Sedimentary Characteristics of Relict Deposits on the Western South Yellow Sea

WEN Chun, LIU Jian,, WANG Hong, XU Gang, QIU Jiandong, and ZHANG Junqiang

1) College of Marine Geo-science, Ocean University of China, Qingdao 266100, P. R. China

2) Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Ministry of Land and Resources, Qingdao 266071, P. R. China

3) Qingdao Institute of Marine Geology, Ministry of Land and Resources, Qingdao 266071, P. R. China

Sedimentary Characteristics of Relict Deposits on the Western South Yellow Sea

WEN Chun1),2),3), LIU Jian2),3),∗, WANG Hong3), XU Gang3), QIU Jiandong1),3), and ZHANG Junqiang1),3)

1) College of Marine Geo-science, Ocean University of China, Qingdao 266100, P. R. China

2) Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Ministry of Land and Resources, Qingdao 266071, P. R. China

3) Qingdao Institute of Marine Geology, Ministry of Land and Resources, Qingdao 266071, P. R. China

Integrated studies of vertical sedimentary sequences, grain sizes, and benthic foraminifera and ostracoda, in combination with AMS14C dating, and210Pb and137Cs analysis were carried out in three vibracores taken from the area of relict deposits on the western South Yellow Sea. The relict sands, which are about 0.4 m thick, overlie on the Early Holocene coastal marsh or tidal flat deposits with an evident erosional interface in between. The middle and upper parts or sometimes the whole of the relict sands have been reworked under the modern dynamic environment. The sedimentation rate varies between 0.20–0.30 cm year−1. The relict sands show a bimodal grain-size distribution pattern in frequency curves, with a sharp peak in the coarse fraction between 3Φ and 4Φ and a secondary peak in the fine fraction of about 7Φ. Of the benthic foraminiferal and ostracod assemblages, the reworked relict sands are characterized by the mixing of the nearshore euryhaline shallow-water species and deeper water species. The erosional interface at the bottom of the relict sands is considered as a regional ravinement surface formed during the transgression in the Early Holocene due to shoreface retreating landwards. The relict sands were accumulated on the ravinement surface during the transgression in the deglaciation period as lag deposits after winnowing and reworking by marine dynamic processes. And the secondary peak of fine fraction in the frequency curve for the relict sands suggests the input of fine-grained sediments during the reworking process. As the conclusion, the relict sands in the study area are interpreted as a type of reworked relict sediments.

relict sediments; sedimentary characteristics; deglaciation period; western South Yellow Sea

1 Introduction

In 1930s, Shepard (1932) discovered that some coarsegrained sediments on the outer shelf of East Asia, which are obviously not in balance with the in situ current dynamic conditions; he, therefore, inferred that these sediments were formed in a littoral environment during the low sea-level period of the last glaciation. Emery (1952) was the first to use the term of ‘relict sediments’ to explain the origin of the medium-coarse brown sands coarser than any other types of sediments on the continental shelf in the southern California, USA; he considered them as the late Pleistocene sheet-like sands exposed at the seafloor and, however, they have not yet been covered by modern sediments. In fact, the relict sediments, when they deposited during the sea level falling in the last glaciations, synchronized well with the environmental conditions then; however, they could not catch up with the succeeding change of environment. Although they still outcrop at the seabed, they are out of balance with the modern environments (Emery, 1968). In contrast to the relict sediments, there are ‘modern sediments’ on the shelf, which are formed in the modern environments after postglacial sea level reached its highest level (Milliman et al., 1987; Guo et al., 1983). Relict sediments have been reworked to a certain degree under the modern dynamics, thus Swift et al. (1971) called them ‘palimpsest sediments’, as the subsequent reworking has made them to have the attributes of both the ancient and modern environments.

Over six decades have passed since the study of relict sediments started in the shelf areas of China seas. The researches have mainly focused on the distribution patterns and the environmental interpretations of relict sediments (Shepard et al., 1949; Niino and Emery, 1961; Qin, 1963; Qin and Zhao, 1985; Chen, 1997; Wang et al., 2002; Xiao et al., 2006; Liu, 1987); they summarized the sedimentary characteristics of relict sediments on the continental shelves of China and pointed out that these relict deposits covering about 30% of the shelf area were formed during 7500–15000 yr BP.

Reports are available on relict sands in the Haizhou Bay, the western South Yellow Sea (Liu, 1987; Liu, 1982; Wang, 1982); however, these reports are only limited to the description of sedimentary characteristics of the surficial relict sands at the sea bottom. The age and subsequent reworking remain to be further studied. Through the analyses of vertical sequences, grain sizes and chronology for the three vibracores from the western South Yellow Sea, this paper aims at revealing sedimentary environment, age and subsequent reworking of relict sands in this area for better understanding their formation and reworking process.

