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OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz
Quaternary International xxx (2014) 1e9
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OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz Jin Cheul Kim*, Tae Soo Chang, Sangheon Yi, Sei Sun Hong, Wook-Hyun Nahm Korea Institute of Geosciences and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon, 305-350, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The comparison of optically stimulated luminescence (OSL) ages of different grain-size fractions has given rise to conflicting results over recent years. To test the reliability and validity of OSL dating of fineand coarse-grained fractions, 25 samples were collected from a 46-m sediment core taken from the southwestern coast of Korea, which contained two typically fine-grained tidal units separated stratigraphically by a coarse-grained sand and gravel layer. A single aliquot regenerative dose (SAR) procedure was applied to chemically separated fine- (4e11 mm, n ¼ 16) and coarse- (90e212 mm, n ¼ 24) grained quartz. The ages of coarse-grained quartz in the upper unit were relatively consistent with those of finegrained quartz in the same unit of Holocene age (12e0 ka). In contrast, in the lower unit the coarse quartz fraction OSL ages are 30e60% (mean 46%) lower than the fine quartz fraction ages. OSL ages for all fine-grained quartz in the lower unit (139e110 ka) were in good agreement with the last interglacial sealevel highstand [Marine Isotope Stage (MIS) 5e]. However, all OSL ages for coarse-grained quartz in this lower unit (90e45 ka) were younger, corresponding broadly to the last glacial period (MIS 4 and 3). OSL ages obtained for fine-grained quartz were in agreement with indirect age controls provided by lithological and palynological data, implying that the use of the SAR protocol to date fine-grained quartz is appropriate for age control of coastal sediments extending to the last interglacial period. The discrepancies between the ages of different grain-size fractions may have arisen from early saturation of the natural signal and, in some cases, from contamination of the feldspar signal in the coarse quartz. However, more research is required to assess the reliability of OSL ages for fine-grained quartz obtained from the high-dose linear region of the dose response curve. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Quartz OSL Grain size Coastal sediment MIS 5e
1. Introduction Optically stimulated luminescence (OSL) dating of quartz has been used successfully since the 1980s. Quartz usually provides a good approximation of age because it does not suffer from anomalous fading (Aitken, 1998). The choice of grain size for equivalent dose (De) determination is governed by availability within the sample. Ideally, the grain size selected for luminescence dating should represent the modal grain size of the deposit (Roberts, 2008). Silt-sized quartz has the advantage of bleaching because of longer suspension prior to deposition. Also, coarse-grained materials are rarely deposited in certain sediment types, such as those of loess or tidal flats. Thus, the widespread application of OSL dating of fine-grained sediments has been attempted despite the difficulty of mineral separation (Roberts and Wintle, 2001; Watanuki and Tsukamoto, 2001; Wang et al., 2006a; Lu et al., 2007; Kim et al., 2010, 2012). However, some studies have reported that the use of * Corresponding author. E-mail address: [email protected] (J.C. Kim).
OSL for fine-grained sediments can lead to the underestimation of age (Fan et al., 2010; Lai, 2010; Lowick et al., 2010; Lowick and Preusser, 2011; Timar-Gabor et al., 2011). Most of these studies have suggested that such underestimation is reliant upon the growth of the linear component beyond the saturating exponential signal. Conversely, other studies have reported that OSL ages of fine-grained quartz were consistently older than those of mediumto coarse-grained quartz (Hu et al., 2010; Zheng et al., 2010; Zhang et al., 2010a, b; Guo et al., 2012), perhaps due to the bleaching characteristics of the differently sized grains. On the other hand, consistent OSL results between coarse- and fine-grained quartz have also been reported (Cao et al., 2012; Constantin et al., 2012). Accordingly, debate regarding these disagreements persists, and further research is required. In this study, the OSL dating method was applied to fine(4e11 mm) and coarse- (90e212 mm) grained quartz in coastal marine sediments from the southwestern coast of the Korean Peninsula. We investigated the luminescence characteristics of these quartz grains and the suitability of OSL dating for different grain sizes. The accuracy of OSL dating results from different quartz
http://dx.doi.org/10.1016/j.quaint.2014.09.001 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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J.C. Kim et al. / Quaternary International xxx (2014) 1e9
grain sizes was evaluated by comparison with lithological and palynological characteristics. 2. Study area and sample preparation The Baeksu coast is a large, muddy, tidal flat area on the southwestern coast of the Korean Peninsula. This southwestern coastline borders the Yellow Sea, which is a semi-enclosed, relatively shallow (mean water depth, 44 m), continental shelf area that connects the Chinese mainland with the Korean Peninsula (Park et al., 1998; Lim et al., 2006; Kim et al., 2012). Stratigraphic studies of this area have demonstrated that Holocene tidal deposits unconformably overlie pre-Holocene deposits (Park et al., 1998; Lim et al., 2003). For this study, a 46-m-long core (11YG C4) was collected from this area (2 m water depth). Most core sediments were fine-grained muddy deposits, and no large grain-size variation was observed throughout the sequence. However, a coarse-grained sandy and gravelly layer was found in the middle part (14e18 m) of the core, separating the finegrained deposits into lower and upper units. The bottom part of the core consisted of unconsolidated basal gravels and gravelly sands (Fig. 1). Twenty-five samples were collected for OSL dating. Fifteen of these samples could be split into fine- and coarse-grained quartz, providing an opportunity for the comparison of OSL characteristics between grain-size fractions. To check the bleaching condition, two modern analogue surface samples were taken from the shoreface near the core site. Under red light, fine- and coarse-grained samples were extracted from the core sediments using two treatment techniques. For the dating of fine-grained samples, chemically purified quartz grains of 4e11 mm in diameter were extracted using sodium pyrophosphate (Na4P2O7$10H2O) to remove any clay, hydrochloric acid (HCl) and
hydrogen peroxide (H2O2) to remove any carbonate and organic matter, respectively, and settling according to Stokes' Law over a depth of 20 cm in a 0.01-M sodium oxalate (Na2C2O4) solution. Finally, the samples were etched in hydrofluorosilicic acid (H2SiF6) for 14 days to chemically remove feldspar (Roberts, 2007; Kim et al., 2009b). For the dating of coarse-grained samples, sediments were treated with HCl and H2O2 and then sieved to isolate the 90e212-mm grain-size fraction. Heavy-liquid separation in sodium polytungstate (2.62 g/cm3) was used to obtain quartz-rich extracts. To remove any remaining feldspar grains and to etch away the outer 10 mm of the quartz grains, a standard treatment (45 min; once or twice) in concentrated hydrofluoric acid (HF) was applied. 3. OSL measurements OSL measurements were undertaken using a Risø TL-DA-20 equipped with a blue light-emitting diode (470 ± 20 nm) stimulation source. Irradiation was provided by a 90Sr/90Y beta source delivering approximately 0.1 Gy s1. An EMI 9635 QA photomultiplier tube and a 7.5-mm-thick U-340 filter were used for photon detection. Radionuclide contents were measured using low-level high-resolution gamma spectrometry. Conversion to dose rates was based on the data presented by Olley et al. (1996). Present water content (weight water/weight dry sediment) was used for dose rate correction. Cosmic ray contributions were calculated using the equations of Prescott and Hutton (1994). The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 and 90e212 mm diameters. The sample was held at 125 C during the 100 s stimulation with blue diodes; the first 2 s of the OSL signal was used for De calculations, and the last 20 s was subtracted as the background. For each sample, 6e15 aliquots were
Fig. 1. The location of the Baeksu coastal area and schematic columnar section of core 11 YG C4. Location map modified from Chang et al. (2014).
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
J.C. Kim et al. / Quaternary International xxx (2014) 1e9
measured for De measurement, and the average of De values was used for age calculation. Sensitivity-corrected doseeresponse curves were fitted by an exponential-plus-linear function. The efficacy of the sensitivity correction was monitored by checking the recycling ratio and recuperation. The rejection criteria of the recycling ratio and recuperation were within 10% of unity and less than 5% of the natural dose, respectively (Murray and Wintle, 2000). The OSL infrared (IR) depletion ratio (Duller, 2003) was used to check for feldspar contamination. In this study, the fine quartz samples show less than 3% of unity by OSL IR depletion ratio. In contrast, coarse quartz samples show more than 10% of unity, even after repeated treatment with HF (twice for 45 min). Using the OSL IR depletion criterion (within 10% of unity) would have rejected a large number of aliquot. Hence, this rejection criterion was not applied in coarse quartz samples. Instead, the double SAR method that uses infra-red (IR) stimulation was used to deplete the signal from feldspar grains, prior to stimulation with blue diodes (Banerjee et al., 2001; Roberts and Wintle, 2001; Wang et al., 2006b; Kim et al., 2009b). This method was applied to some of the coarse-grained samples (C4-14,17, 23) to compare the De values with those obtained using the normal SAR protocol. To determine the appropriate preheating conditions, a preheat plateau test was conducted for core sample C4-14 for both grain sizes using a range of preheat temperatures from 160 C to 300 C in 20 C intervals, with a cut-heat of 160 C. A dose recovery test was performed in the same sample to check the performance of the SAR protocol and the ability to recover a dose (Kim et al., 2012). For the dose recovery test, bleaching was carried out by two blue light stimulations 1000 s each at room temperature (RT), separated by a storage time of 10,000 s at RT. After bleaching, doses ranging from 0 to 700 Gy were administered.
