East China is close to an extensive coastal sedimentary environment. From north to south, three semienclosed continental shelf marginal seas (the Bohai, Yellow, and East China seas) surround the land boundary and form the north–south oriented coastline. With sea level rise and fall, transgression and regression have alternately shifted the coastal belts. The Yangtze (Changjiang) River is one of the world’s largest rivers, flowing eastward to the East China Sea and connecting the delta plain with the shelf–coastal sedimentary systems (
The optically stimulated luminescence (OSL) technique has been applied extensively to late Quaternary sediments since the single-aliquots regenerative (SAR) dose protocol was proposed by Murray and Wintle (2000). It has been proven a robust dating method for coastal and marine deposits all over the world (Jacobs, 2008; Sugisaki
In the current study, we tested the applicability of the OSL dating technique using coarse-grained (CG) quartz (90–200 µm) in a coastal sediment sequence retrieved from core YZ07, which was drilled on the southwestern coastline of the SYS on the northern flank of the Yangtze River delta (
The lower Yangtze River, which is greatly affected by the East Asian summer monsoon, is one of the world’s largest rivers in terms of its hydrological conditions (e.g., suspended sediment load, water discharge, length and drainage area) (Milliman
The Yellow Sea is an epicontinental shallow sea between China and the Korean Peninsula. It is separated into the SYS and the North Yellow Sea by the Shandong Peninsula. The water depth of the SYS is generally less than 100 m (Liu
Overall, 28 OSL samples were collected from the upper 50 m of the core using a 5-cm-long, 3-cm diameter steel cylinder (
All OSL sample preparation and luminescence measurements were performed at the Luminescence Dating Laboratory of NIGLAS (Nanjing, China). In the luminescence laboratory, the sediment at either end of the cylinders was scraped off for dose rate determination and the sediment unexposed to light in the middle part of the cylinder was used for equivalent dose (De) determination. The sediments were initially wet sieved to obtain the CG fraction and then treated with 30% H2O2 and 10% HCl to remove any organic matter and carbonates, respectively. Subsequently, the quartz-rich fraction (2.62–2.70 g/cm3) was extracted by density separation with sodium polytungstate. Finally, the quartz-rich fraction was etched with 40% hydrofluoric acid for one hour to remove the outer alpha-irradiated layer of the quartz grains and to eliminate feldspar contamination, and then the etchedfraction was rinsed with 10% HCI to remove any fluoride. The pure quartz grains were then mounted on 10-mm-diameter steel discs using silicone oil adhesive with a 2-mm-diameter monolayer for measurement.
OSL measurements were performed on an automated luminescence reader (Risø TL/OSL DA-20) equipped with a 90Sr/90Y beta source. Quartz OSL signals were stimulated by blue LEDs (470 nm) and detected through a 7.5-mm Hoya U-340 filter. The SAR protocol was used for all OSL measurements (Murray and Wintle, 2000) and the purity of the isolated quartz for each aliquot was examined using the IR depletion (Duller, 2003) in the SAR sequence.
For dose rate measurement, the material was first dried to determine the water content and then about 10 g of the sample was ground to a homogeneous powder. The concentration of dose rate related elements: uranium (U), thorium (Th) and potassium (K) were measured by neutron activation analysis at the China Institute of Atomic Energy in Beijing. The cosmic ray contribution to dose rate for each sample was estimated as a function of depth, altitude and geomagnetic latitude (Prescott and Hutton, 1994). The water content was measured from the fresh sediments and an uncertainty of 10% was assumed for all samples. Greater consideration regarding the dose rate calculation is presented in Section 5 (Reliability of CG quartz OSL ages).
