurements to the test dose measurement cycle.This lead to a perfectly linear dose response curve (Fig. 4), however, the overestimation was hardly reduced, the recycling ratio remained poor, and the recuperation still varied between 4 % and 10 %.Independent tests of this modified TT-OSL protocol applied on modern test quartz with an artificial beta dose of 206 Gy yielded a perfect linear growth curve, a very small underestimation of about Introduction

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Full 10 % deviation and the recuperation was negligible.This result underlines the general validity of the modified TT-OSL protocol, but also that it is not suitable for the samples of this study.
To avoid the problem of charge transfer a multiple aliquot regenerative dose (MAR) TT-OSL approach was designed and tested.Twelve and 16 discs, respectively, were prepared from two samples (see Table 3).The natural OSL and the natural TT-OSL (step 2-5 from the given SAR protocol, Table 2) were recorded and followed by a hot bleach (300 s blue stimulation at 280 • C).The same discs were then irradiated with 4 dose steps up to 814 Gy and step 2-5 from the SAR protocol were repeated.The first 0.4 s of the natural TT-OSL signal were used for short shine normalisation and the residual level was determined by repeating step 2-5 on the first three discs.D e -values were determined by integrating the first 1.2 s and the average signal of the last 10 s was subtracted for background.The results show a linear growth curve and a good response to the short shine normalisation.Sample 1A1H3 overestimates the expected age range that is inferred from independent age model if the measured water content, which is comparably high, is used for age calculation.A water content of about 70 %, as measured for many of the other samples, would yield an age of about 160 ka.This would match the expected age range and gives a good example for the impact of the water content on the age calculation.The result of sample 1A4H2, however, underestimates the expected age range significantly (see Table 3).Hence, with respect to the independent age control TT-OSL protocols were considered not to yield the required reliability for dating the samples under study here.

SAR-IRSL 50 on polymineral fine grain
The SAR-IRSL 50 protocol (Table 2) proposed by Wallinga et al. (2000) for coarse grain feldspars was slightly modified and applied on the polymineral fine grain fraction.This fraction contains the natural fine silt mineral composition including quartz, though the IR stimulated luminescence emitted in the 410 nm range is dominated by potassiumrich feldspar.Feldspar is known to have a much higher saturation dose than quartz and Introduction

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Full is therefore often used if quartz is not suited for various reasons.However the available age range and the precision of feldspar dating is often hampered by anomalous fading, a spontaneous signal loss (Wintle, 1973) that leads to age underestimations of feldspar ages.Corrections for this signal loss and the corresponding age underestimations have been proposed (Lamothe and Auclair, 1999;Huntley and Lamothe, 2001), but these corrections are not valid for old samples with large palaeodoses and, hence, non-linear dose response curves.IRSL measurements were carried out for 350 s at 50 • C (IRSL 50 ) through an interference filter (410 nm, 5 mm).Dose recovery pre-heat tests were conducted to determine the appropriate pre-heat temperature and to verify the protocol parameters.A set of 15 sub-samples of sample 1A1H2 received a hot bleach, i.e. a 100 s IRSL stimulation at 280 • C in the reader and a subsequent beta dose of 206 Gy.Three discs each were measured at different pre-heat temperatures using the same thermal treatments for the regenerative dose and the test dose.The average D e 's obtained for the different pre-heat temperatures show no plateau (Fig. 5), but a constant decrease with temperature and only a presumably accidental match of the administered dose at a preheat temperature of 230 • C. In addition, the D e is strongly dependent on the integral size used for D e calculation showing a constant increase of about 17 % within the first 25 measurement channels.This performance is usually taken as evidence for signal instability, i.e. for fading, but fading tests revealed only very small fading rates of 1.2 ± 0.4 % decade −1 (1B3H2) and 0.6 ± 0.4 % decade −1 (1A3H1).Summarising these observations, the polymineral fine grains are not suitable for the standard SAR-IRSL 50 dating protocol.With regard to the insufficient pre-heat plateau, any dating approaches were abandoned because the obtainable results would at best be minimum ages which underestimate the true deposition age significantly.Using polymineral fine grain from younger Lake El'gygytgyn sediments, Juschus et al. (2007) and Forman et al. (2007) described only small fading rates and they forebear from doing fading corrections.They obtained a good agreement with the expected age range up to ∼ 160 ka, but underestimate the expected age range significantly beyond 160 ka (Forman et al., 2007).