3
4. Results 4.1. Preheat plateau and dose recovery tests Fig. 2 shows the De values, recycling ratio and recuperation against preheat temperatures in sample C4-14. The De values for coarse-grained samples show a plateau within errors throughout almost the entire temperature ranges, whereas the De value for finegrained samples obtained using a low preheat temperature (160 C) was much higher than those obtained using higher preheat temperatures. The plateau regions of De values between 160 C and 220 C and between 200 C and 240 C indicate the appropriate preheat-temperature ranges for De determination of coarse- and fine-grained samples, respectively. The recycling ratios of both samples were all within 3% of unity. Signal recuperations were <3.5% of the natural signal. A preheat temperature of 220 C was selected by the low scatter in De and low recuperation. The OSL IR depletion ratio values for fine-grained samples were all between 0.95 and 1.0, suggesting that the pretreatment process resulted in complete purification of fine-grained quartz. In contrast, the OSL IR depletion ratio values for coarse-grained samples were relatively low (0.75e0.93), implying that the feldspar signal remained (Fig. 3). Fig. 4 shows the doses recovered using the SAR protocol (preheat temperature of 220 C and a cut-heat of 160 C) for both sample types as a function of the known dose given after bleaching. The recovered doses for coarse grains showed good agreement with given doses less than 300 Gy, but systematic overestimation (~45%) for doses > 300 Gy, which fall beyond the saturation range of the OSL signal for coarsegrained samples. In contrast, all recovered doses for fine grains fell close to the 1:1 line and yielded highly reproducible results. 4.2. Luminescence characteristics A preheat temperature of 220 C for 10 s and a cut-heat of 160 C were selected for De determination based on the results of the preheat plateau test (Fig. 2). The De values for fine- and coarse-grained quartz from modern surface samples show very low De values (less than 0.1 Gy). The fine- and coarse-grained fractions from the upper unit show relatively similar De values ranging from 0.7 to 40 Gy, whereas the De values for the lower unit differ substantially between grain sizes. In samples from the lower unit, De values for the coarse-grained fraction were significantly lower (up to 60%; range, 143e265 Gy) than those for the fine-grained fraction (range, 420e464 Gy). Fig. 5a shows the decay curves of the two grain size (sample C414; lower unit). The decay curve of fine-grained samples shows that
Fig. 2. Preheat plateau test results for fine- and coarse-grained sample C4-14 and the variation of recycling ratio and recuperation with preheat temperatures from 160 to 300 C. Each point shown is the result from 3 aliquots. The errors are calculated using Analyst version 3.24.
Fig. 3. The OSL IR depletion ratio as a function of increasing preheat temperature for sample C4-14.
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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J.C. Kim et al. / Quaternary International xxx (2014) 1e9
Fig. 4. Dose recovery data for fine- and coarse-grained sample (C4-14) over a range of given doses. Each point is the mean of 3e5 aliquots.
the luminescence signal was depleted rapidly during the initial 2 s of stimulation, indicating that the signal was dominated by the fast component. In contrast, slower decay was observed in coarsegrained samples. The DRC (Dose Response Curve) for fine-grained
Fig. 6. The natural OSL decay curves (a) and OSL DRC (b) for the fine quartz OSL, fine quartz post-IR OSL, coarse quartz OSL, and coarse quartz post-IR OSL signals from sample C4-17. All data of the OSL decay curves are shown as raw counts per 0.4 s stimulation normalized to the first data point. All data of the OSL DRC were normalized to 250 Gy regeneration dose. Each point shown in (a) and (b) is the mean of 5 aliquots.
Fig. 5. OSL decay curves (a) and OSL DRC (b) obtained using the SAR procedure for fine- and coarse-grained quartz of sample C4-14. The same test dose (20 Gy) was used for both grain-size fractions.
fraction shows continuous growth up to 800 Gy and fits well with an exponential-plus-linear function. In contrast, the DRC for the coarse-grained fraction from the same sample exhibits a very low slope at high doses (>200 Gy; Fig. 5b). The De value of 470 ± 6 Gy was obtained for fine-grained fraction. The value is obtained in a dose range where the exponential component is saturated. The D0 values of the exponential component are 99 ± 8 Gy and 118 ± 12 Gy for the fine and coarse grain fractions, respectively. The post-IR OSL measurement protocol was used to deplete the signal from feldspar grains prior to stimulation with blue diodes (Roberts and Wintle, 2001, 2003; Kim et al., 2009b). Three samples (C4-14, 17, and 23) were selected for De estimation using the post-IR OSL (500 s IR stimulation at 50 C, followed by blue stimulation), and the obtained De values were compared with those from fine and coarse quartz OSL. De values of 265 ± 33, 335 ± 36, 448 ± 9, and 452 ± 5 Gy were obtained for the coarse quartz OSL, coarse quartz post-IR OSL, fine quartz OSL, and fine quartz post-IR OSL signals from sample C4-17, respectively (Table 2). The De values for coarse quartz OSL from sample C4-17 are ~60% lower compared with those for fine quartz OSL. The De values for coarse quartz post-IR OSL is lower by only ~40%. Fig. 6 shows the patterns of OSL decay and DRC for the two grain sizes using the OSL and post-IR OSL.
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
Table 1 Dose rate information, De values and OSL ages of 11YG C4 core sediments. No. of discs is the number of measurements used for age calculations. O-D (%) is an indication of the scatter from experimental uncertainties. Lab. no
11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG a b
C4-1 C4-2 C4-3 C4-4 C4-5 C4-6 C4-9 C4-10 C4-11 C4-12 C4-13 C4-14 C4-15 C4-16 C4-17 C4-18 C4-19 C4-20 C4-21 C4-22 C4-23 C4-24 C4-25 C4-27 C4-28
Water content (%)a 30.4 32.6 36.2 29.3 27.7 29.9 32.9 15.4 17.3 12.5 39.4 39.7 37.6 40.4 37.2 40.2 43.1 39.2 36.3 45.3 38.8 47.4 21.3 24.7 17.3
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
Depth Alpha dose Beta dose (Gy/ka) (m) (Gy/ka)b Coarse Fine grain grain 1 2 3.3 4.9 6.4 7.9 12.7 13.9 15.8 17 18.6 20.5 22 23.1 24.6 26.1 27.6 29.1 30.6 32.1 33.6 35.1 36.1 39.4 41.6
0.28 ± 0.14 0.30 ± 0.15 0.28 ± 0.14 0.30 ± 0.15
0.38 0.27 0.32 0.25 0.30 0.30 0.33 0.36 0.35 0.36 0.35 0.34
± ± ± ± ± ± ± ± ± ± ± ±
1.98 1.89 1.85 2.04 1.94 2.03 1.97 1.89 2.58 2.14 2.01 1.84 1.78 1.76 1.82 1.89 1.86 1.96 2.06
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.19 0.14 0.16 0.13 0.15 0.15 0.17 0.18 0.18 0.18 0.18 1.90 ± 0.17 1.90 ± 2.38 ± 2.11 ± 1.91 ±
0.11 0.10 0.10 0.11 0.11 0.11 0.11 0.11 0.15 0.13 0.10 0.09 0.09 0.09 0.09 0.10 0.09 0.10 0.11
2.11 ± 0.12 2.32 ± 0.14 2.21 ± 0.13 2.24 ± 0.13
2.29 2.10 2.03 2.01 2.07 2.15 2.12 2.23 2.34 2.21 0.10 2.16 0.09 2.16 0.14 0.12 0.11
± ± ± ± ± ± ± ± ± ± ± ±
0.13 0.12 0.11 0.11 0.12 0.12 0.12 0.12 0.13 0.12 0.12 0.11
Gamma dose (Gy/ka) 1.06 1.02 1.06 1.16 1.08 1.28 1.13 0.87 1.21 0.98 1.30 1.07 1.10 1.00 1.09 1.12 1.16 1.24 1.25 1.25 1.21 1.19 1.38 1.08 0.87
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.06 0.06 0.06 0.06 0.06 0.07 0.06 0.05 0.07 0.06 0.07 0.06 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.06 0.06 0.06 0.08 0.06 0.05
Dose rate (Gy/ka)
Cosmic dose (Gy/ka) 0.18 0.16 0.13 0.11 0.09 0.08 0.05 0.05 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
De (Gy)
Вþgþ
a þ b þ gþ Coarse
cosmic
cosmic
3.21 3.06 3.29 3.31 3.11 3.40 3.14 2.81 3.83 3.15 3.35 2.94 2.91 2.79 2.93 3.03 3.04 3.21 3.33 3.12 3.11 3.78 3.21 2.80
The water content is expressed as the weight of water divided by the weight of dry sediments. Alpha dose rate was calculated using an a-value of 0.04 ± 0.02 (Rees-Jones, 1995).