In order to select suitable preheat conditions for De determination, preheat plateau tests (PHT) (
In order to ensure the suitability of the chosen preheat temperature, we chose 14 samples from the upper part of the sequence for dose recovery tests at the fixed preheat temperature of 180°C. The histograms illustrated in
One valid approach to identify the degree of OSL signal resetting for any sample is to check the equivalent dose distribution measured from single aliquots or single grains, especially for cases where incomplete bleaching is the primary cause of any excess dispersion and the larger contributor to the shape of the dose distribution (Murray
Results of statistical characteristics of the 28 samplesSample ID Depth (m) n De (Gy) OD (%) Skewness Kurtosis mean median KDE. Max CAM NL-588 6.4 24 2.3 2.3 2.3 2.2 ± 0.1 14.3 ± 0.9 0.58 3.11 NL-589 6.9 24 1.9 1.9 1.9 1.9 ± 0.1 20.7 ± 1.4 0.22 2.81 NL-590 7.8 24 3.0 3.1 3.2 3.0 ± 0.1 19.0 ± 1.5 –0.20 2.42 NL-591 9.1 24 2.7 2.6 2.7 2.6 ± 0.1 16.0 ± 1.1 0.76 3.35 NL-593 11.7 24 3.6 3.4 3.5 3.5 ± 0.2 25.4 ± 2.0 0.61 2.73 NL-594 12.2 23 3.4 3.1 3.1 3.3 ± 0.2 21.6 ± 1.7 2.35 8.21 NL-595 13.1 25 3.2 3.2 3.1 3.2 ± 0.1 17.5 ± 1.2 0.37 2.11 NL-596 13.6 27 2.6 2.1 2.1 2.4 ± 0.2 40.7 ± 3.8 2.23 7.50 NL-597 14.7 26 3.0 2.9 3.3 2.9 ± 0.2 29.7 ± 2.4 0.64 3.29 NL-599 18.4 21 3.2 3.3 2.7 3.3 ± 0.1 13.2 ± 0.9 –0.19 3.81 NL-603 22.1 26 3.1 3.2 3.4 6.6 ± 0.3 25.3 ± 1.4 0.35 3.42 NL-604 23.3 30 5.4 5.6 5.8 5.2 ± 0.3 27.2 ± 1.9 –0.18 2.39 NL-606 25.0 25 6.5 6.2 6.5 6.5 ± 0.3 18.4 ± 1.3 0.47 2.56 NL-607 26.1 37 5.6 5.4 5.9 5.5 ± 0.2 25.0 ± 1.5 0.55 2.48 NL-608 26.2 29 6.9 6.6 6.9 6.8 ± 0.2 15.0 ± 0.8 0.74 3.05 NL-609 28.6 24 6.6 6.1 6.8 6.3 ± 0.4 32.2 ± 2.9 0.66 3.82 NL-610 28.9 29 5.1 5.2 5.8 4.8 ± 0.3 32.3 ± 2.7 0.20 2.28 NL-611 29.6 20 5.2 4.5 5.1 4.9 ± 0.3 30.8 ± 2.8 0.88 2.51 NL-612 30.3 32 7.5 7.5 7.3 7.3 ± 0.3 24.9 ± 1.6 1.16 4.36 NL-614 32.2 24 9.0 6.5 9.0 7.8 ± 0.7 46.4 ± 4.6 2.67 9.61 NL-616 34.5 24 10.7 9.2 9.7 9.3 ± 0.9 45.4 ± 4.5 3.48 15.61 NL-618 36. 6 27 9.5 9.2 10.1 9.1 ± 0.5 29.9 ± 2.3 0.91 3.58 NL-619 37.5 32 7.3 6.7 7.8 7.1 ± 0.3 24.0 ± 1.5 1.60 4.96 NL-620 38.5 24 8.9 8.2 9.3 8.6 ± 0.5 26.9 ± 2.1 1.45 4.59 NL-622 41.3 22 15.9 12.0 14.8 13.8 ± 1.5 50.7 ± 5.5 1.71 5.13 NL-626 44.4 23 13.2 10.3 14.4 11.8 ± 1.1 44.0 ± 4.3 1.50 4.81 NL-628 46.5 25 15.3 13.9 16.0 13.3 ± 1.4 52.9 ± 5.5 1.01 2.99 NL-632 50.3 28 56.2 53.3 74.8 51.3 ± 3.0 43.8 ± 2.7 0.37 2.18
The concentrations of U, Th and K elements, water content, total dose rates and OSL ages for all 28 samples are summarized in
CG quartz OSL dating results of the 28 samples.Sample ID Depth (m) U (ppm) Th (ppm) K (%) Water content (%) Total dose rate (Gy/ka) Mean age (ka) CAM age (ka) NL-588 6.4 1.84 ± 0.08 8.71 ± 0.26 1.50 ± 0.05 20.3 ± 10 2.0 ± 0.2 1.2 ± 0.1 1.1 ± 0.1 NL-589 6.9 2.12 ± 0.08 10.40 ± 0.29 1.44 ± 0.05 20.5 ± 10 2.0 ± 0.2 0.9 ± 0.1 0.9 ± 0.1 NL-590 7.8 1.85 ± 0.08 9.38 ± 0.27 1.61 ± 0.05 22.9 ± 10 2.0 ± 0.2 1.5 ± 0.2 1.5 ± 0.2 NL-591 9.1 1.98 ± 0.08 10.10 ± 0.28 1.74 ± 0.05 24.6 ± 