Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea

Marine Geology, 98 (1991) 51-72

51

Elsevier Science Publishers B.V., Amsterdam

Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea C.R. Alexander "'b, D.J. D e M a s t e r c and C.A. Nittrouer a aMarine Sciences Research Center, State University of New York, Stony Brook, N Y 11794-5000, USA bpresent address: Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31416, USA CDept. of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695-8208, USA

(Received March 5, 1990; revision accepted October 3, 1990)

ABSTRACT Alexander, C.R., DeMaster, D.J. and Nittrouer, C.A., 1991. Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea. Mar. Geol., 98: 51-72. Sediment accumulation in the Yellow Sea epicontinental-shelf environment is investigated on 100-yr and 1000-yr time scales using 21°pb and 14C geochronologies. The distribution of modern (21°pb) accumulation rates in the Yellow Sea reveals that the loci of modern Huanghe sediment accumulation are the topset (1-2 mm/yr), foreset (4-9 mm/yr), and proximal bottomset deposits (2-4 mm/yr) of the Shandong subaqueous delta (which extends south from the Shandong Peninsula). 21°pb rates in the distal bottomset deposits of the subaqueous delta (in the central and southern Yellow Sea) are generally low (0.3-0.9 mm/ yr). A sediment budget demonstrates that between 9-15% of the annual Huanghe discharge is accumulating in the Yellow Sea. About two-thirds of this sediment is accumulating in the topset, foreset and proximal bottomset deposits of the Shandong subaqueous delta, with the remaining third accumulating as widespread distal bottomset deposits. ~4C age dates of subaqueous delta sediments indicate that the thick ( ~ 40 m) clinoform structure formed predominantly between 6200 and 4060 yrs B.P. (at rates approaching 20 mm/yr). Observations in the Yellow Sea, as well as on the Amazon and Ganges-Brahmaputra shelves demonstrate that subaqueous-deltaic stratigraphy is the general rule where major rivers enter energetic shelves, whether epicontinental or pericontinental. The development of extensive bottomset deposits may be restricted to epicontinental-shelf environments and may be diagnostic of sedimentation in this type of setting.

Introduction

Epicontinental-shelf seas have been important areas for the accumulation of sediments and the formation of sedimentary rocks throughout much of geologic time (e.g., latest Precambrian to Early Ordovician, Middle Ordovician to Early Devonian, Early Devonian to Early Carboniferous, and Middle Jurassic to Early Paleogene; Sloss and Speed, 1974; Potter et al., 1980). To better interpret the stratigraphic record of such deposits, an increased understanding of the variability in sedimentary processes and products must be derived from modern epicontinental environments. The Yellow Sea, located between the contiguous landmasses of China and Korea, is probably the only modern analogue to these ancient epicontinental 0025-3227/'91/$03.50

environments which has received large amounts ( a b o u t 109 t/yr) of sediment during the late Holocene (past 7500 yrs). The general objective of the research presented here is to examine temporal and spatial variability in sediment accumulation on 100-yr and 1000-yr time scales using 21°Pb and 14C geochronologies. The nature of sedimentary processes in the Yellow Sea can be evaluated by measuring accumulation on modern and Holocene time scales, thereby yielding a better understanding of the rates of formation of sedimentary deposits in an epicontinental setting. In addition, by contrasting present and past sedimentation, insights into the future of the Huanghe dispersal system can be developed. The specific objectives of this paper are: (1) to present the distribution of sediment accumulation rates on a 100-yr time scale,

© 1991 - - Elsevier Science Publishers B.V.

52

(2) to discuss the formation of Huanghe sedimentary deposits in the Yellow Sea during the Holocene, (3) to calculate a sediment budget for the modern Huanghe dispersal system in the Yellow Sea (on a 100-yr time scale), and (4) to contrast the types of deposits expected to form in epicontinental- and pericontinental-shelf sedimentary environments influenced by major rivers.

Background Shelf sediment accumulation Present-day shelf environments are dominantly marginal to the continents, or pericontinental. Thus, studies of 21°pb sediment accumulation rates on continental shelves have been, by default, overwhelmingly dominated by research in pericontinental-shelf environments (i.e., Shokes, 1976; Nittrouer et al., 1979; DeMaster et al., 1985; Kuehl et al., 1986, 1990; Boldrin et al., 1988). From these studies of pericontinental environments, a generalization has emerged that accumulation rates (on a 100-yr time scale) associated with major dispersal systems (> 108 t/yr sediment discharge) are generally centimeters per year, whereas rates associated with smaller dispersal systems (< 108 t/yr) are generally millimeters or less per year (Nittrouer et al., 1985). Accumulation rates decrease from proximal to distal portions of dispersal systems. The three largest dispersal systems (the GangesBrahmaputra, the Huanghe and the Amazon) each discharge over a billion tons of sediment per year, nearly an order of magnitude more than other systems (Milliman and Meade, 1983). Only within the last 5 yrs have sediment accumulation rates associated with the Amazon and Ganges-Brahmaputra pericontinental shelves been reported; this study reports results from the Huanghe epicontinental shelf. Nittrouer et al. (1986b) suggest that subaqueous deltas represent the general case of major rivers (such as the three listed above) entering energetic oceanic regimes. This class of deltas exhibits a clinoform morphology with subaqueous, gently dipping topset, steeply dipping foreset and gently dipping bottomset deposits. Through high-resolution seismic studies, Nittrouer et al. (1986b) and

C.R. ALEXANDER ET AL.

Alexander et al. (1986) describe the form of sediment accumulation on the Amazon shelf as that of a subaqueous delta, with topset, foreset, and bottomset deposits that are prograding across the shelf and along the shelf to the northwest. Kuehl et al. (1986) demonstrate that accumulation rates are low (<0.1 cm/yr) nearshore (<20 m water depth) because of energetic physical conditions that inhibit accumulation, increase to a maximum of about 10 cm/yr on the foreset deposits (water depth 30-60 m) as a result of decreasing wave and current energy, and abruptly decrease on the thin bottomset deposits ( > 70 m water depth) as a result of reduced sediment supply. The bottomset deposits downlap onto coarse, transgressive sediments on the outer shelf, and are restricted to within 20 km of the foreset deposits because of the dominant along-shelf transport. Nearshore and shelf currents transport sediment along the coast to the northwest, extending the Amazon dispersal system 1600 km along the South American coast. The Ganges-Brahmaputra system displays similarities to the Amazon system. Kuehl et al. (1990) report that although no high-resolution seismic profiles are available, the morphology suggests that inner-shelf sediment accumulation for the Ganges-Brahmaputra system is in the form of a subaqueous delta. For reasons similar to those described for the Amazon subaqueous delta, accumulation rates on the Ganges Brahmaputra subaqueous delta are negligible in the topset region (water depth < 20 m), increase seaward to a maximum (about 8 cm/yr) on the foreset deposits (water depth 30-70 m), and rapidly decrease onto thin, spatially restricted bottomset deposits (water depths > 70 m). Bottomset deposits downlap onto coarse, transgressive material and are restricted to within about 20 km of the foreset deposits because of dominant sediment transport along-shelf from east to west. The Yellow Sea Structural setting and Late Quaternary sea level history The Yellow Sea rests in a broad, tectonically stable trough which was submerged by the latest rise in sea level (Fig.l) (Chough, 1983b; Emery