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.12 0.12 0.13 0.13 0.12 0.13 0.12 0.13 0.17 0.15 0.12 0.11 0.11 0.10 0.11 0.11 0.11 0.12 0.13
quartz
3.58 ± 0.19 3.90 ± 0.21 3.66 ± 0.20 3.72 ± 0.21
4.01 3.47 3.47 3.28 3.48 3.60 3.63 3.85 3.96 3.84 ± 0.12 3.74 ± 0.11 3.71 ± 0.16 ± 0.13 ± 0.13
± ± ± ± ± ± ± ± ± ± ± ±
No. of discs Fine quartz
0.9 0.9 2.0 4.1 2.2 4.2 40.8 123.6 188.8 158.6 185.9 206.8 249.5 239.2 264.7 200.3 147.8 143.0 172.3
± 0.2 ± 0.3 ± 0.5 ± 0.8 ± 0.2 ± 0.2 ± 0.4 ± 9.5 ± 15.9 ± 6.6 ± 15.3 ± 10.5 ± 23.2 ± 13.0 ± 33.1 ± 12.2 ± 13.4 ± 12.1 ± 10.4
0.24 0.19 0.21 0.18 0.20 0.20 0.21 0.23 0.23 0.23 0.22 172.8 ± 0.21 201.1 ± 225.6 ± 211.7 ± 179.6 ±
0.7 2.1 2.0
± 0.1 ± 0.0 ± 0.0
40.8
± 0.4
O-D (%)
Age (ka)
Coarsequartz Finequartz Coarsequartz Finequartz Coarse quartz 9/11 10/12 7/14 5/12 9/16 7/10 9/15 10/11 8/10 9/10 12/14 10/13 9/15 12/15 10/15 8/10 8/10 10/10 7/8
440.6 ± 2.9 464.0 ± 7.3 430.5 ± 9.4 455.6 ± 7.4 448.1 ± 9.4 448.3 ± 10.6 431.5 ± 4.2 431.5 ± 4.2 449.4 ± 10.3 454.2 ± 9.6 15.7 420.9 ± 4.9 6/6 25.8 419.8 ± 12.2 6/6 12.2 11/12 13.3 11/12 12.4 12/12
8/8 8/8 8/8 10/10
10/10 7/8 8/8 10/11 9/10 6/6 6/6 6/6 6/6 6/6 6/6 6/6
44 ± 4 67 ± 5 45 ± 5 34 ± 6 19 ± 2 0±0 16 ± 1 21 ± 1 21 ± 2 0±0 20 ± 1 13 ± 1 25 ± 2 16 ± 1 36 ± 3 10 ± 2 25 ± 2 23 ± 2 12 ± 2 18 30 16 17 23
± ± ± ± ±
3 4 1 1 2
8±1 4±1 4±1 2±0
1 4 6 3 5 2 0 1 3 4 1 5
± ± ± ± ± ± ± ± ± ± ± ±
0 0 1 0 0 1 0 1 1 1 1 1
Fine quartz
0.3 ± 0.1 0.3 ± 0.1 0.6 ± 0.1 0.2 ± 0.0 1.2 ± 0.2 0.5 ± 0.0 0.7 ± 0.1 0.5 ± 0.0 1.2 ± 0.1 13.0 ± 0.5 11.0 ± 0.6 43.9 ± 3.9 49.2 ± 4.7 50.3 ± 3.1 55.6 ± 5.0 109.9 ± 6.6 70.4 ± 4.5 133.9 ± 7.6 85.9 ± 8.6 124.0 ± 7.8 85.8 ± 5.7 138.8 ± 7.8 90.3 ± 11.8 128.7 ± 7.8 66.0 ± 4.7 124.7 ± 7.6 48.6 ± 4.8 118.8 ± 7.0 44.5 ± 4.1 112.2 ± 6.8 51.8 ± 3.7 113.4 ± 7.1 118.2 ± 7.4 55.3 ± 5.4 112.6 ± 6.9 64.6 ± 8.6 113.1 ± 7.3 59.7 ± 4.1 66.0 ± 5.0 64.2 ± 5.3
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Table 2 Comparison of De values, ages, recycling ratio and OSL IR depletion ratio obtained for the fine quartz OSL, fine quartz IRSL, fine quartz post-IR OSL, coarse quartz OSL, coarse quartz IRSL and coarse quartz post-IR OSL signals from samples C4-14, C4-17 and C4-23. The De values were obtained from the five aliquots in each sample. De (Gy)
Signal (C4-14) Fine quartz OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL Signal (C4-17) Fine quartz OSL Fine quartz IRSL Fine quartz post-IR OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL Signal (C4-23) Fine quartz OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL
Age (ka)
Recycling ratio
7.3 10.5 15.2 33.6
133.9 ± 7.6 70.4 ± 4.5
1.04 0.99 1.00 1.02
448.1 ± 9.4 79.2 ± 13.0 452.4 ± 4.9
128.7 ± 7.8
464.0 206.8 194.2 283.7
± ± ± ±
264.7 ± 33.1 115.6 ± 19.5 334.9 ± 36.1
420.9 172.8 166.0 291.1
± ± ± ±
4.9 15.7 8.1 41.9
96.5 ± 12.0
129.9 ± 7.6 90.3 ± 11.8 114.2 ± 13.1
112.6 ± 6.9 55.3 ± 5.4 93.2 ± 13.9
0.01 0.05 0.10 0.04
0.98 ± 0.01 0.85 ± 0.04
1.02 ± 0.01 1.59 ± 0.62 1.02 ± 0.01
0.99 ± 0.01
0.99 ± 0.03 1.08 ± 0.10 0.98 ± 0.01
0.85 ± 0.03
± ± ± ±
0.97 ± 0.01 0.69 ± 0.06
1.02 0.92 1.06 0.99
± ± ± ±
OSL IR depletion ratio
0.01 0.08 0.06 0.04
0.99 ± 0.03
1.01 ± 0.01
1.04 ± 0.01
1.01 ± 0.04
4.3. Luminescence ages In the upper unit, OSL ages for both grain-size fractions ranged from 11.5 ± 0.8 to 0.21 ± 0.04 ka (Table 1, Fig. 7). Ages of fine-grained samples (C4-5 and -9) were similar to those of the corresponding coarse-grained samples in the upper unit, but OSL ages of fine-grained samples were more consistent with the stratigraphic order. In contrast, in the lower unit the OSL ages of coarse-grained samples (44 ± 4 to 90 ± 12 ka) showed significant underestimation (up to 60%) compared with those of finegrained samples (110 ± 7 to 139 ± 8 ka). Although there is some age reversals in the lower unit, ages of fine-grained samples were similar and within the range of error, with a weighted mean value of 121 ± 3 ka. The weighted mean age of coarsegrained samples in the lower unit was 65 ± 4 ka. OSL ages for the sandy and gravelly layer between the upper and lower units and the basal gravels and gravelly sand from the lowermost part could be obtained only from coarse-grained quartz due to the coarse-grained nature of the sediments. The sandy and gravelly layer was dated between 50 ± 3 and 44 ± 4 ka, and ages for the sandy layer from the lowermost part were 66 ± 5 and 60 ± 4 ka. 5. Discussion 5.1. Reliability of quartz OSL ages The laboratory parameters such as the recycling ratios and recuperation of both grain-size fractions suggest that their OSL signals are appropriate for De determination (cf. Lowick et al., 2010). However, OSL ages in the lower unit showed clear inconsistency between grain sizes. The age differences between coarse- and finegrained samples could be caused by partial bleaching. Typically, the finer fraction is more likely to be transported in suspension, whereas the coarser fraction is transported mostly as bed load, which would have caused age overestimations. Thus, the fine fraction probably experiences sufficient bleaching during transportation. However, the fine fraction tends to coagulate and form
larger aggregates that are shielded from daylight by the outer grains, and therefore tend to be insufficiently bleached (Fuchs et al., 2005). Mauz et al. (2010) studied the accuracy of optical ages derived from tidal sediments. They concluded that >85% of samples returned accurate ages and that ~13% of optical ages were overestimated, which depended largely on the transport processes. On the other hand, OSL dating has recently been applied successfully to tidal sediments in the Korean Peninsula (Hong et al., 2003; Kim et al., 2012). Kim et al. (2012) suggested that large embayments with tide-dominant conditions on the southwestern coast of the Korean Peninsula, including the current study area, encouraged complete bleaching of the surface sediments. Hong et al. (2003) obtained an OSL date of 41 ± 9 years for a tidal flat surface on the western coast of Korea, which indicated that the tidal surface sample was well bleached at the time of deposition. In this study, the degree of bleaching from surface sediments was measured by the amount of natural, residual signal in modern samples near the core site. Fine quartz showed very low De values (0.08 ± 0.02 Gy). The coarse quartz had a negligible luminescence signal, making the estimation of De values difficult. An age of ~30 years for modern fine sediments was calculated using the average dose rate (3.6 ± 0.2 Gy) of the studied core samples. The ages of coarse-grained modern samples were younger than those of fine sediments. These modern analogues suggest that samples were completely bleached at the time of deposition, regardless of grain size. Assuming that these modern analogues are representative of depositional environments, partial bleaching is not the cause for the age difference between coarse- and fine-grained samples. This supposition is also supported by the consistency of ages with stratigraphic order in the upper unit. The contamination of feldspars in separated quartz samples may give rise to age underestimation because of the anomalous fading of feldspar (Lai and Brückner, 2008; Kim et al., 2009b). The OSL IR depletion ratios near unity and fast decay form for fine-grained samples suggest that the pretreatment process completely purified the fine-grained quartz. In contrast, a relatively low OSL IR depletion ratio coupled with slow signal decay form suggest that feldspar contributed a substantial proportion to the OSL signals of coarse-grained samples compared with that of fine-grained fraction from the same sample. The post-IR OSL decay curve for coarsegrained samples shows a more rapid decay form and similar to that of fine-grained samples. The faster decay after 500 s IR stimulation for coarse-grained samples can be interpreted as an increase in the relative contribution of quartz. The OSL IR depletion ratios near unity for coarse quartz post-IR OSL also suggest that the proportion of quartz component increased after 500 s IR stimulation. However, all post-IR OSL ages for coarse-grained quartz continued to be lower than those obtained from fine quartz OSL. Natural OSL signal close to the saturation level can cause age underestimation. The DRC shape is closely related to the saturation level of quartz. In this study, the DRC of the coarse-grained sample signal usually indicated slower growth than that of the fine-grained signal. The early saturated DRC pattern for coarse-grained samples exhibited a sublinear and negligible growth rate above 300 Gy. In contrast, the fine-grained fraction showed continuous growth to 800 Gy, indicating saturation at much higher doses. A significant difference in De values for fine- and coarse-grained samples was observed at doses greater than ~300 Gy (cf. Constantin et al., 2012). The De values of fine-grained samples from the lower unit were obtained further up the linear part of the DRC (>400 Gy). Previous studies using this linear growth have yielded controversial results. Some studies obtained underestimated age results from that region (e.g. Lowick et al., 2010; Timar et al., 2010; Lowick and Preusser, 2011; Kreutzer et al., 2012). Age underestimations of ~10% (Murray and Funder, 2003) and ~14% (Murray et al., 2007) were
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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7
Fig. 7. Comparison of OSL ages for fine- and coarse-grained quartz samples with depth of 11 YG C4 core sediments.