2 Geologic Environment and Geographic Positions of the Vibracores

The Yellow Sea is a typical epicontinental sea located between the Chinese mainland and Korea Peninsula. The water circulation system consists of the Yellow Sea warm current and several coastal currents. The western South Yellow Sea, that under the joint action of the low- temperature Yellow Sea Coastal Current, Southern Shandong (Lunan) Coastal Current, and Northern Jiangsu (Subei) Coastal Current, is located nearshore within the water depth of 20–30 m and it is also driven by the diluted water from the mixture of river water and nearshore seawater. The Yellow Sea Coastal Current, as a compensation for the Yellow Sea Warm Current, flows in the region of 40– 50 m water depth perennially southwards (Tang et al., 2000; Zang et al., 2001) (Fig.1). Modern sediments on the western South Yellow Sea are primarily supplied by the Yellow River, partly from the materials of modern Yellow River dispersed by the Yellow Sea Coastal Current and partly from the resuspension of the old Yellow River delta in the northern Jiangsu coast (Lan and Shen, 2002; Meng et al., 2000; Xu et al., 2010).

Fig.1 Schematic map of the bathymetry and regional circulation pattern in the Yellow Sea and adjacent areas during wintertime (modified after Su, 1986, and Guan, 1983). Water depth is in meters. The dashed square indicates the scope of Fig.2. SYS, South Yellow Sea; NYS, North Yellow Sea; BS, Bohai Strait; KC, Kuroshio Current; YSWC, Yellow Sea Warm Current; TC, Tsushima Current; TWC, Taiwan Warm Current; YSCC, Yellow Sea Coastal Current; SKCC, South Korean Coastal Current; SSCC, South Shandong Coastal Current; NJCC, North Jiangsu Coastal Current; CDW, Changjiang Diluted Water; ECSCC, East China Sea Coastal Current.

It is known from the latest geological investigation (Fig.2) (Xu et al., 2010) that the shelf area off the northern Jiangsu coast in the western South Yellow Sea, mainly consisting of the large-scale subaqueous Yellow River delta formed during the 1128–1855 AD, is a major supplier of the sediments of silt, sandy silt and mud. To the north, east and northeast of the subaqueous delta, however, the large patches of silty sand or local sand occurred. These sandy sediments were regarded as relict deposits (Liu, 1982). And to the northwest of the abandoned delta, some coarse-grained sediments occur, such as gravels and muddy gravels. They are interpreted in this paper as the palimpsest sediments deposited during the last glaciation.

Three vibracores were collected by Qingdao Institute of Marine Geology during the geologic surveys from Aug. to October, 2008. They are located in a silty sand area on the middle shelf (water depth: 20–50 m) of the western South Yellow Sea (Table 1, Fig.2).

Fig.2 Map of Surface sediments (Xu et al., 2010) with vibracore locations on the western South Yellow Sea.

Table 1 Geographic position of the vibracores from the western South Yellow Sea

3 Sample Analysis and Methods

During the lab analysis, each of the three vibracores (SYSZ-12, SYSZ-31 and SYSZ-17) was split in half lengthwise, half for preservation and another half for observation, description and sampling. Samples were taken every 1–2 cm for grain size and210Pb/137Cs analyses. Eight and three samples were taken from vibracores SYSZ-12 and SYSZ-17 for14C dating, respectively.

Grain sizes were analyzed by the Experiment and Testing Center of Qingdao Institute of Marine Geology. Grain size was measured with a Malvern Mastersizer 2000 laser particle size analyzer (Malvern Instruments Ltd., UK) with a measuring range of 0.02–2000 μm, a deviation of <1% and a repeatability of Φ50<1%. Pretreating of the samples was made with 10% H2O2and 0.1 N HCl to remove organic matter and biogenic carbonate, respectively. Mean grain sizes were calculated by moment statistics (McManus, 1988).

Microfossils (benthic foraminifera and ostracoda) are identified for vibracore SYSZ-12. 50 g of dried samples were taken as the unit for quantitative statistics. The sample is sieved and washed using a standard copper sieve of 250 mesh (0.063 mm). The sample left in the sieve were dried and then put in carbon tetrachloride for concentration of foraminifera. In case it is rich in foraminifera and ostracoda, the sample after sieving would be split at a ratio of 1/2, until the amount is satisfied for identification and statistics. If the sieved sample is less than 50 g, the bulk sample must be taken into account.

