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OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula
Quaternary Geochronology xxx (2015) 1e6
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OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula Jin Cheul Kim a, *, Daekyo Cheong b, Seungwon Shin b, Yong-Hee Park b, Sei Sun Hong a a b
Geological Research Division, Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Division of Geology & Geophysics, Kangwon National University, Chuncheon, Kangwon-do, 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 5 November 2014 Accepted 22 January 2015 Available online xxx
Optically stimulated luminescence (OSL) dating was performed on Late Quaternary deltaic sequences from a 55-m-long core sampled from the Nakdong River estuary, Korea. OSL ages obtained from chemically separated fine (4e11 mm) and coarse (90e212 mm) quartz grains ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka, revealing clear consistency between the grain-size fractions. The De values from the standardized growth curve (SGC) are consistent with those from the single-aliquot regenerative-dose (SAR) procedure, which suggests that the SGC is valid for the Nakdong deltaic sediments. The 14C ages of shells and wood fragments ranged from 11 to 2.9 ka, demonstrating reasonable agreement with the OSL ages, within the error range. However, the limited number and random sampling interval of the 14C age data (10 ages) result in a simple linear and exponential trend in the deptheage curve. In contrast, OSL ages obtained by high-resolution sampling show down-section variations in the deptheage curve, indicating the occurrence of rapid changes in sedimentation rate. It is suggested that the high-samplingresolution OSL ages provide a more realistic and detailed deptheage curve and sedimentation rate. The Nakdong deltaic sediments were divided into five units based on sedimentation rate. The lowest (unit 5) shows a break in sedimentation between the last glacial maximum (LGM) and the Holocene. The sedimentation rate increased in units 4 and 3, presumably corresponding to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate following the regression of the shoreline, until about 2 ka. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during the last 2000 years. © 2015 Elsevier B.V. All rights reserved.
Keywords: Quartz OSL 14 C dating Nakdong River Deltaic sediments Korean Peninsula
1. Introduction Abundant sediment supply and ample accommodation space have led to thick sediment accumulations and the development of the major Late Quaternary Nakdong deltaic sequence in the southeastern coastal area of the Korean Peninsula. This deltaic area is important for reconstructing sea level changes during the late Quaternary. Although several recent studies have examined the Holocene evolution of the Nakdong River delta (Lee and Chung, 2000; Park et al., 2000; Yoo and Park, 2000; Yoo et al., 2011, 2014), little is known about the timing of the major sedimentological changes. A general chronology has been developed for the Nakdong deltaic sediments based on 14C dating. However, ‘old carbon’ effects associated with bulk samples and reservoir effects
* Corresponding author. E-mail address: [email protected] (J.C. Kim).
from marine biological products make it difficult to obtain proper samples and a consistent sampling interval throughout the sequence for 14C dating (Kim et al., 2012). However, a highresolution chronological framework and the reconstruction of coastal environments can be achieved when the collection of dating pez and Thompson, 2012). Optically samples is closely spaced (Lo stimulated luminescence (OSL) dating can provide highly detailed sample data at regular intervals, and it is applicable over a longer time range than 14C dating. OSL has been successfully used for dating the world's major delta sequences (Sanderson et al., 2003, 2007; Zhao et al., 2008; Tamura et al., 2012). However, OSL dating has yet to be successfully applied to deltaic sediments in the Korean Peninsula. In this study, we test the applicability of quartz OSL dating for the Late Quaternary deltaic sequences in the Nakdong River estuary. The OSL results were systematically compared with 14 C dating results. This study provides a new precise chronology and more a detailed accumulation record for the Nakdong deltaic sequences.
http://dx.doi.org/10.1016/j.quageo.2015.01.006 1871-1014/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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2. Study area and sample preparation The southeastern coastal area of the Korean Peninsula is primarily influenced by the Nakdong River, the second largest fluvial system in Korea, which has produced a thick accumulation of deltaic sediments. The Nakdong River drains an area of 23,817 km2, and the main stream is over 525 km long (Park and Lee, 2002). The study area preserves a record of sea level changes. It was
completely exposed during the last glacial maximum (LGM), resulting in subaerial erosion. In contrast, postglacial transgression formed different sedimentary units derived from coastal and estuarine systems. Previous investigations (Park and Yoo, 1988; Suk, 1989; Min, 1994) have shown that glacio-eustatic sea level fluctuations coupled with the sediment discharge from the Nakdong River were significant factors controlling the sedimentary processes in this area during the Late Quaternary.
