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East China Sea tidal regime and its impacts on sediments dispersal and seafloor morphology
Sedimentary Geology 162 (2003) 25 – 38 www.elsevier.com/locate/sedgeo
Late Quaternary evolution of the Yellow/East China Sea tidal regime and its impacts on sediments dispersal and seafloor morphology Katsuto Uehara a,*, Yoshiki Saito b a
Research Institute for Applied Mechanics, Kyushu University, Kasuga Koen 6-1, Kasuga, Fukuoka 816-8580, Japan b MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan
Abstract The evolution of a tidal-current field in the Yellow Sea and East China Sea (YECS) in response to the sea-level rise since the last glacial maximum (LGM) was investigated using a two-dimensional tidal model, with special attention to changes in the sedimentary environment of the continental shelf region. It was found that tidal currents on the YECS shelf have generally been semi-diurnal throughout the post glacial stages, with F-ratio (index of diurnal inequality) and S2/M2 ratio (index of spring – neap cycle) remaining similar to present. On the other hand, the spatial distribution of the tidal bottom stress associated with M2 and M4 tidal currents changed greatly during the transgression. As the sea level rose, two core regions showing high bottom-stress values migrated shoreward from Cheju Island toward the west coast of Korea, and along the retreat path of the paleo-Changjiang Estuary. Distribution of ‘extremely intense’ tidal bottom stress suggested that intense tidal reworking may have occurred at sea levels from 90 to 45 m in the southeast Yellow Sea and at 75 to 60 m off the paleo-Changjiang Estuary, the latter of which agreed with the timing of a large terrigenous organic C flux observed on the shelf slope. Distribution of slightly weaker bottom stress indicated that the sand ridges off the Changjiang Estuary and in the southeast Yellow Sea also might have formed at during those periods. On the other hand, when the sea level was between 45 and 15 m, the tidal bottom stress was generally not strong enough to invoke significant reworking, even though the stress intensity may be stronger than present. Correlation with modern and relict sedimentary features suggested that the tidal process have played a significant role on the reworking and deposition of sediments on the YECS shelf. D 2003 Elsevier B.V. All rights reserved. Keywords: Transgressive sequence; Tidal current; Reworking; Sand ridge; Numerical modeling; Changjiang
1. Introduction After the last glacial maximum (LGM) at about 20 ka, eustatic sea-level rise of more than 100 m inun* Corresponding author. Tel.: +81-92-583-7728; fax: +81-92573-1996. E-mail address: [email protected] (K. Uehara). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0037-0738(03)00234-3
dated large areas and formed the vast continental shelf of the modern Yellow and East China seas (YECS) (Wang and Wang, 1980; Yanagida and Kaizuka, 1982). One of the direct impacts of the postglacial sea-level rise on the sedimentary environment in the YECS was the modification of the tidal-current regime, the nature of which depends greatly on basin shape.
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The tidal current is one of the most energetic oceanic components of the present YECS, and it plays an important role in sedimentary processes such as deposition, erosion, and resuspension. The influence of the tidal current is also found in sedimentary strata and bedform features that were generated during the postglacial transgression. Hori et al. (2001a) recovered successions of sand – mud couplets at 47.5 –48.5 m depth in a borehole sample taken from the modern Changjiang (Yangtze River) delta, which seem to reflect fortnightly cycles of tidal currents. Furthermore, the geological record indicates that the tidalcurrent regime in the YECS might have been different when the sea level was lower than at present. For example, the moribund tidal sand ridges on the shelf of the East China Sea (Fig. 1) are considered to have formed when sea level was lower than present because modern tidal-current conditions are insufficient for
tidal sand-ridge field generation (Yang and Sun, 1988; Yang, 1989; Liu et al., 1998; Saito et al., 1998). In recent years, several numerical simulations have been carried out to verify changes in the YECS tidal regime that occurred in response to postglacial sealevel rise. Oh and Lee (1998) predicted tides and tidal currents by modeling the lowering of the sea level but retaining the present basin configuration. According to their model, tidal amplitudes and the number of tidal amphidromes in the YECS increased as the sea level rose. Wang et al. (1998) considered the shoreline changes caused by sedimentation along the Chinese coast, and their model was extended by Lu¨ et al. (2000) to predict the K1 constituent. Uehara et al. (2002) used paleobathymetry values that took into account the effect of the thick sedimentary strata deposited in the paleo-Changjiang Estuary, and they pointed out that the tidal current on the shelf was generally stronger when sea level was 45 m lower than at present. The purpose of this paper is to elucidate Late Quaternary evolution of the tidal-current field in the YECS and to clarify the effects on sedimentary processes during the postglacial transgression. We emphasize the verification of tidal-current features such as spring –neap cycles and spatial shifts of the region of intense tidal currents that accompanied the sea-level rise because these are essential for interpreting the formation and maintenance of the geological features preserved in the modern YECS.
2. Regional setting
Fig. 1. Map of the study area. Features drawn in the region southeast of the Changjiang Estuary and in the Yellow Sea indicate the location of major sand ridges in the YECS. Numerals denote water depth in meters. Three lines illustrate the location of transects used in Fig. 7.
The Yellow and East China seas form an epicontinental sea enclosed by the Chinese and Korean coasts, and a marginal sea separated from the main Pacific Ocean by the Japanese islands and Taiwan Island, respectively (Fig. 1). The YECS is also connected to the South China Sea by the Taiwan Strait (sill depth, about 60 m) and to the Japan Sea by the Tsushima Strait (sill depth, about 130 m). The YECS has three parts: the Bohai Sea, the Yellow Sea, and the East China Sea. The Bohai Strait separates the Bohai Sea from the Yellow Sea, and a line connecting Cheju Island with the north end of mouth of the Changjiang (Yangtze River) forms the boundary between the Yellow and East China seas. The bathymetry of the
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Fig. 2. Grain-size distribution of sea-bed sediments in the YECS; sand < 4 phi, silt 4 – 8 phi, and clay >8 phi, median grain size, after Saito and Yang (1994). Thick dashed lines denote the bottom-stress contour of 0.35 N/m2 for the present day derived from Fig. 6h.
YECS is characterized by a vast continental shelf, the seventh largest in the world (Fairbridge, 1968). This shelf, which is shallower than 150 m, occupies more than 70% of the area of the YECS. The sea floor of the shelf gradually deepens to the SSE or toward the axis that lies mostly along the east coast of the YECS. The modern YECS shelf is covered by sand and mud (Fig. 2). Muddy sediments in the Bohai Sea are supplied predominantly by the Huanghe (Yellow River), while those along the Chinese coast south of about lat 32jN are supplied by the Changjiang. The sediment discharge from these two Chinese rivers constitutes nearly 90% of the total sediment supply derived from the rivers that flow into the YECS (Saito, 1998a). Muddy sediments on the slope of the inner shelf extending from the Chinese coast, in the central Yellow Sea and in the region south of Cheju Island, are considered to be derived mainly from the Huanghe or the old Huanghe delta at Jiangsu (Milli-
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man et al., 1985a; Saito, 1998a). Sandy sediments with ripples and megaripples are distributed on the inner shelf at depths of 25 – 50 m east of the Changjiang mouth (Butenko et al., 1985; Hori et al., 2002). The sandy sediments on the middle to outer shelf in the East China Sea and along the west coast of Korea are thought to be transgressive sediments deposited since the LGM (Suk, 1989). Suspended sediments are transported mainly by subtidal currents because tidal oscillations contribute little to basin-scale sediment transport over the long term (Naimie et al., 2001). The circulation pattern of the YECS shows an apparent seasonal variation controlled by the East Asian monsoon climate (Fig. 3; after Su, 1998). In general, coastal currents along the Chinese and Korean coasts flow southward in winter and northward in summer in response to prevailing wind directions. The shelf water column is nearly homogeneous in winter, while surface heating, tidal mixing, and freshwater discharge from the Changjiang modify the density structure and flow pattern in summer (Takahashi and Yanagi, 1995; Naimie et al., 2001). Except for the Kuroshio, which flows along the shelf edge with a current speed exceeding 1 m/s, subtidal flows in the YECS are variable in both intensity and position, and they are usually obscured by strong tidal current signals. Erosion and resuspension on the YECS shelf are caused mainly by tidal currents (Milliman et al., 1985b) and storm-generated surface waves (Wells, 1988; Graber et al., 1989). Wind-generated waves in the YECS are fetch-limited; under normal conditions, wave height is less than 2 –3 m in the southern YECS and less than 1 – 2 m in the northern YECS. Waves as high as 4 – 6 m may occur when severe storms or typhoons pass through the YECS region (Japan Meteorological Agency, 1999). Proxy data suggest that the basin geometry of the YECS has changed greatly during the last 20 kyr, primarily as a result of postglacial sea-level changes; sea level is estimated to have been 120 F 5 m at 20 ka, 45 F 5 m at 10 ka, and 0 F 3 m at 6 ka, relative to the present level (Saito, 1998b; Hanebuth et al., 2000). A large portion of the YECS shelf was exposed subaerially during the LGM, which was followed by a large-scale transgression. The incised valley of the paleo-Changjiang, which had formed during the sealevel lowstand, was inundated, creating an elongated
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Fig. 3. Schematic map of the general circulation pattern of the China seas for (a) winter and (b) summer (after Su, 1998). The current features identified are the Bohai Coastal Current (BC), the Yellow Sea Coastal Current (YSCC), the Changjiang River Plume (CRP), the East China Sea Coastal Current (ECSCC), the Taiwan Warm Current (TWC), the Tsushima Current (TC), the Yellow Sea Warm Current (YSWC), the Korean Coastal Current (KCC), the South China Sea Coastal Current (SCSCC), the South China Sea Warm Current (SCSWC), and the Donsha Current (DC). From SU, Jilan, 1998. Circulation dynamics of the China seas north of 18N coastal segment. In: The Sea, Volume 11, Chapter 16, Robinson, A.R. and Brink, K.H. Eds., John Wiley & Sons, Inc., New York, pp. 483 – 505. ISBN 0-471-11545-2. This material is used by permission of John Wiley & Sons, Inc.
paleoestuary, which was 400 km long at about 10 ka. During the last maximum transgression at about 6 ka, the shoreline receded greatly along the western coastline of the Bohai and Yellow seas. Since then, as a result of high sediment discharges from the Huanghe and Changjiang rivers and the stable or slightly falling sea level, the shoreline has migrated seaward to form the present coastline (Saito et al., 2001).
