Evolution of late Quaternary mud deposits and recent sediment budget in the southeastern Yellow Sea

Transcription

1 Marine Geology 17 (2) 271±288 Evolution of late Quaternary mud deposits and recent sediment budget in the southeastern Yellow Sea Soo-Chul Park a, *, Hyun-Hee Lee a, Hyuk-Soo Han a, Gwang-Hoon Lee b, Dae-Chol Kim c, Dong-Geun Yoo d a Department of Oceanography, Chungnam National University, Taejon , South Korea b Department of Oceanography, Kunsan National University, Kunsan , South Korea c Department of Exploration Engineering, Pukyung National University, Pusan South Korea d Petroleum and Marine Resource Division, Korea Institute of Geology, Mining and Materials, Taejon 35-35, South Korea Received 7 March 2; accepted 28 July 2 Abstract Analysis of high-resolution seismic re ection pro les and sediment samples has revealed the evolution and sediment budget of the southeastern Yellow Sea mud belt (SEYSM) along the southwestern Korean Peninsula. The SEYSM, up to 5 m thick, over 25 km long and 2±55 km wide, can be divided into three stratigraphic units (A1, A2, and B, from oldest to youngest). Unit A1, overlying the acoustic basement, comprises the northern part of the SEYSM. Unit A2 comprises the southern part of the SEYSM; much of unit A2 is exposed at the sea oor. Unit B completely covers unit A1 and pinches out southward. 14 C data suggest that evolution of each unit is closely related to the postglacial sea-level changes. Unit A1 consists of estuarine/deltaic or shallow-water muds deposited during the early to middle stage of postglacial sea-level rise (ca. 14,± 7 yr B.P.). Unit A2 corresponds to relict muds deposited during the last, deceleration stage of sea-level rise (ca. 7± 3.5 yr B.P.). Unit B consists of shelf muds deposited during the recent sea-level highstand (ca.,35 yr B.P.). Very low background activities of 21 Pb of the surface sediment of unit A2 suggest that the present-day sediment accumulation is negligible in the southern SEYSM. On the other hand, 21 Pb excess activity pro les in unit B yield an average sediment accumulation rate of 3.9 mm/yr, indicating active sediment accumulation in the northern SEYSM. The annual sink 3: 1 7 tons=yr of ne-grained sediment in unit B is about an order of magnitude greater than can be explained by the sediment input from the Korean rivers alone. We propose that reworking of unit A2 has provided large volumes of muds to unit B, resulting in excessive sediment accumulation in the northern SEYSM. Much of unit A2, in turn, is likely to have originated from erosion of unit A1 in the north. This rather unique erosional/depositional regime of the SEYSM is probably owing to the tidal and regional currents characteristic in the southeastern Yellow Sea. q 2 Elsevier Science B.V. All rights reserved. Keywords: Southeastern Yellow Sea; Mud belt; Late Quaternary evolution; Sediment budget 1. Introduction * Corresponding author. Tel.: ; fax: address: scpark@cnu.ac.kr (S.-C. Park). The Yellow Sea is a tectonically stable, postglacially submerged, shallow (,1 m water depth) epicontinental shelf surrounded by the Chinese mainland and Korean Peninsula (Fig. 1). The Yellow Sea is //$ - see front matter q 2 Elsevier Science B.V. All rights reserved. PII: S ()99-2

2 272 S.-C. Park et al. / Marine Geology 17 (2) 271±288 Huanghe River Shandong Peninsula Sand Muddy Sand Sandy Mud Mud KOREA Han River 38 o N CHINA Keum River CYSM Fig. 2 Youngsan River 35 o N OHDM Old Huanghe River SEYSM Cheju Island Yangtze River N 2 km 32 o N 12 o E 125 o E Fig. 1. Distribution pattern of sediment types in the Yellow Sea, showing the central Yellow Sea mud deposits (CYSM), the old Huanghe delta mud deposits (ODHM) and the southeastern Yellow Sea mud belt (SEYSM) (after Park and Khim, 1992). a depocenter of the clastic sediments derived from surrounding landmass. The major sediment sources are the Huanghe and Yangtze rivers, which annually discharge about 1:1 1 9 and 4:9 1 8 tons of suspended sediments, respectively (Schubel et al., 1984). A maximum of 1:6 1 8 tons of sediments, which is about 15% of the annual Huanghe River discharge, is annually accumulating in the Yellow Sea (Alexander et al., 1991). Approximately 6% of these sediments are deposited in the Shandong subaqueous delta; the remaining sediments are transported further southward and deposited in the central Yellow Sea (Fig. 1). Most sediments from the Yangtze River are transported southward through the East China Sea by the Jiangsu Coastal Current (Milliman et al., 1989). A number of small rivers draining the Korean Peninsula contribute less than 5: 1 6 tons of suspended sediments to the Yellow Sea annually (Schubel et al., 1984). The distribution of the ne-grained sediments in the Yellow Sea is characterized by three distinct zones: (1) the central Yellow Sea mud deposits (CYSM), (2) the old Huangehe deltaic mud patch (OHDM) along the Jiangsu coast of China; and (3) the southeastern Yellow Sea mud belt (SEYSM) along the southwestern Korean Peninsula (Park and Khim, 1992) (Fig. 1). Relative abundance of major clay minerals in surface sediments suggests that the provenance of the CYSM is closely related to the Huanghe River system whereas the SEYSM are largely associated with the Keum and Youngsan rivers in the west coast of Korea (Park et al., 1988; Park and Khim, 1992). The origin and sediment budget of the CYSM and OHDM are relatively well studied (Milliman et al., 1987; Alexander et al., 1991), while those of the SEYSM have remained poorly understood. Lee and Chough (1989) suggested that the amount of sediments accumulated in the SEYSM is tons=yr and the Keum River is the single most important source for the muds in the Holocene. Alexander et