obtained using a single saturating exponential function in which the exponential component was saturated. Conversely, other studies obtained reliable age results from the high-dose linear region (e.g. Murray et al., 2008; Kim et al., 2010). Kim et al. (2009a) suggested that De values from the high-dose linear region (400e650 Gy) were reliable, but reported systematic underestimation (~50%) for doses > 650 Gy in the linear portion. Our OSL De values for fine-grained samples from the lower unit fell into the dose range of 400e450 Gy. Also, the dose recovery test results demonstrate that doses between 400 and 700 Gy, for which the OSL signal is in the linear portion of the response curve, can be recovered more accurately for fine-grained than for coarse-grained samples. Therefore, different DRC and saturation dose level would be one possible reason for the age differences in the lower unit. The De underestimation could be also caused by the unstable medium component (Li and Li, 2006). The coarse-grained samples appear to have a slower OSL decay rate compared to that of the finegrained fraction, indicating that the coarse-grained samples may have a larger proportion of medium component that causing the underestimation. Also, the thermal instability of the fast component may also be a possible source of underestimation (Fan et al., 2011). 5.2. Comparison of OSL ages with lithological and palynological interpretation OSL ages in the lower unit showed clear inconsistency between grain sizes. The age differences may begin in samples older than about 40 ka. The present study shows that fine-grained quartz OSL ages are thought to be more accurate than those from coarsegrained samples because they are not affected by signal saturation and feldspar contamination. However, the accuracy of De determination of fine-grained samples from the linear growth part
for doses >400 Gy remains unclear. Therefore, although luminescence characteristics differed between age results, OSL data alone could not provide an implicit indication of the accuracy of the ages. The OSL ages were compared with lithological characteristics and palynological components. The upper unit is characterized by tidal rhythmites, mottled silty muds, and a coarsening-upward pattern in grain size. OSL ages of both grain-size fractions in the upper unit fell within the Holocene. Thus, lithological characteristics and OSL ages suggest that the upper unit was a transgressive Holocene tidal deposit (cf. Park et al., 1998; Lim et al., 2004; Chang et al., 2006). The Holocene tidal deposit (the upper unit) is separated from the underlying Late Pleistocene deposit (the lower unit) by the coarse-grained sandy and gravelly layer (Fig. 1). This layer (~4 m thick), dated between 50 ± 3 and 44 ± 4 ka by coarse-grained quartz OSL, marks a Late Pleistocene unconformity that might have experienced significant weathering during subaerial exposure of sea-level lowstands (cf. Lim et al., 2004). The lower unit consists mainly of rhythmic sand-mudelaminated couplets and muddominated layers containing wood fragments, fine peats, and rootlets, suggesting that mudflat and saltmarsh environments prevailed during a sea-level highstand (cf. Lim and Park, 2003). All OSL ages from fine-grained quartz in the lower unit (110e139 ka) are in good agreement with the last interglacial sea-level highstand (MIS 5e). However, OSL ages from coarse-grained samples in the lower unit range from 90 to 45 ka, corresponding broadly to the last glacial period (MIS 4 and 3). Sea level could have been at least 5e8 m above the present level during the last interglacial period (MIS 5e) (Muhs et al., 2003), but was ~85 m lower than present sea level during the last glacial period (MIS 4 and 3) (Chappell, 2002; Cutler et al., 2003). The Yellow Sea is a relatively shallow continental shelf area with a mean water depth of 44 m (Lim et al., 2006). Considering mean water depth, the Yellow Sea area would have
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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been subaerially exposed during the last glacial period. The lower unit is distributed between 20 and 38 m below mean sea level. Accordingly, coarse quartz age results do not support the characteristics of a shallow-marine tidal sequence. The palynological composition of the lower unit is consistent with a marine depositional setting. Palynological assemblages from this unit are composed of terrestrial pollen grains and marine dinocysts. Saltmarsh-derived pollen grains predominate throughout the unit, comprising an average of 70% of total palynological assemblages, although the frequency of occurrence varies with depth. Moreover, warm temperate to subtropical deciduous evergreens of the genus Quercus (Cyclobalanopsis) and broadleaved hardwood trees are common, reflecting warm temperate to subtropical climatic conditions at that time. Diverse and abundant marine dinocysts are present throughout the lower unit, particularly the warm-water coastal-living species Lingulodinium filiforme and Tuberculodinium vancampoae. These pollen (e.g. saltmarsh and warm temperate evergreen) and marine dinocyst records may indicate the presence of a warm, shallow marine environment at the time of deposition (Chen et al., 2000; Yi et al., 2004, 2008; Jun et al., 2010). On the basis of the depositional environment, stratigraphic position, and palynological components of the lower unit, OSL ages of fine-grained quartz of MIS 5e appear to be more reliable than those from coarse-grained quartz. 6. Conclusions This study revealed a substantial difference in dating results between grain-size fractions. Coarse-grained (90e212 mm) quartz ages from the upper unit are relatively consistent with those from finegrained (4e11 mm) quartz. Lithological characteristics suggest that the upper unit is a transgressive Holocene tidal deposit. In contrast, coarse-grained samples from the lower unit are up to 60% lower compared with fine-grained samples (45 ± 4 to 90 ± 12 ka vs. 110 ± 7 to 139 ± 8 ka). This apparent age difference was partially caused by feldspar contamination and partially by early saturation of the coarse-grained quartz OSL signal. The ~30% increase in post-IR OSL ages of coarse-grained samples and OSL IR depletion ratios near unity suggests that the contribution of contaminants such as feldspars could be removed from the post-IR OSL measurement protocol. However, different saturation dose level may give rise to underestimated coarse-grained quartz ages even after using the post-IR OSL signal. On the other hand, the dose recovery test results demonstrate that plotting of the OSL signal from fine-grained samples of the lower unit on a linear component of the DRC can be recovered accurately. However, the lack of comparison with an independent age control prevents determination of whether OSL ages of fine-grained quartz based on the linear component beyond the saturating exponential signal are accurate. Nevertheless, the lithological and palynological characteristics of the lower unit suggest that it was deposited as a shallow-marine tidal sequence during a sea-level highstand, supporting the OSL dating of fine-grained quartz to the last interglacial period (MIS 5e) and suggesting that this age is more reliable than that obtained from coarse-grained quartz. Thus, OSL dating of fine-grained quartz has considerable potential in the dating of coastal sediments extending to the last interglacial period. Acknowledgements This project was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea. The authors would like to thank Prof. Noriko Hasebe for valuable discussions and comments on paper.