3. Tidal model We used the two-dimensional tidal model of Uehara et al. (2002) to predict the tidal-current field (M2, S2,
K1, O1, N2, and M4 constituents) of the YECS area shown in Fig. 1 (lat 24jN – 42jN, long 116jE – 131jE; resolution, 1j/12). Eight numerical runs were conducted for different sea levels (15-m intervals from 90 m to 0 m) and bathymetry configurations (Table 1). To minimize the effect of sedimentary strata subsequently deposited along the Chinese coasts, we modeled paleotopographies for two periods: 6 ka (sea level, 0 m) and 10 ka (sea level, 45 m). Special care was taken to reconstruct the paleotopography of the Changjiang Estuary (lat 31jN – 33jN, long 119jE –123jE), which was accomplished by compiling existing borehole data (Hori et al., 2001a,b) to remove the effect of sedimentary strata deposited after the periods being modeled. We also used paleocoastline data along the Jiangsu coast, the Bohai Sea, and Hanzhou Bay for the same reason. For cases other than a00 (present condition), b00 (6 ka), and c45 (10 ka), bathymetry values were obtained by reducing water depth from one of the two modeled paleotopographies. Water depths shallower than 5 m were redefined as 5 m to prevent numerical instabilities. The quadratic friction and horizontal eddy viscosity coefficients were 2 10 3 and 100 m2/s, respectively. For the open boundary values, we used harmonic constants of the M2, S2, K1, O1, and N2 tidal constituents estimated by using a global model based on the present configuration (Le Provost et al., 1998). On the basis of their global paleotidal model, Thomas and Su¨ndermann (1999) suggested that the tidal amplitude change of the M2 constituent along the open boundary was less than 0.05 m during the last 21 kyr except in the Taiwan Strait and limited regions north and south of Kyushu Island. Since that value is within the precision Table 1 Tidal experiments conducted in this study, which assume different relative sea levels (R.S.L.) and model topographies Case a00 b00 b15 b30 c45 c60 c75 c90
R.S.L. (m) 0 0 15 30 45 60 75 90
Model topography present 6 ka 6 ka ( 15 m) 6 ka ( 30 m) 10 ka 10 ka ( 15 m) 10 ka ( 30 m) 10 ka ( 45 m)
In cases b15, b30, c60, c75, and c90, depths were reduced from the original topography by the value shown in parentheses.
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of our model (0.17 m for amplitude and 15 degrees for phase), we used the same boundary values in all cases. Crustal-rebound effects from hydro-isostasy have also given rise to changes in sea level since the LGM (Nakada et al., 1991). However, they were not taken into account by the paleotopography and sea levels used in this study. The model is not applicable for discussing absolute tidal-current intensities in the Bohai Sea, most remote location from the open boundaries, since it systematically underestimate the values observed (e.g., Liu et al., 1998) or those obtained by regional models (e.g., Huang, 1999), which is a common defect of many YECS-scale models (e.g., Yanagi and Inoue, 1994; Blain, 1997; Naimie et al., 2001). As for the Bohai Sea, we only used ratios between tidal constituents for analyses because they agreed well with those predicted by regional models.
4. Results Although results are shown for the YECS as a whole, we refer mostly to the tidal-current structure on the continental shelf (shoreward of the 150-m depth contour, indicated by a dotted line in the figures)
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because our main focus was the tidal-current structure of the shallow shelf waters. 4.1. Diurnal – semidiurnal current amplitude ratio Fig. 4 shows the ratio of diurnal (K1 and O1) to semidiurnal (M2 and S2) tidal-current amplitudes ( F-ratio) in the YECS for cases c90, c45, and a00. This ratio measures the importance of the diurnal component contained in tidal-current signals. Tidal currents on the shelf were dominantly either semidiurnal ( F < 0.25) or mixed (semidiurnal dominated; 0.25 < F< 1.25) in all cases. Even in the case of mixed-type currents, the F-ratio was less than 0.5 except in limited regions. The distribution of semidiurnal and mixedtype tidal currents modeled by using the present shelf configuration (Fig. 4c) was consistent with that reported by Choi (1980). During the early stage of the postglacial transgression, tidal currents in the shelf area were dominantly semidiurnal and the F-ratio was less than 0.4 everywhere on the shelf (Fig. 4a). As the sea level rose, mixed-type (semidiurnal dominated) tidal currents became evident in the Tsushima Strait, the central Yellow Sea, and the Bohai Sea (Fig. 4b and c). Mixedtype currents (diurnal dominated; 1.25 < F <3)
Fig. 4. Distributions of diurnal (K1 and O1 constituents) to semidiurnal (M2 and S2 constituents) ratios of tidal-current amplitudes (F-ratio) for cases (a) c90 (sea level, 90 m), (b) c45 (sea level, 45 m), and (c) a00 (present). The contours shown are 0.25, 0.5, 0.75, 1.00, 1.25, and 3. The dotted line in each panel denotes the 150-m depth contour, which is the approximate shelf edge in the YECS.
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appeared in the southeastern Bohai Strait and southwestern Tsushima Strait during the last phase of the sea-level rise (Fig. 4c). The development of the highF region during the transgression stage may be related to the evolution of the amphidromic system that accompanied the sea-level rise (Oh and Lee, 1998) because the location of the F-ratio maximum corresponds to the current amphidromic point (Xia et al., 1995) of the semidiurnal current. The spatial distribution of the F-ratio suggests that the tidal current on the YECS shelf has had predominantly semidiurnal components throughout the postglacial period, except in the southern parts of the Bohai and Tsushima straits.
30 –45 m lower than at present (Fig. 5b). The value greater than 0.45 was caused by a decrease in the M2 tidal-current constituent in the southern shelf area, which might have been related to the opening of the Taiwan Strait. In most regions on the YECS shelf, temporal variability of the S2/M2 ratio remained within 0.1 even though the water depth increased by 90 m. The results shown here suggest that the spring– neap cycle of the tidal currents occurred even when the sea level was lower than at present, and its intensity was not significantly modified during the postglacial transgression. 4.3. Maximum bottom stress associated with M2 and M4 tidal currents
4.2. S2/M2 ratio The coexistence of M2 and S2 constituents gives rise to a fortnightly modulation of tidal-current amplitudes, which is an important factor affecting sediment transport in coastal regions. The relative intensity of the fortnightly modulation is shown for three sea-level stages in terms of the ratio of S2 to M2 tidal-current amplitudes (Fig. 5). The S2/M2 ratio in the YECS shelf area ranged mostly within 0.25 – 0.45. An exception was that on the East China Sea shelf south of about lat 29jN when the sea level was
To provide lowest-order information regarding the effect of tidal motion on sediment dynamics, the spatial distribution of the maximum bottom stress associated with M2 and M4 tidal currents was calculated by using a formulation of Pingree and Griffiths (1979) and Milliman et al. (1985b) for relative sea levels from 90 to 0 m (Fig. 6). The maximum stress is defined here as the maximum value of the quadratic bottom stress (qCD | u | 2) over a tidal cycle, where q is the density of seawater ( = 1023 kg/m3), CD is the bottom stress coefficient ( = 2.0 10 3),
Fig. 5. Distributions of the ratios of S2 to M2 tidal-current amplitudes. The contour interval is 0.05. The cases are as in Fig. 4.
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Fig. 6. Regional changes in the maximum bottom-stress field associated with M2 and M4 tidal currents in response to the postglacial sea-level rise. The cases shown are (a) c90, (b) c75, (c) c60, (d) c45, (e) b30, (f) b15, (g) b00, and (h) a00 (see Table 1 for the experimental conditions). The contours shown are 0.35, 0.6, 1.0, and 2.0 N/m2, which correspond to tidal-current speeds of 0.41, 0.54, 0.70, and 0.99 m s 1, respectively. It should be noted that the bottom stresses introduced here are based on depth-averaged currents and are not applicable to dynamic relations derived from near-bottom current velocities (e.g., Sternberg et al., 1985).
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and u is the combined M2 and M4 depth-averaged currents. Since CD was assumed to take a constant value, the bottom stress is proportional to the kinetic energy of the tidal current, or the square of the tidalcurrent velocity. Fig. 6 depicts contour distribution of three maximum-stress values that were relevant to specific seabed features on the YECS shelf. The 0.35 N/m2 stressintensity contour for the present-day configuration (case a00; Fig. 6h) roughly coincided with the shoreward edge of the region of mud deposits in the area south of Cheju Island and the central Yellow Sea (Fig. 2), which suggests that that value is a regional limit of tidal influence on fine-sediment deposition. The spatial distribution of megaripple features off the Changjiang Estuary (Butenko et al., 1985; Liu, 1997) and of sand waves off the Keum River (Korea) estuary (Bahng et al., 1994) at the present age corresponds to the 0.6 N/m2 bottom-stress contour (Fig. 6h). Uehara et al. (2002) indicated that high M2 velocity in excess of 0.7 m/s (equivalent to bottom-stress intensity of 1.0 N/m2 for the present study) during the transgression stages coincides roughly with the area where moribund sand ridges exist on the inner to outer shelf of the East China Sea (Fig. 1). The relationship between the bottom-stress distribution and bedform features is examined in the next section. 4.4. Changes in the distribution of bottom stressses during transgression When the sea level was low, tidal currents on the YECS shelf were more energetic and exerted stronger influence on the sea floor than those at present. Values larger than 0.35 N/m2 predominated on the shelf, and values greater than 1.0 N/m2 were observed on the East China Sea shelf, in the region adjacent to Cheju Island, and within the Tsushima Strait (cases c90 and c75; Fig. 6a and b). The core of the high-stress region on the East China Sea shelf was at the mouth of the paleo-Changjiang Estuary, and it migrated northwestward as the Chinese coastline receded (cases c60 and c45; Fig. 6c and d), while the high-stress values around Cheju Island shifted northward along the coast of Korea. In general, high bottom-stress values were observed in shallow waters where the absolute depth was less than 40 m and their spatial extent was reduced as the sea level rose.