Fig. 1. The location of Nakdong deltaic area.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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A 55-m-long core (ND 01) was collected from the study area (Fig. 1: 35 060 04.400 N, 128 54013.200 E, 4.8 m a.s.l.). The lower part of the core (55e35 m) consisted of gravelly sands and laminated sand, the middle part of the core (35e20 m) was mostly homogeneous muddy deposits containing shell fragments, and the upper part of the core (20e8 m) consisted primarily of laminated sand and intercalated muddy layers. Twenty-five samples (NDR series) were collected for OSL dating. Sixteen of these samples could be divided into fine- and coarse-grained quartz sub-samples. For the dating of the fine-grained samples, chemically purified quartz grains 4e11 mm in diameter were extracted using sodium pyrophosphate (Na4P2O7$10H2O) to remove any clay, and hydrochloric acid (HCl) and hydrogen peroxide (H2O2) to remove any carbonates and organic matter. Settling occurred 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., 2012). For the dating of the 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. Concentrated hydrofluoric acid (HF) was applied to remove any remaining feldspar grains and to etch away the outer 10 mm of the quartz grains. Additionally, 10 samples (five of shells and five of wood fragments) were collected from this core for 14C dating. All samples for 14 C dating were dated at the accelerator mass spectrometry laboratory of the Korea Institute of Geoscience and Mineral Resources. All 14C dates were calibrated to calendar years using CALIB 6.0 with a 2s level of reliability (Stuiver and Reimer, 1993; Reimer et al., 2009). A time-dependent global ocean reservoir correction of about 400 years was used for the shells. The difference DR (154) in the reservoir age of the southeastern coastal region was also subtracted to accommodate local reservoir effects (Kong and Lee, 2005).
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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 evaluate feldspar contamination. To determine the appropriate preheating conditions and to assess dose recoverability, the preheat plateau and dose recovery tests were conducted for core samples NDR 7 (17.25e17.35 m) and NDR 16 (45.4e45.5 m) for both grain-size fractions using preheat temperatures of 160e300 C in 20 C intervals, with a cut heat of 160 C. For the dose recovery test, bleaching was performed by two blue light stimulations of 1000 s each at room temperature (RT), separated by a storage time of 10,000 s at RT. 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). A standardized growth curve (SGC) was constructed by calculating the mean sensitivity-corrected luminescence intensity (Lx/ Tx) for each dose point in the growth curve (Roberts and Duller, 2004). A SGC was constructed using regeneration doses of 5, 10, 20, 40, and 80 Gy, with a 4 Gy test dose, using 5 aliquots of each sample. Ln/Tn for each aliquot was analyzed using a conventional SAR approach and applied to a SGC to determine De values.
3. OSL measurements Luminescence signals were measured with a TL/OSL-DA-20 Luminescence Reader (Risø DTU, Denmark) 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 gray (Gy) s1. An EMI 9635 QA photomultiplier tube and a 7.5mm U-340 color glass 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). A present water content (weight water/weight dry sediment) was used for the dose rate correction. Cosmic ray contributions were calculated using the equations of Prescott and Hutton (1994). A mean ɑ-value of 0.04 ± 0.02 for fine-grained quartz was used following Rees-Jones (1995). A beta attenuation factor of 0.88 ± 0.04 was used for coarse-grained quartz (Mejdahl, 1979). The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 and 90e212 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 were used for equivalent dose (De) calculations, and the last 20 s were subtracted as the background. A total of 12e15 aliquots per sample were used for the De measurements for the coarse-grained quartz, and 8e12 aliquots were measured for the fine-grained quartz. Sensitivity-corrected doseeresponse curves were fitted by a saturating exponential function. Sensitivity correction efficacy was assessed by monitoring the recycling ratio and recuperation. The rejection criteria for the recycling ratio and
Fig. 2. Preheat plateau and dose recovery test results for fine- and coarse-grained samples NDR 7 and 16. Dependence of De (a) and of the measured to given dose ratio (b) on 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.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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4. Results and discussion 4.1. Age determination: comparison of methods A rapidly decaying OSL signal and continuous growth to 80 Gy in the doseeresponse curve are typical for different grain sizes of quartz. The luminescence characteristics are suitable for SAR, as all the samples show recycling ratios within 3% of unity, a recuperation of signal that is less than 5% of the natural signal, and OSL IR depletion ratios consistent with unity from both grain-size fractions. Additionally, highly reproducible De values were obtained. OSL ages for both grain-size fractions ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka (Table S1). OSL ages of the fine-grained quartz were similar to those of the corresponding coarse-grained quartz. Determination of the OSL ages from different grain-size fractions enhances confidence in the results (Aitken, 1998). Several studies have reported differences in De values or ages between coarse- and fine-grained samples (Fuchs et al., 2005; Fan et al., 2010; Hu et al., 2010; Kreutzer et al., 2012), perhaps due to the bleaching characteristics of the differently sized grains. Therefore, the effect of partial bleaching on the obtained De values should be minimized by selecting the most bleached grain size for analysis. No modern analogue sample was available for this study because of land reclamation. Additionally, independent age control was not available for the study area. Although highly reproducible De values would result from complete bleaching, the lack of scatter in the De values does not indicate whether the samples were adequately bleached at deposition because of the averaging effect of the high number of grains in each aliquot (cf. Olley et al., 1999; Wallinga, 2002; Duller, 2008). Nevertheless, comparison of the OSL ages obtained from different grain sizes of quartz revealed clear consistency (Fig. 3). Additionally, the quartz ages for both grain sizes are in stratigraphic order and are not affected by signal saturation within this age range. These characteristics support the assumption that the different grain sizes of quartz were fully bleached prior to deposition. The long-distance transport of sediments (over the 500-km length of the main stream) and the multiple opportunities for bleaching during transportation likely resulted in Nakdong deltaic sediments that were completely bleached (cf. Stokes et al., 2001; Rittenour et al., 2005) regardless of grain size. The linear growth demonstrated for doses up to 40 Gy and the similar degrees of reproducibility made it possible to use a SGC for
Fig. 3. Comparison of OSL ages between fine- and coarse-grained quartz samples at the same depth of the core.