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Full  2009) described a high temperature IRSL signal, which is measured at increased temperatures after a first IR shine down.This post-IR IRSL signal appears to be less prone to fading and is more stable than the low-temperature IRSL signal measured at 50 • C (IRSL 50 ).
The pIRIR 290 protocol (Table 2) proposed by Thiel et al. (2011) for middle and upper Pleistocene loess from Austria was applied on 8 of the polymineral fine grain samples.All sub-samples were prepared on aluminium discs and all measurements were made on a Risø TL-DA-12 with a 5 mm 410 nm interference filter.Thiel et al. ( 2011) integrated the first 2.4 s for D e determination.With regard to a weak scatter that was observed at some sub-samples within the first 6-8 channels in the D e vs. stimulation time-plot, the integral of 1-12 s was taken for D e determination and the last 20 s were subtracted as background.In contrast to the SAR-IRSL 50 measurements, which showed an increasing D e with increasing integral, the D e of the high temperature pIRIR 290 does not depend on the size of the integral but shows an extended plateau up to approximately 30 s.With regard to the small number of sub-samples measured (Table 5), the median of the individual measurements was finally used as best representative for the average equivalent dose.Relating to the bleaching experiments, the signal left after bleaching for 3 h under natural sunlight (20 Gy) was subtracted from the pIRIR 290 D e 's (for discussion see also Thiel et al., 2012;Buylaert et al., 2012).All data are given with total uncertainties at a 1-sigma confidence level.Thomsen et al. ( 2008) and Buylaert et al. (2009) have demonstrated that the post-IR IRSL signal is easy to bleach down to a certain residual level of 10-20 Gy, but that it is necessary to verify this assumption, since bleaching characteristics are strongly dependent on the sediment type and mineralogy of the feldspars.A sample set of 1A1H2 was prepared and bleached for 3.15 h and 7.5 h under natural sunlight on a bright cloudless day in June.Another sample set was bleached in a H öhnle solar simulator for 4.5 h, 6.5 h and 36 h.The results (Table 4) confirm that the pIRIR 290 signal is generally easy

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Full to bleach, even if the low temperature IRSL signal (L x ), which is measured prior to the pIRIR 290 signal (see Table 2), is bleached a little faster.After 3.5 h under natural sunlight the pIRIR 290 D e of sample 1A1H2 is already reduced to about 20 Gy and it keeps decreasing with further exposure time.Artificial bleaching of different samples in a H öhnle solar simulator for 4.5, 6.5 h and 36 h gives a residual below 20 Gy.After 36 h in a solar simulator there is still a residual of 14 Gy, but the bleaching level is sample dependent and not only dependent on the bleaching time.However, this experiment illustrates that the post-IR IRSL signal of the lake sediments can be bleached to a reasonable level in a reasonable time.
The impact of these residuals on the ages can be considered to be quite low.For example, the residual of 20 Gy measured after 195 min of sunlight bleaching for sample 1A1H2 (see Table 4), corresponds to 7.7 ka using the dose rate based on the measured water content.The pIRIR 290 age of this sample is 179 ± 20 ka after subtraction of the residual.Thus, the residual is only 4 % and within the range of the 1-sigma error in age.Dose recovery tests at 1A1H2 after a natural sunlight exposure for 450 min and a given dose of ∼ 470 Gy failed to pass the criteria.After subtracting the residual of 14 Gy the measured to given dose ratio was still 1.38±0.09,but a valid protocol should be able to deliver a ratio within 10 % deviation from unity.Another dose recovery test was carried out on five discs of sample 1A3H2 after 390 min light exposure in the solar simulator and an artificial beta dose of ∼ 300 Gy.The measured to given dose ratio was still 1.37 ± 0.06, showing a massive overestimation, too.A further dose recovery test on sample 1A3H1 was carried out after 2160 min bleaching in the solar simulator.
The sub-samples were stored for 6 months after bleaching, then received a beta dose of ∼ 300 Gy and the average measured to given dose ratio was 1.07, which is within the 10 % range and thus acceptable.excellent results, illustrating that the sensitivity correction of the protocol is working well.The average recycling ratio was 1.03 ± 0.02 and the recuperation was low, ranging between 1.0 and 1.5 %.The results obtained for the dose recovery tests using optical bleaching immediately followed by a beta dose seem to indicate some kind of charge transfer after irradiation and during the first pre-heat.Although the natural signal is thoroughly bleached after a few hours under natural or artificial sunlight (Table 4), it is not possible to recover a given dose if measured immediately after bleaching.The resulting D e overestimates the given dose significantly.If the sample is stored for some months after bleaching, the overestimation is reduced and the given dose can be recovered within 10 % deviation.Cleaning out the samples with a hot bleach before the first beta dose yields acceptable dose recovery ratios, suggesting no significant charge transfer or recuperation processes.At this stage, we do not have satisfactory explanation for the different behaviours with and without delay between bleaching and irradiation.We interpret this charge transfer observed after optical bleaching and immediate radiation as a laboratory artefact, which is not relevant for natural samples because under natural conditions bleaching and dosing will happen much slower.However, further experiments with varying pause times between bleaching, artificial irradiation and first pre-heat are necessary to analyse these observations in more detail.