On the other hand, the spatial distribution of bottom-stress values less than 0.35 N/m2 expanded in regions north of Taiwan Island, along the east coast of the Yellow Sea, and off the southern coast of Korea during the transgression stages. This may be ascribed to the evolution of the YECS amphidromic system associated with the postglacial sea-level rise (Oh and Lee, 1998) because the locations of minimum values coincide with current amphidromic points. The existence of degenerate current amphidromic points in the western Yellow Sea during the sea-level stages from 60 to 30 m has given rise to an asymmetric bottom-stress field that extends in a transverse direction (Fig. 6c, d, and e). When sea level was 30 – 15 m lower than at present, the inner shelf off the modern Changjiang mouth was characterized by high bottom stresses with values exceeding 1.0 N/m2, which indicates a strong influence of tidal currents during these periods (cases b30 and b15; Fig. 6e and f). Bottom stresses along the Korean coast were slightly weaker during these periods than during previous stages, and the highstress region migrated shoreward over the inner shelf as the water depth increased. The bottom-stress distribution was similar to that at present at the time of maximum transgression (case b00; Fig. 6g). A notable difference, however, between the time of maximum transgression and the present was that a high-stress region was found at 6 ka in the paleoChangjiang Estuary, which was located to the northwest of the present river mouth.
5. Discussion The previous section suggests that the F-ratio and S2/M2 ratio of tidal currents did not change significantly during the Late Quaternary except at around the Tsushima and Bohai straits. The former value is a measure of diurnal inequality while the latter reflects the intensity of spring –neap cycle. These findings are consistent with stratigraphic records found in the Kyunggi Bay, which indicates that rhythmicities preserved in the Late Pleistocene tidal rhythmites are similar to those of modern tidal cycles (Choi et al., 2001). Rhythmic laminations containing spring– neap cyclicities of the postglacial periods are also recovered from the modern Changjiang Delta (Hori et al.,
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2001a), southeastern Yellow Sea (Jin and Chough, 1998), and East China Sea (Lofi et al., 1999). In contrast, the distribution of maximum bottom stress associated with M2 and M4 tidal currents shows clearly the changes at sea levels from 90 to 0 m. The following text correlates such changes with various modern and relict sedimentary features in the YECS to show that variations in tidal bottom stresses have been an important control on sediment distribution and the development of seafloor morphology. 5.1. Mud patch formation and low tidal bottom stress Comparison of the bottom stress and modern grain-size distribution suggest that mud deposits in the modern YECS form where the tidal bottom stress is less than 0.35 N/m2 (Fig. 2). As the bottom stress decreased with the rise of sea level (Fig. 8b), it is possible to infer the timing when the tidal condition had been transformed to a state favorable for the
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mud deposition. In the mud-patch south of Cheju Island, the bottom stress became less than 0.35 N/m2 at sea levels between 45 and 30 m (ca. 10 –9 14 C kyr BP after Saito, 1998b) in the northeastern part of the mud patch, between 30 and 15 m (ca. 9 – 8 14C kyr BP) in the central area, and between 15 and 0 m (ca. 8– 7 14C kyr BP) in the southwestern region. Radiocarbon ages of the base of muddy sediments in the central part of the mud patch (32jN, 126jE) show less than 8 14C kyr BP (Yang et al., 1995), whereas those of the mud layer in the southwestern area were younger than 6.5 14C kyr BP (Yoo et al., 2002), which indicate that the mud patch was formed after the tidal bottom stress has become sufficiently weak. These results suggest that the weak tidal current accompanied by the postglacial sea-level rise is one of the necessary conditions for the formation and maintenance of the mud patch south of Cheju Island, although there exist other significant pro-
Fig. 7. Lateral extent of the region where the bottom stress associated with M2 and M4 tidal currents exceeded 1.0 N/m2 at sea levels (a) 90, 75, 60, 45, 30, 15, and 0 m, and (b) 75, 60, and 45 m. Thick black lines denote the location of major sand ridges.
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cesses such as supply rate of fine sediments, flow regimes of wind- and density-driven currents, and wave conditions. 5.2. Migration of intense bottom-stress region with the sea-level rise The region of intense bottom stress exceeding 1.0 N/m2 migrated across the shelf mainly through two routes in the YECS when the sea level rose from 90 to 0 m (Fig. 7a): one in the eastern Yellow Sea from around Cheju Island toward the west coast of Korea (Fig. 8a) and another in the East China Sea along the retreat path of the paleo-Changjiang Estuary (Fig. 8c). 5.2.1. Extremely intense bottom stress The bottom stress in the nearshore portion of the intense bottom-stress region (>2.0 N/m2 in extreme cases) was generally higher than that in the offshore region (Figs. 6, 8a, and c). The occurrence of such extremely intense bottom stress at sea levels from
Fig. 9. Changes in area where the tidal bottom stress exceeded (a) 2 N/m2 and (b) 1 N/m2 in the eastern Yellow Sea (32.5jN – 33jN, 125jE – 128jE and 33jN – 40jN, 123.5jE – 128jE) and off the Changjiang Estuary (26jN – 32.5jN, 119jE – 126jE and 32.5jN – 33jN, 119jE – 125jE; 33jN – 35.5jN, 119jE – 123.5jE) for sea levels from 90 to 0 m.
Fig. 8. Evolution of tidal bottom stress associated with M2 and M4 tidal currents along three transects shown in Fig. 1: (a) southern Yellow Sea, (b) south of Cheju Island, and (c) retreat path of the paleo-Changjiang Estuary. Bottom stress values shown on contours are in N/m2.
90 to 0 m is shown in Fig. 9a for the eastern Yellow Sea, southeastern (SE) Yellow Sea, and East China Sea. In the SE Yellow Sea, the intense bottom stress was most extensive during the early transgression stages and reduced its extent after the sea level has reached 75 m. This reduction may be associated with the sparse distribution of sand ridges in the modern nearshore region compared to the offshore area in the SE Yellow Sea, as the sparseness is estimated to have been caused by the reduced supply of sand fed by reworking processes (Jin and Chough, 1998). During the late phase of the sea level rise, on the other hand, the high-energy area was developed rapidly in the Kyunggi Bay and the West Korean Bay (Figs. 6 and 9a) which may be relevant to the evolution of ‘pseudo-tidal’ sand ridges off the Tae-
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an Peninsula blanketed by recently reworked relict (palimpsest) sands (Jung et al., 1998). In the East China Sea, the high-energy area existed near the mouth of the paleo-Changjiang Estuary (Fig. 6) for relatively limited periods (sea level from 75 to 60 m; Fig. 9a), which will be discussed in the following text. 5.2.2. Correlation between extremely intense bottom stress and sediment supply to the continental slope and the Okinawa trough Oguri et al. (2000) examined the time sequence of the stable isotope ratio of organic carbon (y13Corg) for the last 15 14C kyr that was obtained from core samples on the continental slope (F-3PC; 28j40’N, 127j07’E) and in the Okinawa Trough (P-4PC; 28j12’N, 127j14’E) and showed distinct depletion of the y13Corg ratio from 14 to 12 14C kyr BP. This depletion is larger than the stage of low stand of sea level in the LGM and indicates that the large supply of terrestrial organic C from the East China Sea shelf to the slope was occurring during those periods. One possible source of this terrigenous organic C is sediments supplied by the reworking of the shelf, as non-arboreal pollen content in the core F-3PC increased greatly at stages when y13Corg was depleted (Ikehara et al., 1999). Fig. 9a shows that the occurrence of very strong bottom stress (over 2.0 N/m2) on the East China Sea shelf was most extensive when the sea level was between 75 and 60 m, or about 13.5 to 12 14C kyr BP according to the sea-level curve of Saito (1998b). The coincidence in the period of y13Corg depletion and the stress-intensity maximum indicates that an extensive tidal reworking may have occurred during this interval. Furthermore, intense tidal current on the outer shelf may have enhanced sediment transports near the shelf edge. It is therefore suggested that the ‘extremely intense’ tidal bottom stress have played a significant role on the large supply of terrigenous organic C to the shelf slope at 14 –12 14C kyr BP, though other factors such as the increase of river discharge may have been involved in this event. The situation is in contrast to the modern case when strong tidal currents are limited to the nearshore and distant from the shelf edge. At present, across-shelf transport is considered to be maintained primarily by vertical circulation induced by monsoonal winds (Yanagi et al., 1996).
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5.2.3. Intense bottom stress and large-scale bedform features As the location of the sand ridges in the YECS generally falls within the spatial range of the intense bottom stress (exceeding 1.0 N/m2) during postglacial periods (Fig. 7a), the bottom stress intensity of 1.0 N/ m2 is suggested to be the threshold value for the formation of sand ridges. The only exceptions found in Fig. 7a are ridges north of the Bohai Strait and a northern half of the ridges off Jiangsu coast. Discrepancy found in the former area is probably due to the large contribution of diurnal currents not considered here and also to the poor model performance in the Bohai Sea, where nontidal processes such as wave effects may be relevant to the formation of the latter. If we focus on sand ridges in the SE Yellow Sea and East China Sea, their spatial extent seems to agree with that of the intense bottom stress (exceeding 1.0 N/m2) at early transgressive stages when the extremely intense bottom stress (exceeding 2.0 N/ m2) was developed in nearshore areas (cf. Fig. 7b). Therefore, it is suggested that the sand ridges might have formed at sea levels from 90 to 45 m in the SE Yellow Sea and from 75 to 60 m in the East China Sea (cf. Fig. 7b). As suggested by previous examples, the bottom stress exceeding 2.0 N/m2 is likely to provoke intensive reworking. It is therefore indicated that tidal motions are involved in two factors that controls the formation of the sand ridges, the supply rate of sediments, and the intensity of the bottom stress. Though there are no direct dates measured for the sandy ridges, Lofi et al. (1999) reported radiocarbon dates at a bottom set of the ridges in the East China Sea (ca. 29jN, 125jE) which consisted of alternating fine sands and silt beds. They yield 14 C ages of 10550 F 50 and 10680 F 110 yr BP at 1.87 and 6.30 m below the sea floor, respectively. These periods are slightly younger than the formation age we have obtained, though fine sediments might reflect the weaker tidal condition after the ridge has formed. When the sea-level rose to about 45 to 15 m, the area of the intense bottom stress off the Changjiang Estuary extended over a wide inner shelf called Yangtze Shoal (Figs. 6 and 9b). During this period, the area of the extremely intense bottom stress exceeding 2.0 N/m2 was negligible in size (Fig. 9a).
