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Optically stimulated luminescence dating of coastal sediments from southwestern Korea
Quaternary Geochronology 10 (2012) 218e223
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Research paper
Optically stimulated luminescence dating of coastal sediments from southwestern Korea Jin Cheul Kim a, *, Chul Hun Eum b, Sangheon Yi a, Ju Yong Kim a, Sei Sun Hong a, Jin-Young Lee a a b
Geological Research Division, Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Geochemical Analysis Center, Korea Institute of Geoscience 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: Received 4 October 2011 Received in revised form 27 January 2012 Accepted 17 March 2012 Available online 27 March 2012
We tested the applicability of optically stimulated luminescence (OSL) dating to fine grained sediments from coastal (coastal lacustrine and tidal) environments. Eight samples were collected from a 16-m-long core taken from the Yeongam tidal flat on the southwestern coast of the Korean Peninsula. A singlealiquot regenerative-dose (SAR) procedure was applied to chemically purified (H2SiF6) quartz grains of 4 e11 mm in diameter. OSL dating results were compared with ages obtained from 14C dating of shells, wood fragments, and bulk sediments. The suitability of the material for OSL dating was confirmed by the luminescence characteristics. The OSL ages of the coastal sediments ranged from 19 to 1.3 ka, fitting well with the available 14C ages, especially those from wood fragments. Age differences from 500 to 1000 yr were observed between 14C ages of shells and OSL ages, caused by a constant 14C reservoir effect in this tidal area. The 14C ages of the bulk sediments were much older (>2000 yr) than the OSL ages and were not in stratigraphic order. This age discrepancy was most likely caused by incorporation of old and reworked carbon into the bulk sediments. The most reliable materials for 14C dating are wood fragments. The reproducible OSL signal, the narrow distribution of De values, and the consistency of the OSL ages with stratigraphic order indicated that fine quartz grains were not affected by incomplete bleaching. The OSL ages coupled with radiocarbon results show that a short period (between approximately 8 and 6 ka) of very rapid sedimentation (5 mm/yr) was followed by a much lower sedimentation rate since 6 ka. This was probably the result of a rapid rise in sea level in the early Holocene followed by a much slower rate in the late-Holocene. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Quartz OSL 14 C dating Tidal sediments Korean Peninsula
1. Introduction The western part of the Korean Peninsula is one of the predominant tidal flat areas in the world. The evolution of the tidal field has been a response to the sea-level changes since the Last Glacial Maximum (LGM). Coastal sediments, especially tidal sediments, are a marker for defining paleoshorelines. A regional sea level curve has been constructed based on 14C dates from many tidal flat areas on the western coast of the Korean Peninsula (Kim and Kennett, 1998; Lim, 2001; Lim and Park, 2003; Chough et al., 2004). However, a complete sea level curve has not been reconstructed due to the lack of material suitable for radiocarbon age dating. In addition, several factors may complicate radiocarbon dating of tidal samples, including contamination by old carbon and the marine radiocarbon reservoir effect. Optically stimulated
* Corresponding author. Tel.: þ82 10 5099 6150. E-mail addresses: [email protected], [email protected] (J.C. Kim). 1871-1014/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2012.03.008
luminescence (OSL) dating is an alternative dating method that is not affected by these problems. Despite uncertainties involving bleaching of the dosimeter and the dose rate estimate, luminescence dating has recently been successfully applied to tidal sediments (Hong et al., 2003; Madsen et al., 2005, 2007; Mauz and Bungenstock, 2007; Mauz et al., 2010). Previous optical dating work on tidal sediments in the Korean Peninsula used feldspars (Hong et al., 2003). Feldspar has much brighter luminescence than quartz, and therefore it can provide a better estimate for very young samples. However, this approach is complex for sediments of Holocene age because feldspar dating requires correction for anomalous fading, and the OSL signal of feldspars bleaches at least one order of magnitude slower than the OSL signal of quartz (Godfrey-Smith et al., 1988; Huntley and Lamothe, 2001; Mauz and Bungenstock, 2007). Quartz OSL dating can overcome these problems. However, there have been no successful cases in which quartz OSL dating has been applied to tidal sediments in the Korean Peninsula. In this study, the OSL dating method was applied to fine grained quartz (4e11 mm) of
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
tidal and lacustrine sediments from the southwestern part of the Korean Peninsula. We applied both OSL and 14C dating to investigate the suitability of OSL dating for fine grained coastal sediments and to evaluate accuracy by comparison of both dating results (cf. Cheetham et al., 2010; Lee et al., 2011; Long et al., 2011). Wood fragments, shells, and bulk sediments were 14C dated to determine which materials are the most suitable for 14C dating. 2. Study area and sample preparation The southwestern coast of the Korean Peninsula is bordered by the Yellow Sea, which is a semi-enclosed, relatively shallow, continental shelf area between the Korean Peninsula and China (Lim et al., 2006). Numerous coastal embayments exist in this region and are major depocenters for fine grained sediment transported from surrounding landmasses (Lim and Park, 2003). Yeongam Bay is a large area of muddy tidal flats along the southwestern coast of the Korean Peninsula (Fig. 1), although many of these tidal flats have been reclaimed as farmland. A 16-m-long core was collected in this area. As the upper part of the core contained reclaimed sediments, core subsamples were taken only from depths greater than 2 m below the surface. The sediments of this area are mainly fine grained mud deposits, and large grain size variation was not observed throughout the sequence. However, shells and wood fragments were found separately in the middle and lower parts of core. Shells of Proclava pfefferi (Dunker), a species that lives in the euneritic fascia of embayments were dominant in the middle part (5e10 m), whereas wood fragments were dominant in the lower part (11e15 m). Authigenic vivianite, a mineral associated with lacustrine sediments (Fagel et al., 2005), was found in the lower 12-m horizon. Considering the occurrence