the Nakdong deltaic sediments (Fig. 4). The greatest advantage of a SGC is the simplification of the measurement procedure. Interpolating sensitivity-corrected natural luminescence intensity (Ln/Tn) to the SGC allows the measurement of a large number of samples in a relatively short time, resulting in high sampling resolution for OSL dating (Shen and Mauz, 2011). The De values estimated using a conventional SAR and those based on the SGCs were compared for both fine- and coarse-grained quartz sample fractions (Fig. 5). The De values obtained from SGC were similar to those obtained from conventional SAR. The average ratios of the De derived from SAR relative to those derived from the SGC were 1.08 ± 0.01 for fine quartz and 1.06 ± 0.03 for coarse quartz. This suggests that De determination from the SGC is valid for the Nakdong deltaic sediments. However, the variation in SGC characteristics from the different quartz grain sizes suggests that it is only applicable within the same grain-size ranges. Previous studies indicated that De determinations from SGC were consistent with those from SAR not only from the same section, but also from other sections (Roberts and Duller, 2004; Lai, 2006). However, Stevens et al. (2007) demonstrated that different samples from different sites did not share doseeresponse characteristics. Therefore, further research is required to investigate the applicability of a SGC for other core samples in the Nakdong deltaic area. Calibrated 14C dating results are presented in Fig. 6 and Table S2. Five 14C ages from shells (NDC 2, 3, 5, 7, and 9) ranged from 9.5 to 3.2 ka, and five from wood fragments (NDC 1, 4, 6, 9, and 10) ranged from 11 to 2.9 ka. All 14C ages were consistent with stratigraphic order and demonstrated good agreement with the OSL ages, within the error range. This suggests that terrestrial wood and shells are reliable materials for 14C dating in the Nakdong deltaic sediments.
4.2. Sediment accumulation rates The limited number and random interval of the 14C age data (10 ages) led to a simple linear and exponential trend in the deptheage curve (Fig. 6: green (in the web version) dotted line), implying a uniform sedimentation rate. However, it is difficult to determine whether the linear relationship between age and depth throughout the sequence is realistic. In contrast, a relatively high number of OSL ages formed significant down-section variation in the deptheage curve, indicating the occurrence of rapid changes in sedimentation rate (Fig. 6: red (in the web version) line). In general, coastal areas
Fig. 4. Comparison of standardized growth curves of the fine- and coarse-grained quartz samples of ND 01 core sediments. A SGC was constructed by averaging 5 aliquots of each sample using 4 Gy test dose.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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Fig. 5. De values determined using a SGC and the SAR protocol. The dark line is the 1:1 line. The empty squares are from fine-grained quartz, and the empty triangles are from coarse-grained quartz.
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are highly dynamic systems influenced by stream flow and marine transgression or regression, and sedimentation rates may vary accordingly over time. Sedimentation must be discontinuous on large time scales even in a continuously subsiding delta because deltaic sediments accumulate at or near sea level, which is presumed to represent sea-level variation (Worm et al., 1998). Therefore, variable sedimentation rates based on OSL ages are more realistic. The 29.4e0.4 ka OSL age estimates based on fine- and coarsegrained quartz fractions are in reasonable agreement with the stratigraphic order of the sediments. The Nakdong deltaic sediments were divided into five units based on sedimentation rates: the lower terrestrial/fluvial facies (units 4 and 5), the middle brackish/marine facies (units 2 and 3), and the upper fluvial facies (unit 1). A relatively low sedimentation rate of 0.18 m/kyr occurred in unit 5 during 26.9e11 ka. The sedimentation rate gradually increased in unit 4 (6.97 m/kyr) during 11e7.9 ka. A considerably higher accumulation rate of 3.79 m/kyr in unit 3 occurred during 7.9e5.2 ka. The sedimentation rate then gradually decreased to about 1.51 m/kyr until 2 ka (unit 2). An abrupt increase in sedimentation rate (7.73 m/kyr) occurred in the last 2000 years. Previous determinations of the regional sea level curve near the Korean Peninsula showed that the lowest sea level during the LGM was about 130 m less than the present level (Park et al., 2000; Hori et al., 2002). The study area was completely exposed during the LGM, and erosional processes were dominant. This resulted in a break in sedimentation between the LGM and the Holocene, corresponding to unit 5. Units 4 and 3 presumably correspond to the early to middle Holocene sea level rise and high stand. Sea level reached its present level around 6 ka, and has remained relatively stable (cf. Hori et al., 2002; Yoo et al., 2014). The increased strength of rainfall associated with the East Asian summer monsoon at around 6.3e5 ka in the Korean Peninsula (Nahm and Hong, 2014) likely increased the sedimentation rate in unit 3 in addition to sea level change. Shoreline regression began at around 5 ka. The sedimentation rate gradually decreased in the subsequent regressions until about 2 ka, corresponding to unit 2. Terrigenous sediments derived from the Nakdong River began to increase, and in turn, the Nakdong River mouth prograded seaward with continuous accumulation of deltaic sediments. The warm climate combined with sea level changes increased the supply of fluvial sediments from the Nakdong River, which may have contributed to the rapid accumulation of unit 1 during the last 2000 years (cf. Jang, 1990; Lee and Chung, 2000; Yoo et al., 2014). These chronological and sedimentological characteristics are coincident with global coastal changes during the Late Pleistocene and Holocene. 5. Conclusion
Fig. 6. OSL ages for fine- and coarse-grained quartz samples and 14C ages with depth of ND 01 core sediments.