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These results suggest that the intense tidal reworking did not substantially occur during this period even though tidal currents were stronger than present. The absence of significant reworking in this area is also reported by previous studies (e.g., Liu, 1997). On the other hand, Liu (1997) inferred that the lack of sand ridges on the Yangtze Shoal is due to the rotational nature of tidal currents in this area. However, our model results indicate that intense and reciprocal tidal currents may have existed in the southern part of the shoal when the sea level was 30 m (not shown). It is therefore suggested that the absence of sand ridges on the Yangtze Shoal might be ascribed to the shortage of sands to be supplied by the reworking process.
Sea and off the paleo-Changjiang Estuary during the early transgression stages, as summarized in Fig. 10. This result indicates that the Late Quaternary depositional sequences reported in these areas characterized by transgressive surface of erosion and overlying ridge-and-swale topography (Jin and Chough, 1998; Saito et al., 1998; Berne´ et al., 2002) are formed largely by tidal processes. The low occurrence of ‘extremely intense’ bottom stress during higher sealevel stages (Fig. 9a) suggests that the transgressive reworking process might have been weaker when the sea level was higher than 45 m, which could reflect to the lack of large-scale sedimentary features in the Yangtze Shoal.
5.2.4. Relation with Late Quaternary depositional sequences The distribution of ‘extremely intense’ and ‘intense’ bottom stresses suggests that transgressive reworking and subsequent formation of large-scale bedforms have occurred mainly at the SE Yellow
6. Conclusion
Fig. 10. Schematic picture summarizing the role of tidal stress on the sedimentary environment in the YECS region during the postglacial transgression.
(1) Tidal currents on the YECS shelf have generally been semi-diurnal throughout the postglacial stages, with intensities of spring – neap modulation and diurnal inequality remaining similar to present. (2) On the other hand, the spatial distribution of the maximum bottom stress changed greatly as the sea level rose. The region of intense bottom stress migrated across the shelf mainly through two routes: one from around Cheju Island toward the southwestern coast of Korea and another along the retreat path of the paleo-Changjiang Estuary. Coincidence between the location of these migrating paths and the distribution of transgressive deposits suggests that the ‘extremely intense’ and ‘intense‘ tidal bottom stress seems to have played a significant role on the formation of transgressive sequences found in these areas. (3) The area of extremely intense tidal bottom stress (over 2.0 N/m2) emerged most extensively when the sea level was between 90 and 45 m in the SE Yellow Sea, and 75 and 60 m in the East China Sea, the latter of which might be relevant to the large supply of terrigenous organic C to the shelf slope observed at similar periods. Sand ridges on these shelf regions were also estimated to have formed when the reworking processes were active. (4) Though estimated stress intensity on the Yangtze Shoal at low-sea-level stages was stronger than
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present, it was not strong enough to provoke significant tidal reworking of relict sediments. Absence of sand ridges in this area might be ascribed to the lack of tidally reworked sediments. (5) The region of weak bottom stress evolved in the central Yellow Sea and on the outer shelf of the East China Sea during the later phase of the sealevel rise. The weak bottom stress seems to be a requisite condition for the formation and maintenance of the offshore mud deposition.
Acknowledgements The authors would like to thank Dr. J. Okuno for valuable suggestions regarding the paleotopography in the YECS. We thank Dr. S.L. Goodbred, Jr., for encouraging us to contribute to this special issue. Valuable comments by two anonymous reviewers helped improving the manuscript. One of the authors (K.U.) was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture, and Technology of Japan. References Bahng, H.K., Lee, C.W., Oh, J.-K., 1994. Origin and characteristics of sand ridges in the western continental shelf of Korean Peninsula. J. Korean Soc. Oceanogr. 29, 217 – 227 (in Korean with English abstract). Berne´, S., Vagner, P., Guichard, F., Lericolais, G., Liu, Z., Trentesaux, A., Yin, P., Yi, H.I., 2002. Pleistocene forced regressions and tidal sand ridges in the East China Sea. Mar. Geol. 188, 293 – 315. Blain, C.A., 1997. Development of a data sampling strategy for semienclosed seas using a shallow water model. J. Atmos. Ocean Tech. 14, 1157 – 1173. Butenko, J., Milliman, J.D., Ye, Y., 1985. Geomorphology, shallow structure, and geological hazards in the East China Sea. Cont. Shelf Res. 4, 121 – 141. Choi, B.H., 1980. A tidal model of Yellow Sea and Eastern China Sea. Korea Ocean Research and Development Institute Rep. 80 – 02. 72 pp. Choi, K.S., Kim, B.O., Park, Y.A., 2001. Late Pleistocene tidal rhythmites in the Kyunggi Bay, west coast of Korea: a comparison with simulated rhythmites based on modern tides and implications for intertidal positioning. J. Sediment. Res. 71, 680 – 691. Fairbridge, R.W., 1968. Continents and oceans. In: Fairbridge, R.W. (Ed.), The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 177 – 186. Graber, H.C., Beardsley, R.C., Grant, W.D., 1989. Storm-generated
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surface waves and sediment resuspension in the East China and Yellow Seas. J. Phys. Oceanogr. 19, 1039 – 1059. Hanebuth, T., Stattegger, K., Grootes, P.M., 2000. Rapid flooding of the Sunda Shelf: a late-glacial sea-level record. Science 288, 1033 – 1035. Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Sato, Y., Li, C., 2001a. Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression. Mar. Geol. 177, 331 – 351. Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Sato, Y., Li, C., 2001b. Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China. Geomorphology 41, 233 – 248. Hori, K., Saito, Y., Zhao, Q., Wang, P., 2002. Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China. Sediment. Geol. 146, 249 – 264. Huang, D., Su, J., Backhaus, J.O., 1999. Modelling the seasonal thermal stratification and baroclinic circulation in the Bohai Sea. Cont. Shelf Res. 19, 1485 – 1505. Ikehara, K., Oshima, H., Katayama, H., Saito, Y., 1999. Palynological evidence on Latest Pleistocene – Holocene paleoenvironmental change in the East China Sea. In: Saito, Y., Ikehara, K., Katayama, H. (Eds.), Proceedings of an International Workshop on Sediment Transport and Storage in Coastal Sea – Ocean System. STA (JISTEC) and Geol. Survey of Japan, Tsukuba, Japan, pp. 183 – 188. Japan Meteorological Agency, 1999. Annual Report on Ocean Waves, vol. 4. Japan Meteorological Agency, Tokyo. 344 pp. Jin, J.H., Chough, S.K., 1998. Partitioning of transgressive deposits in the southeastern Yellow Sea: a sequence stratigraphic interpretation. Mar. Geol. 149, 79 – 92. Jung, W.Y., Suk, B.C., Min, G.H., Lee, Y.K., 1998. Sedimentary structure and origin of a mud-cored pseudo-tidal sand ridge, eastern Yellow Sea, Korea. Mar. Geol. 151, 73 – 88. Le Provost, C., Lyard, F., Molines, J.M., Genco, M.L., Rabilloud, F., 1998. A hydrodynamic ocean tide model improved by assimilating a satellite altimeter-derived data set. J. Geophys. Res. 103, 5513 – 5529. Liu, Z.X., 1997. Yangtze Shoal—a modern tidal sand sheet in the northwestern part of the East China Sea. Mar. Geol. 137, 321 – 330. Liu, Z.X., Xia, D.X., Berne, S., Wang, K.Y., Marsset, T., Tang, Y.X., Bourillet, J.F., 1998. Tidal deposition systems of China’s continental shelf, with special reference to the eastern Bohai Sea. Mar. Geol. 145, 225 – 253. Lofi, J., Vagner, P., Berne´, S., Weber, O., 1999. High frequency cyclicities in a tidal sand ridge of the East China Sea. In: Saito, Y., Ikehara, K., Katayama, H. (Eds.), Proceedings of an International Workshop on Sediment Transport and Storage in Coastal Sea – Ocean System. STA (JISTEC) and Geol. Survey of Japan, Tsukuba, Japan, pp. 162 – 167. Lu¨, G., Lin, H., Song, Z., Jia, J., 2000. Modeling of the tide wave system changes in the marginal seas adjacent to eastern China since the last full glacial period. Mar. Geol. Quat. Geol. 20, 1 – 7 (in Chinese with English abstract). Milliman, J.D., Shen, H.-T., Yang, Z.-S., Meade, R.H., 1985a. Transport and deposition of river sediment in the Changjiang
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estuary and adjacent continental shelf. Cont. Shelf Res. 4, 37 – 45. Milliman, J.D., Beardsley, R.C., Yang, Z.-S., Limeburner, R., 1985b. Modern Huanghe-derived muds on the outer shelf of the East China Sea. Cont. Shelf Res. 4, 175 – 188. Naimie, C.E., Blain, C.A., Lynch, D.R., 2001. Seasonal mean circulation in the Yellow Sea—a model-generated climatology. Cont. Shelf Res. 21, 667 – 695. Nakada, M., Yonekura, N., Lambeck, K., 1991. Late Pleistocene and Holocene sea-level changes in Japan: implications for tectonic histories and mantle rheology. Palaeogeogr. Palaeoclimatol. Palaeoecol. 85, 107 – 122. Oguri, K., Matsumoto, E., Saito, Y., Honda, M.C., Harada, N., Kusakabe, M., 2000. Evidence for the offshore transport of terrestrial organic matter due to the rise of sea level: the case of the East China Sea continental shelf. Geophys. Res. Lett. 27, 3893 – 3896. Oh, I.S., Lee, D.E., 1998. Tides and tidal currents of the Yellow and East China Seas during the last 13,000 years. J. Korean Soc. Oceanogr. 33, 137 – 145. Pingree, R.D., Griffiths, D.K., 1979. Sand transport paths around the British Isles resulting from M2 and M4 tidal interactions. J. Mar. Biol. Assoc. U. K. 59, 497 – 513. Saito, Y., 1998a. Sedimentary environment and budget in the East China Sea. Bull. Coast. Oceanogr. 36, 43 – 58 (in Japanese with English abstract). Saito, Y., 1998b. Sea levels of the last glacial in the East China Sea continental shelf. Quat. Res. (Jpn) (Daiyonki-Kenkyu) 37, 235 – 242 (in Japanese with English abstract). Saito, Y., Yang, Z., 1994. The Huanghe River: its water discharge, sediment discharge and sediment budget. J. Sediment. Soc. Japan 40, 7 – 17 (in Japanese with English abstract). Saito, Y., Katayama, H., Ikehara, K., Kato, Y., Matsumoto, E., Oguri, K., Oda, M., Yumoto, M., 1998. Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea. Sediment. Geol. 122, 217 – 232. Saito, Y., Yang, Z., Hori, K., 2001. The Huanghe (Yellow River) and Changjiang (Yangtze River) deltas: a review on their characteristics, evolutions and sediment discharge during the Holocene. Geomorphology 41, 219 – 231. Sternberg, R.W., Larsen, L.H., Miao, Y.T., 1985. Tidally driven sediment transport on the East China Sea continental shelf. Cont. Shelf Res. 4, 105 – 120. Su, J., 1998. Circulation dynamics of the China Seas north of 18jN. In: Robinson, A.R., Brink, K.H. (Eds.), The Sea, vol. 11. Wiley, New York, pp. 483 – 505. Suk, B.-C., 1989. Sedimentology and history of sea level changes in the East China Sea and adjacent seas. In: Taira, A., Masuda, F. (Eds.), Sedimentary Facies in the Active Plate Margin. Terra Sci. Publ., Tokyo, pp. 215 – 231.