3 S.-C. Park et al. / Marine Geology 17 (2) 271± KOREA Study Area Fig. 6c KOREA 35 O ' Fig. 6a Fig. 6b Fig. 7c Fig. 7a 34 O ' N Fig. 7b 5 km Airgun 3.5 khz Chirp Cheju Island 125 O ' 126 O ' 127 O ' Fig. 2. Track lines of high-resolution seismic survey on the southeastern Yellow Sea mud belt. Heavy lines denote position of pro les shown in Figs. 6 and 7. al. (1991), on the other hand, estimated that 4: :7 1 7 tons of sediments are deposited in the SEYSM annually. Deep cores drilled by the Korea Institute of Geology, Mining and Materials (KIGAM) (KIGAM, 1996) indicate that the SEYSM is as thick as 5 m and mainly consists of Holocene muds. Jin and Chough (1998) described the sequence stratigraphy of the transgressive deposits in the southeastern Yellow Sea. In the present study, we interpret high-resolution seismic re ection pro les (3.5 khz, chirp, and airgun pro les) (Fig. 2) to describe the acoustic character and to map the sediment thickness and distribution of the SEYSM. We also analyzed sediment cores (Fig. 3) from the SEYSM to estimate the sediment accumulation rates and sediment sink. We further discuss the evolution and depositional history of the SEYSM in relation to the late Quaternary sea-level changes. 2. Oceanographic setting The Yellow Sea is affected strongly by semi-diurnal tides and tidal currents (Choi and Fang, 1993; Fang, 1994). The tidal ranges are from 1.5 to 8 m with the maximum amplitudes found in the Kyonggi Bay, western central coast of Korea. Tidal current velocity is over 1 cm/s in the Kyonggi Bay and near the southwestern tip of the Korean Peninsula. It

4 274 S.-C. Park et al. / Marine Geology 17 (2) 271±288 3 YS-C19 6 YS-C18 KOREA 35 O ' 6 YS-C15 3 YS-C13 YSDP-13 3 YS-C N YS-C7 YS-C5 YS-C6 YS-C4 9 YSDP-12 5 km YS-C O ' Cheju Island 125 O ' 126 O ' 127 O ' Fig. 3. The extent of the SEYSM with core locations (dots) superimposed (solid lines, clear boundary; dotted lines, unidenti ed boundary). Open circles indicate the deep drill cores (YSDP-13 and YSDP-12) obtained by the Korea Institute of Geology, Mining and Material (1996) (see Fig. 4). Water depth in meters. exceeds 2 cm/s in many narrows along the western and southwestern coasts of Korea. The large tidal ranges and strong tidal currents along the west coast of Korea produce a complex and dynamic hydraulic regime in terms of sediment erosion and deposition, especially in the nearshore areas (Song et al., 1983; Adams et al., 199; Alexander et al., 1991; Wells and Park, 1992). Because of shallow water depth, wave action in the Yellow Sea is strong and important in redistributing coastal sediments derived from the rivers (Kang and Choi, 1984; Wells, 1988; Booth and Winters, 1991). The Yellow Sea is affected by the monsoon; the southerly and southwesterly winds are dominant in the summer and the northerly and northwesterly winds prevail in the winter. The northerly winds are persistent with an average speed of 8±9 m/s whereas the southerly winds are weaker and less persistent. Wells and Park (1992) suggested that the dominant sediment source to the west coast of Korea during the winter is the tidal ats; sediment in ux from the rivers is relatively low. During the summer, most