The OSL characteristics of the fine- (4e11 mm) and coarsegrained (90e212 mm) quartz fractions from the Nakdong deltaic sediments are suitable for the SAR protocol. OSL ages are in reasonable agreement for both grain-size fractions, ranging from 29.4 ± 2.6 to 0.4 ± 0.04 ka. All 14C ages from the Nakdong deltaic sediments are consistent with stratigraphic order and show good agreement with the OSL ages, within the error range. However, the lack of dateable materials to determine the 14C ages results in a simple linear and exponential trend in the deptheage curve. In contrast, the deptheage curve derived from OSL ages estimates based on fine- and coarse-grained quartz fractions is more variable. This suggests that a high sampling resolution for age dating would provide a more detailed and realistic deptheage curve and sedimentation rate. The reasonable agreement among OSL dates measured from different quartz grain-size fractions indicates that OSL dating could be applicable for the whole sedimentary sequence
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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regardless of grain size. The SGC approach for the Nakdong deltaic sediments is also suitable as an alternative dating method with high sampling resolution. The Nakdong deltaic sediments were divided into five units based on sedimentation rates. The lowest part (unit 5) shows a break in sedimentation between the LGM and the Holocene. The increased sedimentation rates of units 4 and 3 presumably correspond to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate until about 2 ka, following regression of the shoreline. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during last 2000 years. Acknowledgments This research has been supported by Basic Research Program through the National Research Foundation of Korea (NRF-20100023036, NRF-2012R1A1A2038789) and by the Korea Institute of Geoscience and Mineral Resources (15-3113). Dr. J.S. Lim and Dr. J.Y. Lee are thanked for valuable discussions. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quageo.2015.01.006. References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiat. Meas. 37, 161e165. Duller, G.A.T., 2008. Single-grain optical dating of Quaternary sediments: why aliquot size matters in luminescence dating. Boreas 37, 589e612. Fan, Y.X., Zhao, H., Chen, F.H., 2010. The equivalent dose of different grain size quartz fractions from lakeshore sediments in the arid region of north China. Quat. Geochronol. 5, 205e211. € ller, L., 2005. Residual luminescence signals of recent river Fuchs, M., Straub, J., Zo flood sediments: a comparison between quartz and feldspar of fine- and coarse-grain sediments. Anc. TL 23, 25e30. Hori, K., Saito, Y., Zhao, Q., Wang, P., 2002. Evolution of the coastal depositional systems of the Changjiang (Yangtze) River in response to Late PleistoceneeHolocene sea-level changes. J. Sediment. Res. 72, 884e897. Hu, G., Zhang, J.-F., Qiu, W.-L., Zhou, L.-P., 2010. Residual OSL signals in modern fluvial sediments from the Yellow River (HuangHe) and the implications for dating young sediments. Quat. Geochronol. 5, 187e193. Jang, K.M., 1990. Sedimentation and Fine-grained Sediments on the Inner Shelf off the Southeastern Coast of Korea (Master's thesis). Chungnam National University, Korea, p. 113. Kim, J.C., Eum, C.H., Yi, S., Kim, J.Y., Hong, S.S., Lee, J.-Y., 2012. Optically stimulated luminescence dating of coastal sediments from southwestern Korea. Quat. Geochronol. 10, 218e223. Kong, G.S., Lee, C.W., 2005. Marine reservoir corrections (DR) for southern coastal waters of Korea. J. Korean Soc. Oceanogr. 10, 124e128. Kreutzer, S., Fuchs, M., Meszner, S., Faust, D., 2012. OSL chronostratigraphy of a loess-palaeosol sequence in Saxony/Germany using quartz of different grain sizes. Quat. Geochronol. 10, 102e109. Lai, Z.P., 2006. Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau. Radiat. Meas. 41, 9e16. Lee, C.S., Chung, Y.H., 2000. Late Quaternary sedimentation in the Kadeok region, Korea. Geo-Mar. Lett. 20, 72e79. pez, G.I., Thompson, J.W., 2012. OSL and sediment accumulation rate models: Lo understanding the history of sediment deposition. Quat. Geochronol. 10, 175e179. Mejdahl, V., 1979. Thermoluminescence dating: beta-dose attenuation in quartz grains. Archaeometry 21, 61e72. Min, G.H., 1994. Seismic Stratigraphy and Depositional History of PlioceneeHolocene Deposits in the Southeastern Shelf, Korean Peninsula (Ph.D. thesis). Seoul National University, Korea, p. 196. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57e73.