SEDIMENT ACCUMULATION

53

IN THE YELLOW SEA

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128 °

Fig.1. Bathymetricchart of the Yellow Sea showing the geographic locations of features referred to in text. Note that the 100 m isobath does not enter the Yellow Sea basin. General circulation is northward along the eastern basin margin, and southward along the western basin margin. Major structural features in the Yellow Sea are the Shantung-LaoyehlingMassif (SLM) and the Fukien-Reinan Massif (FRM). Four deltaic deposits are located on the map: (1) the modern Huanghe delta (MHD, active 1855-present),(2) the abandoned Huanghe delta (AHD, active 1128-1855A.D.), (3) the Shandong subaqueous delta (SSD) near the tip of the Shandong Peninsula, and (4) the early Holocene delta (EHD, active 7500-8500 B.P.).

and Aubrey, 1986). At its northern extremity, the Yellow Sea is separated from the Gulf of Bohai by the Shandong Peninsula (Shantung-Laoyehling Massif) and to the south is bordered by the East China Sea and Fukien-Reinan Massif. Water depths in the Yellow Sea are everywhere less than 100 m. The distribution of subsurface sedimentary strata in the Yellow Sea is controlled by the presence of northeast-southwest trending ridges that separate depositional basins (Wageman et al., 1970). These ridges have been traced in the subsurface of the Yellow Sea using geophysical and magnetic methods (Zhang and Yang, 1983). Because the structures can be mapped as joining analogous geologic units on land, the Yellow Sea rests in a semi-enclosed, depressed area of conti-

nental margin, and is, by definition, a true epicontinental sea. During the last lowstand of sea level, the entire Yellow Sea basin was subaerially exposed (Geng, 1982; Peng et al., 1984; Butenko et al., 1985). With the onset of the Holocene transgression, sea level advanced rapidly across the exposed shelf, at a horizontal rate approaching 80 m/yr, because of the very shallow gradient in the Yellow Sea basin ( < l m / k m ) (Milliman et al., in press). Sea level reached its present position about 5000 yrs B.P., and has apparently fluctuated within 1-2 m of the present level since that time. These conclusions are based on '4C dating of peats, cheniers and shell ridges found on the Chinese mainland (Geng, 1982; Chen et al., 1985; Qin and Zhao, 1987; Milliman et al., 1987).

Physical regime The climatic and oceanographic conditions in the Yellow Sea have been reviewed by Niino and Emery (1961), Emery et al. (1969) and Wageman et al. (1970). Wind patterns are monsoonal, with wind stress and swell direction primarily toward the north in summer. During the winter, storms are common, winds and waves intensify (with wave heights reaching 4 m) and are directed toward the south. The general circulation in the Yellow Sea is characterized as a counterclockwise gyre, with northward inflow of the Yellow Sea Warm Current along the eastern margin of the basin. The Yellow Sea Coastal Current and Jiangsu Coastal Current flow southward along the western margin of the basin (Beardsley et al., 1985; Milliman et al., in press) and are strongest in winter. A smaller, semipermanent, counterclockwise gyre is formed southwest of Cheju Island (Mao et al., 1983). Semi-diurnal tides in the Yellow Sea range from 1.5 m to 8 m (Wang, 1983; Chough, 1983b). Tidal currents (50-100 cm/s) may play an important role in the redistribution of sediment in the Yellow Sea, especially in nearshore areas (Sternberg et al., 1985; Milliman et al., in press). Suspended sediment profiles show that higher suspended sediment concentrations relative to the rest of the Yellow Sea are observed near the Shandong Peninsula and along the Jiangsu coastline (Milliman et al., 1986). Wintertime suspended sediment concentrations are

54

often an order of magnitude greater than those observed in summer. Wells and Huh (1984) report similarly elevated suspended sediment concentrations in a band within 50 km of the southwestern Korean coast. A strong front between nearshore, turbid waters and the Yellow Sea Warm Current inhibits seaward advection of this sediment. The central Yellow Sea exhibits relatively low suspended sediment concentrations throughout the year, probably because of the cold bottom water (the Yellow Sea Bottom Cold Water) which causes stable stratification (Park, 1986; Seung, 1987). Sediment sources and patterns in the Yellow Sea The major sources of fine-grained sediment to the Yellow Sea are the Huanghe (Yellow River) and the Changjiang (Yangtze River), both of which rank among the top four of the world's rivers in terms of annual sediment discharge (Fig.l). The Changjiang annually discharges about 5 × 108 tons of sediment (Milliman and Meade, 1983) but the bulk of this sediment is transported southward through the East China Sea by the Jiangsu Coastal Current (Milliman et al., 1985b). Thus, the Changjiang is not a significant source of fine-grained sediment to the Yellow Sea when compared to the Huanghe, which annually discharges 1.1 × 109 tons of sediment (Milliman and Meade, 1983; MiUiman et al., 1985b). Further evidence for the Huanghe dispersal system being the dominant source of sediment to the Yellow Sea comes from the presence of detrital calcite, which is observed throughout the Yellow Sea. This material is present in the loess which makes up over 90% of the Huanghe's sediment load, but is not observed in the Changjiang sediment (Milliman et al., 1985a). A number of smaller rivers which drain China and the Korean Peninsula may contribute sediment to the Yellow Sea, but together these rivers are thought to discharge only about 4 x 106 tons of sediment annually (Schubel et al., 1984). Lee and Chough (1989) calculate a rough sediment budget for the Huanghe using preliminary data taken from Alexander et al. (1987), and suggest, that 20% of the Huanghe discharge is presently accumulating in the Yellow Sea. However, their estimate is inaccurate because they ignore the fact that many of the rates reported in Alexander et al. (1987) are maximum estimates

C.R. ALEXANDER

of the actual accumulation rates. In addition, they erroneously assign an accumulation rate of 1 mm/ yr to the extensive fine-grained deposit near the southwest Korean Peninsula, where the accumulation rate is actually ~ 1 cm/yr. The eastern third of the Yellow Sea is floored dominantly with coarse-grained transgressive sands (Fig.2) (Niino and Emery, 1961; Wageman et al., 1970; Milliman et al., submitted), but because of the large sediment supply from the Huanghe, the western two-thirds of the Yellow Sea is covered dominantly with fine-grained sediment (mean grain size 7-8 ~b; Ren and Shi, 1986; Alexander et al., submitted). There are several exceptions to this general distribution. Lag deposits of the abandoned Huanghe delta (active between 1128 and 1855A.D.; Fig.l) extend offshore about 150 km, and are characterized by sand deposits along the North Jiangsu coast (Wang, 1983; Milliman et al., 1987; Alexander et al., submitted). North and east of the abandoned Huanghe delta, an area of very coarse, relict material (sand and gravel) is exposed at the seabed south of Qingdao (Milliman et al., submitted). This latter area represents the erosional remnants of an early Holocene delta (active between about 8500 and 7500 yrs

120 °

122 °

124 °

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Fig.2. Map of surficial sediment distribution in the Yellow Sea (redrawn from Milliman et al., submitted). Coarse, sandy deposits flank the fine-grained Huanghe sediments in the central Yellow Sea. Coarse sediments along the Jiangsu coast are from erosion of Holocene deltaic deposits; coarse sediments in the southwestern, southern and eastern portions of the Yellow Sea were formed by the Holocene transgression. Note the separate fine-grained deposit flanking the southwest Korean peninsula.