AMS14C datings of the shells from vibracores SYSZ-12 and SYSZ-17 were measured by Beta Analytic Inc. using accelerator mass spectrometry (AMS) and 11 dates were obtained (Table 2). Directly dated ages adopt 5568 year as half life; and δ13C value of the sample is also measuredsimultaneously. After correcting the fractionation effect, we obtained conventional ages. Calendar ages were from the conventional ages after the correction by CALIB5.0.1 (Stuiver et al., 2005). The regional marine reservoir effect (∆R) is determined from the two dating data from thewestern (Map No. 416) and eastern South Yellow Sea (Map No. 417) supplied by CALIB5.0.1, and the acquired∆R mean value is −100±36 yr. Ages calibrated in this manuscript are reported as calendar14C ages before AD 1950 (cal yr BP).

Table 2 AMS14C dating results from vibracores SYSZ-12 and SYSZ-17

210Pb,226Ra and137Cs of the SYSZ-12, SYSZ-31 and SYSZ-17 core samples were analyzed using the BE3830 γ-spectrometer (made in Canberra Company of USA) in the laboratory of Qingdao Institute of Marine Geology, following a procedure similar to that of Xia et al. (2011). The sample was ground into powder after dried at low temperature and weighed after compaction in a standard measuring apparatus. Then put the sample into an airtight lead chamber to measure the specific activities of210Pb,226Ra and137Cs nuclides. Excess210Pb specific radioactivity (210Pbex) was the difference between210Pb specific radioactivity and226Ra specific radioactivity. Clay content was used to eliminate the effect of grain sizes on210Pbex(Palinkas et al., 2006; Fan et al., 2000). Sedimentation rates were calculated by use of210Pb constant sedimentary energy model (CIC) (Oldfield, 1978).

4 Results

4.1 Sedimentary Characteristics and Age of Vibracore SYSZ-12

According to lithologic characters and contact relationship between the sedimentary units, vibracore SYSZ-12 can be divided into the upper (0–44 cm) and the lower (44–210 cm) segments by an evident erosional interface (Fig.3).

Fig.3 Sedimentary succession, the changes of grain-size and dating data of vibracore SYSZ-12 in depth.

The lower segment (44–210 cm) dark gray clayey silt-sandy silt intercalating fine sand-silt lenses or laminas with lenticular and flaser bedding, rich in shell fragments, intensively bioturbated, and with wood debris occasionally. Generally, the grain sizes of sediments are fining up wards with the decreasing of fine sand-silt lenses or laminas. From 210 cm upwards to 133 cm, the mean of grain size reduces from 4.6 to 7.0 Φ. Further upwards, the mean of grain size remains stable, roughly fluctuating around 6–7 Φ. The abundance of benthic foraminifera fluctuates between 2424 and 8128, with 5727 on average. Simple diversity varies from 11 to 22, averaging 14.9 (Fig.4). Benthic foraminifera are dominated by euryhaline nearshore shallow water species represented by Ammonia beccarii (Linné) vars. and Elphidium magellanicum Heron-Allen and Earland. A. beccarii var. gradually decreases upwards from 80% in content at the bottom to about 40% at the top of the sediments, while E. magellanicum ranges roughly from 1.5% to 9.5% fluctuating around a mean of 4.8%, but increases in the top part from about 70 to 44 cm. At the top of the segment (about 70–44 cm), inner- or middle-shelf representative species of benthic foraminifera, such as Ammonia compressiuscula (Brady), Protelphidium tuberculatum (d’Orbigny) and Ammonia ketienziensis (Ishizaki) appear for the first time or increase evidently in content. The abundance of ostracoda varies between 344 and 1968 with 1048.4 as the average, and the simple diversity changes from 5 to 11, averaging 7.1 (Fig.4). The ostracoda assemblage is dominated by euryhaline nearshore shallow water species or inner-shelf common species represented by Sinocytheridea impressa (Brady) and Keijella bisanensis (Ocubo) respectively.

Five AMS14C ages vary from 10227 to 10764 cal yr BP. in this segment. The characteristics of sediments suggest that the lower segment was deposited in the littoral marsh, tidal flat and shallow subtidal environments during the early Holocene, with water depth increasing ascendingly.

Fig. 4 Distribution pattern of foraminifera (a–h) and ostracoda (i–o) in vibracore SYSZ-12.