Takahashi, S., Yanagi, T., 1995. A numerical study on the formation of circulations in the Yellow Sea during summer. La mer 33, 135 – 147. Thomas, M., Su¨ndermann, J., 1999. Tides and tidal torques of the world ocean since the last glacial maximum. J. Geophys. Res. 104, 3159 – 3183. Uehara, K., Saito, Y., Hori, K., 2002. Paleotidal regime in the Changjiang (Yangtze) Estuary, the East China Sea, and the Yellow Sea at 6 ka and 10 ka estimated from a numerical model. Mar. Geol. 183, 179 – 192. Wang, J., Wang, P., 1980. Relationship between sea-level and climatic fluctuation in East China since late Pleistocene. Acta Geogr. Sin. 35, 299 – 312 (in Chinese with English abstract). Wang, J., Lu, G., Lin, H., Song, Z., Jia, J., 1998. Developing process and mechanism of tidal sand ridges off the coast of Jiangsu Province. J. Nanjing Normal Univ. (Natural Sci.) 21, 95 – 108 (in Chinese with English abstract). Wells, J.T., 1988. Distribution of suspended sediment in the Korea Strait and southwestern Yellow Sea: onset of winter monsoons. Mar. Geol. 83, 273 – 284. Xia, Z., Carbajal, N., Su¨ndermann, J., 1995. Tidal current amphidromic system in semi-enclosed basins. Cont. Shelf Res. 15, 219 – 240. Yanagi, T., Inoue, K., 1994. Tide and tidal currents in the Yellow/ East China Seas. La mer 32, 153 – 165. Yanagi, T., Takahashi, S., Hoshika, A., Tanimoto, T., 1996. Seasonal variation in the transport of suspended matter in the East China Sea. J. Oceanogr. 52, 539 – 552. Yanagida, M., Kaizuka, S., 1982. Recent Chinese studies on the sea level change since the last interglacial in the Sea of Bohai, Yellow Sea, and East China Sea. Quat. Res. (Jpn) (Daiyonki-Kenkyu) 21, 115 – 122 (in Japanese with English abstract). Yang, C.-S., 1989. Active, moribund and buried tidal sand ridges in the East China Sea and the Southern Yellow Sea. Mar. Geol. 88, 97 – 115. Yang, C.-S., Sun, J.-S., 1988. Tidal sand ridges on the East China Sea shelf. In: de Boer, P.L., van Gelder, A., Nio, S.D. (Eds.), Tide-Influenced Sedimentary Environments and Facies. Reidel, Dordrecht, pp. 23 – 38. Yang, Z., Saito, Y., Guo, Z., Gui, Q., 1995. Distal mud area as a material sink in the East China Sea. In: Tsunogai, S., Iseki, K., Koike, I., Oba, T. (Eds.), Global Fluxes of Carbon and Its Related Substances in the Coastal Sea – Ocean – Atmosphere System. M and J International, Yokohama, pp. 1 – 6. Yoo, D.G., Lee, C.W., Kim, S.P., Jin, J.H., Kim, J.K., Han, H.C., 2002. Late Quaternary transgressive and highstand systems tracts in the northern East China Sea mid-shelf. Mar. Geol. 187, 313 – 328.
Late Quaternary evolution of the Yellow/East China Sea tidal regime and its impacts on sediments dispersal and seafloor morphology Katsuto Uehara a,*, Yoshiki Saito b a
Research Institute for Applied Mechanics, Kyushu University, Kasuga Koen 6-1, Kasuga, Fukuoka 816-8580, Japan b MRE, Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan
Abstract The evolution of a tidal-current field in the Yellow Sea and East China Sea (YECS) in response to the sea-level rise since the last glacial maximum (LGM) was investigated using a two-dimensional tidal model, with special attention to changes in the sedimentary environment of the continental shelf region. It was found that tidal currents on the YECS shelf have generally been semi-diurnal throughout the post glacial stages, with F-ratio (index of diurnal inequality) and S2/M2 ratio (index of spring – neap cycle) remaining similar to present. On the other hand, the spatial distribution of the tidal bottom stress associated with M2 and M4 tidal currents changed greatly during the transgression. As the sea level rose, two core regions showing high bottom-stress values migrated shoreward from Cheju Island toward the west coast of Korea, and along the retreat path of the paleo-Changjiang Estuary. Distribution of ‘extremely intense’ tidal bottom stress suggested that intense tidal reworking may have occurred at sea levels from 90 to 45 m in the southeast Yellow Sea and at 75 to 60 m off the paleo-Changjiang Estuary, the latter of which agreed with the timing of a large terrigenous organic C flux observed on the shelf slope. Distribution of slightly weaker bottom stress indicated that the sand ridges off the Changjiang Estuary and in the southeast Yellow Sea also might have formed at during those periods. On the other hand, when the sea level was between 45 and 15 m, the tidal bottom stress was generally not strong enough to invoke significant reworking, even though the stress intensity may be stronger than present. Correlation with modern and relict sedimentary features suggested that the tidal process have played a significant role on the reworking and deposition of sediments on the YECS shelf. D 2003 Elsevier B.V. All rights reserved. Keywords: Transgressive sequence; Tidal current; Reworking; Sand ridge; Numerical modeling; Changjiang
1. Introduction After the last glacial maximum (LGM) at about 20 ka, eustatic sea-level rise of more than 100 m inun* Corresponding author. Tel.: +81-92-583-7728; fax: +81-92573-1996. E-mail address: [email protected] (K. Uehara). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0037-0738(03)00234-3
dated large areas and formed the vast continental shelf of the modern Yellow and East China seas (YECS) (Wang and Wang, 1980; Yanagida and Kaizuka, 1982). One of the direct impacts of the postglacial sea-level rise on the sedimentary environment in the YECS was the modification of the tidal-current regime, the nature of which depends greatly on basin shape.
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The tidal current is one of the most energetic oceanic components of the present YECS, and it plays an important role in sedimentary processes such as deposition, erosion, and resuspension. The influence of the tidal current is also found in sedimentary strata and bedform features that were generated during the postglacial transgression. Hori et al. (2001a) recovered successions of sand – mud couplets at 47.5 –48.5 m depth in a borehole sample taken from the modern Changjiang (Yangtze River) delta, which seem to reflect fortnightly cycles of tidal currents. Furthermore, the geological record indicates that the tidalcurrent regime in the YECS might have been different when the sea level was lower than at present. For example, the moribund tidal sand ridges on the shelf of the East China Sea (Fig. 1) are considered to have formed when sea level was lower than present because modern tidal-current conditions are insufficient for
tidal sand-ridge field generation (Yang and Sun, 1988; Yang, 1989; Liu et al., 1998; Saito et al., 1998). In recent years, several numerical simulations have been carried out to verify changes in the YECS tidal regime that occurred in response to postglacial sealevel rise. Oh and Lee (1998) predicted tides and tidal currents by modeling the lowering of the sea level but retaining the present basin configuration. According to their model, tidal amplitudes and the number of tidal amphidromes in the YECS increased as the sea level rose. Wang et al. (1998) considered the shoreline changes caused by sedimentation along the Chinese coast, and their model was extended by Lu¨ et al. (2000) to predict the K1 constituent. Uehara et al. (2002) used paleobathymetry values that took into account the effect of the thick sedimentary strata deposited in the paleo-Changjiang Estuary, and they pointed out that the tidal current on the shelf was generally stronger when sea level was 45 m lower than at present. The purpose of this paper is to elucidate Late Quaternary evolution of the tidal-current field in the YECS and to clarify the effects on sedimentary processes during the postglacial transgression. We emphasize the verification of tidal-current features such as spring –neap cycles and spatial shifts of the region of intense tidal currents that accompanied the sea-level rise because these are essential for interpreting the formation and maintenance of the geological features preserved in the modern YECS.
2. Regional setting
Fig. 1. Map of the study area. Features drawn in the region southeast of the Changjiang Estuary and in the Yellow Sea indicate the location of major sand ridges in the YECS. Numerals denote water depth in meters. Three lines illustrate the location of transects used in Fig. 7.