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OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula Jin Cheul Kim a, *, Daekyo Cheong b, Seungwon Shin b, Yong-Hee Park b, Sei Sun Hong a a b
Geological Research Division, Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea Division of Geology & Geophysics, Kangwon National University, Chuncheon, Kangwon-do, 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 5 November 2014 Accepted 22 January 2015 Available online xxx
Optically stimulated luminescence (OSL) dating was performed on Late Quaternary deltaic sequences from a 55-m-long core sampled from the Nakdong River estuary, Korea. OSL ages obtained from chemically separated fine (4e11 mm) and coarse (90e212 mm) quartz grains ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka, revealing clear consistency between the grain-size fractions. The De values from the standardized growth curve (SGC) are consistent with those from the single-aliquot regenerative-dose (SAR) procedure, which suggests that the SGC is valid for the Nakdong deltaic sediments. The 14C ages of shells and wood fragments ranged from 11 to 2.9 ka, demonstrating reasonable agreement with the OSL ages, within the error range. However, the limited number and random sampling interval of the 14C age data (10 ages) result in a simple linear and exponential trend in the deptheage curve. In contrast, OSL ages obtained by high-resolution sampling show down-section variations in the deptheage curve, indicating the occurrence of rapid changes in sedimentation rate. It is suggested that the high-samplingresolution OSL ages provide a more realistic and detailed deptheage curve and sedimentation rate. The Nakdong deltaic sediments were divided into five units based on sedimentation rate. The lowest (unit 5) shows a break in sedimentation between the last glacial maximum (LGM) and the Holocene. The sedimentation rate increased in units 4 and 3, presumably corresponding to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate following the regression of the shoreline, until about 2 ka. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during the last 2000 years. © 2015 Elsevier B.V. All rights reserved.
Keywords: Quartz OSL 14 C dating Nakdong River Deltaic sediments Korean Peninsula
1. Introduction Abundant sediment supply and ample accommodation space have led to thick sediment accumulations and the development of the major Late Quaternary Nakdong deltaic sequence in the southeastern coastal area of the Korean Peninsula. This deltaic area is important for reconstructing sea level changes during the late Quaternary. Although several recent studies have examined the Holocene evolution of the Nakdong River delta (Lee and Chung, 2000; Park et al., 2000; Yoo and Park, 2000; Yoo et al., 2011, 2014), little is known about the timing of the major sedimentological changes. A general chronology has been developed for the Nakdong deltaic sediments based on 14C dating. However, ‘old carbon’ effects associated with bulk samples and reservoir effects
* Corresponding author. E-mail address: [email protected] (J.C. Kim).
from marine biological products make it difficult to obtain proper samples and a consistent sampling interval throughout the sequence for 14C dating (Kim et al., 2012). However, a highresolution chronological framework and the reconstruction of coastal environments can be achieved when the collection of dating pez and Thompson, 2012). Optically samples is closely spaced (Lo stimulated luminescence (OSL) dating can provide highly detailed sample data at regular intervals, and it is applicable over a longer time range than 14C dating. OSL has been successfully used for dating the world's major delta sequences (Sanderson et al., 2003, 2007; Zhao et al., 2008; Tamura et al., 2012). However, OSL dating has yet to be successfully applied to deltaic sediments in the Korean Peninsula. In this study, we test the applicability of quartz OSL dating for the Late Quaternary deltaic sequences in the Nakdong River estuary. The OSL results were systematically compared with 14 C dating results. This study provides a new precise chronology and more a detailed accumulation record for the Nakdong deltaic sequences.
http://dx.doi.org/10.1016/j.quageo.2015.01.006 1871-1014/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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2. Study area and sample preparation The southeastern coastal area of the Korean Peninsula is primarily influenced by the Nakdong River, the second largest fluvial system in Korea, which has produced a thick accumulation of deltaic sediments. The Nakdong River drains an area of 23,817 km2, and the main stream is over 525 km long (Park and Lee, 2002). The study area preserves a record of sea level changes. It was
completely exposed during the last glacial maximum (LGM), resulting in subaerial erosion. In contrast, postglacial transgression formed different sedimentary units derived from coastal and estuarine systems. Previous investigations (Park and Yoo, 1988; Suk, 1989; Min, 1994) have shown that glacio-eustatic sea level fluctuations coupled with the sediment discharge from the Nakdong River were significant factors controlling the sedimentary processes in this area during the Late Quaternary.
Fig. 1. The location of Nakdong deltaic area.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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A 55-m-long core (ND 01) was collected from the study area (Fig. 1: 35 060 04.400 N, 128 54013.200 E, 4.8 m a.s.l.). The lower part of the core (55e35 m) consisted of gravelly sands and laminated sand, the middle part of the core (35e20 m) was mostly homogeneous muddy deposits containing shell fragments, and the upper part of the core (20e8 m) consisted primarily of laminated sand and intercalated muddy layers. Twenty-five samples (NDR series) were collected for OSL dating. Sixteen of these samples could be divided into fine- and coarse-grained quartz sub-samples. For the dating of the fine-grained samples, chemically purified quartz grains 4e11 mm in diameter were extracted using sodium pyrophosphate (Na4P2O7$10H2O) to remove any clay, and hydrochloric acid (HCl) and hydrogen peroxide (H2O2) to remove any carbonates and organic matter. Settling occurred 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., 2012). For the dating of the 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. Concentrated hydrofluoric acid (HF) was applied to remove any remaining feldspar grains and to etch away the outer 10 mm of the quartz grains. Additionally, 10 samples (five of shells and five of wood fragments) were collected from this core for 14C dating. All samples for 14 C dating were dated at the accelerator mass spectrometry laboratory of the Korea Institute of Geoscience and Mineral Resources. All 14C dates were calibrated to calendar years using CALIB 6.0 with a 2s level of reliability (Stuiver and Reimer, 1993; Reimer et al., 2009). A time-dependent global ocean reservoir correction of about 400 years was used for the shells. The difference DR (154) in the reservoir age of the southeastern coastal region was also subtracted to accommodate local reservoir effects (Kong and Lee, 2005).