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of authigenic vivianite, abundant wood fragments, and the absence of shell in the lower part of the core, the lower sediments are potentially lacustrine in origin. Therefore, sedimentary environments of the Yeongam core sediments can be interpreted as coastal lacustrine to tidal flat throughout the sequence. Eight OSL samples (YAL 10-1, 10-2, 10-3, 10-4, 10-5, 10-6, 10-7, and 10-8) were collected at approximately 2-m intervals from 2.5 m down to 15.3 m. Under red light, chemically purified quartz grains 4e11 mm in diameter were extracted from the samples using sodium pyrophosphate (Na4P2O7$10H2O) to remove clays, HCl and H2O2 to remove carbonates and organics, and settling according to Stokes’ Law over a depth of 20 cm in 0.01 M solution of sodium oxalate (Na2C2O4) to isolate the 4e11 mm diameter grains. Finally the samples were etched in H2SiF6 for 14 days to chemically remove feldspar and then re-settled to remove grains <4 mm (Roberts, 2007; Kim et al., 2009, 2010). Additionally, 12 samples (five of shells, four of wood fragments, and three of bulk sediment) were collected from this core for 14C dating. All samples for 14C dating were dated at the National Science Foundation-Accelerator Mass Spectrometry Laboratory of the University of Arizona, USA. All radiocarbon dates were calibrated to calendar years using CALIB 6.0 with a 2s level of reliability (Stuiver and Reimer, 1993; Reimer et al., 2009). 3. OSL measurements OSL measurements were undertaken using a Risø TL-DA-20, equipped with a blue LED (470 20 nm) stimulation source. Aliquots of fine quartz grains were prepared by settling 1 mg of material in acetone onto an aluminum disc. Irradiation was provided by a 90Sr/90Y beta source delivering about 0.1 Gy s1. An
Fig. 1. The location of Yeongam area, Korea.
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Fig. 2. Histograms, weighted histogram and radial plots of De distribution of sample YAL 10-4.
EMI 9635QA photomultiplier tube and a 7.5-mm-thick U-340 filter were used for photon detection. Dose rates were measured using high-resolution gamma spectrometry. Conversion to dose rates used the data presented by Olley et al. (1996). Cosmic ray contributions were calculated using Prescott and Hutton’s (1994) equations. The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 mm in diameter. 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 dating, and the last 20 s was subtracted as the background. The efficacy of the sensitivity correction was monitored by checking the recycling ratio and recuperation. The ‘OSL IR depletion ratio’ (Duller, 2003) was used to check for feldspar contamination. To determine the appropriate preheating conditions, a preheat plateau test was conducted for sample YAL 10-4 using a range of preheat temperatures from 160 to 300 C in 20 C intervals, with a cut-heat of 160 C. Additionally, a dose recovery test was performed to check the performance of the SAR protocol. The results of the preheat plateau and dose recovery tests are plotted in Figs. S1, S2, and S3. From the results of the preheat plateau test, a preheat temperature of 220 C for 10 s and a cut-heat of 160 C were selected for equivalent dose (De) determination. The dose recovery test showed highly reproducible results across a large plateau region (160e260 C preheat), indicating successful correction of sensitivity changes. 4. Results 4.1. OSL dating A typical natural OSL decay curve and a dose response curve are shown in Fig. S4. In the decay curve the luminescence signal drops
rapidly during the initial 2 s of stimulation, indicating that the signal is dominated by the fast component. The dose response curve shows continuous growth to 100 Gy, and fits well with an exponential-plus-linear component. The dose response curves demonstrate the excellent reproducibility of the OSL data. The recycling ratios are within 5% of unity, recuperation is less than 4% of the natural signal, and the OSL-IR depletion ratio is consistent with unity in all samples indicating that the luminescence characteristics of the material are suitable for OSL dating of fine grained coastal sediments. A typical distribution of De values is shown in Fig. 2 from sample YAL 10-4. Most De values are centered near the mean value (25.9 Gy), which is indistinguishable from the median values (25.8 Gy). The radial plot of the De values from 22 aliquots shows that 21 aliquots (96%) fall within the 2-sigma region. OSL dating results are listed in Table 1, and plotted against depth in Fig. 3. The OSL ages range from 19 2 to 1.3 0.1 ka and are in stratigraphic order throughout the sequence.
4.2.
14
C dating
Calibrated 14C dating results are presented in Fig. 3 and Table 2. The measured 14C ages have not been corrected for isotopic fractionation and reservoir effects because there are not enough data to correct. There are five 14C ages from shells (YAC-1e5), four from wood fragments (YAC-6e9), and three from bulk sediments (YAC10e12). The 14C ages of the shells ranged from 7.9 to 8.5 14C cal ka and are stratigraphically consistent. 14C ages of the wood fragments ranged from 8.5 to 19 ka and are also consistent with stratigraphic order. However, bulk sediment 14C ages are older (>2000 yr) than those of shells from the same horizon and show an age inversion between YAC-11 and 12.
Table 1 Dose rate information, De values and OSL ages of Yeongam tidal sediments. Lab. no
Depth (cm)
Water content (%)a
YAL10-1 YAL10-2 YAL10-3 YAL10-4 YAL10-5 YAL10-6 YAL10-7 YAL 10-8
250 450 650 850 1050 1250 1430 1530
46.8 42.4 42.0 38.9 37.7 30.2 12.7 10.0
a b
5 5 5 5 5 5 5 5
Alpha dose (Gy/ka)b 0.337 0.342 0.344 0.358 0.399 0.424 0.503 0.728
0.169 0.172 0.173 0.180 0.201 0.214 0.254 0.368
Beta dose (Gy/ka) 1.941 2.028 2.022 2.014 2.063 1.906 2.591 3.611
0.105 0.111 0.111 0.113 0.116 0.114 0.170 0.236
Gamma dose (Gy/ka) 1.130 1.135 1.132 1.150 1.202 1.187 1.461 2.024
0.058 0.059 0.059 0.061 0.064 0.067 0.090 0.126
Cosmic dose (Gy/ka) 0.146 0.116 0.093 0.075 0.062 0.052 0.045 0.041
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 (Ree-Jones, 1995).