5 S.-C. Park et al. / Marine Geology 17 (2) 271± YSDP-13 C Z S G YSDP-12 C Z S G 4,2 _+ 53 (C) UNIT B 1 2,393 _+ 8 (A) 1 4,72 _+ 38 (C) Depth (m) 2 > 3, (C) 8,311 _+ 68 (A) 12,8 _+ 185 (C) 2 8,36 _+ 5 (C) 6,8 _+ 6 (A) UNIT A1 11,78 _+ 12 (A) 13,43 _+ 14 (A) UNIT A2 3 13,99 _+ 129 (C) 3 13,83 _+ 17 (A) 6,91 _+ 11 (A) laminated bioturbated sand layer 4 5 7,95 _+ 71 (C) 1,16 _+ 5 (C) 6,67 _+ 7 (A) 6,48 _+ 1 (A) 5,649 _+ 93 (A) 6,5 _+ 1 (A) 6,44 _+ 1 (A) Fig. 4. Lithology and 14 C dates (yr B.P.) of the KIGAM's two drill cores after KIGAM (1996); Jin and Chough (1998) (A, the dates measured by the accelerator mass spectrometry; C, the dates measured by conventional scintillation method). YSDP-13 is well laminated, whereas YSDP- 12 is generally bioturbated (C, clay; Z, silt; S, sand; G, gravel). The upper and lower sections of YSDP-13 are correlated with units B and A1, respectively, while the core section of YSDP-12 corresponds to unit A2 (see Fig. 5b). For core locations, see Fig. 3. ne-grained sediments derived from the rivers are deposited in the tidal ats and coastal bays. Northward owing currents dominate the circulation around the Korean Peninsula (Lie and Cho, 1994; Guan, 1994). They are the Yellow Sea Warm Current in the central Yellow Sea and the Tsushima Warm Current in the Korea (Tsushima) Strait. Southward owing currents are the Jiangsu Coastal Current along the Chinese coast and the Korean Coastal Current along the Korean coast. The Korean Coastal Current plays an important role in the southward transport of the ne-grained sediments derived from the rivers, especially during the winter (Wells, 1988). 3. Background 14 C data Fig. 4 shows the lithology of the KIGAM's two drill cores (YSDP-13 and 12, for location see Fig. 3) taken from the SEYSM, with a number of 14 C dates reported by the KIGAM (1996) and Jin and Chough (1998). The upper (±18.2 m) and lower (18.6± 32.4 m) sections of YSDP-13 correspond to units B and A1, respectively, and the core section of YSDP- 12 is correlated with unit A2 of our study (Fig. 5b). The 14 C ages were measured either by the conventional scintillation method or by the accelerator mass spectrometry (AMS). The ages from the

6 276 S.-C. Park et al. / Marine Geology 17 (2) 271±288 a) N KOREA 35 O ' N 34 O ' km S Cheju Island 125 O ' 126 O ' 127 O ' b) N S Depth (m) 5 A1 B YSDP-13 A2 YSDP O 'N 34 O 3'N 34 O 'N Fig. 5. (a) Isopach map showing the total thickness of the SEYSM (contour in meters). The heavy solid line denotes the N±S transact across the SEYSM as shown in (b). (b) The SEYSM is divided into three units (A1, A2 and B, from oldest to youngest). Units A1 and B comprise the lower and upper part of the northern SEYSM, respectively. Unit A2 occupies the southern SEYSM. YSDP-13 and YSDP-12 represent the KIGAM's two drill cores.