SEDIMENTACCUMULATIONIN THEYELLOWSEA

55

B.P.), formed when the Huanghe discharged directly into the Yellow Sea. These remnants presently constitute a bathymetric high on which there is no modern sediment accumulation. There is also an anomalous fine-grained deposit located southwest of Cheju Island. This deposit is thought to be the result both of the influence of the semipermanent gyre present there and of accumulation of sediment transported into the area by the Jiangsu Coastal Current (Mao et al., 1983; Milliman et al., 1986, in press).

The Huanghe dispersal system Holocene

during the

Because of its large sediment load, the Huanghe rapidly aggrades its channel. This rapid aggrada-

tion leads to avulsion; the river has changed its course frequently during the Holocene. There have been eight recorded shifts of the river since 2278 B.C. (inset Fig.3; Wang, 1983). Twice during this period, the Huanghe has discharged south of the Shandong Peninsula directly into the Yellow Sea. However, Chinese historical records show that the Huanghe has discharged into the Yellow Sea for only 727 of the last 4267 yrs, or about 17% of the time. Based on stratigraphic evidence presented in Milliman et al. (submitted), the Huanghe discharged into the Yellow Sea for only 727 of the last 7500 yrs or 10% of the late Holocene. Between 1128 and 1855 A.D., the Huanghe directly discharged its sediment load into the Yellow Sea south of the Shandong Peninsula (along

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Fig.3. Holocene sediment thickness map (redrawn from Milliman et al., 1987) (isopachs are in meters, and dashed lines are inferred). Thick accumulations of Holocene sediment are observed in the Gulf of Bohai (the modern Huanghe delta, 20 m thick), near the tip of the Shandong peninsula (the Shandong subaqueous delta, 40 m thick), and along the Jiangsu coast (the abandoned Huanghe delta, 30 m thick). Note the lack of Holocene sediment south of Qingdao and east of the northern Jiangsu coast. The thick sediment accumulation near the mouth of the Changjiang was supplied by that river. Inset shows the historical path of the Huanghe over the past 4267 yrs, during which the river has discharged into the Gulf of Bohai about 83% of the time. Shading indicates zone of meandering river paths (modified from Wang, 1983 and Milliman et al., 1987).

56

C.R, ALEXANDER

the Jiangsu coastline), forming a delta which prograded 90 km seaward. The depocenter for these deltaic deposits is offset southward from the abandoned river mouth because of the southerly current regime (Milliman et al., submitted). After the Huanghe changed its course from the Yellow Sea back to the Gulf of Bohai in 1855, the northern Jiangsu coastline retreated by as much as 30 m per yr (Wang, 1983). However, the Jiangsu coastline has been relatively stable since 1970 (Liu and Walker, 1989). Although large amounts of sediment have been eroded from the abandoned delta, suspended sediment in the Jiangsu Coastal Current is confined to nearshore areas and transported to the southeast (Milliman et al., 1986, 1987, in press). Milliman et al. (1985a, 1986, in press) suggest that the presence of the fine-grained deposit southwest of Cheju Island is the result, in large part, of the transport and accumulation of sediment eroded from the abandoned Huanghe delta. The deposit southwest of Cheju Island has been studied in detail by Nittrouer et al. (1984b) and DeMaster e t a l . (1985). They suggest that this deposit represents the most distal end of the Huanghe dispersal system where fine-grained, Holocene sediments are prograding over the coarser transgressive material, and where bimodal sediments are formed (Nittrouer et al., 1984b). Estimates of sediment accumulation rates in this deposit, based on 21°pb geochronologies, range from 2-3 mm/yr (DeMaster et al., 1985).

A Holocene sediment isopach map of the Yellow Sea (Fig.3), developed by Milliman et al. (1987), shows that there are three major depocenters for Huanghe-derived sediment: the modern Huanghe delta in the Gulf of Bohai (about 20 m thick); the abandoned Huanghe delta along the Jiangsu coast (about 30 m thick); and a subaqueous delta associated with long-term dispersal out of the Gulf of Bohai, termed here the Shandong subaqueous delta (about 40 m thick, Fig.4) (Alexander et al., 1987). Opinions differ as to the age of the Shandong subaqueous delta, and even as to whether it is an active or relict feature (e.g., Qin and Li, 1983; Milliman et al., 1986, 1987, in press; Alexander et al., 1987). Southeastward from the Shandong subaqueous delta, Holocene sediments thin from about 40m to about 5m in thickness over a distance of about 200 km. These sediments are about 2 m thick throughout the central Yellow Sea, and increase again to about 5 m in thickness in the fine-grained deposit southwest of Cheju Island.

The post-1855 Huanghe dispersal system Since 1855, the Huanghe has discharged into the Gulf of Bohai and has formed a subaerial delta which extends eastward into the gulf about 50 km; the subaqueous portion of this delta extends another 15 km seaward (Fig. 1) (Milliman and Meade, 1983; Keller and Prior, 1986; Ren and Shi, 1986). The Gulf of Bohai is connected with the Yellow

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Fig.4. High-resolution (3.5-kHz) seismic profile of the Shandong subaqueous delta. This feature exhibits a clinoform morphology and the internal stratification characteristic o f a subaqueous delta (gently dipping topset, steeply dipping foreset, and gently dipping, acoustically transparent bottomset deposits). Profile extends along a west-east transect about 37~N (see Fig.5 for location).

SEDIMENT ACCUMULATIONIN THE YELLOW SEA

57

Sea through a narrow strait at the eastern end of the Gulf, and thus has functioned as a trap for Huanghe sediment since 1855. Recent estimates are that 10-36% of the annual Huanghe discharge is not accumulating in the active delta lobe, and thus is available for accumulation farther along the dispersal system (Pang and Si, 1980; Bornhold et al., 1986). Individual delta lobes are active for about 10yrs before being abandoned (Wang, 1983). Methods

Field methods During cruises in August 1985 (YS8508, on Chinese R.V. Science 1), January 1986 (TT193, on R.V. Thompson) and July 1986 (PPTU-10C, on R.V. Washington), Yellow Sea sediments were col-

120';

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lected extensively with a variety of coring devices. A total of 15 box cores (20 × 30 cm cross section, 2 5 - 5 0 c m long), 31 kasten cores (12.7× 12.7cm cross section, 0 . 2 5 - 3 m long), 18 gravity cores (4-cm diameter, 0.25-1.5 m long), and 10 piston cores (6.60-cm I.D. on TT193 and 8.13-cm I.D. on PPTU-10C, 2-8 m long) were collected during the three cruises (Fig.5). Samples for radiochemical analyses (1 cm in vertical thickness) were removed from box and kasten cores and homogenized. 14C samples were removed from at least the top, middle and bottom of each kasten and piston core, and were immediately frozen or dried on board ship, as facilities allowed. Piston and gravity cores were cut to manageable lengths (1-1.5 m) with a pipe cutter, capped on board ship, and returned for laboratory examination. Positioning was generally accomplished by satellite navigation.

123 °

124 °

125 °

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Fig.5. Location of cores collected for this project (74 in total) in relation to the general distribution of fine-grained sediments in the Yellow Sea. Heavy line marked P is the location o f seismic profile shown in Fig.4. Radiochemical profiles are presented in this paper for stations indicated by large dots.