The Yellow and East China seas form an epicontinental sea enclosed by the Chinese and Korean coasts, and a marginal sea separated from the main Pacific Ocean by the Japanese islands and Taiwan Island, respectively (Fig. 1). The YECS is also connected to the South China Sea by the Taiwan Strait (sill depth, about 60 m) and to the Japan Sea by the Tsushima Strait (sill depth, about 130 m). The YECS has three parts: the Bohai Sea, the Yellow Sea, and the East China Sea. The Bohai Strait separates the Bohai Sea from the Yellow Sea, and a line connecting Cheju Island with the north end of mouth of the Changjiang (Yangtze River) forms the boundary between the Yellow and East China seas. The bathymetry of the
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Fig. 2. Grain-size distribution of sea-bed sediments in the YECS; sand < 4 phi, silt 4 – 8 phi, and clay >8 phi, median grain size, after Saito and Yang (1994). Thick dashed lines denote the bottom-stress contour of 0.35 N/m2 for the present day derived from Fig. 6h.
YECS is characterized by a vast continental shelf, the seventh largest in the world (Fairbridge, 1968). This shelf, which is shallower than 150 m, occupies more than 70% of the area of the YECS. The sea floor of the shelf gradually deepens to the SSE or toward the axis that lies mostly along the east coast of the YECS. The modern YECS shelf is covered by sand and mud (Fig. 2). Muddy sediments in the Bohai Sea are supplied predominantly by the Huanghe (Yellow River), while those along the Chinese coast south of about lat 32jN are supplied by the Changjiang. The sediment discharge from these two Chinese rivers constitutes nearly 90% of the total sediment supply derived from the rivers that flow into the YECS (Saito, 1998a). Muddy sediments on the slope of the inner shelf extending from the Chinese coast, in the central Yellow Sea and in the region south of Cheju Island, are considered to be derived mainly from the Huanghe or the old Huanghe delta at Jiangsu (Milli-
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man et al., 1985a; Saito, 1998a). Sandy sediments with ripples and megaripples are distributed on the inner shelf at depths of 25 – 50 m east of the Changjiang mouth (Butenko et al., 1985; Hori et al., 2002). The sandy sediments on the middle to outer shelf in the East China Sea and along the west coast of Korea are thought to be transgressive sediments deposited since the LGM (Suk, 1989). Suspended sediments are transported mainly by subtidal currents because tidal oscillations contribute little to basin-scale sediment transport over the long term (Naimie et al., 2001). The circulation pattern of the YECS shows an apparent seasonal variation controlled by the East Asian monsoon climate (Fig. 3; after Su, 1998). In general, coastal currents along the Chinese and Korean coasts flow southward in winter and northward in summer in response to prevailing wind directions. The shelf water column is nearly homogeneous in winter, while surface heating, tidal mixing, and freshwater discharge from the Changjiang modify the density structure and flow pattern in summer (Takahashi and Yanagi, 1995; Naimie et al., 2001). Except for the Kuroshio, which flows along the shelf edge with a current speed exceeding 1 m/s, subtidal flows in the YECS are variable in both intensity and position, and they are usually obscured by strong tidal current signals. Erosion and resuspension on the YECS shelf are caused mainly by tidal currents (Milliman et al., 1985b) and storm-generated surface waves (Wells, 1988; Graber et al., 1989). Wind-generated waves in the YECS are fetch-limited; under normal conditions, wave height is less than 2 –3 m in the southern YECS and less than 1 – 2 m in the northern YECS. Waves as high as 4 – 6 m may occur when severe storms or typhoons pass through the YECS region (Japan Meteorological Agency, 1999). Proxy data suggest that the basin geometry of the YECS has changed greatly during the last 20 kyr, primarily as a result of postglacial sea-level changes; sea level is estimated to have been 120 F 5 m at 20 ka, 45 F 5 m at 10 ka, and 0 F 3 m at 6 ka, relative to the present level (Saito, 1998b; Hanebuth et al., 2000). A large portion of the YECS shelf was exposed subaerially during the LGM, which was followed by a large-scale transgression. The incised valley of the paleo-Changjiang, which had formed during the sealevel lowstand, was inundated, creating an elongated
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Fig. 3. Schematic map of the general circulation pattern of the China seas for (a) winter and (b) summer (after Su, 1998). The current features identified are the Bohai Coastal Current (BC), the Yellow Sea Coastal Current (YSCC), the Changjiang River Plume (CRP), the East China Sea Coastal Current (ECSCC), the Taiwan Warm Current (TWC), the Tsushima Current (TC), the Yellow Sea Warm Current (YSWC), the Korean Coastal Current (KCC), the South China Sea Coastal Current (SCSCC), the South China Sea Warm Current (SCSWC), and the Donsha Current (DC). From SU, Jilan, 1998. Circulation dynamics of the China seas north of 18N coastal segment. In: The Sea, Volume 11, Chapter 16, Robinson, A.R. and Brink, K.H. Eds., John Wiley & Sons, Inc., New York, pp. 483 – 505. ISBN 0-471-11545-2. This material is used by permission of John Wiley & Sons, Inc.
paleoestuary, which was 400 km long at about 10 ka. During the last maximum transgression at about 6 ka, the shoreline receded greatly along the western coastline of the Bohai and Yellow seas. Since then, as a result of high sediment discharges from the Huanghe and Changjiang rivers and the stable or slightly falling sea level, the shoreline has migrated seaward to form the present coastline (Saito et al., 2001).
3. Tidal model We used the two-dimensional tidal model of Uehara et al. (2002) to predict the tidal-current field (M2, S2,
K1, O1, N2, and M4 constituents) of the YECS area shown in Fig. 1 (lat 24jN – 42jN, long 116jE – 131jE; resolution, 1j/12). Eight numerical runs were conducted for different sea levels (15-m intervals from 90 m to 0 m) and bathymetry configurations (Table 1). To minimize the effect of sedimentary strata subsequently deposited along the Chinese coasts, we modeled paleotopographies for two periods: 6 ka (sea level, 0 m) and 10 ka (sea level, 45 m). Special care was taken to reconstruct the paleotopography of the Changjiang Estuary (lat 31jN – 33jN, long 119jE –123jE), which was accomplished by compiling existing borehole data (Hori et al., 2001a,b) to remove the effect of sedimentary strata deposited after the periods being modeled. We also used paleocoastline data along the Jiangsu coast, the Bohai Sea, and Hanzhou Bay for the same reason. For cases other than a00 (present condition), b00 (6 ka), and c45 (10 ka), bathymetry values were obtained by reducing water depth from one of the two modeled paleotopographies. Water depths shallower than 5 m were redefined as 5 m to prevent numerical instabilities. The quadratic friction and horizontal eddy viscosity coefficients were 2 10 3 and 100 m2/s, respectively. For the open boundary values, we used harmonic constants of the M2, S2, K1, O1, and N2 tidal constituents estimated by using a global model based on the present configuration (Le Provost et al., 1998). On the basis of their global paleotidal model, Thomas and Su¨ndermann (1999) suggested that the tidal amplitude change of the M2 constituent along the open boundary was less than 0.05 m during the last 21 kyr except in the Taiwan Strait and limited regions north and south of Kyushu Island. Since that value is within the precision Table 1 Tidal experiments conducted in this study, which assume different relative sea levels (R.S.L.) and model topographies Case a00 b00 b15 b30 c45 c60 c75 c90
R.S.L. (m) 0 0 15 30 45 60 75 90
Model topography present 6 ka 6 ka ( 15 m) 6 ka ( 30 m) 10 ka 10 ka ( 15 m) 10 ka ( 30 m) 10 ka ( 45 m)
In cases b15, b30, c60, c75, and c90, depths were reduced from the original topography by the value shown in parentheses.
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of our model (0.17 m for amplitude and 15 degrees for phase), we used the same boundary values in all cases. Crustal-rebound effects from hydro-isostasy have also given rise to changes in sea level since the LGM (Nakada et al., 1991). However, they were not taken into account by the paleotopography and sea levels used in this study. The model is not applicable for discussing absolute tidal-current intensities in the Bohai Sea, most remote location from the open boundaries, since it systematically underestimate the values observed (e.g., Liu et al., 1998) or those obtained by regional models (e.g., Huang, 1999), which is a common defect of many YECS-scale models (e.g., Yanagi and Inoue, 1994; Blain, 1997; Naimie et al., 2001). As for the Bohai Sea, we only used ratios between tidal constituents for analyses because they agreed well with those predicted by regional models.
4. Results Although results are shown for the YECS as a whole, we refer mostly to the tidal-current structure on the continental shelf (shoreward of the 150-m depth contour, indicated by a dotted line in the figures)
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because our main focus was the tidal-current structure of the shallow shelf waters. 4.1. Diurnal – semidiurnal current amplitude ratio Fig. 4 shows the ratio of diurnal (K1 and O1) to semidiurnal (M2 and S2) tidal-current amplitudes ( F-ratio) in the YECS for cases c90, c45, and a00. This ratio measures the importance of the diurnal component contained in tidal-current signals. Tidal currents on the shelf were dominantly either semidiurnal ( F < 0.25) or mixed (semidiurnal dominated; 0.25 < F< 1.25) in all cases. Even in the case of mixed-type currents, the F-ratio was less than 0.5 except in limited regions. The distribution of semidiurnal and mixedtype tidal currents modeled by using the present shelf configuration (Fig. 4c) was consistent with that reported by Choi (1980). During the early stage of the postglacial transgression, tidal currents in the shelf area were dominantly semidiurnal and the F-ratio was less than 0.4 everywhere on the shelf (Fig. 4a). As the sea level rose, mixed-type (semidiurnal dominated) tidal currents became evident in the Tsushima Strait, the central Yellow Sea, and the Bohai Sea (Fig. 4b and c). Mixedtype currents (diurnal dominated; 1.25 < F <3)
Fig. 4. Distributions of diurnal (K1 and O1 constituents) to semidiurnal (M2 and S2 constituents) ratios of tidal-current amplitudes (F-ratio) for cases (a) c90 (sea level, 90 m), (b) c45 (sea level, 45 m), and (c) a00 (present). The contours shown are 0.25, 0.5, 0.75, 1.00, 1.25, and 3. The dotted line in each panel denotes the 150-m depth contour, which is the approximate shelf edge in the YECS.