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OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz Jin Cheul Kim*, Tae Soo Chang, Sangheon Yi, Sei Sun Hong, Wook-Hyun Nahm Korea Institute of Geosciences and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon, 305-350, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The comparison of optically stimulated luminescence (OSL) ages of different grain-size fractions has given rise to conflicting results over recent years. To test the reliability and validity of OSL dating of fineand coarse-grained fractions, 25 samples were collected from a 46-m sediment core taken from the southwestern coast of Korea, which contained two typically fine-grained tidal units separated stratigraphically by a coarse-grained sand and gravel layer. A single aliquot regenerative dose (SAR) procedure was applied to chemically separated fine- (4e11 mm, n ¼ 16) and coarse- (90e212 mm, n ¼ 24) grained quartz. The ages of coarse-grained quartz in the upper unit were relatively consistent with those of finegrained quartz in the same unit of Holocene age (12e0 ka). In contrast, in the lower unit the coarse quartz fraction OSL ages are 30e60% (mean 46%) lower than the fine quartz fraction ages. OSL ages for all fine-grained quartz in the lower unit (139e110 ka) were in good agreement with the last interglacial sealevel highstand [Marine Isotope Stage (MIS) 5e]. However, all OSL ages for coarse-grained quartz in this lower unit (90e45 ka) were younger, corresponding broadly to the last glacial period (MIS 4 and 3). OSL ages obtained for fine-grained quartz were in agreement with indirect age controls provided by lithological and palynological data, implying that the use of the SAR protocol to date fine-grained quartz is appropriate for age control of coastal sediments extending to the last interglacial period. The discrepancies between the ages of different grain-size fractions may have arisen from early saturation of the natural signal and, in some cases, from contamination of the feldspar signal in the coarse quartz. However, more research is required to assess the reliability of OSL ages for fine-grained quartz obtained from the high-dose linear region of the dose response curve. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Quartz OSL Grain size Coastal sediment MIS 5e
1. Introduction Optically stimulated luminescence (OSL) dating of quartz has been used successfully since the 1980s. Quartz usually provides a good approximation of age because it does not suffer from anomalous fading (Aitken, 1998). The choice of grain size for equivalent dose (De) determination is governed by availability within the sample. Ideally, the grain size selected for luminescence dating should represent the modal grain size of the deposit (Roberts, 2008). Silt-sized quartz has the advantage of bleaching because of longer suspension prior to deposition. Also, coarse-grained materials are rarely deposited in certain sediment types, such as those of loess or tidal flats. Thus, the widespread application of OSL dating of fine-grained sediments has been attempted despite the difficulty of mineral separation (Roberts and Wintle, 2001; Watanuki and Tsukamoto, 2001; Wang et al., 2006a; Lu et al., 2007; Kim et al., 2010, 2012). However, some studies have reported that the use of * Corresponding author. E-mail address: [email protected] (J.C. Kim).
OSL for fine-grained sediments can lead to the underestimation of age (Fan et al., 2010; Lai, 2010; Lowick et al., 2010; Lowick and Preusser, 2011; Timar-Gabor et al., 2011). Most of these studies have suggested that such underestimation is reliant upon the growth of the linear component beyond the saturating exponential signal. Conversely, other studies have reported that OSL ages of fine-grained quartz were consistently older than those of mediumto coarse-grained quartz (Hu et al., 2010; Zheng et al., 2010; Zhang et al., 2010a, b; Guo et al., 2012), perhaps due to the bleaching characteristics of the differently sized grains. On the other hand, consistent OSL results between coarse- and fine-grained quartz have also been reported (Cao et al., 2012; Constantin et al., 2012). Accordingly, debate regarding these disagreements persists, and further research is required. In this study, the OSL dating method was applied to fine(4e11 mm) and coarse- (90e212 mm) grained quartz in coastal marine sediments from the southwestern coast of the Korean Peninsula. We investigated the luminescence characteristics of these quartz grains and the suitability of OSL dating for different grain sizes. The accuracy of OSL dating results from different quartz
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Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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grain sizes was evaluated by comparison with lithological and palynological characteristics. 2. Study area and sample preparation The Baeksu coast is a large, muddy, tidal flat area on the southwestern coast of the Korean Peninsula. This southwestern coastline borders the Yellow Sea, which is a semi-enclosed, relatively shallow (mean water depth, 44 m), continental shelf area that connects the Chinese mainland with the Korean Peninsula (Park et al., 1998; Lim et al., 2006; Kim et al., 2012). Stratigraphic studies of this area have demonstrated that Holocene tidal deposits unconformably overlie pre-Holocene deposits (Park et al., 1998; Lim et al., 2003). For this study, a 46-m-long core (11YG C4) was collected from this area (2 m water depth). Most core sediments were fine-grained muddy deposits, and no large grain-size variation was observed throughout the sequence. However, a coarse-grained sandy and gravelly layer was found in the middle part (14e18 m) of the core, separating the finegrained deposits into lower and upper units. The bottom part of the core consisted of unconsolidated basal gravels and gravelly sands (Fig. 1). Twenty-five samples were collected for OSL dating. Fifteen of these samples could be split into fine- and coarse-grained quartz, providing an opportunity for the comparison of OSL characteristics between grain-size fractions. To check the bleaching condition, two modern analogue surface samples were taken from the shoreface near the core site. Under red light, fine- and coarse-grained samples were extracted from the core sediments using two treatment techniques. For the dating of fine-grained samples, chemically purified quartz grains of 4e11 mm in diameter were extracted using sodium pyrophosphate (Na4P2O7$10H2O) to remove any clay, hydrochloric acid (HCl) and
hydrogen peroxide (H2O2) to remove any carbonate and organic matter, respectively, and settling according to Stokes' Law over a depth of 20 cm in a 0.01-M sodium oxalate (Na2C2O4) solution. Finally, the samples were etched in hydrofluorosilicic acid (H2SiF6) for 14 days to chemically remove feldspar (Roberts, 2007; Kim et al., 2009b). For the dating of coarse-grained samples, sediments were treated with HCl and H2O2 and then sieved to isolate the 90e212-mm grain-size fraction. Heavy-liquid separation in sodium polytungstate (2.62 g/cm3) was used to obtain quartz-rich extracts. To remove any remaining feldspar grains and to etch away the outer 10 mm of the quartz grains, a standard treatment (45 min; once or twice) in concentrated hydrofluoric acid (HF) was applied. 3. OSL measurements OSL measurements were undertaken using a Risø TL-DA-20 equipped with a blue light-emitting diode (470 ± 20 nm) stimulation source. Irradiation was provided by a 90Sr/90Y beta source delivering approximately 0.1 Gy s1. An EMI 9635 QA photomultiplier tube and a 7.5-mm-thick U-340 filter were used for photon detection. Radionuclide contents were measured using low-level high-resolution gamma spectrometry. Conversion to dose rates was based on the data presented by Olley et al. (1996). Present water content (weight water/weight dry sediment) was used for dose rate correction. Cosmic ray contributions were calculated using the equations of Prescott and Hutton (1994). The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 and 90e212 mm diameters. The sample was held at 125 C during the 100 s stimulation with blue diodes; the first 2 s of the OSL signal was used for De calculations, and the last 20 s was subtracted as the background. For each sample, 6e15 aliquots were
Fig. 1. The location of the Baeksu coastal area and schematic columnar section of core 11 YG C4. Location map modified from Chang et al. (2014).
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
J.C. Kim et al. / Quaternary International xxx (2014) 1e9
measured for De measurement, and the average of De values was used for age calculation. Sensitivity-corrected doseeresponse curves were fitted by an exponential-plus-linear function. The efficacy of the sensitivity correction was monitored by checking the recycling ratio and recuperation. The rejection criteria of the recycling ratio and recuperation were within 10% of unity and less than 5% of the natural dose, respectively (Murray and Wintle, 2000). The OSL infrared (IR) depletion ratio (Duller, 2003) was used to check for feldspar contamination. In this study, the fine quartz samples show less than 3% of unity by OSL IR depletion ratio. In contrast, coarse quartz samples show more than 10% of unity, even after repeated treatment with HF (twice for 45 min). Using the OSL IR depletion criterion (within 10% of unity) would have rejected a large number of aliquot. Hence, this rejection criterion was not applied in coarse quartz samples. Instead, the double SAR method that uses infra-red (IR) stimulation was used to deplete the signal from feldspar grains, prior to stimulation with blue diodes (Banerjee et al., 2001; Roberts and Wintle, 2001; Wang et al., 2006b; Kim et al., 2009b). This method was applied to some of the coarse-grained samples (C4-14,17, 23) to compare the De values with those obtained using the normal SAR protocol. To determine the appropriate preheating conditions, a preheat plateau test was conducted for core sample C4-14 for both grain sizes using a range of preheat temperatures from 160 C to 300 C in 20 C intervals, with a cut-heat of 160 C. A dose recovery test was performed in the same sample to check the performance of the SAR protocol and the ability to recover a dose (Kim et al., 2012). For the dose recovery test, bleaching was carried out by two blue light stimulations 1000 s each at room temperature (RT), separated by a storage time of 10,000 s at RT. After bleaching, doses ranging from 0 to 700 Gy were administered.