7 S.-C. Park et al. / Marine Geology 17 (2) 271± conventional method have generally larger error ranges than those from the AMS method. The dates measured from the conventional method are mainly based on organic materials (KIGAM, 1996); the larger error ranges are most likely due to reworking of sediments. The AMS dates were measured mainly on foraminifera tests. The 14 C dates in the lower section (unit A1) of YSDP-13 generally decrease upward. The sediment sample (31.6 m depth) overlying the basal sand layer was dated about 13,8 yr B.P. and that (18.4 m) below the boundary of units A1 and B about 83 yr B.P. The ages from the upper section (unit B) of YSDP-13 are much younger than those of the lower section (unit A1), suggesting a signi cant time gap or hiatus between units A1 and B. The 14 C dates of YSDP-12 include erratic age sequences at some core horizons; the age distribution is somewhat random from bottom to top. The AMS dates are between about 56 and 69 yr B.P., whereas the dates from the conventional method range from 47 to about 1,1 yr B.P. The age data of YSDP-12 suggest that unit A2 was deposited in a relatively short time interval. 4. Materials and methods We collected approximately 62 km of high-resolution subbottom pro les (Fig. 2) over the SEYSM in 1997 using a chirp pro ling system (Datasonics CAP- 6W) on board R.V. Tamyang of the Pukyong National University. Additional seismic re ection data include air-gun and 3.5-kHz seismic re ection pro les (Fig. 2) provided by the KIGAM. Precision depth data were simultaneously collected using an echo sounder (Simrad 5). Along with the seismic survey, sediment cores were collected at ten stations (Fig. 3) using a piston corer. A combination of GPS and radar navigation systems was used for shipboard navigation. Ship speed was maintained at about 7± 8 knots. The seismic re ection data were interpreted to de ne acoustic characteristics and thickness and distribution of the SEYSM. Time-to-depth conversion was done using a sediment sound velocity of 155 m/s (Kim et al., 1992). The cores were analyzed for texture, structure and sediment accumulation rates. The core liners were opened and split lengthwise into two halves. One half of each core was described visually. X-radiographs were taken on the sediment slabs cm to examine the sedimentary structures and lithology. Grain size analysis was conducted using standard sieves and a particle size analyzer (Sedigraph 5 ET) for sand and mud fractions, respectively. 21 Pb geochronologies were determined on selected cores following the technique outlined by Nittrouer et al. (1979). One-centimeter-thick subsamples were taken at a number of depths in each core and dried to determine porosity. About 5±1 g of dried samples were ground and spiked with a known amount of manmade 28 Po for yield determination. Then, the samples were leached and brought to dryness three times in the presence of concentrated HNO 3 and 6 N HCl. After being brought up to volume with dilute HCl, the solution was separated from the residual solids by centrifuging. The dissolved polonium isotopes were plated spontaneously onto a silver planchet for each sample. 21 Pb activity was determined at the Korea Basic Science Institute (KBSI) by measuring the alpha activity of its granddaughter, 21 Po with a siliconsurface-barrier detector coupled to a multi-channel analyzer. The 226 Ra activity was also determined throughout the core length by measuring the gamma ray emission of 214 Pb and 214 Bi in leached samples at the KBSI. 5. Results 5.1. Acoustic character of the SEYSM The SEYSM is over 25 km long and 2±55 km wide, occupying an area of more than 81 km 2 (Fig. 5a). The SEYSM extends toward the Cheju Island in the south; the northern limit is not clearly identi ed due to the lack of seismic pro les. Various seismic facies including parallel re ectors, prograding clinoforms, acoustic turbidity, transparent re ection pattern, and erosional surfaces are recognized in the SEYSM. The SEYSM is up to 5 m thick (Fig. 5a) and can be divided into three units (A1, A2, and B, from oldest to youngest) based on erosional surfaces or unconformities (Fig. 5b). Units A1 and B comprise the northern two-thirds of the SEYSM. Unit B completely covers

8 278 S.-C. Park et al. / Marine Geology 17 (2) 271±288 NE a SW 25 α-reflector Depth (m) 75 AB A1 β-reflector B 1km SSW b NNE 25 Depth (m) 75 β-reflector α-reflector AT AB B A1 1km SSW c NNE Depth (m) 25 B AT 75 1km Fig. 6. High-resolution seismic (chirp) pro les showing the SEYSM above the acoustic basement (AB) de ned by b-re ector (a, b). The SEYSM can be divided into lower (A1) and upper (B) units separated by a-re ector. Unit A1 is characterized by parallel and distinct re ections. Unit B is acoustically transparent (a) or indistinctly layered (b). Unit B is also characterized by the acoustic turbidity (AT) at shallow depths, which masks the underlying sedimentary layer (b, c). For pro le locations, see Fig. 2.

9 S.-C. Park et al. / Marine Geology 17 (2) 271± Fig. 7. High-resolution chirp (a, c) and air-gun (b) pro les showing unit A2, directly overlying the acoustic basement (AB) de ned by b- re ector. Unit A2 is characterized by inclined clinoforms and is exposed at the sea oor (AT, acoustic turbidity). For pro le locations, see Fig. 2. unit A1 and pinches out southward. The southern SEYSM is composed entirely of unit A2. Unit A2 partly covers the southern tip of unit A1. Units A1 and B are 1±2 m and 1±15 m thick, respectively. Unit A2 is up to 5 m thick. The mid-re ector (a) forms the boundary between units A1 and B and is the rst subbottom re ector of regional extent in the northern SEYSM (Fig. 6a and b). The lower boundary (re ector b) of unit A1 is irregular and characterized by a large acoustic-impedance contrast (Fig. 6a and b). It coincides with the acoustic basement and is exposed at the sea oor beyond the outer boundary of the SEYSM. Unit A1 is characterized by continuous, parallel to subparallel

10 28 YS-C1 YS-C4 YS-C5 YS-C6 YS-C7 C Z S C Z S C Z S C Z S C Z S YS-C11 YS-C13 YS-C15 YS-C18 C Z S C Z S YS-C19 C Z S C Z S C Z S Core length (cm) Sand Muddy sand Homogeneous mud S.-C. Park et al. / Marine Geology 17 (2) 271±288 Unit A laminated mud Shell Burrow Shell fragment Unit B Fig. 8. Lithologic description of cores based on X-radiographs and sediment texture (C, clay; Z, silt; S, sand). For core locations, see Fig. 3.