58

Laboratory methods and analytical procedures 21°pb sediment geochronologies were determined following the technique outlined in Nittrouer et al. (1979). Sediment samples were spiked with a a°Spo tracer (for yield determination) and were then sequentially leached in concentrated HNO3 and in 6N HC1. 21°pb activities (in dpm/g dry weight) were corrected for salt content and porosity variations before accumulation rates were calculated. zl°Pb accumulation rates were verified by measurement of the depth of 137Cs penetration in cores. 137Cs activities were directly determined by gamma spectroscopic measurement of its 662-keV gamma peak (Kuehl et al., 1986). 137Cs is an impulse tracer (produced from atmospheric nuclear tests) which was first introduced into the environment in significant amounts about 35 years ago. The depth of 137Cs penetration into the seabed can be predicted (given a calculated 21°pb accumulation rate) as the sum of the mixed-layer thickness and the amount of sediment accumulation since the first appearance of 137Cs (calculated accumulation rate x 35 yr). If sediment accumulation dominates a 21°pb profile, 137Cs penetrates no deeper than predicted. If the observed depth of 137Cs penetration is deeper than the predicted depth, then the radiochemical profile is affected by deep sediment mixing (e.g., deep bioturbation), and the calculated accumulation rate is a maximum estimate of the actual accumulation rate. In areas where 137Cs is observed to penetrate deeper than predicted, the contributions of sediment accumulation and deep mixing to a 2~°pb profile can be estimated by comparing the observed depth of 137Cs with the depths predicted by lower accumulation rates and complementary mixing rates (see DeMaster et al., 1985). For a detailed discussion of the discrimination between mixing and accumulation from 2~°pb and 137Csprofiles, see Nittrouer et al. (1984a) and DeMaster et al. (1985). 14C ages were determined from the organiccarbon fraction of kasten and piston core samples using techniques described in Noakes et al. (1965) and Kuehl et al. (1986). Samples were pretreated with cold 0.6N HC1 to remove CaCO 3. All ages

C.R. ALEXANDER

were calculated assuming a 5730 yr half-life for 14C and using the NBS oxalic acid standard. Results

Distribution of accumulation rates 21°pb accumulation rates and 137Cs geochronologies Six distinct areas of sediment accumulation rate are identified in the Yellow Sea (Fig.6). These areas are defined primarily based on the distribution of apparent accumulation rate (rate determined from radionuclide gradient assuming no bioturbation). Where radiochemical data is not available to define boundaries of an area, sedimentologic criteria (i.e., textural boundaries between modern sediment and the transgressive sands or old deltaic deposits) and geographic location are used. Three areas have high apparent accumulation rates (> 3 mm/yr; > 0.2 g/cm2/yr). The most northern of these areas, Area 2, flanks the southern margin of the Shandong Peninsula, and lies at intermediate water depths (30-65 m; Fig.6). Apparent accumulation rates in this area range from 3.4-8.6mm/yr (0.23-0.93 g/cm2/yr). The second area of high apparent accumulation rate, Area 4, flanks the abandoned Huanghe delta along the Jiangsu coast, and exhibits apparent rates of 4.26.9 mm/yr (0.46-0.95 g/cmZ/yr; Fig.6). The third area of rapid apparent accumulation, Area 5, is a poorly defined region near the southwestern Korean coast, which is based on only one core (PPTU10C KC55), but where measured apparent rates are extremely rapid (18mm/yr; 1.11g/cmZ/yr; Fig.6). The boundaries of Area 5 are delineated by the extent of silty sediments seaward of the west Korean coast (Fig.2). These sediments are representative of sediments retrieved in core KC55 (Alexander et al., submitted). For cores in Area 2, 13VCs penetrates only as deeply as predicted. Modelling of 137Cs penetration demonstrates that radiochemical profiles in this area are unaffected by deep mixing (Fig.7A). In Area 4, however, the penetration depth of 137Cs is significantly greater than predicted from the calculated accumulation rates in this area. In Area 5, the precise depth of 137Cs penetration could not

59

SEDIMENT ACCUMULATION IN THE YELLOW SEA

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Fig.6. Distribution of apparent accumulation rates (mm/yr) and the six accumulation-rate areas in the Yellow Sea based on :l°pb analyses of 33 cores. Rates are low in Areas 1, 3 and 6 (<3 mm/yr; <0.2 g/cmZ/yr): Area 1 represents the topset deposits of the Shandong subaqueous delta, whereas Areas 3 and 6 encompass the distal bottomset deposits. Rates in Areas 2, 4 and 5 are high (>3 mm/yr; >0.2g/cm2/yr): Area 2 receives sediment transported out of the Gulf of Bohai and encompasses the foreset and proximal bottomset deposits of the subaqueous delta, Area 4 receives significant amounts of sediment from the abandoned Huanghe delta along the Jiangsu coast, and Area 5 receives sediment from a variety of undifferentiated sources, although it is not a part of the Huanghe dispersal system. Accumulation-rate areas are defined by magnitude of accumulation rate, sedimentological criteria, and geographic location.

be d e t e r m i n e d because of a 20-cm spacing between samples below 1 0 0 c m in core KC55. W i t h a calculated a c c u m u l a t i o n rate of 18 m m / y r , the predicted depth o f 137Cs p e n e t r a t i o n is a b o u t 90 cm. A low 1 3 7 C s activity is detected in the 100103 cm interval, a n d n o activity is observed in the 120-123 cm interval (Fig.7B).

Three areas in the Yellow Sea exhibit slow a p p a r e n t sediment a c c u m u l a t i o n ( < 3 m m / y r ; < 0 . 2 g/cm2/yr). T w o of these areas have been studied by this project: Area 1, at the n o r t h e r n m o s t extent o f the study area (near the tip of the S h a n d o n g P e n i n s u l a ) in shallow water depths, which is defined by the 30 m isobath (Fig.6), a n d

60

C.R. ALEXANDER

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Profile of seabed 21°Pb activity in a kasten core from Area 2 (the foreset deposits of the subaqueous delta). Comparison between the predicted and observed depths of 137Cs penetration (30 cm vs. 32 cm) demonstrates that the 21°pb profile predominantly is a result of sediment accumulation. (B) Profile of seabed 21°pb activity in a kasten core from Area 5 (near the Korean Peninsula). The precise depth of 'aTCs penetration could not be determined because of the large sampling interval below I00 cm (see text for explanation). Fig.7. ( A )

Area 3, in the central Yellow Sea, between 65-90 m water depths (Fig.6). Because of the shallow water depths in Area 1, and restrictions on coring near the Chinese mainland, only one core was collected from this area. The 21°pb profile from Area 1 is shown in Fig.8A. Apparent accumulation rates in Area 3 (Fig.8B) increase from 0.3 mm/yr (0.02 g/cmZ/yr) to 2.7 mm/yr (0.22 g/cm2/yr) from east to west across the central Yellow Sea. 137Cs in cores from both these areas penetrates much deeper into the seabed than can be explained by sediment accumulation alone (Figs.8A and B).