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appeared in the southeastern Bohai Strait and southwestern Tsushima Strait during the last phase of the sea-level rise (Fig. 4c). The development of the highF region during the transgression stage may be related to the evolution of the amphidromic system that accompanied the sea-level rise (Oh and Lee, 1998) because the location of the F-ratio maximum corresponds to the current amphidromic point (Xia et al., 1995) of the semidiurnal current. The spatial distribution of the F-ratio suggests that the tidal current on the YECS shelf has had predominantly semidiurnal components throughout the postglacial period, except in the southern parts of the Bohai and Tsushima straits.
30 –45 m lower than at present (Fig. 5b). The value greater than 0.45 was caused by a decrease in the M2 tidal-current constituent in the southern shelf area, which might have been related to the opening of the Taiwan Strait. In most regions on the YECS shelf, temporal variability of the S2/M2 ratio remained within 0.1 even though the water depth increased by 90 m. The results shown here suggest that the spring– neap cycle of the tidal currents occurred even when the sea level was lower than at present, and its intensity was not significantly modified during the postglacial transgression. 4.3. Maximum bottom stress associated with M2 and M4 tidal currents
4.2. S2/M2 ratio The coexistence of M2 and S2 constituents gives rise to a fortnightly modulation of tidal-current amplitudes, which is an important factor affecting sediment transport in coastal regions. The relative intensity of the fortnightly modulation is shown for three sea-level stages in terms of the ratio of S2 to M2 tidal-current amplitudes (Fig. 5). The S2/M2 ratio in the YECS shelf area ranged mostly within 0.25 – 0.45. An exception was that on the East China Sea shelf south of about lat 29jN when the sea level was
To provide lowest-order information regarding the effect of tidal motion on sediment dynamics, the spatial distribution of the maximum bottom stress associated with M2 and M4 tidal currents was calculated by using a formulation of Pingree and Griffiths (1979) and Milliman et al. (1985b) for relative sea levels from 90 to 0 m (Fig. 6). The maximum stress is defined here as the maximum value of the quadratic bottom stress (qCD | u | 2) over a tidal cycle, where q is the density of seawater ( = 1023 kg/m3), CD is the bottom stress coefficient ( = 2.0 10 3),
Fig. 5. Distributions of the ratios of S2 to M2 tidal-current amplitudes. The contour interval is 0.05. The cases are as in Fig. 4.
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Fig. 6. Regional changes in the maximum bottom-stress field associated with M2 and M4 tidal currents in response to the postglacial sea-level rise. The cases shown are (a) c90, (b) c75, (c) c60, (d) c45, (e) b30, (f) b15, (g) b00, and (h) a00 (see Table 1 for the experimental conditions). The contours shown are 0.35, 0.6, 1.0, and 2.0 N/m2, which correspond to tidal-current speeds of 0.41, 0.54, 0.70, and 0.99 m s 1, respectively. It should be noted that the bottom stresses introduced here are based on depth-averaged currents and are not applicable to dynamic relations derived from near-bottom current velocities (e.g., Sternberg et al., 1985).
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and u is the combined M2 and M4 depth-averaged currents. Since CD was assumed to take a constant value, the bottom stress is proportional to the kinetic energy of the tidal current, or the square of the tidalcurrent velocity. Fig. 6 depicts contour distribution of three maximum-stress values that were relevant to specific seabed features on the YECS shelf. The 0.35 N/m2 stressintensity contour for the present-day configuration (case a00; Fig. 6h) roughly coincided with the shoreward edge of the region of mud deposits in the area south of Cheju Island and the central Yellow Sea (Fig. 2), which suggests that that value is a regional limit of tidal influence on fine-sediment deposition. The spatial distribution of megaripple features off the Changjiang Estuary (Butenko et al., 1985; Liu, 1997) and of sand waves off the Keum River (Korea) estuary (Bahng et al., 1994) at the present age corresponds to the 0.6 N/m2 bottom-stress contour (Fig. 6h). Uehara et al. (2002) indicated that high M2 velocity in excess of 0.7 m/s (equivalent to bottom-stress intensity of 1.0 N/m2 for the present study) during the transgression stages coincides roughly with the area where moribund sand ridges exist on the inner to outer shelf of the East China Sea (Fig. 1). The relationship between the bottom-stress distribution and bedform features is examined in the next section. 4.4. Changes in the distribution of bottom stressses during transgression When the sea level was low, tidal currents on the YECS shelf were more energetic and exerted stronger influence on the sea floor than those at present. Values larger than 0.35 N/m2 predominated on the shelf, and values greater than 1.0 N/m2 were observed on the East China Sea shelf, in the region adjacent to Cheju Island, and within the Tsushima Strait (cases c90 and c75; Fig. 6a and b). The core of the high-stress region on the East China Sea shelf was at the mouth of the paleo-Changjiang Estuary, and it migrated northwestward as the Chinese coastline receded (cases c60 and c45; Fig. 6c and d), while the high-stress values around Cheju Island shifted northward along the coast of Korea. In general, high bottom-stress values were observed in shallow waters where the absolute depth was less than 40 m and their spatial extent was reduced as the sea level rose.
On the other hand, the spatial distribution of bottom-stress values less than 0.35 N/m2 expanded in regions north of Taiwan Island, along the east coast of the Yellow Sea, and off the southern coast of Korea during the transgression stages. This may be ascribed to the evolution of the YECS amphidromic system associated with the postglacial sea-level rise (Oh and Lee, 1998) because the locations of minimum values coincide with current amphidromic points. The existence of degenerate current amphidromic points in the western Yellow Sea during the sea-level stages from 60 to 30 m has given rise to an asymmetric bottom-stress field that extends in a transverse direction (Fig. 6c, d, and e). When sea level was 30 – 15 m lower than at present, the inner shelf off the modern Changjiang mouth was characterized by high bottom stresses with values exceeding 1.0 N/m2, which indicates a strong influence of tidal currents during these periods (cases b30 and b15; Fig. 6e and f). Bottom stresses along the Korean coast were slightly weaker during these periods than during previous stages, and the highstress region migrated shoreward over the inner shelf as the water depth increased. The bottom-stress distribution was similar to that at present at the time of maximum transgression (case b00; Fig. 6g). A notable difference, however, between the time of maximum transgression and the present was that a high-stress region was found at 6 ka in the paleoChangjiang Estuary, which was located to the northwest of the present river mouth.
5. Discussion The previous section suggests that the F-ratio and S2/M2 ratio of tidal currents did not change significantly during the Late Quaternary except at around the Tsushima and Bohai straits. The former value is a measure of diurnal inequality while the latter reflects the intensity of spring –neap cycle. These findings are consistent with stratigraphic records found in the Kyunggi Bay, which indicates that rhythmicities preserved in the Late Pleistocene tidal rhythmites are similar to those of modern tidal cycles (Choi et al., 2001). Rhythmic laminations containing spring– neap cyclicities of the postglacial periods are also recovered from the modern Changjiang Delta (Hori et al.,
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2001a), southeastern Yellow Sea (Jin and Chough, 1998), and East China Sea (Lofi et al., 1999). In contrast, the distribution of maximum bottom stress associated with M2 and M4 tidal currents shows clearly the changes at sea levels from 90 to 0 m. The following text correlates such changes with various modern and relict sedimentary features in the YECS to show that variations in tidal bottom stresses have been an important control on sediment distribution and the development of seafloor morphology. 5.1. Mud patch formation and low tidal bottom stress Comparison of the bottom stress and modern grain-size distribution suggest that mud deposits in the modern YECS form where the tidal bottom stress is less than 0.35 N/m2 (Fig. 2). As the bottom stress decreased with the rise of sea level (Fig. 8b), it is possible to infer the timing when the tidal condition had been transformed to a state favorable for the
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mud deposition. In the mud-patch south of Cheju Island, the bottom stress became less than 0.35 N/m2 at sea levels between 45 and 30 m (ca. 10 –9 14 C kyr BP after Saito, 1998b) in the northeastern part of the mud patch, between 30 and 15 m (ca. 9 – 8 14C kyr BP) in the central area, and between 15 and 0 m (ca. 8– 7 14C kyr BP) in the southwestern region. Radiocarbon ages of the base of muddy sediments in the central part of the mud patch (32jN, 126jE) show less than 8 14C kyr BP (Yang et al., 1995), whereas those of the mud layer in the southwestern area were younger than 6.5 14C kyr BP (Yoo et al., 2002), which indicate that the mud patch was formed after the tidal bottom stress has become sufficiently weak. These results suggest that the weak tidal current accompanied by the postglacial sea-level rise is one of the necessary conditions for the formation and maintenance of the mud patch south of Cheju Island, although there exist other significant pro-
Fig. 7. Lateral extent of the region where the bottom stress associated with M2 and M4 tidal currents exceeded 1.0 N/m2 at sea levels (a) 90, 75, 60, 45, 30, 15, and 0 m, and (b) 75, 60, and 45 m. Thick black lines denote the location of major sand ridges.
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cesses such as supply rate of fine sediments, flow regimes of wind- and density-driven currents, and wave conditions. 5.2. Migration of intense bottom-stress region with the sea-level rise The region of intense bottom stress exceeding 1.0 N/m2 migrated across the shelf mainly through two routes in the YECS when the sea level rose from 90 to 0 m (Fig. 7a): one in the eastern Yellow Sea from around Cheju Island toward the west coast of Korea (Fig. 8a) and another in the East China Sea along the retreat path of the paleo-Changjiang Estuary (Fig. 8c). 5.2.1. Extremely intense bottom stress The bottom stress in the nearshore portion of the intense bottom-stress region (>2.0 N/m2 in extreme cases) was generally higher than that in the offshore region (Figs. 6, 8a, and c). The occurrence of such extremely intense bottom stress at sea levels from
Fig. 9. Changes in area where the tidal bottom stress exceeded (a) 2 N/m2 and (b) 1 N/m2 in the eastern Yellow Sea (32.5jN – 33jN, 125jE – 128jE and 33jN – 40jN, 123.5jE – 128jE) and off the Changjiang Estuary (26jN – 32.5jN, 119jE – 126jE and 32.5jN – 33jN, 119jE – 125jE; 33jN – 35.5jN, 119jE – 123.5jE) for sea levels from 90 to 0 m.
Fig. 8. Evolution of tidal bottom stress associated with M2 and M4 tidal currents along three transects shown in Fig. 1: (a) southern Yellow Sea, (b) south of Cheju Island, and (c) retreat path of the paleo-Changjiang Estuary. Bottom stress values shown on contours are in N/m2.