11 Unit A2 YS-C4 21 Pb Activity (dpm/g) YS-C5 21 Pb Activity (dpm/g) YS-C6 21 Pb Activity (dpm/g) YS-C7 21 Pb Activity (dpm/g) Core depth (cm) Unit B YS-C11 21 Pb Activity (dpm/g) Core depth (cm) S.R. =.39.7 (cm/yr) R 2 =.94 Total activity Excess activity YS-C15 21 Pb Activity (dpm/g) SML S.R. = (cm/yr) R 2 = Ra activity YS-C18 21 Pb Activity (dpm/g) SML S.R. =.39.7 (cm/yr) R 2 =.88 YS-C19 21 Pb Activity (dpm/g) SML S.R. = (cm/yr) R 2 =. 7 Fig. 9. Total and excess 21 Pb activity pro les of selected cores from unit A2 and B (SML, surface mixed layer). 226 Ra activity is also measured in some core horizons to estimate background 21 Pb activity. Sediment accumulation rate (S.R.) is determined from the slope of excess 21 Pb activity pro les (R 2, determinant coef cient). For more explanation, see text. S.-C. Park et al. / Marine Geology 17 (2) 271±

12 282 S.-C. Park et al. / Marine Geology 17 (2) 271±288 re ections, whereas unit B is indistinctively layered or almost re ection-free, suggesting uniform lithology. In some places, acoustic turbid layers (AT) are present a few meters below the sea oor, partly or completely masking the underlying sedimentary layers and the acoustic basement (Fig. 5b and c). The re ector b also forms the lower boundary of unit A2 (Fig. 7). Unit A2 exhibits inclined clinoforms, prograding mainly to the east and south along the seismic tracklines (Fig. 7). Unit A2 downlaps onto the acoustic basement and the top surface of unit A2 is uneven and irregular, indicating active erosion. Acoustic turbid layers are also present within unit A2 (Fig. 7a and c). The extensive occurrence of acoustic turbid layers in the study area is interpreted to be due to the entrapped gas bubbles (mainly methane gas) produced by biochemical degradation of organic matter in shallow sediments, which scatter and attenuate the acoustic energy (Lee, 1999) Sediment cores Five cores were taken each from unit A2 (YS-C1, C4, C5, C6, and C7) and unit B (YS-C11, C13, C15, C18, and C19) (Figs. 3 and 8). Penetration depths range from 14 to 356 cm. Surface sediments (±2 cm) in each core consist mainly of silt and clay; silt and clay contents range from 39.5 to 79.6% and 19.6 to 46.9%, respectively. Sand-sized materials, comprising.8±21.8% of the surface sediments, are mostly biogenic. Mean grain sizes and sorting values of the surface sediments range from 5.78 to 7.4f and from 1.98 to 2.83f, respectively. Below the surface sediments, the cores are generally homogeneous or well laminated with some shells and shell fragments (Fig. 8). YS-C1 contains a homogeneous mud layer in the upper section (ca. 1 cm) with shell fragments. Sand is dominant in the lower section (12±237 cm) of YS-C1; burrows with diameters of.5±1 cm are seen near the bottom of the core. The upper sections of YS-C4, YS-C5 and YS-C7 are characterized by faintly or well-laminated structures, whereas the lower sections are homogeneous. YS-C6 is also homogeneous throughout the section except for the muddy sand layers and shell fragments. YS-C11, YS-C13, and YS-C18 are characterized by parallel-laminated structures in the upper section and homogeneous mud in the lower section. Thickness of individual laminae in the upper section ranges from less than 1 to over 5 mm. YS-C15 and YS-C19 are generally homogeneous with some interbeddings of laminated mud and sand layers; bioturbation is common Pb pro les Total and excess 21 Pb activity pro les for eight cores, four cores each from units A2 and B, are shown in Fig. 9. Excess 21 Pb activity was determined by subtracting 226 Ra-supported 21 Pb activity (background activity) from the total activity. The background activity was estimated from 21 Pb activities measured in the cores where excess activity had decayed to constant low levels. Signi cant differences were not observed between the constant background 21 Pb ( 226 Ra-supported) activities and the measured 226 Ra activities (Fig. 9). The excess 21 Pb activity was used to compute sediment accumulation rates following the simpli ed equation of Nittrouer et al. (1979) and DeMaster et al. (1985). The equation is given by: S ˆ lz= ln A =A z where S is sediment accumulation rate (cm/yr), l the decay constant of 21 Pb (.31/yr), z the sediment depth, and A and A z are the unsupported excess 21 Pb activity at the sediment surface (dpm/g) and the unsupported excess 21 Pb activity at depth z, respectively. YS-C4, YS-C5, YS-C6 and YS-C7, taken from unit A2, reveal constant low 21 Pb activities downcore, indicating the background level of 21 Pb supported by 226 Ra. 226 Ra activities in some levels of the cores are consistent with total 21 Pb activity. Excess 21 Pb activities may be seen in the surface layer (±5 cm) of YS-C6; however, the near absence of excess 21 Pb in the seabed suggests negligible sediment accumulation over a 1-yr time scale in unit A2. YS-C18 from unit B exhibits a typical 21 Pb pro le characterized by three distinct regions: (1) a surface mixed layer (±3 cm) with homogeneous activities; (2) a middle region (3±7 cm) where activities decrease logarithmically with depth; and (3) a lower