0.007 0.006 0.005 0.004 0.003 0.003 0.002 0.002
Dose rate (Gy/ka) 3.555 3.621 3.590 3.598 3.726 3.569 4.599 6.416
0.207 0.213 0.214 0.221 0.241 0.251 0.318 0.455
De (Gy) 4.63 23.27 25.86 25.82 28.40 38.15 63.77 121.89
Age (ka) 0.06 0.33 0.65 0.32 0.71 0.26 0.49 10.18
1.30 6.43 7.20 7.18 7.62 10.69 13.87 19.00
0.08 0.39 0.47 0.45 0.53 0.77 0.97 2.08
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
Fig. 3. The OSL and Yeongam core.
14
C ages of the eight OSL and twelve
14
C samples with depth of
5. Discussion The most widely applied dating method for tidal and lacustrine sediments is radiocarbon dating, as these sediments usually have suitably high organic carbon content and abundant organic materials. However, 14C dating can be prone to errors. Therefore, the comparison of age results between 14C dating and other dating techniques such as OSL is necessary. Also, some materials may be more suitable for 14C dating than others. In this study, the OSL and 14 C ages of wood fragments were in concordance, while most of the 14 C ages from shells were consistently older (500e1000 yr) than those determined by OSL. The 14C ages of bulk samples from core sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order (Figs. 3 and 4). The age difference (500e1000 yr) between 14C ages of the shells and OSL ages is probably a constant 14C reservoir effect in this tidal area. A
221
previous study of the marine reservoir effect showed that the 14C ages of molluscs from the southwestern and southeastern coasts of Korea that lived in AD 1942 were 350 45 and 307 35 14C yr BP, respectively (Kong and Lee, 2005). This suggested that the regional reservoir 14C ages of these areas were lower than the mean global reservoir 14C age, indicating that these areas were influenced by freshwater inflow. Accordingly, regional reservoir 14C ages were not used as an appropriate reservoir correction for the Yeongam sediments. We did not estimate the 14C reservoir ages in the Yeongam tidal area on the basis of the 14C content of shells. Nevertheless, based on the difference between the 14C ages of the shells and the OSL ages, the reservoir 14C ages of the Yeongam area could be higher than previous results. The consistent dates between OSL and 14 C ages of wood fragments in this study suggest that 14C dates from wood fragments do not need to be corrected for a reservoir effect. The 14C ages of bulk samples from tidal sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order. The discrepancy in the 14C ages from bulk sediments may have been caused by the incorporation of old reworked carbon in the bulk sediments. Old reworked carbon may have been derived from the Yeongsan River, which is a main river in southwestern Korea. The reversals in the 14C ages of bulk samples could be related to quantitative changes in terrestrial organic materials containing old carbon derived from this river (cf. Edwards and Whittington, 2001; Kortekaas et al., 2007; Long et al., 2011). The error from the old carbon effect is much larger than that from the reservoir effect in the Yeongam sediments. Thus, chronological interpretation based on the 14C ages of bulk sediments is still problematic in this tidal environment (cf. Hutchinson et al., 2004). In contrast, 14C dating of wood fragments from lacustrine sediments does not have these problems because direct fixation of atmospheric CO2 occurs during the lifetime of plants (Watanabe et al., 2010). The good agreement between the OSL and 14C ages of the wood fragments in this study supports this interpretation, indicating that the most reliable materials for 14C dating in the Yeongam area are wood fragments such as small twigs (cf. Davidson et al., 2004; Hutchinson et al., 2004; Marshall et al., 2007). However, it is sometimes necessary to date shells because terrestrial wood fragments are rarely found in tidal sediments or because reworked carbon material makes 14C dating more complex. In this case, 14C ages from shells should be corrected with the appropriate marine reservoir correction factor for a given tidal area. The reservoir effect could vary temporally and spatially. Therefore, it is necessary to determine the 14C reservoir age throughout the whole sediment core. Also, evaluating and calibrating other sediment dating techniques such as OSL dating are valuable to correct the 14C reservoir effect (cf. Zhou et al., 2009). OSL dating of fine grained quartz is an alternative method for dating to overcome the problems associated with 14C methods.
Table 2 Radiocarbon dating results for twelve samples from Yeongam tidal sediments. Lab. Code
Sample ID
Depth (cm)
Material
d13C
14
AA92855 AA92856 AA92857 AA92858 AA92859 AA92860 AA92861 AA92862 AA92864 AA92865 AA92866 AA92867
YAC-1 YAC-2 YAC-3 YAC-4 YAC-5 YAC-6 YAC-7 YAC-8 YAC-9 YAC-10 YAC-11 YAC-12
490 640 752 860 1050 1150 1248 1418 1530 640 860 1050
Shell Shell Shell Shell Shell Wood fragments Wood fragments Wood fragments Wood fragments Bulk sediment Bulk sediment Bulk sediment
0.6 0.7 0.3 0.1 10.6 28 28 27.2 23.5 25.5 24.1 26
7,040 7,202 7,493 7,583 7,726 7,754 7,881 11,382 15,909 9,681 10,467 9,046
C age (year BP)
Calibrated age ranges 2s (year BP) 120 48 57 57 50 51 51 68 89 58 60 54
7,889 8,052 8,292 8,376 8,505 8,523 8,767 13,254 19,119 11,006 12,352 10,156
266 108 102 161 86 103 210 135 270 219 220 217
222
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
Fig. 4. Comparison of OSL ages with 14C ages at the same depth of the core. Open squares represent OSL ages against 14C ages from the wood fragments, and open triangles and open diamonds represent OSL ages against 14C ages from the bulk sediments and the shell, respectively.