The upper segment (0–44 cm): dark yellowish gray silty sand. The mean of grain size generally coarsens upwards; the content of sand is gradually increased and the contents of silt and clay are decreased. According to the grain size variation, the upper segment can be further divided into two layers. The lower layer (30–44 cm) is poorly sorted and comprised of silty sand that is rich in shell fragments and that has an erosional surface as the boundary with the underlying segment. The mean of grain size changes between 4.25–4.81 Φ. The upper layer from 0 to 30 cm is poorly sorted and composed of silty sand with few shell fragments. The mean of grain size varies between 4.34–4.66 Φ. The abundance of benthic foraminifera in the lower layer (16064 on average) is significantly higher than that either in the lower segment or in the upper layer (6915 on average) of the upper segment.The simple diversity (averaging 21.8) is slightly higher than that in the lower segment but lower than that in the upper layer (28.5 as the average value) of the upper segment. In the lower layer, benthic foraminifera are characterized by the mixing of A. beccarii vars. (30.7% on an average) and E. magellanicum (14.8%) of euryhaline nearshore shallow water species, A. compressiuscula (10.6%) of inner-shelf common species, P. tuberculatum (8.2%) of nearshore-inner shelf psychrophilic common species, and Epistominella naraensis (Kuwano) (15.1%) of estuarine shallow water species. On the other hand, in the upper layer, there is an evident decrease in A. beccarii vars. (4.3%) and A. compressiuscula (2.7%) and obvious increase in P. tuberculatum (19.3%) and A. ketienziensis (Ishizaki) (7.2%, a middle-shelf representative species). E. magellanicum (19.4%) and E. naraensis (9.1%) are also high in content. Therefore, it is unmistakable that in the upper layer the euryhaline nearshore shallow water species of benthic foraminifera have been mixed with deeper water species (inner- and middle-shelf common species). The abundance of ostracoda (128 on the average) in the upper segment is evidently lower than that in the lower segment; however the simple diversity (11.9 as the mean) is clearly opposite. The lower layer of the upper segment differs from the upper layer in the domination of ostracod species. The K. bisanensis (38.8%), S. impressa (8.2%) and Munseyella japonica (Hanai) (22.2%, an inner-shelf common species) occur mainly in the lower layer, while M. japonica (31.1%) and the inner- and middle-shelf common species, such as M. pupilla Chen and Kobayashiina donghaiensis Zhao (8.2%), dominate in the upper layer; in addition, the S. impressa (16%) is also high. The ostracod assemblage is similar to the benthic foraminifera that euryhaline nearshore shallow water species coexist with relatively deep water species (inner- and middleshelf common species) in the upper layer.

The frequency curve of grain size of the upper segment is always in a bimodal manner (Fig.5). The main peak is very sharp roughly between 3 and 4 Φ, with kurtosis value generally over 3 and the second peak is quite low and occurs at about 7 Φ. Since the skewness value is more than 1, thus, the major grain sizes (main peak) are inclined to the coarse grain side with a low tail on the fine grain side. The grain size distribution pattern indicates a coastal environment intensively influenced by sediment dynamics, which is obviously different from the modern environment in which the sediment now occurs. The fine grained part corresponding to the second peak in the frequency curve is probably an allochthonous material which was added into the original sediments during the subsequent reworking process.

Dating data reveal that the sediment at 32 cm was deposited at 10398 cal yr BP; the sediment at 40 cm was deposited at 10550 cal yr BP; however, the sediment in the middle of the segment (23 cm) is only 7638 cal yr BP in age.

From the dating data mentioned above, the sediments in the lower layer (30–44 cm) of the upper segment were deposited in the early Holocene. Both the depositional and lithologic features of the lower layer (silty sand rich in shell fragments) indicate a sandy coastal origin; the upper layer (0–30 cm) has the mixture of coastal and inner-shelf sediments, which is possibly the result of subsequent (modern) reworking.

Fig.5 Frequency curves of sandy sediments in the upper segment of vibracore SYSZ-12.

4.2 Sedimentary Characteristics of Core SYSZ-31

Same as vibracore SYSZ-12, the strata of vibracore SYSZ-31 could also be divided into upper and lower segments, with an erosional interface in between (Fig.6c). Samples from 0–100 cm were selected for analysis.

The lower segment (41–100 cm): dark gray clayey silt, with mean of grain size mostly between 5.5 and 7.5 Φ (Figs.6a–b), rich in shell fragments and wood debris. Similar in lithologic character to the lower segment of vibracore SYSZ-12, this segment is regarded as the coastal marsh and muddy tidal-flat sediment.