13 S.-C. Park et al. / Marine Geology 17 (2) 271± kyr B.P B A2 Stage 3 Stage 2 A1 2 4 Stage Water Depth (m) Fig. 1. Sea-level curve (Park, 1992) during the last 3, yr in the eastern Yellow Sea. Stages 1, 2 and 3 indicate the evolutionary stage of units A1, A2 and B, respectively, as shown in Fig. 11. background region of constant low activities (7± 9 cm). Assuming that sediment mixing is restricted to the surface mixed layer (i.e. mixing coef cient is zero below the surface mixed layer), the least-square ts for the logarithmic decrease of the 21 Pb excess activity in the middle region give a sediment accumulation rate of.39 cm/yr. In YS-C11, taken from the southernmost part of unit B, the surface mixed layer is not observed and the slope of the 21 Pb excess activity in the upper layer (±15 cm) has a very high determinant coef cient r 2 ˆ :94 ; yielding the same sediment accumulation rate as that in YS-C18. Because the upper parts of these two cores show well-laminated structures without erosional features or bioturbation, the estimated sediment accumulation rates are probably close to the true values. The total 21 Pb activities below the surface mixed layer in YS-C15 and YS-C19 decrease slightly with depth. Because the total 21 Pb activities of these cores do not reach the low background level, the 21 Pb excess activities were calculated by subtracting the averaged 226 Ra activity from the total activity. Then, the estimated sediment accumulation rates of YS-C15 and YS-C19 are 4.29 and 2.48 cm/yr, respectively. These are much greater than those of YS-C11 and YS-C18. YS-C15 and YS-C19 show extensive bioturbation, most samples being homogeneous and lacking primary sedimentary structures (Fig. 8). The impact of biogenic (or physical) mixing below the surface mixed layer presumably causes the 21 Pb sediment accumulation rates to be much higher than the true values in these cores. 6. Discussion 6.1. Budget of recent muds The excess 21 Pb activity pro les and the 14 C data of unit B suggest that sediment accumulation is actively occurring at present in the northern SEYSM. The mass accumulation rate in the northern SEYSM can be computed from sediment accumulation rates and dry bulk density of unit B. The average sediment accumulation rate of unit B, estimated from the 21 Pb pro les of YS-C11 and YS-C18, is 3.9 mm/ yr. The 14 C age (24 yr B.P.) at the depth of 13 m of YSDP-13 (Fig. 4), determined from foraminifera tests by the AMS method, provides an average longterm (1-yr time scale) sediment accumulation rate of 5.4 mm/yr in unit B, which is quite comparable with the short-term (1-yr time scale) sediment accumulation rates determined from 21 Pb pro les of cores. The average water content and wet bulk density

14 284 S.-C. Park et al. / Marine Geology 17 (2) 271±288 N Stage 1: 14-7 yrs B.P. S.L. S Water Depth (m) 5 1 A1 Active formation of A1 N Stage 2: 7-35 yrs B.P. S.L. S Water Depth (m) 5 1 A1 Erosion of A1 Formation of A2 A2 N Stage 3: 35 yrs B.P. - present S.L. S Water Depth (m) 5 1 B A1 Erosion of A2 Deposition of B A2 35 O 'N 35 O 'N 35 O 'N Fig. 11. A conceptual model for evolution of the SEYSM, in relation to the postglacial sea-level changes (Fig. 1). Unit A1 represents the early to middle stage of the postglacial sea-level rise (stage 1). Unit A2 was deposited during the last, deceleration stage of the postglacial sea-level rise (stage 2). Much of unit A2 was probably originated from erosion of unit A1. Unit B represents the recent muds deposited during the last 35 yr when sea level was close to the present level (stage 3). Reworking of mud deposits in unit A2 is one of the main sources for the muds in unit B. of unit B were assumed to be 62% and 1.68 g/cm 3, respectively (Lee, 1999). Because unit B occupies an area of approximately 58 km 2, the total accumulation of mud in the surface area of unit B is about 3: 1 7 tons=yr: Chough and Kim (1981) suggested that the discharge of sediment from the Keum River on the west coast of Korea is about 5:6 1 6 tons=yr and the Youngsan River in the south is a negligible sediment source. Schubel et al. (1984) estimated that approximately 1:3 1 6 tons of suspended sediment are delivered annually to the Yellow Sea by the Keum River. Yoo (1986) attributed the very turbid water mass found on the inner shelf of the southwestern Korea to the suspensates from the Keum River. Park and Khim (1992), based on clay mineralogy, also suggested that the Keum River is the primary source for the ne-grained sediments in the SEYSM. Alexander et al. (1991), on the other hand, suggested that the SEYSM consists of a highly complex mixture