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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 evaluate feldspar contamination. To determine the appropriate preheating conditions and to assess dose recoverability, the preheat plateau and dose recovery tests were conducted for core samples NDR 7 (17.25e17.35 m) and NDR 16 (45.4e45.5 m) for both grain-size fractions using preheat temperatures of 160e300 C in 20 C intervals, with a cut heat of 160 C. For the dose recovery test, bleaching was performed by two blue light stimulations of 1000 s each at room temperature (RT), separated by a storage time of 10,000 s at RT. 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). A standardized growth curve (SGC) was constructed by calculating the mean sensitivity-corrected luminescence intensity (Lx/ Tx) for each dose point in the growth curve (Roberts and Duller, 2004). A SGC was constructed using regeneration doses of 5, 10, 20, 40, and 80 Gy, with a 4 Gy test dose, using 5 aliquots of each sample. Ln/Tn for each aliquot was analyzed using a conventional SAR approach and applied to a SGC to determine De values.
3. OSL measurements Luminescence signals were measured with a TL/OSL-DA-20 Luminescence Reader (Risø DTU, Denmark) 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 gray (Gy) s1. An EMI 9635 QA photomultiplier tube and a 7.5mm U-340 color glass 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). A present water content (weight water/weight dry sediment) was used for the dose rate correction. Cosmic ray contributions were calculated using the equations of Prescott and Hutton (1994). A mean ɑ-value of 0.04 ± 0.02 for fine-grained quartz was used following Rees-Jones (1995). A beta attenuation factor of 0.88 ± 0.04 was used for coarse-grained quartz (Mejdahl, 1979). The single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000) was applied to chemically purified quartz grains of 4e11 and 90e212 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 were used for equivalent dose (De) calculations, and the last 20 s were subtracted as the background. A total of 12e15 aliquots per sample were used for the De measurements for the coarse-grained quartz, and 8e12 aliquots were measured for the fine-grained quartz. Sensitivity-corrected doseeresponse curves were fitted by a saturating exponential function. Sensitivity correction efficacy was assessed by monitoring the recycling ratio and recuperation. The rejection criteria for the recycling ratio and
Fig. 2. Preheat plateau and dose recovery test results for fine- and coarse-grained samples NDR 7 and 16. Dependence of De (a) and of the measured to given dose ratio (b) on 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.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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4. Results and discussion 4.1. Age determination: comparison of methods A rapidly decaying OSL signal and continuous growth to 80 Gy in the doseeresponse curve are typical for different grain sizes of quartz. The luminescence characteristics are suitable for SAR, as all the samples show recycling ratios within 3% of unity, a recuperation of signal that is less than 5% of the natural signal, and OSL IR depletion ratios consistent with unity from both grain-size fractions. Additionally, highly reproducible De values were obtained. OSL ages for both grain-size fractions ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka (Table S1). OSL ages of the fine-grained quartz were similar to those of the corresponding coarse-grained quartz. Determination of the OSL ages from different grain-size fractions enhances confidence in the results (Aitken, 1998). Several studies have reported differences in De values or ages between coarse- and fine-grained samples (Fuchs et al., 2005; Fan et al., 2010; Hu et al., 2010; Kreutzer et al., 2012), perhaps due to the bleaching characteristics of the differently sized grains. Therefore, the effect of partial bleaching on the obtained De values should be minimized by selecting the most bleached grain size for analysis. No modern analogue sample was available for this study because of land reclamation. Additionally, independent age control was not available for the study area. Although highly reproducible De values would result from complete bleaching, the lack of scatter in the De values does not indicate whether the samples were adequately bleached at deposition because of the averaging effect of the high number of grains in each aliquot (cf. Olley et al., 1999; Wallinga, 2002; Duller, 2008). Nevertheless, comparison of the OSL ages obtained from different grain sizes of quartz revealed clear consistency (Fig. 3). Additionally, the quartz ages for both grain sizes are in stratigraphic order and are not affected by signal saturation within this age range. These characteristics support the assumption that the different grain sizes of quartz were fully bleached prior to deposition. The long-distance transport of sediments (over the 500-km length of the main stream) and the multiple opportunities for bleaching during transportation likely resulted in Nakdong deltaic sediments that were completely bleached (cf. Stokes et al., 2001; Rittenour et al., 2005) regardless of grain size. The linear growth demonstrated for doses up to 40 Gy and the similar degrees of reproducibility made it possible to use a SGC for
Fig. 3. Comparison of OSL ages between fine- and coarse-grained quartz samples at the same depth of the core.