234Th studies of the mixing coefficient (Ob) in the uppermost seabed of these two areas yield values between 25-40 cm2/yr (DeMaster et al., 1985; Alexander et al., submitted). X-radiographs of cores from these areas exhibit mottled to homogenous sedimentary structure, also indicating that sediment mixing is an important process in these areas (Alexander et al., 1987; submitted). The finegrained deposit southwest of Cheju Island (Area 6) has been delineated by combining apparent accumulation rate data from this project and data reported in DeMaster et al. (1985). Apparent accu-

Pb- 210 Activity (dpm/gm) ......

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Fig.8. Seabed 21°pb profiles from kasten cores collected in Areas 1 and 3. In cores from Areas 1, 3, 4 and 6, '37Cs is observed much deeper than predicted, indicating that 21°pb profiles from these areas are affected by deep biologic mixing, and that accumulation rates are maximum estimates of the actual accumulation rates. (A) Profile from Area 1 (the topset deposits of the subaqueous delta). (B) Profile from Area 3 (bottomset deposits of the subaqueous delta).

61

SEDIMENT ACCUMULATION IN THE YELLOW SEA

mulation rates in Area 6 are 2.7-3.1 mm/yr (0.160.25 g/cmZ/yr). In this area, as in the other two areas of relatively low apparent accumulation rate (Areas 1 and 3), ~37Cs penetrates much deeper into the seabed than can be explained by sediment accumulation alone. An axial transect of bathymetry and accumulation rate (which begins on the topset deposits of the subaqueous delta (YK7) and which extends southward along the axis of the modern Huanghe dispersal system in the Yellow Sea) (Fig.9) shows that apparent accumulation rates are low on the topset deposits of the subaqueous delta, abruptly increase on the foreset deposits, decrease onto the bottomset deposits and are consistently low southward along the dispersal system, increasing slightly under the influence of the semi-permanent gyre in the southeastern Yellow Sea. A transect oriented obliquely to the dispersal system (which trends southeast) demonstrates that bottomset accumulation is restricted to the south and southwest on the subaqueous delta (Fig.9). Rates abruptly decrease to < 0.3 mm/yr at the base of the subaque8.01 ~ {~ [ ~'\ =~6"°1////

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ous delta and become undetectable out on the transgressive sand. 14C accumulation rates

Radiocarbon dating of cores was performed to evaluate temporal variability of sediment accumulation in the Yellow Sea and along the Huanghe dispersal system (Table 1). Sample intervals were dependent on the core lengths because samples were removed from at least the top, middle and bottom of each core. Core YK7, from the topset deposits of the subaqueous delta, has a surfacesediment age of 2207 +_ 158 yrs B.P. (reported relative to t950). This core yields a t4C accumulation rate (for the upper 60 cm) of about 0.2 mm/ yr, which compares with the 21°pb accumulation rate of 1.6 mm/yr (on a 100-yr time scale) calculated for the upper 25 cm of this core. In contrast, the 14C rate for the interval 60-253 cm is 1.7 mm/ yr (on a 1000-yr time scale). This apparent discrepancy is addressed in the discussion section. The clinoform deposits at 249-253 cm depth have a radiocarbon age of 6266 _ 147 yrs B.P., indicating that the bulk of this 40-m thick deposit is greater than about 4060 years old (6260 yrs B.P. age minus 2200 yrs B.P. surface age). Core PK48, from the distal bottomset deposits has a ~4C age of 2909 + 146 yrs B.P. for the interval 20-26 cm and a ~4C age of 8793 _+ 144 yrs B.P. for the interval 191-204 cm. Two calculations of the accumulation rate in PK48, one over the 0-26 cm interval and the other over the 26-204 cm interval yield similar rates of about 0.3 mm/yr for both these intervals (assuming a surface age similar to that observed

70~

TABLE 1 0

100

200

300

400

80 500

Distance Along Transect (km) Fig.9. Transects of bathymetry and apparent 21°pb accumulation rate extending southward (along the dispersal system) and southeastward (oblique to the dispersal system) from the topset deposits of the Shandong subaqueous delta. These transects demonstrate that accumulation rates are low ( ~ !.5 mm/yr) on the topset deposits, increase to a maximum ( ~ 9 mm/yr) on the foreset deposits, and rapidly decrease ( < 2 mm/yr) onto the bottomset deposits. Rates increase slightly south of Cheju Island because of the input of sediment from the Jiangsu coast. In the southeast portion of the Yellow Sea, bottomset deposits are less extensive, indicating that the direction of progradation of the subaqueous delta is dominantly southward.

14C ages in the Yellow Sea (core locations in Fig.5) Core

Location

YK7

Topset, north Yellow Sea

PK48 G8024"

Depth (cm)

0-2 57 60 249-253 Distal bottomset 20-26 central Yellow Sea 191-204 Distal bottomset, 30 55 southwest of I00 120 Cheju Island

*Denotes data from You et al. (1983)

14C age (yrs B.P.) 2207+ 158 5115_+ 148 6266_+147 2909 _+ 146 8793_+144 4900+ 150 6450-+140

62

C.R. A L E X A N D E R

in Core YK7). Measurement of 613C on some samples analyzed for 14C indicate that the organic carbon fraction is dominantly derived from marine sources (613C values are about -19.8%; Harden, pers. commun., 1989). Discussion Accumulation rate areas

The following sections discuss each of the six areas and their inferred sediment sources, starting with the most northerly area. The distribution of apparent accumulation rates in relation to the six different areas in the study region is shown in Fig.6. Area 1 - - modern Huanghe sediment, slow accumulation

The apparent accumulation rate in Area 1 is 1.6 mm/yr, based on the single core collected in this area (Fig.6). This area of low accumulation rate corresponds to the topset deposit of the Shandong subaqueous delta, which are found in 20-30 m of water. 137Cs is observed deeper in the seabed than predicted by accumulation alone, indicating that the zl°Pb profile is affected by sediment mixing, and that the calculated rate is a maximum estimate of the actual accumulation rate (Fig.8A). Physical processes in this relatively shallow area probably inhibit sediment accumulation, an observation also reported from similar water depths on the Amazon subaqueous delta (Nittrouer et al., 1986a; Kuehl et al., 1986). Sediments in Area 1 are unimodal coarse to medium silts, with <25% fine silt and clay (Alexander et al., submitted). Because the Huanghe discharges approximately 75% fine silt and clay (Ren and Shi, 1986), the low abundance of these sizes in Area 1 further suggests that physical processes are relatively energetic. This area of low accumulation rate probably extends into the Gulf of Bohai and Liazhou Bay (Fig.l), because bathymetric charts show this to be a wide terrace at a water depth of about 20 30m which flanks the northern side of the Shandong Peninsula and extends westward (Milliman et al., 1987). Unfortunately, research was prohibited near the strategic Bohai Straits.