10 2.1 ± 0.2 1.3 ± 0.2 1.3 ± 0.2 NL-593 11.7 2.08 ± 0.08 12.20 ± 0.34 2.06 ± 0.06 29.0 ± 10 2.3 ± 0.3 1.6 ± 0.2 1.5 ± 0.2 NL-594 12.2 2.06 ± 0.08 11.1 ± 0.31 1.69 ± 0.05 23.7 ± 10 2.1 ± 0.3 1.6 ± 0.2 1.5 ± 0.2 NL-595 13.1 2.27 ± 0.09 13.01 ± 0.34 2.04 ± 0.06 26.1 ± 10 2.4 ± 0.3 1.3 ± 0.2 1.3 ± 0.2 NL-596 13.6 2.46 ± 0.09 14.80 ± 0.39 2.31 ± 0.06 30.5 ± 10 2.5 ± 0.3 1.0 ± 0.2 0.9 ± 0.1 NL-597 14.7 1.91 ± 0.08 10.01 ± 0.27 1.74 ± 0.05 20.7 ± 10 2.2 ± 0.3 1.4 ± 0.2 1.3 ± 0.2 NL-599 18.4 1.61 ± 0.07 8.92 ± 0.26 1.53 ± 0.05 19.7 ± 10 1.9 ± 0.2 1.7 ± 0.2 1.7 ± 0.2 NL-603 22.1 1.96 ± 0.08 9.36 ± 0.27 1.82 ± 0.05 21.2 ± 10 2.2 ± 0.3 3.3 ± 0.5 3.1 ± 0.4 NL-604 23.3 1.73 ± 0.07 8.68 ± 0.26 1.74 ± 0.05 20.7 ± 10 2.1 ± 0.3 2.6 ± 0.3 2.5 ± 0.3 NL-606 25.0 1.54 ± 0.07 8.01 ± 0.24 1.72 ± 0.05 19.4 ± 10 2.0 ± 0.2 3.3 ± 0.4 3.2 ± 0.4 NL-607 26.1 2.80 ± 0.10 13.40 ± 0.36 1.5 ± 0.05 18.8 ± 10 2.4 ± 0.3 2.5 ± 0.3 2.3 ± 0.3 NL-608 26.2 1.86 ± 0.08 10.60 ± 0.29 1.6 ± 0.05 20.0 ± 10 2.1 ± 0.3 3.3 ± 0.4 3.3 ± 0.4 NL-609 28.6 1.47 ± 0.07 7.74 ± 0.24 1.74 ± 0.05 18.8 ± 10 2.0 ± 0.2 3.3 ± 0.5 3.1 ± 0.4 NL-610 28.9 1.27 ± 0.06 6.61 ± 0.21 1.82 ± 0.05 14.7 ± 10 2.1 ± 0.3 2.4 ± 0.3 2.3 ± 0.3 NL-611 29.6 1.19 ± 0.06 6.38 ± 0.21 1.68 ± 0.05 19.9 ± 10 1.8 ± 0.2 2.8 ± 0.4 2.7 ± 0.4 NL-612 30.3 1.71 ± 0.07 9.30 ± 0.26 1.87 ± 0.05 25.1 ± 10 2.0 ± 0.2 3.7 ± 0.5 3.6 ± 0.5 NL-614 32.2 1.87 ± 0.08 9.02 ± 0.27 1.66 ± 0.05 22.3 ± 10 2.0 ± 0.2 4.6 ± 0.9 3.9 ± 0.6 NL-616 34.5 2.09 ± 0.08 8.32 ± 0.25 1.71 ± 0.05 18.0 ± 10 2.1 ± 0.3 5.0 ± 1.1 4.3 ± 0.7 NL-618 36.6 1.50 ± 0.07 8.05 ± 0.24 1.72 ± 0.05 17.4 ± 10 2.1 ± 0.3 4.6 ± 0.6 4.4 ± 0.6 NL-619 37.5 2.15 ± 0.08 8.71 ± 0.26 1.73 ± 0.05 20.7 ± 10 2.1 ± 0.3 3.5 ± 0.5 3.4 ± 0.4 NL-620 37.5 2.30 ± 0.09 11.90 ± 0.33 1.89 ± 0.05 21.1 ± 10 2.4 ± 0.3 3.7 ± 0.5 3.6 ± 0.5 NL-622 41.3 2.23 ± 0.08 11.50 ± 0.32 1.88 ± 0.05 23.3 ± 10 2.3 ± 0.3 7.0 ± 1.3 6.1 ± 1.0 NL-626 44.4 1.70 ± 0.07 8.50 ± 0.26 1.87 ± 0.05 20.8 ± 10 2.1 ± 0.3 6.2 ± 1.0 6.0 ± 1.0 NL-628 46.5 2.22 ± 0.08 11.20 ± 0.31 1.71 ± 0.05 18.3 ± 10 2.3 ± 0.3 6.6 ± 1.1 6.0 ± 1.0 NL-632 50.3 1.76 ± 0.08 9.80 ± 0.28 1.83 ± 0.05 21.5 ± 10 2.1 ± 0.3 26.2 ± 3.5 23.9 ± 3.2
The reliability of the CG quartz OSL ages is illustrated in the following. First, we evaluate the intrinsic characteristics of luminescence. As described in Section 4 (OSL pretests and luminescence characteristics), the quartz grains had fast-dominated OSL signals. Furthermore, they produced ideal growth curves, satisfied the recycling ratio (0.9–1.1) and recuperation (<5%), as well as the expected recovery ratio, and they produced good dose recovery curves up to 70 Gy. These luminescence characteristics provide a robust warranty for CG quartz OSL dating. Furthermore, for each sample, the De values derived from different statistical models were generally consistent, indicating the samples might have been well bleached before deposition. Therefore, we concluded that optical dating could be considered reliable for establishing the chronology of the studied core.