Discussion on pIRIRSL 290 results
From the comparison of the luminescence dates with the independent age model it becomes evident that only the post-IR IRSL protocol yielded reliable dating results significantly beyond 200 ka (Table 6).The pIRIR 290 D e -values (Table 5) show -in contrast to the IRSL values measured at 50 • C -a constant increase with depth.Three of the samples are in very good agreement with the expected age range, if the measured water content is used for age calculation.Samples from the age range between 200 and 300 ka overestimate the expected age range significantly (Fig. 6) by about 30 %.Many reasons are conceivable for this, such as saturation effects as visible for sample

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Full 1A3H2, erroneous water contents or dose rate underestimation by means of disequilibria or even insufficient bleaching.Juschus et al. (2007) described decreasing water contents with depth for their sediment core with minimum values around 50 % at the base.There is no such trend visible for the samples of this study and the question remains if the measured water content represents the environmental conditions during the geological times since deposition.
The samples from the lower part of the profile are in good agreement with the expected age and even sample 1A9H2 from the B/M boundary with an expected natural dose of > 2700 Gy gave a D e of more than 2400 Gy (Fig. 7) and an age of > 700 yr (Fig. 6).The dose response curves are best fitted to an exponential plus linear function and the natural sensitivity corrected signals exceed the highest regenerated dose point of 2430 Gy.The D e obtained for 1A9H2 lies in the extrapolated part of the dose response curve and the true saturation level was not reached.D e and age are hence just given as minimum values.In this part of the core section, the natural signal of the post-IR IRSL is slowly approaching the saturation level, but the dose response curve is rather steep compared to sample 1A6H1B (Fig. 7).Saturation dose experiments with up to 7 regeneration dose points and a maximum dose of 2500 Gy carried out on sample 1A6H1B and 1A4H2 still allow to fit an exponential plus linear function to the data but the natural pIRIR 290 signal lies in the upper slow rising linear part of the dose response curve and is close to saturation (Fig. 8).This slow rise considerably limits the reliability of the three samples 1A9H2, 1A4H2B and 1A3H2 and is taken as one of the reasons for the comparably large relative standard deviation observed for these samples.Polymineral fine grain samples usually do not show a significant scatter because a large number of grains on the sample disc are emitting luminescence signals.The pIRIR 290 D e data sets of these samples show RSD-values between 18 % and 29 % (Table 5).Even if the number of aliquots is small and the validity is limited, this scatter is most likely ascribed to the shape of the dose response curve, indicating a natural dose close to saturation level.The scatter for the IRSL measurements at 50 • C is much smaller.Introduction