Area 2 - - modern Huanghe sediment, rapid accumulation

The area south of the Shandong Peninsula in water depths between 30 and 65 m exhibits apparent accumulation rates of 3.4-8.6 mm/yr (0.230.85 g/cmZ/yr). The depth of 13VCspenetration verifies that the Zx°pb profiles are not affected by deep mixing, and that calculated rates are good estimates of the actual accumulation rates (Fig.7A). Because of the proximity of this deposit to the Gulf of Bohai and the counterclockwise circulation pattern in the Yellow Sea (which would inhibit sediment from being transported northward on the western side of the basin), sediment supplied to this area is probably Huanghe sediment, which is not accumulating within the active lobe of the modern delta (10-36% of annual discharge, see background section) and is eroded from abandoned lobes of the modern delta (rates of shoreline retreat can reach 100-300 m/yr; Li, 1986). Lobes are abandoned on decadal time scales (see background section), thereby providing a relatively constant source of available sediment. The highest accumulation rates are observed on the foreset deposits of the subaqueous delta, with lower rates on the proximal bottomset deposits to the south and southwest of the Shandong Peninsula (Figs.6 and 9). This distribution of accumulation rates produces aggradation and southward progradation of the subaqueous delta. The highest rates of sediment accumulation are associated with unimodal clayey silt observed between 30-60 m water depth on the subaqueous delta (Alexander et al., 1987, submitted). Unimodal silt gives way to unimodal clay along a sharp boundary in about 60 to 65 m of water at the junction between the foreset and proximal bottomset deposits, and accumulation rates abruptly decrease. These observations indicate that the Shandong subaqueous delta is an actively accreting structure, and that significant amounts of sediment are presently escaping the Gulf of Bohai. Rapid rates of accumulation are present on the southern side of the peninsula most likely because this area is relatively protected during intense winter storms. Area 3 - - modern and resuspended Huanghe sediment, slow accumulation

Area 3, which encompasses much of the central Yellow Sea, exhibits low apparent accumulation

63

SEDIMENT ACCUMULATION IN THE YELLOW SEA

rates of 0.3-2.7 mm/yr (0.02-0.22 g/cm2/yr). In all cores from this area, ~3~Cs is observed deeper than predicted by sediment accumulation alone, indicating that z~°Pb activity profiles in this large area are primarily controlled by sediment mixing, and that calculated sediment accumulation rates are maximum estimates of the actual rates (Fig.8B). Rates increase from east to west across the central basin (Fig.6). Relatively high rates along the western margin of this area are consistent with eastward transport of sediment supplied from the abandoned Huanghe delta on the Jiangsu coast. Low rates on the eastern side of this area probably represent low levels of sediment input solely from the modern Huanghe dispersal system, because the stable Yellow Sea Bottom Cold Water inhibits significant supply of sediment from the west (Park, 1986; Seung, 1987). The dominant bottom sediment in the eastern two-thirds of this area is a unimodal clay that exhibits a progressively greater amount of silt toward the west (Alexander et al., submitted). Area 4 - - resuspended sediment, rapid accumulation

Apparent accumulation rates in Area 4 are high as a result of significant input of sediment eroded from the abandoned Huanghe delta (4.2-6.9 mm/ yr; 0.46-0.95 g/cmZ/yr). The depth of 13VCs penetration in this area indicates that 21°pb profiles are dominantly a result of sediment mixing, and that calculated 21°pb accumulation rates are maximum estimates of the actual accumulation rates. Sediment mixing may be a result of physical, rather than biologic processes because of the relatively shallow, tidally energetic environment present. Hard-packed, lag sands characterize the shallow topset region of the abandoned delta (Milliman et al., 1986; Alexander et al., submitted). Because this sand could not be cored, cores collected within this area are predominantly distributed along the foreset and bottomset slopes of the abandoned delta. The bathymetry seaward of the delta is irregular, and thus both proximity to the delta front and local seafloor morphology control the rate and type of sediment accumulation. Parallel to the coast, no trends in grain size are discernible. From west to east, sediments exhibit a decrease in

coarser (sand and silt) material (Alexander et al., submitted), indicating an increase in distance from the source of the sediment. Much of the sand presently accumulating within the southern part of this area may be supplied by tidal resuspension of the transgressive deposits (Sternberg et al., 1985; Milliman et al., 1987). The basinward bulge in the boundary of this area corresponds to the greatest seaward extent of the subaqueous deposits of the abandoned delta. Area 5 - - m i x e d source, rapid accumulation

Based on the one core obtained from Area 5 (KC55), the apparent sediment accumulation rate in this area is the highest observed in the Yellow Sea (18 mm/yr; 1.11 g/cmZ/yr). The depth of 137Cs penetration in this core could only be resolved to be between 103-119 cm because samples were collected every 20 cm below 100 cm in the core. The predicted depth of 13~Cs in KC55 is 90 cm; low 13VCs activity is measured in the 100-103 cm interval, but is not detected in the 120-123cm interval (the next interval sampled). Because of the imprecisely known depth of 137Cs penetration, modelling of the 13~Cs penetration depth (DeMaster et al., 1985) does not provide a precise result. These calculations can only demonstrate that the actual accumulation rate in KC55 is between 10 and 17mm/yr (0.60 and 1.07g/cm2/yr, respectively). However, the benthic community in the Yellow Sea would not be expected to cause biologic mixing (resembling an eddy-diffusion process) to depths greater than about 20 cm. Therefore, the high accumulation rate is probably correct. The X-radiographs from this core exhibit laminated sedimentary structure, further indicating that mixing is not the dominant contributor to the 21°pb profile, and that the accumulation rate is relatively high (Alexander et al., submitted). The apparent accumulation rate in this area may actually be much higher than 18 mm/yr. The slope of the line through the excess zl°Pb data is strongly influenced by the lowermost datum, which falls significantly off the trend of the other data (Fig.7A). Excluding the lowermost excess 21°Pb datum, the calculated apparent accumulation rate is about 33 mm/yr (1.98 g/cm2/yr). This higher accumulation rate would be in much better

64

agreement with the observed penetration depth of

C.R. ALEXANDER

TABLE 2

137Cs '

This area is not part of the Huanghe dispersal system. Sediments in this area, which exhibit neither the Huanghe detrital calcite signature nor the typical clay mineralogy of Huanghe sediments, seem to be derived from the Korean Peninsula (Chough and Kim, 1981; Chough, 1983a; Ren and Shi, 1986; Park, 1986). The dominant clay-mineral signature in these deposits is that of the rocks in the Korean K e u m River basin (Chough and Kim, 1981; Park et al., 1986). However, the K e u m River does not discharge enough sediment to support these rates alone (Schubel et al., 1984). Additionally, Wells and H u h (1984) and Wells (1988) document transport of suspended sediment along the west Korean coast (dominantly during the winter) which is on the order of 1-10 × 1 0 7 t/yr - - at least an order of magnitude above the amount thought to be supplied each year by the smaller rivers. Thus, this deposit probably consists of a highly complex mixture of sediments derived from the smaller Chinese and Korean rivers entering the eastern Yellow Sea, resuspended Huanghe deposits from the central basin, and suspended sediment carried to the area from the south by the Yellow Sea Warm Current. Assuming that core KC55 is representative of the total area (and using the range in accumulation rate calculated above), 4.9 8.7 × 10 v t of sediment are annually accumulating in Area 5 (Table 2); this area may be an important sink for sediment being transported southward along the Korean coast.

Area 6 - - mixed source, slow accumulation A sixth area of sediment accumulation rate, not studied during this project but important for understanding sediment dispersal in the Yellow Sea, is the fine-grained deposit southwest of Cheju Island (see background section). Accumulation rates are moderate (2-3 mm/yr), but are m a x i m u m estimates because of biologic mixing of the seabed (DeMaster et al., 1985). Sediments in this area are probably derived both from the modern Huanghe dispersal system, and from erosion of the abandoned Huanghe delta along the Jiangsu coast.