However, a potential source of uncertainty in OSL dating is whether the mineral grains were completely bleached at deposition. The efficiency of bleaching in tidal flats decreases with increasing sedimentation rate and increasing water depth (cf. Mauz et al., 2010). To the north of our study site, on the central west coast of the Korean Peninsula, a lack of large embayments leads to wavedominated conditions. That coast is dominated by sandy, nonbarred open coast tidal flats, with a moderately high rate of sedimentation. These wave-dominated conditions tend to create a steep coastal gradient and deep water conditions (Yang et al., 2006). In contrast, in the current study area, located on the southwestern coast of the Korean Peninsula, large embayments exist with tide-dominant conditions. In tide-dominant conditions, the average water depth is lower than that of wave-dominant conditions, and the period of subaerial exposure and the frequency of sediment re-suspension will be longer and higher than those of wave-dominant conditions. This will encourage complete bleaching of the surface sediments. The De distributions are typified by unimodal, symmetric De dispersion and very low (1.8 0.2%) overdispersion values. In a radial plot, De values fall within a relatively narrow band. The OSL ages are consistent with stratigraphic order. In addition, if the mean sedimentation rate (39 cm/ka) between samples YAL10-250 and YAL10-450 is extrapolated this gives an age close to zero for the surface sediments (2 m below the surface excluding reclaimed sediments). These lines of evidence support the assertion that the Yeongam sediments had been fully bleached at deposition, and this allows successful application of OSL dating method to these coastal sediments. The OSL ages between 450 and 1050 cm core depths are very similar to each other, indicating a high sedimentation rate (5 mm/ yr) due to a rapid transgression 8e6 ka ago. The limited sediment record between 6 and 1 ka is interpreted to reflect a low sedimentation rate, rather than enhanced erosion, because there is no physical evidence of erosion. On the basis of these OSL ages, the Yeongam sediments show an early Holocene rapid sea level rise followed by a late-Holocene reduction in the rate of sea level rise. 6. Conclusions The OSL characteristics of the fine quartz grains (4e11 mm diameter) from Yeongam coastal (coastal lacustrine and tidal)
sediments are suitable for application of the SAR protocol. The sediments from the Yeongam tidal area received sufficient bleaching before burial to allow successful application of OSL dating. The OSL ages from the Yeongam coastal sediments are consistent with stratigraphic order and show good agreement with 14 C ages derived from wood fragments. In contrast, 14C ages of bulk sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order. The 14C ages of shells are consistently older (500e1000 yr) than the OSL ages. These age differences are mainly from ‘old carbon’ effects associated with bulk samples and the reservoir effects from shells. Terrestrial wood fragments are the most reliable material for 14C dating in the Yeongam tidal area. The OSL dating of fine quartz grains is an alternative method that avoids the problems inherent in 14C dating of coastal sediments. The OSL dates indicate that the southwestern coast of the Korean Peninsula has experienced sea level which has risen rapidly from 8 ka to 6 ka, leading to a sedimentation rate of 5 mm/yr, followed by a low sedimentation rate between 6 ka and 1 ka, which might have been influenced by a reduction in the rate of sea level rise. Acknowledgments This project was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM). The authors would like to thank Prof. Geoff Duller for helpful review of the manuscript. Dr. J. Lim and Prof. Y.G. Lee are thanked for valuable discussions and an analysis of shell species. Editorial handling by: Dr. R. Grun Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quageo.2012.03.008. References Cheetham, M., Keene, A., Erskine, W., Bush, R., Fitzsimmons, K., Jacobsen, G., Fallon, S., 2010. Resolving the Holocene alluvial record in southeastern Australia using luminescence and radiocarbon techniques. Journal of Quaternary Science 25, 1160e1168. Chough, S.K., Lee, H.J., Chun, S.S., Shinn, Y.J., 2004. Depositional processes of late Quaternary sediments in the Yellow Sea: a review. Geoscience Journal 8, 211e264. Davidson, G.R., Carnley, M., Lange, T., Galicki, S.J., Douglas, A., 2004. Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agricultural use: a 210Pb, 137Cs, and 14C study in Mississippi, USA. Radiocarbon 46, 755e764. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiation Measurements 37, 161e165. Edwards, K.J., Whittington, G., 2001. Lake sediments, erosion and landscape change during the Holocene in Britain and Ireland. Catena 42, 23e73. Fagel, N., Alleman, L.Y., Granina, L., Hatert, F., Thamo-Bozso, E., Cloots, R., André, L., 2005. Vivianite formation and distribution in Lake Baikal sediments. Global and Planetary Change 46, 315e336. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.-H., 1988. Optical dating studies of quartz and feldspar sediment extracts. Quaternary Science Reviews 7, 373e380. Hong, D.-G., Choi, M.S., Han, J.-H., Cheong, C.-S., 2003. Determination of sedimentation rate of a recently deposited tidal flat, western coast of Korea, using IRSL dating. Quaternary Science Reviews 22, 1185e1189. Huntley, D., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars, and the measurement and correction for it in optical dating. Canadian Journal of Earth Sciences 38, 1093e1106. Hutchinson, I., James, T.S., Reimer, P.J., Bornhold, B.D., Clague, J.J., 2004. Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial environments in Georgia Basin and Puget Lowland, British Columbia, Canada and Washington, USA. Quaternary Research 61, 193e203. Kim, J.C., Roberts, H.M., Duller, G.A.T., Lee, Y.I., Yi, S.B., 2009. Assessment of diagnostic tests for evaluating the reliability of SAR De values from polymineral and quartz fine grains. Radiation Measurements 44, 149e157. Kim, J.C., Duller, G.A.T., Roberts, H.M., Wintle, A.G., Lee, Y.I., Yi, S.B., 2010. Re-evaluation of the chronology of the palaeolithic site at Jeongokri, Korea, using OSL and TT-OSL signals from quartz. Quaternary Geochronology 5, 365e370.