3
4. Results 4.1. Preheat plateau and dose recovery tests Fig. 2 shows the De values, recycling ratio and recuperation against preheat temperatures in sample C4-14. The De values for coarse-grained samples show a plateau within errors throughout almost the entire temperature ranges, whereas the De value for finegrained samples obtained using a low preheat temperature (160 C) was much higher than those obtained using higher preheat temperatures. The plateau regions of De values between 160 C and 220 C and between 200 C and 240 C indicate the appropriate preheat-temperature ranges for De determination of coarse- and fine-grained samples, respectively. The recycling ratios of both samples were all within 3% of unity. Signal recuperations were <3.5% of the natural signal. A preheat temperature of 220 C was selected by the low scatter in De and low recuperation. The OSL IR depletion ratio values for fine-grained samples were all between 0.95 and 1.0, suggesting that the pretreatment process resulted in complete purification of fine-grained quartz. In contrast, the OSL IR depletion ratio values for coarse-grained samples were relatively low (0.75e0.93), implying that the feldspar signal remained (Fig. 3). Fig. 4 shows the doses recovered using the SAR protocol (preheat temperature of 220 C and a cut-heat of 160 C) for both sample types as a function of the known dose given after bleaching. The recovered doses for coarse grains showed good agreement with given doses less than 300 Gy, but systematic overestimation (~45%) for doses > 300 Gy, which fall beyond the saturation range of the OSL signal for coarsegrained samples. In contrast, all recovered doses for fine grains fell close to the 1:1 line and yielded highly reproducible results. 4.2. Luminescence characteristics A preheat temperature of 220 C for 10 s and a cut-heat of 160 C were selected for De determination based on the results of the preheat plateau test (Fig. 2). The De values for fine- and coarse-grained quartz from modern surface samples show very low De values (less than 0.1 Gy). The fine- and coarse-grained fractions from the upper unit show relatively similar De values ranging from 0.7 to 40 Gy, whereas the De values for the lower unit differ substantially between grain sizes. In samples from the lower unit, De values for the coarse-grained fraction were significantly lower (up to 60%; range, 143e265 Gy) than those for the fine-grained fraction (range, 420e464 Gy). Fig. 5a shows the decay curves of the two grain size (sample C414; lower unit). The decay curve of fine-grained samples shows that
Fig. 2. Preheat plateau test results for fine- and coarse-grained sample C4-14 and the variation of recycling ratio and recuperation with preheat temperatures from 160 to 300 C. Each point shown is the result from 3 aliquots. The errors are calculated using Analyst version 3.24.
Fig. 3. The OSL IR depletion ratio as a function of increasing preheat temperature for sample C4-14.
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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J.C. Kim et al. / Quaternary International xxx (2014) 1e9
Fig. 4. Dose recovery data for fine- and coarse-grained sample (C4-14) over a range of given doses. Each point is the mean of 3e5 aliquots.
the luminescence signal was depleted rapidly during the initial 2 s of stimulation, indicating that the signal was dominated by the fast component. In contrast, slower decay was observed in coarsegrained samples. The DRC (Dose Response Curve) for fine-grained
Fig. 6. The natural OSL decay curves (a) and OSL DRC (b) for the fine quartz OSL, fine quartz post-IR OSL, coarse quartz OSL, and coarse quartz post-IR OSL signals from sample C4-17. All data of the OSL decay curves are shown as raw counts per 0.4 s stimulation normalized to the first data point. All data of the OSL DRC were normalized to 250 Gy regeneration dose. Each point shown in (a) and (b) is the mean of 5 aliquots.
Fig. 5. OSL decay curves (a) and OSL DRC (b) obtained using the SAR procedure for fine- and coarse-grained quartz of sample C4-14. The same test dose (20 Gy) was used for both grain-size fractions.
fraction shows continuous growth up to 800 Gy and fits well with an exponential-plus-linear function. In contrast, the DRC for the coarse-grained fraction from the same sample exhibits a very low slope at high doses (>200 Gy; Fig. 5b). The De value of 470 ± 6 Gy was obtained for fine-grained fraction. The value is obtained in a dose range where the exponential component is saturated. The D0 values of the exponential component are 99 ± 8 Gy and 118 ± 12 Gy for the fine and coarse grain fractions, respectively. The post-IR OSL measurement protocol was used to deplete the signal from feldspar grains prior to stimulation with blue diodes (Roberts and Wintle, 2001, 2003; Kim et al., 2009b). Three samples (C4-14, 17, and 23) were selected for De estimation using the post-IR OSL (500 s IR stimulation at 50 C, followed by blue stimulation), and the obtained De values were compared with those from fine and coarse quartz OSL. De values of 265 ± 33, 335 ± 36, 448 ± 9, and 452 ± 5 Gy were obtained for the coarse quartz OSL, coarse quartz post-IR OSL, fine quartz OSL, and fine quartz post-IR OSL signals from sample C4-17, respectively (Table 2). The De values for coarse quartz OSL from sample C4-17 are ~60% lower compared with those for fine quartz OSL. The De values for coarse quartz post-IR OSL is lower by only ~40%. Fig. 6 shows the patterns of OSL decay and DRC for the two grain sizes using the OSL and post-IR OSL.
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
Table 1 Dose rate information, De values and OSL ages of 11YG C4 core sediments. No. of discs is the number of measurements used for age calculations. O-D (%) is an indication of the scatter from experimental uncertainties. Lab. no
11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG 11YG a b
C4-1 C4-2 C4-3 C4-4 C4-5 C4-6 C4-9 C4-10 C4-11 C4-12 C4-13 C4-14 C4-15 C4-16 C4-17 C4-18 C4-19 C4-20 C4-21 C4-22 C4-23 C4-24 C4-25 C4-27 C4-28
Water content (%)a 30.4 32.6 36.2 29.3 27.7 29.9 32.9 15.4 17.3 12.5 39.4 39.7 37.6 40.4 37.2 40.2 43.1 39.2 36.3 45.3 38.8 47.4 21.3 24.7 17.3
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
Depth Alpha dose Beta dose (Gy/ka) (m) (Gy/ka)b Coarse Fine grain grain 1 2 3.3 4.9 6.4 7.9 12.7 13.9 15.8 17 18.6 20.5 22 23.1 24.6 26.1 27.6 29.1 30.6 32.1 33.6 35.1 36.1 39.4 41.6
0.28 ± 0.14 0.30 ± 0.15 0.28 ± 0.14 0.30 ± 0.15
0.38 0.27 0.32 0.25 0.30 0.30 0.33 0.36 0.35 0.36 0.35 0.34
± ± ± ± ± ± ± ± ± ± ± ±
1.98 1.89 1.85 2.04 1.94 2.03 1.97 1.89 2.58 2.14 2.01 1.84 1.78 1.76 1.82 1.89 1.86 1.96 2.06
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.19 0.14 0.16 0.13 0.15 0.15 0.17 0.18 0.18 0.18 0.18 1.90 ± 0.17 1.90 ± 2.38 ± 2.11 ± 1.91 ±
0.11 0.10 0.10 0.11 0.11 0.11 0.11 0.11 0.15 0.13 0.10 0.09 0.09 0.09 0.09 0.10 0.09 0.10 0.11
2.11 ± 0.12 2.32 ± 0.14 2.21 ± 0.13 2.24 ± 0.13
2.29 2.10 2.03 2.01 2.07 2.15 2.12 2.23 2.34 2.21 0.10 2.16 0.09 2.16 0.14 0.12 0.11
± ± ± ± ± ± ± ± ± ± ± ±
0.13 0.12 0.11 0.11 0.12 0.12 0.12 0.12 0.13 0.12 0.12 0.11
Gamma dose (Gy/ka) 1.06 1.02 1.06 1.16 1.08 1.28 1.13 0.87 1.21 0.98 1.30 1.07 1.10 1.00 1.09 1.12 1.16 1.24 1.25 1.25 1.21 1.19 1.38 1.08 0.87
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.06 0.06 0.06 0.06 0.06 0.07 0.06 0.05 0.07 0.06 0.07 0.06 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.06 0.06 0.06 0.08 0.06 0.05
Dose rate (Gy/ka)
Cosmic dose (Gy/ka) 0.18 0.16 0.13 0.11 0.09 0.08 0.05 0.05 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
De (Gy)
Вþgþ
a þ b þ gþ Coarse
cosmic
cosmic
3.21 3.06 3.29 3.31 3.11 3.40 3.14 2.81 3.83 3.15 3.35 2.94 2.91 2.79 2.93 3.03 3.04 3.21 3.33 3.12 3.11 3.78 3.21 2.80
The water content is expressed as the weight of water divided by the weight of dry sediments. Alpha dose rate was calculated using an a-value of 0.04 ± 0.02 (Rees-Jones, 1995).