The upper segment (0–41 cm): poorly sorted grayish yellow silty sand. The deviation of grain size is small, with the mean grain sizes of 4.18–4.91 Φ (Figs.5a–b). It contains abundant shell fragments. The lithology of the segment is similar to the upper segment of vibracore SYSZ-12, thus it is considered as a sort of coastal sandy sediment formed in the early Holocene and reworked later in the modern environment.

4.3 Sedimentary Characteristics and Age of Core SYSZ-17

Vibracore SYSZ-17 also has the upper and lower segments separated by an erosional boundary (Fig.6d).

Lower segment (37–100 cm): dark gray clayey silt, with silty-fine sandy lenses (Fig.6d). It contains few shell fragments and is weak in bioturbation.

Upper segment (0–37 cm): poorly sorted grayish yellow silty sand (Fig.6d). There are some 1–2 mm thick gray clay bands in its lower part (15–37 cm) bearing shell fragments,and the upper part (0–15 cm) contains more shell fragments with black carbonaceous spots on the top.

The low segment at 42 cm is dated to 9778 cal yr BP, while the bottom at 34 cm and the top at 11 cm of the upper segment are dated to 9440 and 177 cal yr BP, respectively.

Similar to vibracores SYSZ-12 and SYSZ-31, the lower segment of vibracore SYSZ-17 is considered as coastal marsh or muddy tidal-flat deposits of the early Holocene. The sandy sediment of the upper segment was formed in a coastal environment of the early Holocene as well, but its middle and upper parts have been reworked under modern conditions and thus show the younger age.

4.4210Pb and137Cs Geochronology

Fig.6 Grain sizes (a) and mean of grain size (b) of 0–50 cm of vibracore SYSZ-31, and geological columns of 0–100 cm of vibracore SYSZ-31 (c), and 0–100 cm of vibracore SYSZ-17 (d).

Fig.7210Pbexand137Cs sections of vibracore SYSZ-12 (a), vibracore SYSZ-31 (b) and vibracore SYSZ-17 (c) (210Pbex: solid black dot;137Cs: open circle).

The210Pb and137Cs profiles of vibracores of SYSZ-12, SYSZ-31 and SYSZ-17 are shown in Fig.7. The vertical changes in210Pbexof the top sandy sediment (0–44 cm) ofvibracore SYSZ-12 can be divided into three intervals. The top interval of 0–2 cm is the surface mixing layer (SML), in which the210Pbexhas approximately uniform distribution without obvious change due to bioturbation (Oldfield et al., 1978; Fan et al., 2000); the210Pbexin the middle interval of 2–25 cm is in decline logarithmically with the sedimentation rate of 0.23 cm year−1; and the lower interval from 25 cm downwards is assumed as the background of210Pbex. The137Cs anomaly is clear in the interval of 0–12 cm. If the first appearance of137Cs is assumed as the record of the nuclear test for hydrogen bomb in 1954 (Neill and Allison, 2005), then, the sedimentation rate of the interval from 0 to 12 cm is approximately 0.22 cm year−1, very close to the result calculated from the210Pbexchanges. It suggests that the two results are rather reliable.

The top sandy sediment (0–41 cm) of vibracore SYSZ-31 could be divided into two intervals according to the210Pbexchanges. The top interval of 0–9 cm is the surface mixing layer (SML); the210Pbexin the lower interval of 9–41 cm shows a logarithmical decline trend with a sedimentation rate of 0.30 cm year−1. No137Cs anomaly has been detected.

In the upper part of the sandy sediment (0–37 cm) of core SYSZ-17, the top 0–4 cm is obviously the deposits of the surface mixing layer (SML). From 4 cm down to 30 cm,210Pbexlogarithmically decreases downwards with the sedimentation rate of 0.30 cm year−1. Below 30 cm,210Pbexreach the background.137Cs is detected in the 0–9 cm sandy layer, and the sedimentation rate is 0.17 cm year−1, lower than the average value (0.30 cm year−1) of the 4–30 cm sandy sediment, but close to the sedimentation rate (0.19 cm year−1, 0–9 cm ) calculated by using210Pbex. It means that the two results are both credible.