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Research paper
Optically stimulated luminescence dating of coastal sediments from southwestern Korea Jin Cheul Kim a, *, Chul Hun Eum b, Sangheon Yi a, Ju Yong Kim a, Sei Sun Hong a, Jin-Young Lee a a b
Geological Research Division, Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Geochemical Analysis Center, Korea Institute of Geoscience 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: Received 4 October 2011 Received in revised form 27 January 2012 Accepted 17 March 2012 Available online 27 March 2012
We tested the applicability of optically stimulated luminescence (OSL) dating to fine grained sediments from coastal (coastal lacustrine and tidal) environments. Eight samples were collected from a 16-m-long core taken from the Yeongam tidal flat on the southwestern coast of the Korean Peninsula. A singlealiquot regenerative-dose (SAR) procedure was applied to chemically purified (H2SiF6) quartz grains of 4 e11 mm in diameter. OSL dating results were compared with ages obtained from 14C dating of shells, wood fragments, and bulk sediments. The suitability of the material for OSL dating was confirmed by the luminescence characteristics. The OSL ages of the coastal sediments ranged from 19 to 1.3 ka, fitting well with the available 14C ages, especially those from wood fragments. Age differences from 500 to 1000 yr were observed between 14C ages of shells and OSL ages, caused by a constant 14C reservoir effect in this tidal area. The 14C ages of the bulk sediments were much older (>2000 yr) than the OSL ages and were not in stratigraphic order. This age discrepancy was most likely caused by incorporation of old and reworked carbon into the bulk sediments. The most reliable materials for 14C dating are wood fragments. The reproducible OSL signal, the narrow distribution of De values, and the consistency of the OSL ages with stratigraphic order indicated that fine quartz grains were not affected by incomplete bleaching. The OSL ages coupled with radiocarbon results show that a short period (between approximately 8 and 6 ka) of very rapid sedimentation (5 mm/yr) was followed by a much lower sedimentation rate since 6 ka. This was probably the result of a rapid rise in sea level in the early Holocene followed by a much slower rate in the late-Holocene. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Quartz OSL 14 C dating Tidal sediments Korean Peninsula
1. Introduction The western part of the Korean Peninsula is one of the predominant tidal flat areas in the world. The evolution of the tidal field has been a response to the sea-level changes since the Last Glacial Maximum (LGM). Coastal sediments, especially tidal sediments, are a marker for defining paleoshorelines. A regional sea level curve has been constructed based on 14C dates from many tidal flat areas on the western coast of the Korean Peninsula (Kim and Kennett, 1998; Lim, 2001; Lim and Park, 2003; Chough et al., 2004). However, a complete sea level curve has not been reconstructed due to the lack of material suitable for radiocarbon age dating. In addition, several factors may complicate radiocarbon dating of tidal samples, including contamination by old carbon and the marine radiocarbon reservoir effect. Optically stimulated
* Corresponding author. Tel.: þ82 10 5099 6150. E-mail addresses: [email protected], [email protected] (J.C. Kim). 1871-1014/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2012.03.008
luminescence (OSL) dating is an alternative dating method that is not affected by these problems. Despite uncertainties involving bleaching of the dosimeter and the dose rate estimate, luminescence dating has recently been successfully applied to tidal sediments (Hong et al., 2003; Madsen et al., 2005, 2007; Mauz and Bungenstock, 2007; Mauz et al., 2010). Previous optical dating work on tidal sediments in the Korean Peninsula used feldspars (Hong et al., 2003). Feldspar has much brighter luminescence than quartz, and therefore it can provide a better estimate for very young samples. However, this approach is complex for sediments of Holocene age because feldspar dating requires correction for anomalous fading, and the OSL signal of feldspars bleaches at least one order of magnitude slower than the OSL signal of quartz (Godfrey-Smith et al., 1988; Huntley and Lamothe, 2001; Mauz and Bungenstock, 2007). Quartz OSL dating can overcome these problems. However, there have been no successful cases in which quartz OSL dating has been applied to tidal sediments in the Korean Peninsula. In this study, the OSL dating method was applied to fine grained quartz (4e11 mm) of
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
tidal and lacustrine sediments from the southwestern part of the Korean Peninsula. We applied both OSL and 14C dating to investigate the suitability of OSL dating for fine grained coastal sediments and to evaluate accuracy by comparison of both dating results (cf. Cheetham et al., 2010; Lee et al., 2011; Long et al., 2011). Wood fragments, shells, and bulk sediments were 14C dated to determine which materials are the most suitable for 14C dating. 2. Study area and sample preparation The southwestern coast of the Korean Peninsula is bordered by the Yellow Sea, which is a semi-enclosed, relatively shallow, continental shelf area between the Korean Peninsula and China (Lim et al., 2006). Numerous coastal embayments exist in this region and are major depocenters for fine grained sediment transported from surrounding landmasses (Lim and Park, 2003). Yeongam Bay is a large area of muddy tidal flats along the southwestern coast of the Korean Peninsula (Fig. 1), although many of these tidal flats have been reclaimed as farmland. A 16-m-long core was collected in this area. As the upper part of the core contained reclaimed sediments, core subsamples were taken only from depths greater than 2 m below the surface. The sediments of this area are mainly fine grained mud deposits, and large grain size variation was not observed throughout the sequence. However, shells and wood fragments were found separately in the middle and lower parts of core. Shells of Proclava pfefferi (Dunker), a species that lives in the euneritic fascia of embayments were dominant in the middle part (5e10 m), whereas wood fragments were dominant in the lower part (11e15 m). Authigenic vivianite, a mineral associated with lacustrine sediments (Fagel et al., 2005), was found in the lower 12-m horizon. Considering the occurrence
219
of authigenic vivianite, abundant wood fragments, and the absence of shell in the lower part of the core, the lower sediments are potentially lacustrine in origin. Therefore, sedimentary environments of the Yeongam core sediments can be interpreted as coastal lacustrine to tidal flat throughout the sequence. Eight OSL samples (YAL 10-1, 10-2, 10-3, 10-4, 10-5, 10-6, 10-7, and 10-8) were collected at approximately 2-m intervals from 2.5 m down to 15.3 m. Under red light, chemically purified quartz grains 4e11 mm in diameter were extracted from the samples using sodium pyrophosphate (Na4P2O7$10H2O) to remove clays, HCl and H2O2 to remove carbonates and organics, and settling according to Stokes’ Law over a depth of 20 cm in 0.01 M solution of sodium oxalate (Na2C2O4) to isolate the 4e11 mm diameter grains. Finally the samples were etched in H2SiF6 for 14 days to chemically remove feldspar and then re-settled to remove grains <4 mm (Roberts, 2007; Kim et al., 2009, 2010). Additionally, 12 samples (five of shells, four of wood fragments, and three of bulk sediment) were collected from this core for 14C dating. All samples for 14C dating were dated at the National Science Foundation-Accelerator Mass Spectrometry Laboratory of the University of Arizona, USA. All radiocarbon dates were calibrated to calendar years using CALIB 6.0 with a 2s level of reliability (Stuiver and Reimer, 1993; Reimer et al., 2009). 3. OSL measurements OSL measurements were undertaken using a Risø TL-DA-20, equipped with a blue LED (470 20 nm) stimulation source. Aliquots of fine quartz grains were prepared by settling 1 mg of material in acetone onto an aluminum disc. Irradiation was provided by a 90Sr/90Y beta source delivering about 0.1 Gy s1. An
Fig. 1. The location of Yeongam area, Korea.