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.12 0.12 0.13 0.13 0.12 0.13 0.12 0.13 0.17 0.15 0.12 0.11 0.11 0.10 0.11 0.11 0.11 0.12 0.13
quartz
3.58 ± 0.19 3.90 ± 0.21 3.66 ± 0.20 3.72 ± 0.21
4.01 3.47 3.47 3.28 3.48 3.60 3.63 3.85 3.96 3.84 ± 0.12 3.74 ± 0.11 3.71 ± 0.16 ± 0.13 ± 0.13
± ± ± ± ± ± ± ± ± ± ± ±
No. of discs Fine quartz
0.9 0.9 2.0 4.1 2.2 4.2 40.8 123.6 188.8 158.6 185.9 206.8 249.5 239.2 264.7 200.3 147.8 143.0 172.3
± 0.2 ± 0.3 ± 0.5 ± 0.8 ± 0.2 ± 0.2 ± 0.4 ± 9.5 ± 15.9 ± 6.6 ± 15.3 ± 10.5 ± 23.2 ± 13.0 ± 33.1 ± 12.2 ± 13.4 ± 12.1 ± 10.4
0.24 0.19 0.21 0.18 0.20 0.20 0.21 0.23 0.23 0.23 0.22 172.8 ± 0.21 201.1 ± 225.6 ± 211.7 ± 179.6 ±
0.7 2.1 2.0
± 0.1 ± 0.0 ± 0.0
40.8
± 0.4
O-D (%)
Age (ka)
Coarsequartz Finequartz Coarsequartz Finequartz Coarse quartz 9/11 10/12 7/14 5/12 9/16 7/10 9/15 10/11 8/10 9/10 12/14 10/13 9/15 12/15 10/15 8/10 8/10 10/10 7/8
440.6 ± 2.9 464.0 ± 7.3 430.5 ± 9.4 455.6 ± 7.4 448.1 ± 9.4 448.3 ± 10.6 431.5 ± 4.2 431.5 ± 4.2 449.4 ± 10.3 454.2 ± 9.6 15.7 420.9 ± 4.9 6/6 25.8 419.8 ± 12.2 6/6 12.2 11/12 13.3 11/12 12.4 12/12
8/8 8/8 8/8 10/10
10/10 7/8 8/8 10/11 9/10 6/6 6/6 6/6 6/6 6/6 6/6 6/6
44 ± 4 67 ± 5 45 ± 5 34 ± 6 19 ± 2 0±0 16 ± 1 21 ± 1 21 ± 2 0±0 20 ± 1 13 ± 1 25 ± 2 16 ± 1 36 ± 3 10 ± 2 25 ± 2 23 ± 2 12 ± 2 18 30 16 17 23
± ± ± ± ±
3 4 1 1 2
8±1 4±1 4±1 2±0
1 4 6 3 5 2 0 1 3 4 1 5
± ± ± ± ± ± ± ± ± ± ± ±
0 0 1 0 0 1 0 1 1 1 1 1
Fine quartz
0.3 ± 0.1 0.3 ± 0.1 0.6 ± 0.1 0.2 ± 0.0 1.2 ± 0.2 0.5 ± 0.0 0.7 ± 0.1 0.5 ± 0.0 1.2 ± 0.1 13.0 ± 0.5 11.0 ± 0.6 43.9 ± 3.9 49.2 ± 4.7 50.3 ± 3.1 55.6 ± 5.0 109.9 ± 6.6 70.4 ± 4.5 133.9 ± 7.6 85.9 ± 8.6 124.0 ± 7.8 85.8 ± 5.7 138.8 ± 7.8 90.3 ± 11.8 128.7 ± 7.8 66.0 ± 4.7 124.7 ± 7.6 48.6 ± 4.8 118.8 ± 7.0 44.5 ± 4.1 112.2 ± 6.8 51.8 ± 3.7 113.4 ± 7.1 118.2 ± 7.4 55.3 ± 5.4 112.6 ± 6.9 64.6 ± 8.6 113.1 ± 7.3 59.7 ± 4.1 66.0 ± 5.0 64.2 ± 5.3
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Table 2 Comparison of De values, ages, recycling ratio and OSL IR depletion ratio obtained for the fine quartz OSL, fine quartz IRSL, fine quartz post-IR OSL, coarse quartz OSL, coarse quartz IRSL and coarse quartz post-IR OSL signals from samples C4-14, C4-17 and C4-23. The De values were obtained from the five aliquots in each sample. De (Gy)
Signal (C4-14) Fine quartz OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL Signal (C4-17) Fine quartz OSL Fine quartz IRSL Fine quartz post-IR OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL Signal (C4-23) Fine quartz OSL Coarse quartz OSL Coarse quartz IRSL Coarse quartz post-IR OSL
Age (ka)
Recycling ratio
7.3 10.5 15.2 33.6
133.9 ± 7.6 70.4 ± 4.5
1.04 0.99 1.00 1.02
448.1 ± 9.4 79.2 ± 13.0 452.4 ± 4.9
128.7 ± 7.8
464.0 206.8 194.2 283.7
± ± ± ±
264.7 ± 33.1 115.6 ± 19.5 334.9 ± 36.1
420.9 172.8 166.0 291.1
± ± ± ±
4.9 15.7 8.1 41.9
96.5 ± 12.0
129.9 ± 7.6 90.3 ± 11.8 114.2 ± 13.1
112.6 ± 6.9 55.3 ± 5.4 93.2 ± 13.9
0.01 0.05 0.10 0.04
0.98 ± 0.01 0.85 ± 0.04
1.02 ± 0.01 1.59 ± 0.62 1.02 ± 0.01
0.99 ± 0.01
0.99 ± 0.03 1.08 ± 0.10 0.98 ± 0.01
0.85 ± 0.03
± ± ± ±
0.97 ± 0.01 0.69 ± 0.06
1.02 0.92 1.06 0.99
± ± ± ±
OSL IR depletion ratio
0.01 0.08 0.06 0.04
0.99 ± 0.03
1.01 ± 0.01
1.04 ± 0.01
1.01 ± 0.04
4.3. Luminescence ages In the upper unit, OSL ages for both grain-size fractions ranged from 11.5 ± 0.8 to 0.21 ± 0.04 ka (Table 1, Fig. 7). Ages of fine-grained samples (C4-5 and -9) were similar to those of the corresponding coarse-grained samples in the upper unit, but OSL ages of fine-grained samples were more consistent with the stratigraphic order. In contrast, in the lower unit the OSL ages of coarse-grained samples (44 ± 4 to 90 ± 12 ka) showed significant underestimation (up to 60%) compared with those of finegrained samples (110 ± 7 to 139 ± 8 ka). Although there is some age reversals in the lower unit, ages of fine-grained samples were similar and within the range of error, with a weighted mean value of 121 ± 3 ka. The weighted mean age of coarsegrained samples in the lower unit was 65 ± 4 ka. OSL ages for the sandy and gravelly layer between the upper and lower units and the basal gravels and gravelly sand from the lowermost part could be obtained only from coarse-grained quartz due to the coarse-grained nature of the sediments. The sandy and gravelly layer was dated between 50 ± 3 and 44 ± 4 ka, and ages for the sandy layer from the lowermost part were 66 ± 5 and 60 ± 4 ka. 5. Discussion 5.1. Reliability of quartz OSL ages The laboratory parameters such as the recycling ratios and recuperation of both grain-size fractions suggest that their OSL signals are appropriate for De determination (cf. Lowick et al., 2010). However, OSL ages in the lower unit showed clear inconsistency between grain sizes. The age differences between coarse- and finegrained samples could be caused by partial bleaching. Typically, the finer fraction is more likely to be transported in suspension, whereas the coarser fraction is transported mostly as bed load, which would have caused age overestimations. Thus, the fine fraction probably experiences sufficient bleaching during transportation. However, the fine fraction tends to coagulate and form
larger aggregates that are shielded from daylight by the outer grains, and therefore tend to be insufficiently bleached (Fuchs et al., 2005). Mauz et al. (2010) studied the accuracy of optical ages derived from tidal sediments. They concluded that >85% of samples returned accurate ages and that ~13% of optical ages were overestimated, which depended largely on the transport processes. On the other hand, OSL dating has recently been applied successfully to tidal sediments in the Korean Peninsula (Hong et al., 2003; Kim et al., 2012). Kim et al. (2012) suggested that large embayments with tide-dominant conditions on the southwestern coast of the Korean Peninsula, including the current study area, encouraged complete bleaching of the surface sediments. Hong et al. (2003) obtained an OSL date of 41 ± 9 years for a tidal flat surface on the western coast of Korea, which indicated that the tidal surface sample was well bleached at the time of deposition. In this study, the degree of bleaching from surface sediments was measured by the amount of natural, residual signal in modern samples near the core site. Fine quartz showed very low De values (0.08 ± 0.02 Gy). The coarse quartz had a negligible luminescence signal, making the estimation of De values difficult. An age of ~30 years for modern fine sediments was calculated using the average dose rate (3.6 ± 0.2 Gy) of the studied core samples. The ages of coarse-grained modern samples were younger than those of fine sediments. These modern analogues suggest that samples were completely bleached at the time of deposition, regardless of grain size. Assuming that these modern analogues are representative of depositional environments, partial bleaching is not the cause for the age difference between coarse- and fine-grained samples. This supposition is also supported by the consistency of ages with stratigraphic order in the upper unit. The contamination of feldspars in separated quartz samples may give rise to age underestimation because of the anomalous fading of feldspar (Lai and Brückner, 2008; Kim et al., 2009b). The OSL IR depletion ratios near unity and fast decay form for fine-grained samples suggest that the pretreatment process completely purified the fine-grained quartz. In contrast, a relatively low OSL IR depletion ratio coupled with slow signal decay form suggest that feldspar contributed a substantial proportion to the OSL signals of coarse-grained samples compared with that of fine-grained fraction from the same sample. The post-IR OSL decay curve for coarsegrained samples shows a more rapid decay form and similar to that of fine-grained samples. The faster decay after 500 s IR stimulation for coarse-grained samples can be interpreted as an increase in the relative contribution of quartz. The OSL IR depletion ratios near unity for coarse quartz post-IR OSL also suggest that the proportion of quartz component increased after 500 s IR stimulation. However, all post-IR OSL ages for coarse-grained quartz continued to be lower than those obtained from fine quartz OSL. Natural OSL signal close to the saturation level can cause age underestimation. The DRC shape is closely related to the saturation level of quartz. In this study, the DRC of the coarse-grained sample signal usually indicated slower growth than that of the fine-grained signal. The early saturated DRC pattern for coarse-grained samples exhibited a sublinear and negligible growth rate above 300 Gy. In contrast, the fine-grained fraction showed continuous growth to 800 Gy, indicating saturation at much higher doses. A significant difference in De values for fine- and coarse-grained samples was observed at doses greater than ~300 Gy (cf. Constantin et al., 2012). The De values of fine-grained samples from the lower unit were obtained further up the linear part of the DRC (>400 Gy). Previous studies using this linear growth have yielded controversial results. Some studies obtained underestimated age results from that region (e.g. Lowick et al., 2010; Timar et al., 2010; Lowick and Preusser, 2011; Kreutzer et al., 2012). Age underestimations of ~10% (Murray and Funder, 2003) and ~14% (Murray et al., 2007) were
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
J.C. Kim et al. / Quaternary International xxx (2014) 1e9
7
Fig. 7. Comparison of OSL ages for fine- and coarse-grained quartz samples with depth of 11 YG C4 core sediments.