5 Discussions

5.1 Age and Sedimentary Environment of Submarine Relict Sandy Deposits

The results of sedimentary facies analyses and AMS14C dating of three vibracores show that, in the study area, submarine sandy deposits directly overlie the early Holocene coastal marsh, muddy tidal-flat or shallow subtidal sediments as the result of early Holocene transgression. Previous studies of sedimentary environmental evolution in the late Quaternary in the western South Yellow Sea have suggested that, from the last glacial maximum to the early Holocene, the study area gradually transitioned from prevailed terrestrial environments to a shallow-water continental shelf environment through incised valley system, estuary, coast, and shallow sea (Liu et al., 2010). The depositional sequences of SYSZ-12 and SYSZ-17 reveal that coastal marsh, muddy tidal flat or shallow subtidal sediment started from approximately 44 m bpsl (below the present sea level) 9400–11000 cal yr BP basically in consistent with the position of the sea level in eastern China during the early Holocene (Liu et al., 2004). Then, with the sea level up rising, the western South Yellow Sea is gradually altered from the coastal marsh, tidal flat and nearshore to the shallow sea. The shoreface retreated landwards during the sea-level rise, resulting in a regional transgressive ravinement surface. The transgression induced marine sediment dynamics to winnow and rework the coastal sediments, leaving behind relatively coarse grain sandy deposits on the transgressive surface (Duncan et al., 2000; Xue et al., 2010) as the sheet-like deposits that revealed by the above vibracores. Such sheet-like relict sands are thin in thickness, generally less than 0.5 m in the study area and mostly over 1 m in other shelf areas of China (Liu, 1987). In principle, this transgressive surface is diachronous in the western South Yellow Sea. The sandy sediment on the surface should gradually become younger from east to west, roughly from as early as 13000 cal yr BP (Liu et al., 2010) up to about 7000 cal yr BP when the sea level reached its highest position. Vibracores SYSZ-12 and SYSZ-17 were located on the east side of the western shelf, and the ages for the bottom of the sandy sediment of transgression origin are about 9400–11000 cal yr BP, coordinated with the early Holocene transgression process in this area.

Like the western South Yellow Sea, the shelf areas of East China Sea and South China Sea are also partially covered by sandy sediments, which were formed during the post-glacial transgression and can also be considered as relict sediments (Liu, 1987).

5.2 Subsequent Reworking of Relict Sands

The post-glacial sea level reached its maximum around 7-6 cal kyr BP in the Yellow Sea and East China Sea (Liu et al., 2004). Afterwards, the sea level only had small fluctuations (under 5 m), and the sedimentary environment has remained stable (Liu et al., 2007). The Yellow Sea as well as the East China Sea entered ‘modern environment’ since the middle Holocene (7-6 cal kyr BP). Even though the sea level rose sharply during the postglacial period, the transgressive sandy sediments formed during the early Holocene on the western South Yellow Sea (largely in the northern side of the shelf in the water depths from >50 m to about 20 m) are still exposed and never covered by modern fine-grained sediment. In this regard, these sandy materials should belong to ‘relict sediment’. It, however, can be seen from210Pb analyses that the middle and upper parts (0–25 cm of vibracore SYSZ-12 and 0–30 cm of vibracore SYSZ-17) or the whole part (the entire top segment of vibracore SYSZ-31) of such thin-layered (about 40 cm thick) sandy sediments has been reworked under modern dynamics (Fig.8). And the distribution pattern of foraminiferal and ostracod assemblages suggests that the sediment of 0–30 cm of vibracore SYSZ-12 has been reworked; the sediments between 25–30 cm could be deposited prior to about 150 cal yr BP because the210Pbexin the same part shows a background value (Fig.7a). AMS14C dating results also support the explanation that the above-mentioned sandy sediments were subsequently reworked. For instance, the shell is aged to 7638 cal yr BP at the 23 cm of vibracoreSYSZ-12, evidently much younger than the sediment in the bottom of the sandy sequence (10550 cal yr BP). Therefore, the shell used for dating at the 23 cm might be an allochthonous component coming from another place during the reworking process. In vibracore SYSZ-17, the age of the shell at 11 cm is dated to 177 cal yr BP, much younger than the basal sandy sediments. Centainly, it is an allochthonous component brought in by subsequent reworking. The benthic foraminiferal and ostracod assemblages in the sandy layer in 0–30 cm of vibracore SYSZ-12 are characterized by the mixing of euryhaline nearshore shallow water species and relatively deep water species (common species in inner and middle shelf areas), which is another evidence that the sandy layer of vibracore SYSZ-12 has been reworked under modern conditions.

Relict sandy sediment in the western South Yellow Sea is mostly developed below the fair wave base about 20 m bpsl. We believe that storm wave was the main power to rework the relict sands, based on the fact that strong waves often lash over the western shelf area in the modern days. In this regard, such relict sands can be regarded as the ‘reworked relict sediments’.

Fig.8 Correlation of reworked sandy layers at the tops of vibracores of SYS-12, SYS-31 and SYS-17 (the reworked layers are above the red dashed line).