90 to 0 m is shown in Fig. 9a for the eastern Yellow Sea, southeastern (SE) Yellow Sea, and East China Sea. In the SE Yellow Sea, the intense bottom stress was most extensive during the early transgression stages and reduced its extent after the sea level has reached 75 m. This reduction may be associated with the sparse distribution of sand ridges in the modern nearshore region compared to the offshore area in the SE Yellow Sea, as the sparseness is estimated to have been caused by the reduced supply of sand fed by reworking processes (Jin and Chough, 1998). During the late phase of the sea level rise, on the other hand, the high-energy area was developed rapidly in the Kyunggi Bay and the West Korean Bay (Figs. 6 and 9a) which may be relevant to the evolution of ‘pseudo-tidal’ sand ridges off the Tae-
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an Peninsula blanketed by recently reworked relict (palimpsest) sands (Jung et al., 1998). In the East China Sea, the high-energy area existed near the mouth of the paleo-Changjiang Estuary (Fig. 6) for relatively limited periods (sea level from 75 to 60 m; Fig. 9a), which will be discussed in the following text. 5.2.2. Correlation between extremely intense bottom stress and sediment supply to the continental slope and the Okinawa trough Oguri et al. (2000) examined the time sequence of the stable isotope ratio of organic carbon (y13Corg) for the last 15 14C kyr that was obtained from core samples on the continental slope (F-3PC; 28j40’N, 127j07’E) and in the Okinawa Trough (P-4PC; 28j12’N, 127j14’E) and showed distinct depletion of the y13Corg ratio from 14 to 12 14C kyr BP. This depletion is larger than the stage of low stand of sea level in the LGM and indicates that the large supply of terrestrial organic C from the East China Sea shelf to the slope was occurring during those periods. One possible source of this terrigenous organic C is sediments supplied by the reworking of the shelf, as non-arboreal pollen content in the core F-3PC increased greatly at stages when y13Corg was depleted (Ikehara et al., 1999). Fig. 9a shows that the occurrence of very strong bottom stress (over 2.0 N/m2) on the East China Sea shelf was most extensive when the sea level was between 75 and 60 m, or about 13.5 to 12 14C kyr BP according to the sea-level curve of Saito (1998b). The coincidence in the period of y13Corg depletion and the stress-intensity maximum indicates that an extensive tidal reworking may have occurred during this interval. Furthermore, intense tidal current on the outer shelf may have enhanced sediment transports near the shelf edge. It is therefore suggested that the ‘extremely intense’ tidal bottom stress have played a significant role on the large supply of terrigenous organic C to the shelf slope at 14 –12 14C kyr BP, though other factors such as the increase of river discharge may have been involved in this event. The situation is in contrast to the modern case when strong tidal currents are limited to the nearshore and distant from the shelf edge. At present, across-shelf transport is considered to be maintained primarily by vertical circulation induced by monsoonal winds (Yanagi et al., 1996).
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5.2.3. Intense bottom stress and large-scale bedform features As the location of the sand ridges in the YECS generally falls within the spatial range of the intense bottom stress (exceeding 1.0 N/m2) during postglacial periods (Fig. 7a), the bottom stress intensity of 1.0 N/ m2 is suggested to be the threshold value for the formation of sand ridges. The only exceptions found in Fig. 7a are ridges north of the Bohai Strait and a northern half of the ridges off Jiangsu coast. Discrepancy found in the former area is probably due to the large contribution of diurnal currents not considered here and also to the poor model performance in the Bohai Sea, where nontidal processes such as wave effects may be relevant to the formation of the latter. If we focus on sand ridges in the SE Yellow Sea and East China Sea, their spatial extent seems to agree with that of the intense bottom stress (exceeding 1.0 N/m2) at early transgressive stages when the extremely intense bottom stress (exceeding 2.0 N/ m2) was developed in nearshore areas (cf. Fig. 7b). Therefore, it is suggested that the sand ridges might have formed at sea levels from 90 to 45 m in the SE Yellow Sea and from 75 to 60 m in the East China Sea (cf. Fig. 7b). As suggested by previous examples, the bottom stress exceeding 2.0 N/m2 is likely to provoke intensive reworking. It is therefore indicated that tidal motions are involved in two factors that controls the formation of the sand ridges, the supply rate of sediments, and the intensity of the bottom stress. Though there are no direct dates measured for the sandy ridges, Lofi et al. (1999) reported radiocarbon dates at a bottom set of the ridges in the East China Sea (ca. 29jN, 125jE) which consisted of alternating fine sands and silt beds. They yield 14 C ages of 10550 F 50 and 10680 F 110 yr BP at 1.87 and 6.30 m below the sea floor, respectively. These periods are slightly younger than the formation age we have obtained, though fine sediments might reflect the weaker tidal condition after the ridge has formed. When the sea-level rose to about 45 to 15 m, the area of the intense bottom stress off the Changjiang Estuary extended over a wide inner shelf called Yangtze Shoal (Figs. 6 and 9b). During this period, the area of the extremely intense bottom stress exceeding 2.0 N/m2 was negligible in size (Fig. 9a).
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These results suggest that the intense tidal reworking did not substantially occur during this period even though tidal currents were stronger than present. The absence of significant reworking in this area is also reported by previous studies (e.g., Liu, 1997). On the other hand, Liu (1997) inferred that the lack of sand ridges on the Yangtze Shoal is due to the rotational nature of tidal currents in this area. However, our model results indicate that intense and reciprocal tidal currents may have existed in the southern part of the shoal when the sea level was 30 m (not shown). It is therefore suggested that the absence of sand ridges on the Yangtze Shoal might be ascribed to the shortage of sands to be supplied by the reworking process.
Sea and off the paleo-Changjiang Estuary during the early transgression stages, as summarized in Fig. 10. This result indicates that the Late Quaternary depositional sequences reported in these areas characterized by transgressive surface of erosion and overlying ridge-and-swale topography (Jin and Chough, 1998; Saito et al., 1998; Berne´ et al., 2002) are formed largely by tidal processes. The low occurrence of ‘extremely intense’ bottom stress during higher sealevel stages (Fig. 9a) suggests that the transgressive reworking process might have been weaker when the sea level was higher than 45 m, which could reflect to the lack of large-scale sedimentary features in the Yangtze Shoal.
5.2.4. Relation with Late Quaternary depositional sequences The distribution of ‘extremely intense’ and ‘intense’ bottom stresses suggests that transgressive reworking and subsequent formation of large-scale bedforms have occurred mainly at the SE Yellow
6. Conclusion
Fig. 10. Schematic picture summarizing the role of tidal stress on the sedimentary environment in the YECS region during the postglacial transgression.
(1) Tidal currents on the YECS shelf have generally been semi-diurnal throughout the postglacial stages, with intensities of spring – neap modulation and diurnal inequality remaining similar to present. (2) On the other hand, the spatial distribution of the maximum bottom stress changed greatly as the sea level rose. The region of intense bottom stress migrated across the shelf mainly through two routes: one from around Cheju Island toward the southwestern coast of Korea and another along the retreat path of the paleo-Changjiang Estuary. Coincidence between the location of these migrating paths and the distribution of transgressive deposits suggests that the ‘extremely intense’ and ‘intense‘ tidal bottom stress seems to have played a significant role on the formation of transgressive sequences found in these areas. (3) The area of extremely intense tidal bottom stress (over 2.0 N/m2) emerged most extensively when the sea level was between 90 and 45 m in the SE Yellow Sea, and 75 and 60 m in the East China Sea, the latter of which might be relevant to the large supply of terrigenous organic C to the shelf slope observed at similar periods. Sand ridges on these shelf regions were also estimated to have formed when the reworking processes were active. (4) Though estimated stress intensity on the Yangtze Shoal at low-sea-level stages was stronger than
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present, it was not strong enough to provoke significant tidal reworking of relict sediments. Absence of sand ridges in this area might be ascribed to the lack of tidally reworked sediments. (5) The region of weak bottom stress evolved in the central Yellow Sea and on the outer shelf of the East China Sea during the later phase of the sealevel rise. The weak bottom stress seems to be a requisite condition for the formation and maintenance of the offshore mud deposition.
Acknowledgements The authors would like to thank Dr. J. Okuno for valuable suggestions regarding the paleotopography in the YECS. We thank Dr. S.L. Goodbred, Jr., for encouraging us to contribute to this special issue. Valuable comments by two anonymous reviewers helped improving the manuscript. One of the authors (K.U.) was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture, and Technology of Japan. References Bahng, H.K., Lee, C.W., Oh, J.-K., 1994. Origin and characteristics of sand ridges in the western continental shelf of Korean Peninsula. J. Korean Soc. Oceanogr. 29, 217 – 227 (in Korean with English abstract). Berne´, S., Vagner, P., Guichard, F., Lericolais, G., Liu, Z., Trentesaux, A., Yin, P., Yi, H.I., 2002. Pleistocene forced regressions and tidal sand ridges in the East China Sea. Mar. Geol. 188, 293 – 315. Blain, C.A., 1997. Development of a data sampling strategy for semienclosed seas using a shallow water model. J. Atmos. Ocean Tech. 14, 1157 – 1173. Butenko, J., Milliman, J.D., Ye, Y., 1985. Geomorphology, shallow structure, and geological hazards in the East China Sea. Cont. Shelf Res. 4, 121 – 141. Choi, B.H., 1980. A tidal model of Yellow Sea and Eastern China Sea. Korea Ocean Research and Development Institute Rep. 80 – 02. 72 pp. Choi, K.S., Kim, B.O., Park, Y.A., 2001. Late Pleistocene tidal rhythmites in the Kyunggi Bay, west coast of Korea: a comparison with simulated rhythmites based on modern tides and implications for intertidal positioning. J. Sediment. Res. 71, 680 – 691. Fairbridge, R.W., 1968. Continents and oceans. In: Fairbridge, R.W. (Ed.), The Encyclopedia of Geomorphology. Reinhold Book, New York, pp. 177 – 186. Graber, H.C., Beardsley, R.C., Grant, W.D., 1989. Storm-generated
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