obtained using a single saturating exponential function in which the exponential component was saturated. Conversely, other studies obtained reliable age results from the high-dose linear region (e.g. Murray et al., 2008; Kim et al., 2010). Kim et al. (2009a) suggested that De values from the high-dose linear region (400e650 Gy) were reliable, but reported systematic underestimation (~50%) for doses > 650 Gy in the linear portion. Our OSL De values for fine-grained samples from the lower unit fell into the dose range of 400e450 Gy. Also, the dose recovery test results demonstrate that doses between 400 and 700 Gy, for which the OSL signal is in the linear portion of the response curve, can be recovered more accurately for fine-grained than for coarse-grained samples. Therefore, different DRC and saturation dose level would be one possible reason for the age differences in the lower unit. The De underestimation could be also caused by the unstable medium component (Li and Li, 2006). The coarse-grained samples appear to have a slower OSL decay rate compared to that of the finegrained fraction, indicating that the coarse-grained samples may have a larger proportion of medium component that causing the underestimation. Also, the thermal instability of the fast component may also be a possible source of underestimation (Fan et al., 2011). 5.2. Comparison of OSL ages with lithological and palynological interpretation OSL ages in the lower unit showed clear inconsistency between grain sizes. The age differences may begin in samples older than about 40 ka. The present study shows that fine-grained quartz OSL ages are thought to be more accurate than those from coarsegrained samples because they are not affected by signal saturation and feldspar contamination. However, the accuracy of De determination of fine-grained samples from the linear growth part
for doses >400 Gy remains unclear. Therefore, although luminescence characteristics differed between age results, OSL data alone could not provide an implicit indication of the accuracy of the ages. The OSL ages were compared with lithological characteristics and palynological components. The upper unit is characterized by tidal rhythmites, mottled silty muds, and a coarsening-upward pattern in grain size. OSL ages of both grain-size fractions in the upper unit fell within the Holocene. Thus, lithological characteristics and OSL ages suggest that the upper unit was a transgressive Holocene tidal deposit (cf. Park et al., 1998; Lim et al., 2004; Chang et al., 2006). The Holocene tidal deposit (the upper unit) is separated from the underlying Late Pleistocene deposit (the lower unit) by the coarse-grained sandy and gravelly layer (Fig. 1). This layer (~4 m thick), dated between 50 ± 3 and 44 ± 4 ka by coarse-grained quartz OSL, marks a Late Pleistocene unconformity that might have experienced significant weathering during subaerial exposure of sea-level lowstands (cf. Lim et al., 2004). The lower unit consists mainly of rhythmic sand-mudelaminated couplets and muddominated layers containing wood fragments, fine peats, and rootlets, suggesting that mudflat and saltmarsh environments prevailed during a sea-level highstand (cf. Lim and Park, 2003). All OSL ages from fine-grained quartz in the lower unit (110e139 ka) are in good agreement with the last interglacial sea-level highstand (MIS 5e). However, OSL ages from coarse-grained samples in the lower unit range from 90 to 45 ka, corresponding broadly to the last glacial period (MIS 4 and 3). Sea level could have been at least 5e8 m above the present level during the last interglacial period (MIS 5e) (Muhs et al., 2003), but was ~85 m lower than present sea level during the last glacial period (MIS 4 and 3) (Chappell, 2002; Cutler et al., 2003). The Yellow Sea is a relatively shallow continental shelf area with a mean water depth of 44 m (Lim et al., 2006). Considering mean water depth, the Yellow Sea area would have
Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001
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J.C. Kim et al. / Quaternary International xxx (2014) 1e9
been subaerially exposed during the last glacial period. The lower unit is distributed between 20 and 38 m below mean sea level. Accordingly, coarse quartz age results do not support the characteristics of a shallow-marine tidal sequence. The palynological composition of the lower unit is consistent with a marine depositional setting. Palynological assemblages from this unit are composed of terrestrial pollen grains and marine dinocysts. Saltmarsh-derived pollen grains predominate throughout the unit, comprising an average of 70% of total palynological assemblages, although the frequency of occurrence varies with depth. Moreover, warm temperate to subtropical deciduous evergreens of the genus Quercus (Cyclobalanopsis) and broadleaved hardwood trees are common, reflecting warm temperate to subtropical climatic conditions at that time. Diverse and abundant marine dinocysts are present throughout the lower unit, particularly the warm-water coastal-living species Lingulodinium filiforme and Tuberculodinium vancampoae. These pollen (e.g. saltmarsh and warm temperate evergreen) and marine dinocyst records may indicate the presence of a warm, shallow marine environment at the time of deposition (Chen et al., 2000; Yi et al., 2004, 2008; Jun et al., 2010). On the basis of the depositional environment, stratigraphic position, and palynological components of the lower unit, OSL ages of fine-grained quartz of MIS 5e appear to be more reliable than those from coarse-grained quartz. 6. Conclusions This study revealed a substantial difference in dating results between grain-size fractions. Coarse-grained (90e212 mm) quartz ages from the upper unit are relatively consistent with those from finegrained (4e11 mm) quartz. Lithological characteristics suggest that the upper unit is a transgressive Holocene tidal deposit. In contrast, coarse-grained samples from the lower unit are up to 60% lower compared with fine-grained samples (45 ± 4 to 90 ± 12 ka vs. 110 ± 7 to 139 ± 8 ka). This apparent age difference was partially caused by feldspar contamination and partially by early saturation of the coarse-grained quartz OSL signal. The ~30% increase in post-IR OSL ages of coarse-grained samples and OSL IR depletion ratios near unity suggests that the contribution of contaminants such as feldspars could be removed from the post-IR OSL measurement protocol. However, different saturation dose level may give rise to underestimated coarse-grained quartz ages even after using the post-IR OSL signal. On the other hand, the dose recovery test results demonstrate that plotting of the OSL signal from fine-grained samples of the lower unit on a linear component of the DRC can be recovered accurately. However, the lack of comparison with an independent age control prevents determination of whether OSL ages of fine-grained quartz based on the linear component beyond the saturating exponential signal are accurate. Nevertheless, the lithological and palynological characteristics of the lower unit suggest that it was deposited as a shallow-marine tidal sequence during a sea-level highstand, supporting the OSL dating of fine-grained quartz to the last interglacial period (MIS 5e) and suggesting that this age is more reliable than that obtained from coarse-grained quartz. Thus, OSL dating of fine-grained quartz has considerable potential in the dating of coastal sediments extending to the last interglacial period. Acknowledgements This project was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea. The authors would like to thank Prof. Noriko Hasebe for valuable discussions and comments on paper.
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Please cite this article in press as: Kim, J.C., et al., OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz, Quaternary International (2014), http://dx.doi.org/10.1016/j.quaint.2014.09.001