6 Conclusions

From the integrated study of sedimentary characteristics and the dating data of the three vibracores from the western South Yellow Sea, we come to the following conclusions:

1) The submarine relict sandy deposits are thin in thickness (about 0.4 m) and directly overlie the early Holocene coastal marsh or tidal flat sediment. Between the two sequences, there is an erosional interface, which is a regional transgressive ravinement surface formed due to shoreface retreating landwards with sea level rising duirng the early Holocene. During the transgression, due to the winnowing (or sorting) and reworking of the old coastal sediments, coarse-grained sands left behind and settled down onto the transgressive surface, and thus the relict sandy sediments formed.

2) Under modern environmental conditions, the middle and upper parts or sometimes the whole part of the relict sandy deposits were reworked by storm waves, and therefore they can be called the ‘reworked relict sediments’, which were formed at a rate of 0.20–0.30 cm year-1.

3) The size-frequency curve of the relict sands is bimodal in nature, with a sharp main peak in the coarse end between 3 and 4 Φ and a smooth peak in the fine end at about 7Φ. The smooth peak is interpreted to be brought in from outside during the reworking process in the modern environments.

Acknowledgements

This study was jointly funded by the National Natural Science Foundation of China (Grant Nos. 41330964 and 40876034) and by the China Geological Survey (Grant No. 1212010611401). We thank Profs. Qixiang He and Jianhua Ma for their help in the preparation of this manuscript.

Chen, F., 1997. A comparative study on depositional characteristics of the coastal sand and the continental shelf sand of Southeast China and its environmental significance. Quarter-nary Sciences, 17 (4): 365-375 (in Chinese).

Duncan, C. S., Goff, J. A., and Austin, J., 2000. Tracking the last sea-level cycle:seafloor morphology and shallow stratigraphy of the latest Quaternary New Jersey middle continental shelf. Marine Geology, 170: 395-424.

Emery, K. O., 1952. Continental shelf sediments off southern California. Bulletin of the Geological Society of America, 63: 1105-1269.

Emery, K. O., 1968. Relict sediments on continental shelves of the world. The American Association of Petroleum Geologists Bulletin, 52: 445-464.

Fan, D. J., Yang, Z. S., and Guo, Z. G., 2000. Review of210Pb dating in the continental shelf of China. Advance in Earth Science, 15 (3): 297-302 (in Chinese with English abstract).

Guan, B., 1983. A sketch of the current structures and eddy characteristics in the East China Sea. In: Proceedings of the International Symposium on Sedimentation on Continental Shelf with Special Reference to the East China Sea, China Ocean Press, Beijing, 52-73.

Guo, Y. S., Han, Y. S., Yang, G. F., and Zhang, M. H., 1983. The discussion on Shandong peninsula Pleistocene high-stand sea level. Acta Oceanologica Sinica, 5 (4): 480-489 (in Chinese).

Lan, X. H., and Shen, S. X., 2002. Geochemical characteristics of rare earth elements of sediment cores from the central South Yellow Sea. Marine Science Bulletin, 21 (5): 46-53 (in Chinese with English abstract).

Liu, J., Yoshiki, S., Kong, X. H., Wang, H., Wen, C., Yang, Z. G., and Nakashima, R., 2010. Delta development and channel incision during marine isotope stages 3 and 2 in the western South Yellow Sea. Marine Geology, 278: 54-76.

Liu, J., Yoshiki, S., Wang, H., Yang, Z. G., and Nakashima, R., 2007. Sedimentary evolution of the holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea. Marine Geology, 236: 165-187.

Liu, J. P., Milliman, J. D., Gao, S., and Cheng, P., 2004. Holocene development of the Yellow River’s subaqueous delta, North Yellow Sea. Marine Geology, 209: 45-67.

Liu, X. Q., 1987. Relict sediments in China continental shelf. Marine Geology and Quaternary Geology, 7 (1): 1-14 (in Chinese with English abstract).

Liu, Z. X., 1982. The distribution of surficial sediment in the Yellow Sea. Marine Science Bulletin, 1: 43-51 (in Chinese with English abstract).

McManus, J., 1988. Grain size determination and interpretation. In: Techniques in Sedimentology. Tucker, M., ed., Blackwell, Oxford, 63-85.

Meng, X. W., Du, D. W., and Wang, X. Q., 2000. Stepwise discriminant analysis used for discriminant sediment source of the south of the Yellow Sea. Geological Review, 46 (suppl.): 269-273 (in Chinese with English abstract).