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J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
Fig. 2. Histograms, weighted histogram and radial plots of De distribution of sample YAL 10-4.
EMI 9635QA photomultiplier tube and a 7.5-mm-thick U-340 filter were used for photon detection. Dose rates were measured using high-resolution gamma spectrometry. Conversion to dose rates used the data presented by Olley et al. (1996). Cosmic ray contributions were calculated using Prescott and Hutton’s (1994) equations. The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 mm in diameter. 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 dating, and the last 20 s was subtracted as the background. The efficacy of the sensitivity correction was monitored by checking the recycling ratio and recuperation. The ‘OSL IR depletion ratio’ (Duller, 2003) was used to check for feldspar contamination. To determine the appropriate preheating conditions, a preheat plateau test was conducted for sample YAL 10-4 using a range of preheat temperatures from 160 to 300 C in 20 C intervals, with a cut-heat of 160 C. Additionally, a dose recovery test was performed to check the performance of the SAR protocol. The results of the preheat plateau and dose recovery tests are plotted in Figs. S1, S2, and S3. From the results of the preheat plateau test, a preheat temperature of 220 C for 10 s and a cut-heat of 160 C were selected for equivalent dose (De) determination. The dose recovery test showed highly reproducible results across a large plateau region (160e260 C preheat), indicating successful correction of sensitivity changes. 4. Results 4.1. OSL dating A typical natural OSL decay curve and a dose response curve are shown in Fig. S4. In the decay curve the luminescence signal drops
rapidly during the initial 2 s of stimulation, indicating that the signal is dominated by the fast component. The dose response curve shows continuous growth to 100 Gy, and fits well with an exponential-plus-linear component. The dose response curves demonstrate the excellent reproducibility of the OSL data. The recycling ratios are within 5% of unity, recuperation is less than 4% of the natural signal, and the OSL-IR depletion ratio is consistent with unity in all samples indicating that the luminescence characteristics of the material are suitable for OSL dating of fine grained coastal sediments. A typical distribution of De values is shown in Fig. 2 from sample YAL 10-4. Most De values are centered near the mean value (25.9 Gy), which is indistinguishable from the median values (25.8 Gy). The radial plot of the De values from 22 aliquots shows that 21 aliquots (96%) fall within the 2-sigma region. OSL dating results are listed in Table 1, and plotted against depth in Fig. 3. The OSL ages range from 19 2 to 1.3 0.1 ka and are in stratigraphic order throughout the sequence.
4.2.
14
C dating
Calibrated 14C dating results are presented in Fig. 3 and Table 2. The measured 14C ages have not been corrected for isotopic fractionation and reservoir effects because there are not enough data to correct. There are five 14C ages from shells (YAC-1e5), four from wood fragments (YAC-6e9), and three from bulk sediments (YAC10e12). The 14C ages of the shells ranged from 7.9 to 8.5 14C cal ka and are stratigraphically consistent. 14C ages of the wood fragments ranged from 8.5 to 19 ka and are also consistent with stratigraphic order. However, bulk sediment 14C ages are older (>2000 yr) than those of shells from the same horizon and show an age inversion between YAC-11 and 12.
Table 1 Dose rate information, De values and OSL ages of Yeongam tidal sediments. Lab. no
Depth (cm)
Water content (%)a
YAL10-1 YAL10-2 YAL10-3 YAL10-4 YAL10-5 YAL10-6 YAL10-7 YAL 10-8
250 450 650 850 1050 1250 1430 1530
46.8 42.4 42.0 38.9 37.7 30.2 12.7 10.0
a b
5 5 5 5 5 5 5 5
Alpha dose (Gy/ka)b 0.337 0.342 0.344 0.358 0.399 0.424 0.503 0.728
0.169 0.172 0.173 0.180 0.201 0.214 0.254 0.368
Beta dose (Gy/ka) 1.941 2.028 2.022 2.014 2.063 1.906 2.591 3.611
0.105 0.111 0.111 0.113 0.116 0.114 0.170 0.236
Gamma dose (Gy/ka) 1.130 1.135 1.132 1.150 1.202 1.187 1.461 2.024
0.058 0.059 0.059 0.061 0.064 0.067 0.090 0.126
Cosmic dose (Gy/ka) 0.146 0.116 0.093 0.075 0.062 0.052 0.045 0.041
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 (Ree-Jones, 1995).