Sediment budget for the Yellow Sea. Accumulation-rate Areas 1, 2, and a combined area (consisting of Areas 3, 4 and 6) represent the region of modern Huanghe sediment presently accumulating in the Yellow Sea. Area 5 is not a part of the Huanghe dispersal system. The two accumulation rates reported for Area 5 represent upper and lower estimates for the actual accumulation rate based on 21°Pb and 137Cs data Area (cm2)

Accumulationrate Total (g/cmZ/yr) (t/yr)

Percent of total

Area 1 Area 2 Area 3+4+6 Total

5.0 × l013 0.17 1.4× 1014 0.68 1.5× 1015 0.04

8.5 × 106 4 9.5x 107 58 6.0× 107 38

1.7× I0is

1.6× 10s 100

Area 5

8.1 ×

1013

0.60

1.07

4.9 × 107 8.7 × 107

The Huanghe dispersal system in the Yellow Sea Proximal dispersal system 14C geochronologies in Yellow Sea sediments document the rates of long-term sediment accumulation. Given a surface sediment age of about 2200 yr B.P. (Table 1), the present 14C data from Core YK7 on the Shandong subaqueous delta indicate that sediments at 57-60 cm in this feature are about 2900 yrs old (Table 1). A 14C accumulation rate calculated for the upper 60 cm is 0.2 ram/ yr. This rate is about an order of magnitude below that measured on Zl°Pb time scales from the same core (1.6 mm/yr), probably because the 21°pb profile in this region is dominated by sediment mixing. Because the calculated 21°pb accumulation rate (applicable to the 0-25 cm interval) is a m a x i m u m estimate of the actual accumulation rate, the actual rate is probably lower, and may be similar to the lower 14C accumulation rate (applicable to the 0-60 cm interval). In addition, the resolution of the ~4C accumulation rate is limited; the ~4C rate is calculated from only two dates. Alternatively, this discrepancy between the 14C and 21°pb rates is consistent with the idea that the shift of the river to the south between l 1 2 8 - 1 8 5 5 A . D , not only decreased accumulation rates on the subaqueous delta during this time, but allowed erosion of some topset deposits to take place. The 2200 yr surface sediment age measured for

SEDIMENT ACCUMULATION IN THE YELLOW SEA

65

> 60 m water depth (Milliman et al., submitted) at maximum rates of 3 mm/yr (DeMaster et al., 1985; this paper). Thus, these bottomset deposits represent a minimum of ca. 1300 yrs of accumulation in a subtidal environment (4000 mm/3 mm/ yr). Sea level did not reach the Shandong Peninsula until about 10,000 yrs B.P. and reached - 3 0 m (and entered the Gulf of Bohai) about 9000 yrs B.P. (Geng, 1982). The Huanghe discharged directly into the Yellow Sea, south of Qingdao, between about 7500-8500 yrs B.P. (Milliman et al., 1987, submitted). The bottomset muds could not have accumulated prior to the Holocene transgression, nor probably when the river discharged south of the peninsula (given the present cyclonic circulation pattern in the Yellow Sea). Sediment discharged into the Yellow Sea south of the Shandong Peninsula would be transported southward into the East China Sea, and not accumulate northward near the Shandong Peninsula. Thus, the bottomset deposits probably formed during the subsequent 1300 yrs after the Huanghe shifted back into the Gulf of Bohai about 7500 yrs B.P. The period during which the bulk of the Shandong

the 0-2 cm interval in Core YK7 is assumed to represent the radiocarbon age of modern organic carbon being buried with sediments in the Yellow Sea. Data presented here suggest that this assumption is reasonable for at least the northern and central Yellow Sea, because extrapolating the 14C age dates in Core PK48 to the surface also yields an age of approximately 2200 yrs. In addition, the source of organic carbon buried in Yellow Sea sediments is assumed to be constant. This assumption is reasonable for at least Core YK7 because of similar carbon isotope values measured in the two lower samples from YK7 (&13C-19.8 and -19.7%).

Formation of the Shandong subaqueous delta Rates of processes associated with formation of the Shandong subaqueous delta also can be determined from the 14C ages in Core YK7 (Fig.10A). The age of the clinoform deposits at 249-253 cm within the seabed is about 4060 yrs (Table 1). The subaqueous delta rests upon about 4 m of acoustically transparent, Holocene bottomset deposits (Fig.5; Alexander et al., 1987). Similar sediments are presently being deposited in the Yellow Sea in Core YK7

Age

A

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Years B.P. Fig.10. Model for the formation of the Shandong subaqueous delta at the location of core YK7. (A) Cartoon of the subaqueous delta showing the relative location of Core YK7. Composite column shows horizons dated by 14C; the lower two horizon ages (6200 and 7500 yrs B.P.) are inferred from geophysical, sea-level and radiochemical data (see background section). (B) Hypothetical plot of accumulation rate versus time at location of Core YK7. Rates were low (maximum of 3 mm/yr) during bottomset accumulation, increased during foreset accumulation (to about 20 mm/yr), and have decreased to about 1-2 mm/yr during topset deposition. Nondeposition and erosion occur when the Huanghe discharges south of the Shandong Peninsula, directly into the Yellow Sea.

66 subaqueous delta accreted is thus constrained between about 6200 yrs B.P. (7500 yrs B.P. minus 1300 yrs) and 4060 yrs B.P. (the age of the sediment 249-253cm within the Shandong subaqueous delta). The average accumulation rate would have been about 20 mm/yr to construct the subaqueous delta (40 m thick) over this time period. If the underlying bottomsets accumulated at an average rate lower than 3 mm/yr, the 4-m bottomset deposits could represent a longer period of accumulation, and thus the accumulation rate during formation of the subaqueous delta may have been higher than 20 mm/yr. These accumulation rates are reasonable because similarly high rates of fine-grained sediment accumulation have been reported from the modern Amazon subaqueous delta (> 100 mm/ yr) (Kuehl et al., 1986). After rapid accretion of the subaqueous delta, rates decreased to values similar to those measured today using 21°pb geochronologies. This decrease results from the progradational nature of the subaqueous delta. As the zone of rapid accumulation associated with the foreset environment moves seaward, the topset environment, with lower accumulation rates, progrades over the foreset environment and accumulation rates at a specific site decrease (Fig. 10B). Using the estimate of Milliman et al. (1987) that 510 km 3 of sediment resides within the Shandong subaqueous delta, and assuming all the Huanghe discharge (about 109 t/yr) accumulated in the subaqueous delta (with a wet bulk density of 1.4 g/cm3), volume calculations show that about 720 yr would be necessary to construct the subaqueous delta, well within the 2140 yrs time deduced for the formation of the subaqueous delta in the previous paragraph. Several researchers have presented evidence for sediment discharge rates two-thirds to one-third of the present rate during the time in which the delta was forming (e.g., Ye et al., 1983; Ren and Shi, 1986). If discharge rates were one-third of their present rate, about 2100 yrs would be required to build the subaqueous delta, still within the time-frame calculated. The data presented here for the formation of the Shandong subaqueous delta do not support the hypothesis that sediment discharge rates were an order of

C.R.ALEXANDER magnitude lower during 3000-7000yrs B.P., as Milliman et al. (1987) suggest.