Milliman, J. D., Broadous, J. M., and Gable, F., 1987. 1989 Environmental and economic impact of rising sea level an subsiding delta: The Nile and Bengal. Ambio, 18: 340-345.

Neill, C. F., and Allison, M. A., 2005. Subaqueous deltaic formation on the Atchafalaya Shelf, Louisiana. Marine Geology, 214: 411-430.

Niino, H., and Emery, K. O., 1961. The sediments of shallow position of South and East China Sea. Bulletin of the Geological Society of America, 72: 5.

Oldfield, F., Appleby, P. G., and Battarbee, R. W., 1978. Alternative210Pb dating: Results from the New Guinea Highlands and Lough Erne. Nature, 271: 339-342.

Palinkas, C. M., Nittrouer, C. A., and Walsh, J. P., 2006. Innershelf sedimentation in the Gulf of Papua, New Guinea: A mud-rich shallow shelf setting. Journal of Coastal Research, 22 (4): 760-772.

Qin, Y. S., 1963. Preliminary study on the sea floor topography and sediment distribution in Chinese shelves. Oceanologia et Limnologia Sinica, 5 (1): 71-86 (in Chinese).

Qin, Y. S., and Zhao, S. L., 1985. The sedimentary model and Quaternary transoressions in the continental shelf of China. Quaternary Sciences, 6 (1): 27-34 (in Chinese with English abstract).

Shepard, F. P., 1932. Sediments on the continental shelves. Bulletin of the Geological Society of America, 43: 1017-1034.

Shepard, F. P., Emery, K. O., and Gould, H. R., 1949. Distribution of Sediments on East Asiatic Continental Shelf. University of Southern California Press, Los Angeles, 64pp.

Stuiver, M., Reimer, P. J., and Reimer, R. W., 2005. CALIB 5.0.1. WWW program and documentation retrieved from http://calib.qub.ac.uk/calib/.

Su, Y.S., 1986. A survey of geographic environment, circulation system and the central fishing grounds in the Huanghai Sea and East China Sea. Journal of Shandong College of Oceanology, 16 (1): 12-28 (in Chinese with English abstract).

Swift, D. J. P., Stanley, D. J., and Curray, J. R., 1971. Relict sediments on continental shelves: A reconsideration. The Journal of Geology, 79: 322-346.

Tang, Y. X., Zou, E. M., Lie, H.-J., and Lie, J.-H., 2000. Some features of circulation in the southern Huanghai Sea. Acta Oceanologica Sinica, 22 (1): 1-16 (in Chinese with English abstract).

Wang, Z. H., Guo, Z. Y., and Chen, Z. Y., 2002. Relict sediment and its environmental implication in Pingbei Area, East China Sea continental shelf. Journal of East China Normal University (Natural Science), 1 (3): 81-86 (in Chinese with English abstract).

Wang, Z. Y., 1982. Discussion on the characteristics and origin of the relict sand of Western South Yellow Sea. Marine Geological Research, 2 (3): 63-70 (in Chinese with English abstract).

Xia, P., Meng, X. W., Yin, P., Cao, Z. M., and Wang, X. Q., 2011. Eighty-year sedimentary record of heavy metal inputs in the intertidal sediments from the Nanliu River estuary, Beibu Gulf of South China Sea. Environmental Pollution, 159: 92-99.

Xiao, S. B., Chen, M. H., Lu, J., Chen, Z., Xiang, R., Zheng, Y., Xie, Q., and Chen, L., 2006. New evidence for remnant deposits recorded by columnar sediments in the shelf of the Northern South China Sea. Marine Geology and Quaternary Geology, 26 (3): 1-5 (in Chinese with English abstract).

Xu, G., Liu, J., Wen, C., and Kong, X. H., 2010. Sedimentary characteristics and provenance of surficial sediments in the South Yellow Sea. Marine Geology and Quaternary Geology, 30 (4): 49-56 (in Chinese with English abstract).

Xue, C. T., Liu, J., and Kong, X. H., 2010. Preliminary study of Holocene Huaihe River delta on west coastal plain of Yellow Sea, China. Quaternary Sciences, 30 (5): 892-901 (in Chinese with English abstract).

Zang, J. Y., Tang, Y. X., Zou, E. M., and Lie, H.-J., 2001. The analysis of the circulation of the Yellow Sea. Chinese Science Bulletin, 46 (suppl.): 7-15 (in Chinese).

(Edited by Xie Jun)

(Received March 12, 2012; revised March 28, 2012; accepted July 25, 2012)

© Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

* Corresponding author. Tel: 0086-532-85755832

E-mail: liujian0550@vip.sina.com