0.007 0.006 0.005 0.004 0.003 0.003 0.002 0.002
Dose rate (Gy/ka) 3.555 3.621 3.590 3.598 3.726 3.569 4.599 6.416
0.207 0.213 0.214 0.221 0.241 0.251 0.318 0.455
De (Gy) 4.63 23.27 25.86 25.82 28.40 38.15 63.77 121.89
Age (ka) 0.06 0.33 0.65 0.32 0.71 0.26 0.49 10.18
1.30 6.43 7.20 7.18 7.62 10.69 13.87 19.00
0.08 0.39 0.47 0.45 0.53 0.77 0.97 2.08
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
Fig. 3. The OSL and Yeongam core.
14
C ages of the eight OSL and twelve
14
C samples with depth of
5. Discussion The most widely applied dating method for tidal and lacustrine sediments is radiocarbon dating, as these sediments usually have suitably high organic carbon content and abundant organic materials. However, 14C dating can be prone to errors. Therefore, the comparison of age results between 14C dating and other dating techniques such as OSL is necessary. Also, some materials may be more suitable for 14C dating than others. In this study, the OSL and 14 C ages of wood fragments were in concordance, while most of the 14 C ages from shells were consistently older (500e1000 yr) than those determined by OSL. The 14C ages of bulk samples from core sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order (Figs. 3 and 4). The age difference (500e1000 yr) between 14C ages of the shells and OSL ages is probably a constant 14C reservoir effect in this tidal area. A
221
previous study of the marine reservoir effect showed that the 14C ages of molluscs from the southwestern and southeastern coasts of Korea that lived in AD 1942 were 350 45 and 307 35 14C yr BP, respectively (Kong and Lee, 2005). This suggested that the regional reservoir 14C ages of these areas were lower than the mean global reservoir 14C age, indicating that these areas were influenced by freshwater inflow. Accordingly, regional reservoir 14C ages were not used as an appropriate reservoir correction for the Yeongam sediments. We did not estimate the 14C reservoir ages in the Yeongam tidal area on the basis of the 14C content of shells. Nevertheless, based on the difference between the 14C ages of the shells and the OSL ages, the reservoir 14C ages of the Yeongam area could be higher than previous results. The consistent dates between OSL and 14 C ages of wood fragments in this study suggest that 14C dates from wood fragments do not need to be corrected for a reservoir effect. The 14C ages of bulk samples from tidal sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order. The discrepancy in the 14C ages from bulk sediments may have been caused by the incorporation of old reworked carbon in the bulk sediments. Old reworked carbon may have been derived from the Yeongsan River, which is a main river in southwestern Korea. The reversals in the 14C ages of bulk samples could be related to quantitative changes in terrestrial organic materials containing old carbon derived from this river (cf. Edwards and Whittington, 2001; Kortekaas et al., 2007; Long et al., 2011). The error from the old carbon effect is much larger than that from the reservoir effect in the Yeongam sediments. Thus, chronological interpretation based on the 14C ages of bulk sediments is still problematic in this tidal environment (cf. Hutchinson et al., 2004). In contrast, 14C dating of wood fragments from lacustrine sediments does not have these problems because direct fixation of atmospheric CO2 occurs during the lifetime of plants (Watanabe et al., 2010). The good agreement between the OSL and 14C ages of the wood fragments in this study supports this interpretation, indicating that the most reliable materials for 14C dating in the Yeongam area are wood fragments such as small twigs (cf. Davidson et al., 2004; Hutchinson et al., 2004; Marshall et al., 2007). However, it is sometimes necessary to date shells because terrestrial wood fragments are rarely found in tidal sediments or because reworked carbon material makes 14C dating more complex. In this case, 14C ages from shells should be corrected with the appropriate marine reservoir correction factor for a given tidal area. The reservoir effect could vary temporally and spatially. Therefore, it is necessary to determine the 14C reservoir age throughout the whole sediment core. Also, evaluating and calibrating other sediment dating techniques such as OSL dating are valuable to correct the 14C reservoir effect (cf. Zhou et al., 2009). OSL dating of fine grained quartz is an alternative method for dating to overcome the problems associated with 14C methods.
Table 2 Radiocarbon dating results for twelve samples from Yeongam tidal sediments. Lab. Code
Sample ID
Depth (cm)
Material
d13C
14
AA92855 AA92856 AA92857 AA92858 AA92859 AA92860 AA92861 AA92862 AA92864 AA92865 AA92866 AA92867
YAC-1 YAC-2 YAC-3 YAC-4 YAC-5 YAC-6 YAC-7 YAC-8 YAC-9 YAC-10 YAC-11 YAC-12
490 640 752 860 1050 1150 1248 1418 1530 640 860 1050
Shell Shell Shell Shell Shell Wood fragments Wood fragments Wood fragments Wood fragments Bulk sediment Bulk sediment Bulk sediment
0.6 0.7 0.3 0.1 10.6 28 28 27.2 23.5 25.5 24.1 26
7,040 7,202 7,493 7,583 7,726 7,754 7,881 11,382 15,909 9,681 10,467 9,046
C age (year BP)
Calibrated age ranges 2s (year BP) 120 48 57 57 50 51 51 68 89 58 60 54
7,889 8,052 8,292 8,376 8,505 8,523 8,767 13,254 19,119 11,006 12,352 10,156
266 108 102 161 86 103 210 135 270 219 220 217
222
J.C. Kim et al. / Quaternary Geochronology 10 (2012) 218e223
Fig. 4. Comparison of OSL ages with 14C ages at the same depth of the core. Open squares represent OSL ages against 14C ages from the wood fragments, and open triangles and open diamonds represent OSL ages against 14C ages from the bulk sediments and the shell, respectively.