15 S.-C. Park et al. / Marine Geology 17 (2) 271± of sediments from the smaller Korean rivers, resuspended Huanghe deposits from the central Yellow Sea, and suspended sediments carried to the area from the south by the Yellow Sea Warm Current. Our estimate of the annual mass accumulation rate of unit B is about an order of magnitude greater than that can be attributed to the Korean rivers alone, suggesting various sediment sources for the SEYSM. Very low background activities of 21 Pb in the cores from unit A2 indicate negligible sediment accumulation in the southern SEYSM at the present time. The erosional truncation along the top surface of unit A2 further indicates that unit A2 is in a state of erosion today. We propose that the ne-grained sediments eroded from unit A2 are being redeposited in the north; these sediments, together with the suspended sediments from the Korean rivers, actively form unit B. It is not clear, however, why the southern SEYSM is an area of erosion whereas active sediment accumulation is currently underway in the northern SEYSM. The study area is characterized by a distinct front between the northward owing Yellow Sea Warm Current and the southward owing Korean Coastal Current (Guan, 1994; Pang and Hyun, 1998). We speculate that strong tidal currents in this area (Choi and Fang, 1993), combined with complicated regional currents, are probably responsible for these differentiated erosional or depositional processes. Direct hydrographic and oceanographic observations over the southeastern Yellow Sea are required to understand better the sedimentary processes in the SEYSM Late Quaternary evolution of the SEYSM The regional sea-level data from the Yellow Sea continental shelf (Feng, 1983; Park, 1992; Park et al., 1993) suggest that the sea level during the last glacial maximum (LGM) was about 14±15 m below the present level (Fig. 1), which is about 2±3 m lower than the glacio-eustatic sea-level position estimated by Fairbanks (1989). This discrepancy may be due to the isostatic subsidence associated with the thick accumulation of sediments in the Yellow Sea (Wang, 1993). During the LGM, much of the Yellow Sea shelf was subaerially exposed and the coastline was located along the present-day outer shelf, connecting the Chinese mainland and Korean Peninsula. During the Holocene transgression, sea level rose rapidly to about 1±15 m below the present level at around 65±7 yr B.P. (Park et al., 1993), followed by a gradual rise to the present level at about 35 yr B.P. We propose a simpli ed conceptual model (Fig. 11) of the late Quaternary evolution of the SEYSM based on the 14 C data (KIGAM, 1996) and the general sealevel curve in the eastern Yellow Sea (Park et al., 1993). Evolution of the SEYSM can be divided into three stages. Stage 1 corresponds to the early to middle stage (about 14,±7 yr B.P.) of the postglacial sea-level rise, during which unit A1 was deposited. The base level for deposition of unit A1, estimated from the water depth and sediment thickness at the site of YSDP-13, is about 9 m below the present sea level (Fig. 5b). The sea level rose to approximately 9 m below the present level at about 14, yr B.P. (Fig. 1). This age coincides well with the date of 13,8 yr B.P., obtained at the base of unit A1 (Fig. 4). The uppermost part of unit A1 at the site of YSDP-13 was dated about 83 yr B.P. (Fig. 4), at which the sea level rose to about 25 m below the present level (Fig. 1). Foraminiferal and oxygen isotopic data from YSDP-13 indicate continuous presence of brackish water species in unit A1 (Kong, 1998), indicating freshwater discharge from the land. The study area was probably a depocenter for continuous accumulation of mud in the coastal or shallow-water environment during the early to middle stage of the postglacial transgression period. Thus, we interpret that unit A1 represents deltaic/estuarine or shallow-water muds. The average accumulation rate of unit A1 during 13,8±83 yr B.P., estimated from YSDP-13, is about 2.4 m/1 yr. Accumulation of mud probably continued until about 7 yr B.P. when sea-level rise began to decelerate. Stage 2 represents the last, deceleration stage (about 7±35 yr B.P.) of the postglacial sealevel rise, during which unit A2 was deposited. The short time interval (56±69 yr B.P. on the basis of the AMS dates, Fig. 4) relative to the thickness suggests extremely rapid deposition of unit A2. Clinoforms in unit A2, prograding mainly toward the south and east, indicate sediment input from the north and/ or northwest. We interpret that strong currents eroded large volumes of sediments from unit A1 and redeposited them in the south, forming unit A2. The