Second, the dose rate is another major factor that could directly affect the accuracy of the OSL ages. Especially for water-laid sediments, dose rate estimation is always complex because of the uncertainties of the disequilibria in the uranium decay series, cosmic contribution and water content variation. These uncertainties might cause large deviations for realistic dose rates. When OSL samples are collected from marine, coastal beach or deep lake environments containing plenty of organic matter and precipitated carbonates, the likelihood of disequilibria in the uranium decay series should be considered (Olley
Third, the OSL chronology was compared with 10 independent 14C ages obtained using the accelerator mass spectrometry (AMS) dating technique (unpublished data). The 10 AMS 14C ages were dated using different materials from the sediments in the YZ07 core,
Based on all 28 OSL ages, the age–depth relationship of the uppermost 50 m of core YZ07 was established (
According to Hori
We noted that the age–depth pattern and its sedimentary implications in the present study were very common when compared with other deltas in the marginal seas of the western Pacific Ocean, especially the coastal and deltaic sedimentary environments of adjacent areas. For example, in the eastern part of the Yellow sea, as reported by Kim
This study tested the applicability of the quartz OSL dating technique to the coastal sediment sequence of core YZ07, retrieved from the southwestern coast of the SYS, on the northern flank of the Yangtze River delta. The OSL characteristics indicate that using CG quartz as a dosimeter is suitable for the SAR protocol. The reliability of the OSL ages was demonstrated by both intrinsic luminescence features and comparison with independent ages derived from 10 AMS 14C samples. Finally, a 24-ka sedimentary record for the uppermost 50 m of the YZ07 core was reconstructed.
This sedimentary sequence recorded the relationship of the Yangtze River delta evolution with its depositional accumulation history since the LGM. Because of the relatively low sea level before the Holocene, the decline of the base level of the palaeo-Yangtze River stream and strengthened erosion could have caused the formation of the incised valley in the study area, which might have contributed to a depositional hiatus from ~24 to 8 ka. Subsequently, with the sea level rise during the early Holocene, a significant transgression occurred in the study area at ~8 ka. The incised valley was first filled as the depocenter migrated landward. Thereafter, during the process of aggradation and progradation, the successive occurrence of estuarine sand bars and abandoned channels, constituting a subdelta, gradually merged and formed the Yangtze River delta. Consequently, our study core revealed a history of rapid sediment accumulation of the Yangtze River delta since 8 ka. Based on the age– depth results, we calculated the accumulation thickness was up to ~45 m and the sedimentation rate was ~6 m/ka from the mid- to late Holocene (~8 to 1 ka).