However, a potential source of uncertainty in OSL dating is whether the mineral grains were completely bleached at deposition. The efficiency of bleaching in tidal flats decreases with increasing sedimentation rate and increasing water depth (cf. Mauz et al., 2010). To the north of our study site, on the central west coast of the Korean Peninsula, a lack of large embayments leads to wavedominated conditions. That coast is dominated by sandy, nonbarred open coast tidal flats, with a moderately high rate of sedimentation. These wave-dominated conditions tend to create a steep coastal gradient and deep water conditions (Yang et al., 2006). In contrast, in the current study area, located on the southwestern coast of the Korean Peninsula, large embayments exist with tide-dominant conditions. In tide-dominant conditions, the average water depth is lower than that of wave-dominant conditions, and the period of subaerial exposure and the frequency of sediment re-suspension will be longer and higher than those of wave-dominant conditions. This will encourage complete bleaching of the surface sediments. The De distributions are typified by unimodal, symmetric De dispersion and very low (1.8 0.2%) overdispersion values. In a radial plot, De values fall within a relatively narrow band. The OSL ages are consistent with stratigraphic order. In addition, if the mean sedimentation rate (39 cm/ka) between samples YAL10-250 and YAL10-450 is extrapolated this gives an age close to zero for the surface sediments (2 m below the surface excluding reclaimed sediments). These lines of evidence support the assertion that the Yeongam sediments had been fully bleached at deposition, and this allows successful application of OSL dating method to these coastal sediments. The OSL ages between 450 and 1050 cm core depths are very similar to each other, indicating a high sedimentation rate (5 mm/ yr) due to a rapid transgression 8e6 ka ago. The limited sediment record between 6 and 1 ka is interpreted to reflect a low sedimentation rate, rather than enhanced erosion, because there is no physical evidence of erosion. On the basis of these OSL ages, the Yeongam sediments show an early Holocene rapid sea level rise followed by a late-Holocene reduction in the rate of sea level rise. 6. Conclusions The OSL characteristics of the fine quartz grains (4e11 mm diameter) from Yeongam coastal (coastal lacustrine and tidal)
sediments are suitable for application of the SAR protocol. The sediments from the Yeongam tidal area received sufficient bleaching before burial to allow successful application of OSL dating. The OSL ages from the Yeongam coastal sediments are consistent with stratigraphic order and show good agreement with 14 C ages derived from wood fragments. In contrast, 14C ages of bulk sediments are much older (>2000 yr) than the corresponding OSL ages and are not in stratigraphic order. The 14C ages of shells are consistently older (500e1000 yr) than the OSL ages. These age differences are mainly from ‘old carbon’ effects associated with bulk samples and the reservoir effects from shells. Terrestrial wood fragments are the most reliable material for 14C dating in the Yeongam tidal area. The OSL dating of fine quartz grains is an alternative method that avoids the problems inherent in 14C dating of coastal sediments. The OSL dates indicate that the southwestern coast of the Korean Peninsula has experienced sea level which has risen rapidly from 8 ka to 6 ka, leading to a sedimentation rate of 5 mm/yr, followed by a low sedimentation rate between 6 ka and 1 ka, which might have been influenced by a reduction in the rate of sea level rise. Acknowledgments This project was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM). The authors would like to thank Prof. Geoff Duller for helpful review of the manuscript. Dr. J. Lim and Prof. Y.G. Lee are thanked for valuable discussions and an analysis of shell species. Editorial handling by: Dr. R. Grun Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quageo.2012.03.008. References Cheetham, M., Keene, A., Erskine, W., Bush, R., Fitzsimmons, K., Jacobsen, G., Fallon, S., 2010. Resolving the Holocene alluvial record in southeastern Australia using luminescence and radiocarbon techniques. Journal of Quaternary Science 25, 1160e1168. Chough, S.K., Lee, H.J., Chun, S.S., Shinn, Y.J., 2004. Depositional processes of late Quaternary sediments in the Yellow Sea: a review. Geoscience Journal 8, 211e264. Davidson, G.R., Carnley, M., Lange, T., Galicki, S.J., Douglas, A., 2004. Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agricultural use: a 210Pb, 137Cs, and 14C study in Mississippi, USA. Radiocarbon 46, 755e764. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiation Measurements 37, 161e165. Edwards, K.J., Whittington, G., 2001. Lake sediments, erosion and landscape change during the Holocene in Britain and Ireland. Catena 42, 23e73. Fagel, N., Alleman, L.Y., Granina, L., Hatert, F., Thamo-Bozso, E., Cloots, R., André, L., 2005. Vivianite formation and distribution in Lake Baikal sediments. Global and Planetary Change 46, 315e336. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.-H., 1988. Optical dating studies of quartz and feldspar sediment extracts. Quaternary Science Reviews 7, 373e380. Hong, D.-G., Choi, M.S., Han, J.-H., Cheong, C.-S., 2003. Determination of sedimentation rate of a recently deposited tidal flat, western coast of Korea, using IRSL dating. Quaternary Science Reviews 22, 1185e1189. Huntley, D., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars, and the measurement and correction for it in optical dating. Canadian Journal of Earth Sciences 38, 1093e1106. Hutchinson, I., James, T.S., Reimer, P.J., Bornhold, B.D., Clague, J.J., 2004. Marine and limnic radiocarbon reservoir corrections for studies of late- and postglacial environments in Georgia Basin and Puget Lowland, British Columbia, Canada and Washington, USA. Quaternary Research 61, 193e203. Kim, J.C., Roberts, H.M., Duller, G.A.T., Lee, Y.I., Yi, S.B., 2009. Assessment of diagnostic tests for evaluating the reliability of SAR De values from polymineral and quartz fine grains. Radiation Measurements 44, 149e157. Kim, J.C., Duller, G.A.T., Roberts, H.M., Wintle, A.G., Lee, Y.I., Yi, S.B., 2010. Re-evaluation of the chronology of the palaeolithic site at Jeongokri, Korea, using OSL and TT-OSL signals from quartz. Quaternary Geochronology 5, 365e370.
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