16 286 S.-C. Park et al. / Marine Geology 17 (2) 271±288 erosional lag facies consisting of the sand layer in YSDP-13 and the strong mid-re ector (a) along the upper boundary of unit A1 probably indicate extensive erosion. A time break of about 19 yr between units A1 and A2 as well as the inconsistent age sequence of YSDP-12 strongly support our interpretation. Stage 3 represents the recent highstand stage of sea level (about,35 yr B.P.), in which unit B has been deposited. The 14 C age (24 yr B.P.) at the depth of 13 m of YSDP-13 yields an average sediment accumulation rate of 5.4 m/1 yr in unit B. Extrapolating at a constant rate, we obtained an age of about 33 yr B.P. at the lower boundary of unit B, which agrees well with the termination of the sea-level rising (Park et al., 1993). As discussed in the previous section, the sediment budget in unit B, together with the acoustic and age data, suggest that a large amount of reworked, ne-grained sediments from unit A2 has been redeposited in unit B. Smectite is an important clay mineral to distinguish different sediment sources in the Yellow Sea (Park and Khim, 1991, 1992). Smectite contents in the surface sediments of the SEYSM are less than 1% whereas they are as high as 11±13% in the CYSM which received sediments from the Huanghe River (Park and Khim, 1991, 1992). The surface sediments of the SEYSM appear to have been derived from the Korean rivers (Park and Khim, 1991, 1992). However, smectite contents of units A1 and A2 are much higher than those reported by Park and Khim (1991, 1992), ranging from 5.4 to 12.4% and from 3.1 to 9.8%, respectively (KIGAM, 1996). We interpret that unit A1 was affected primarily by the Huanghe River system, which supplied large volumes of sediments to the Yellow Sea prior to about 7 yr B.P. (Alexander et al., 1991). Between 7 and 4 yr B.P., corresponding to unit A2, sediments from the Huanghe River were trapped near the Shandong Peninsula (Alexander et al., 1991). Thus, the high smectite content in unit A2 supports our interpretation that much of unit A2 consists of sediments eroded from unit A1. The smectite contents (2.3 to 9.5%) of unit B (KIGAM, 1996) are also higher than those of the surface sediments of the SEYSM, suggesting that the Korean rivers have provided only small amounts of sediments to the SEYSM. 7. Summary The SEYSM can be divided into three units (A1, A2 and B) and evolution of each unit is closely related to the postglacial sea-level changes and sediment erosion and reworking. Unit A1, comprising the northern lower part of the SEYSM, represents continuous accumulation of estuarine/deltaic or shallowwater muds during the early to middle stage of the postglacial sea-level rise (ca. 14,±7 yr B.P.), with an average long-term sediment accumulation rate of 2.4 m/1 yr. Unit A2, occupying the southern SEYSM, was deposited during the last, deceleration stage (ca. 7±35 yr B.P.) of the postglacial sea-level rise. The inconsistent 14 C data and the short time interval (ca. 56±69 yr B.P.) relative to the thickness suggest that unit A2 was deposited very rapidly and much of its sediment was not directly from the Korean rivers. Erosion of unit A1 probably provided a significant amount of sediment to the south, resulting in rapid sediment accumulation in unit A2. Unit B, comprising the upper part of the northern SEYSM, consists of the recent muds deposited during the last 35 yr when sea level was close to the present level. The annual sink of the recent muds (about 3: 1 7 tons is an order of magnitude greater than the sediment discharge from the Korean rivers. Thus, unit B may consist of a highly complex mixture of sediments from the Korean rivers and other parts of the Yellow Sea. Reworking of mud deposits in unit A2 in the south is probably one of the main sources for the muds in unit B. Erosional truncation along the top surface of unit A2 and very low background activities of 21 Pb in the surface sediments of unit A2 indicate that the southern SEYSM is in a state of erosion today. Acknowledgements This research was funded by the Korea Science and Engineering Foundation through a grant to S.C. Park (Grant No ). We express our appreciation to the of cers and crew of the R/V Tamyang, Pukyung National University for providing support for eld survey. We also thank the Korea Institute of Geology, Mining and Materials for providing seismic data. Drs D.J.W. Piper, C. Alexander and S.A.

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