the Nakdong deltaic sediments (Fig. 4). The greatest advantage of a SGC is the simplification of the measurement procedure. Interpolating sensitivity-corrected natural luminescence intensity (Ln/Tn) to the SGC allows the measurement of a large number of samples in a relatively short time, resulting in high sampling resolution for OSL dating (Shen and Mauz, 2011). The De values estimated using a conventional SAR and those based on the SGCs were compared for both fine- and coarse-grained quartz sample fractions (Fig. 5). The De values obtained from SGC were similar to those obtained from conventional SAR. The average ratios of the De derived from SAR relative to those derived from the SGC were 1.08 ± 0.01 for fine quartz and 1.06 ± 0.03 for coarse quartz. This suggests that De determination from the SGC is valid for the Nakdong deltaic sediments. However, the variation in SGC characteristics from the different quartz grain sizes suggests that it is only applicable within the same grain-size ranges. Previous studies indicated that De determinations from SGC were consistent with those from SAR not only from the same section, but also from other sections (Roberts and Duller, 2004; Lai, 2006). However, Stevens et al. (2007) demonstrated that different samples from different sites did not share doseeresponse characteristics. Therefore, further research is required to investigate the applicability of a SGC for other core samples in the Nakdong deltaic area. Calibrated 14C dating results are presented in Fig. 6 and Table S2. Five 14C ages from shells (NDC 2, 3, 5, 7, and 9) ranged from 9.5 to 3.2 ka, and five from wood fragments (NDC 1, 4, 6, 9, and 10) ranged from 11 to 2.9 ka. All 14C ages were consistent with stratigraphic order and demonstrated good agreement with the OSL ages, within the error range. This suggests that terrestrial wood and shells are reliable materials for 14C dating in the Nakdong deltaic sediments.
4.2. Sediment accumulation rates The limited number and random interval of the 14C age data (10 ages) led to a simple linear and exponential trend in the deptheage curve (Fig. 6: green (in the web version) dotted line), implying a uniform sedimentation rate. However, it is difficult to determine whether the linear relationship between age and depth throughout the sequence is realistic. In contrast, a relatively high number of OSL ages formed significant down-section variation in the deptheage curve, indicating the occurrence of rapid changes in sedimentation rate (Fig. 6: red (in the web version) line). In general, coastal areas
Fig. 4. Comparison of standardized growth curves of the fine- and coarse-grained quartz samples of ND 01 core sediments. A SGC was constructed by averaging 5 aliquots of each sample using 4 Gy test dose.
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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Fig. 5. De values determined using a SGC and the SAR protocol. The dark line is the 1:1 line. The empty squares are from fine-grained quartz, and the empty triangles are from coarse-grained quartz.
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are highly dynamic systems influenced by stream flow and marine transgression or regression, and sedimentation rates may vary accordingly over time. Sedimentation must be discontinuous on large time scales even in a continuously subsiding delta because deltaic sediments accumulate at or near sea level, which is presumed to represent sea-level variation (Worm et al., 1998). Therefore, variable sedimentation rates based on OSL ages are more realistic. The 29.4e0.4 ka OSL age estimates based on fine- and coarsegrained quartz fractions are in reasonable agreement with the stratigraphic order of the sediments. The Nakdong deltaic sediments were divided into five units based on sedimentation rates: the lower terrestrial/fluvial facies (units 4 and 5), the middle brackish/marine facies (units 2 and 3), and the upper fluvial facies (unit 1). A relatively low sedimentation rate of 0.18 m/kyr occurred in unit 5 during 26.9e11 ka. The sedimentation rate gradually increased in unit 4 (6.97 m/kyr) during 11e7.9 ka. A considerably higher accumulation rate of 3.79 m/kyr in unit 3 occurred during 7.9e5.2 ka. The sedimentation rate then gradually decreased to about 1.51 m/kyr until 2 ka (unit 2). An abrupt increase in sedimentation rate (7.73 m/kyr) occurred in the last 2000 years. Previous determinations of the regional sea level curve near the Korean Peninsula showed that the lowest sea level during the LGM was about 130 m less than the present level (Park et al., 2000; Hori et al., 2002). The study area was completely exposed during the LGM, and erosional processes were dominant. This resulted in a break in sedimentation between the LGM and the Holocene, corresponding to unit 5. Units 4 and 3 presumably correspond to the early to middle Holocene sea level rise and high stand. Sea level reached its present level around 6 ka, and has remained relatively stable (cf. Hori et al., 2002; Yoo et al., 2014). The increased strength of rainfall associated with the East Asian summer monsoon at around 6.3e5 ka in the Korean Peninsula (Nahm and Hong, 2014) likely increased the sedimentation rate in unit 3 in addition to sea level change. Shoreline regression began at around 5 ka. The sedimentation rate gradually decreased in the subsequent regressions until about 2 ka, corresponding to unit 2. Terrigenous sediments derived from the Nakdong River began to increase, and in turn, the Nakdong River mouth prograded seaward with continuous accumulation of deltaic sediments. The warm climate combined with sea level changes increased the supply of fluvial sediments from the Nakdong River, which may have contributed to the rapid accumulation of unit 1 during the last 2000 years (cf. Jang, 1990; Lee and Chung, 2000; Yoo et al., 2014). These chronological and sedimentological characteristics are coincident with global coastal changes during the Late Pleistocene and Holocene. 5. Conclusion
Fig. 6. OSL ages for fine- and coarse-grained quartz samples and 14C ages with depth of ND 01 core sediments.