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Full A similar dating approach by Thiel et al. (2011) on polymineral fine grains extracted from Austrian loess from below the B/M boundary was not successful because the natural of the pIRIR 290 signal was in the saturating part of the dose response curve, which they defined as > 1600 Gy.In our study the pIRIR 290 saturation dose was not reached up to 2400 Gy for sample 1A9H2, which is correlated to the B/M boundary.D e -values derived for the low temperature IRSL signal measured as part of the pIRIR 290 measurement sequence (see Table 2) do not exceed a maximum value of 540 Gy (see Table 5).Whereas pIRIR 290 D e -values continue to rise with depth, the low temperature IRSL ages show no increase with depth, which further illustrates the unreliability of these estimates (Table 6).A similar trend was described by Forman et al. (2007), who reported on significant underestimations beyond 160 ka for multiple aliquot low temperature IRSL measurements on polymineral fine grains.Finally, we deduce that the predominant agreement with the age model and the continuous age increase with depth (see Table 6) seem to speak for the validity of the pIRIR 290 data.
No fading corrections were made, neither for the IRSL 50 nor the pIRIR 290 D e -values.Juschus et al. (2007) reported on small fading ratios between 0.84 and 0.99 and followed Forman et al. (2007), who determined fading ratios between 0.92 and 0.99 and did not apply fading corrections.The same trend was observed for two samples from this study, which gave fading rates of 1.2 ± 0.4 % decade −1 (1B3H2) and 0.6 ± 0.4 % decade −1 (1A3H1) for IRSL 50 .Although these rates are small, an influence cannot be excluded.However, the introduced uncertainties associated with fading corrections in the considered age range are supposed to exceed any advantage of correction here.Buylaert et al. (2012) calculated fading rates for low temperature IRSL and post-IR IRSL at samples of different origins and concluded that fading rates of 1-1.5 % decade −1 are most likely artefacts of measurement procedures.We, hence, have abandoned any fading corrections for IRSL 50 and pIRIR 290 dose estimates.

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Conclusions
The samples from Lake El'gygytgyn turned out to be very challenging for luminescence dating and the reasons therefore are manifold.Although the general luminescence properties of the sample material, like signal intensity, recuperation, dose response, sensitivity, and others, are acceptable for the fine-grained quartz and polymineral frac- have passed all validation tests and result in ages that increase with depth down to the B/M boundary.Four out of eight samples are in fairly good agreement with the ages calculated from the magnetic polarity, though their errors are large.However, the results obtained for the uppermost sample and the samples from the MIS 7 sediment record seem to show systematic overestimations but still an increase in age with depth.
It is not possible to verify if this overestimation is partly due to insufficient bleaching.Polymineral fine grain measurements do not allow conclusions about the bleaching level prior to deposition.Thus, for verification of the dating results obtained in this study it might be interesting for further projects to analyse the effective bleaching of a modern sample from the delta of one of the small inlet streams of Lake El'gygytgyn.Introduction

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Fig. 1 .Fig. 2 .
Fig. 1.Location of Lake El'gygytgyn in Northeastern Russia (inserted map) and schematic cross-section of the El'gygytgyn basin stratigraphy showing the location of ICDP Sites 5011-1 and 5011-3.Lz1024 is a 16-m long percussion piston core taken in 2003 that fills the stratigraphic gap between the lake sediment surface and the top of drill cores 1A and 1B (from Melles et al., 2012).
tions, respectively, the SAR protocol for OSL and IRSL 50 dating techniques failed to deliver satisfactory and reliable dating results.Quartz OSL results underestimate the palaeomagnetic time frame significantly, for example date the sample from the 780 ka B/M boundary to only about 150 ka.A single aliquot TT-OSL protocol was inappropriate because insufficient sensitivity corrections prevented a reliable curve fit.A multiple aliquot TT-OSL protocol for fine grain quartz was tested, but failed for the older samples as well.The low temperature IRSL at 50 • C measured prior the high temperature IRSL at 290 • C during the pIRIR 290 sequence gave a maximum age of 180 ka, showing no age increase with depth.Only post-IR IRSL measurements with 290 • C stimulation temperature (pIRIR 290 )

Table 2 .
Compilation of measurement protocols used in this study.

Table 3 .
SAR-OSL dating results and multiple aliquot regenerative dose (MAR) TT-OSL dating results obtained on fine grain quartz (4-11 µm).Ages and dose rates are given for different assumed and the measured water content (weight-%) to point out the significance of the water content on the age estimates.All analytical results are presented with their 1-sigma error.

Table 5 .
Equivalent doses calculated for the post-IR IRSL signal measured at 290 • C and the IRSL signal measured at 50• C in the course of the pIRIR 290 measurement sequence (see Table2).All analytical results are presented with their 1-sigma error.Number of discs taken for D e determination and total number of discs measured.b Relating to the bleaching experiments, the residual signal equivalent to 20 Gy was subtracted from the D e .Introduction a