The OSL characteristics of the fine- (4e11 mm) and coarsegrained (90e212 mm) quartz fractions from the Nakdong deltaic sediments are suitable for the SAR protocol. OSL ages are in reasonable agreement for both grain-size fractions, ranging from 29.4 ± 2.6 to 0.4 ± 0.04 ka. All 14C ages from the Nakdong deltaic sediments are consistent with stratigraphic order and show good agreement with the OSL ages, within the error range. However, the lack of dateable materials to determine the 14C ages results in a simple linear and exponential trend in the deptheage curve. In contrast, the deptheage curve derived from OSL ages estimates based on fine- and coarse-grained quartz fractions is more variable. This suggests that a high sampling resolution for age dating would provide a more detailed and realistic deptheage curve and sedimentation rate. The reasonable agreement among OSL dates measured from different quartz grain-size fractions indicates that OSL dating could be applicable for the whole sedimentary sequence
Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006
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regardless of grain size. The SGC approach for the Nakdong deltaic sediments is also suitable as an alternative dating method with high sampling resolution. The Nakdong deltaic sediments were divided into five units based on sedimentation rates. The lowest part (unit 5) shows a break in sedimentation between the LGM and the Holocene. The increased sedimentation rates of units 4 and 3 presumably correspond to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate until about 2 ka, following regression of the shoreline. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during last 2000 years. Acknowledgments This research has been supported by Basic Research Program through the National Research Foundation of Korea (NRF-20100023036, NRF-2012R1A1A2038789) and by the Korea Institute of Geoscience and Mineral Resources (15-3113). Dr. J.S. Lim and Dr. J.Y. Lee are thanked for valuable discussions. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quageo.2015.01.006. References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiat. Meas. 37, 161e165. Duller, G.A.T., 2008. Single-grain optical dating of Quaternary sediments: why aliquot size matters in luminescence dating. Boreas 37, 589e612. Fan, Y.X., Zhao, H., Chen, F.H., 2010. The equivalent dose of different grain size quartz fractions from lakeshore sediments in the arid region of north China. Quat. Geochronol. 5, 205e211. € ller, L., 2005. Residual luminescence signals of recent river Fuchs, M., Straub, J., Zo flood sediments: a comparison between quartz and feldspar of fine- and coarse-grain sediments. Anc. TL 23, 25e30. Hori, K., Saito, Y., Zhao, Q., Wang, P., 2002. Evolution of the coastal depositional systems of the Changjiang (Yangtze) River in response to Late PleistoceneeHolocene sea-level changes. J. Sediment. Res. 72, 884e897. Hu, G., Zhang, J.-F., Qiu, W.-L., Zhou, L.-P., 2010. Residual OSL signals in modern fluvial sediments from the Yellow River (HuangHe) and the implications for dating young sediments. Quat. Geochronol. 5, 187e193. Jang, K.M., 1990. Sedimentation and Fine-grained Sediments on the Inner Shelf off the Southeastern Coast of Korea (Master's thesis). Chungnam National University, Korea, p. 113. Kim, J.C., Eum, C.H., Yi, S., Kim, J.Y., Hong, S.S., Lee, J.-Y., 2012. Optically stimulated luminescence dating of coastal sediments from southwestern Korea. Quat. Geochronol. 10, 218e223. Kong, G.S., Lee, C.W., 2005. Marine reservoir corrections (DR) for southern coastal waters of Korea. J. Korean Soc. Oceanogr. 10, 124e128. Kreutzer, S., Fuchs, M., Meszner, S., Faust, D., 2012. OSL chronostratigraphy of a loess-palaeosol sequence in Saxony/Germany using quartz of different grain sizes. Quat. Geochronol. 10, 102e109. Lai, Z.P., 2006. Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau. Radiat. Meas. 41, 9e16. Lee, C.S., Chung, Y.H., 2000. Late Quaternary sedimentation in the Kadeok region, Korea. Geo-Mar. Lett. 20, 72e79. pez, G.I., Thompson, J.W., 2012. OSL and sediment accumulation rate models: Lo understanding the history of sediment deposition. Quat. Geochronol. 10, 175e179. Mejdahl, V., 1979. Thermoluminescence dating: beta-dose attenuation in quartz grains. Archaeometry 21, 61e72. Min, G.H., 1994. Seismic Stratigraphy and Depositional History of PlioceneeHolocene Deposits in the Southeastern Shelf, Korean Peninsula (Ph.D. thesis). Seoul National University, Korea, p. 196. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57e73.
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Please cite this article in press as: Kim, J.C., et al., OSL chronology and accumulation rate of the Nakdong deltaic sediments, southeastern Korean Peninsula, Quaternary Geochronology (2015), http://dx.doi.org/10.1016/j.quageo.2015.01.006