Distal Huanghe dispersal system Distal Huanghe sediment accumulation is revealed by the 14C ages of sediment from the central and southern Yellow Sea. Core PK48, from the fine-grained deposits in the central Yellow Sea (Fig. 5), yields accumulation rates of about 0.3 mm/ yr when rates are calculated over both the upper interval (assuming a surface age of about 2200 yrs B.P.), and the lower interval between the two dated horizons. 14C ages of fine-grained sediment from southwest of Cheju Island have been reported from Core G8024 by You et al. (1983) (Table 1). The accumulation rate between dated horizons is about 0.4mm/yr. Apparent rates measured by 21°pb geochronologies in the same core are almost an order of magnitude greater. Modern rates probably have been greatly enhanced by sediment input from the rapidly eroding, abandoned delta along the Jiangsu coast (Milliman et al., 1987, in press). These 14C rates are similar to accumulation rates (0.3-0.9 mm/yr) measured by 21°pb geochronologies in areas of the central Yellow Sea which have not received sediment from the abandoned Huanghe delta, and may represent typical distal bottomset accumulation rates in the late Holocene. Although comparison between 14C and 21°pb accumulation rates can provide significant insight into temporal changes in sediment accumulation, in areas where sediment mixing significantly affects /l°pb profiles the comparisons do not provide unequivocable results. Because sediment mixing is dominantly controlling the 2~°pb profiles in the distal mud deposit, it may be that the true accumulation rate at present is only tenths of millimeters per year, and the ~4C and 21°pb rates may be in agreement. A calculation of sediment accumulation rate for the Holocene gives results similar to those derived from ~4C ages. Sea level transgressed beyond the location of the southern fine-grained deposit (G8024) about 12,000 yrs B.P. (Geng, 1982), and the thickness of Holocene sediment is about 5 m (Milliman et al., 1987). Thus (from data in Table 1), the lower 3.8 m of the deposit represent

S E D I M E N T A C C U M U L A T I O N IN T H E Y E L L O W S E A

about 7750 yrs of sediment accumulation, which yields an average accumulation rate of about 0.5 mm/yr. Once again, this rate is similar to present rates in the central Yellow Sea. Most of the sediment from the Huanghe was probably accumulating in the Shandong subaqueous delta during this time period, and was not available to accumulate farther along the dispersal system. Additional 14C dates from the central Yellow Sea are needed to further test this hypothesis.

Sediment budget In conjunction with zl°Pb sediment accumulation rates, a simple budget to evaluate sediment accumulation in the Yellow Sea associated with dispersal out of the Gulf of Bohai during the last 100 years can be calculated using Areas 1 and 2, and a combination of Areas 3, 4 and 6 (Table 2). For Areas 1 and 2, average rates measured within these areas are used in the calculation. In the large, combined area (Areas 3, 4 and 6), however, the sediment contribution from the abandoned Huanghe delta and/or the transgressive deposit must be removed so that the average accumulation rate in this region will reflect accurately the amount of modern Huanghe sediment which is entering the Yellow Sea from the Gulf of Bohai. Because modern Huanghe sand and coarse silt from the Gulf of Bohai are not transported beyond the foreset deposits of the Shandong subaqueous delta (Alexander et al., submitted), the sand and coarse silt contained within the sediments in Areas 3, 4 and 6 are probably derived from the abandoned Huanghe delta and/or the transgressive deposit. After mathematically removing the proportion of sand and coarse silt in these sediments (known for each core analyzed; see Alexander et al., submitted), accumulation rates in this large region are similar to those measured in the eastern Yellow Sea (0.03-0.23 g/cmZ/yr) and an average value (0.04 g/cmZ/yr) is calculated for the combined area based on these data. The subsequent budget calculation demonstrates that a maximum of 1.6x 10st of sediment is annually accumulating in the Yellow Sea (about 15% of the annual Huanghe discharge). The major area of accumulation is Area 2 (the foreset and

67

proximal bottomset deposits of the subaqueous delta), where calculated accumulation rates are good estimates of the actual accumulation rates (on a 100-yr time scale), and where sediments are obviously derived from the post-1855 Huanghe dispersal system. Area 2 accounts for 9.5 x 10vt (58%) of the total sediment accumulating in the Yellow Sea, or 9% of the annual Huanghe discharge. In contrast, the very extensive region in the central and southern Yellow Sea (Areas 3, 4 and 6; the distal bottomset deposits of the subaqueous delta) represents about 90% of the area, but contributes a maximum of 6.0 x 10v t (38%) of sediment to the budget (after the contribution from sources other than the modern Huanghe are removed). Accumulation rates in this region are maximum accumulation rates (see results section), and actual accumulation rates may be considerably lower. The contribution from Area 1 (the topset deposits of the subaqueous delta) is relatively minor (about 8.5 x 10 6 t annually) as a result of slow accumulation and the restricted extent of these deposits in the Yellow Sea (similar deposits may extend northwestward into the Gulf of Bohai). Together, Areas 1 and 2, which represent the topset, foreset, and proximal bottomset deposits of the Shandong subaqueous delta (and about 10% of the area covered by fine-grained sediment in the Yellow Sea), account for about 1.0 x 108 t (62%) of sediment presently accumulating in the Yellow Sea. The major sources of error to the budget come from errors in delineating the boundaries of the accumulation-rate areas in the Yellow Sea, and errors introduced because zl°Pb accumulation rates used in calculations are maximum estimates of the actual accumulation rates in a large part of the study area. The most current sedimentologic data were employed to help constrain the area boundaries. The western boundary of Area 2 is defined by sedimentological data (Milliman et al., submitted) because coring was restricted near the Chinese coastline. Similarly, beyond the limits of core coverage, the boundaries of the combined region (Areas 3, 4 and 6) are sedimentologicaily constrained to the west, south and east, where coarse, sandy sediments surround the fine-grained deposits (Milliman et al., submitted). The low

68 accumulation rates in the combined region would cause small errors in the boundaries of this region to have little effect on the budget. Because accumulation rates in the combined region are maximum estimates of the actual accumulation rates, the contribution of this area to the budget is difficult to quantify, and may be small. The uncertainty in the contribution of this combined area to the sediment budget (+ 38%) probably represents the greatest source of error associated with the sediment budget calculation (Table 2). However, even if we assume that the contribution of the combined region is negligible, 9% of the annual Huanghe discharge is calculated to be accumulating in the foreset and proximal bottomset deposits of the subaqueous delta. Thus, at least 9% and as much as 15% of the annual Huanghe sediment discharge appears to be accumulating in the Yellow Sea.

The future Huanghe dispersal system The importance of the Shandong subaqueous delta as a depocenter of Huanghe sediment in the Yellow Sea will increase with time. Extensive diking of the lower reaches of the Huanghe by the Chinese government has led to stabilization of the river and a greatly decreased likelihood of a major shift in the river's course (Ren and Shi, 1986). Sediment input to the Yellow Sea from erosion of the abandoned delta (along the Jiangsu coast) will decrease as the coastline slows its retreat and reaches equilibrium with current oceanographic conditions (Liu and Walker, 1989). The Gulf of Bohai is shallow (average depth 18 m), small (about 7.7 x 104 km2), and physically energetic (Fig.l). Ren and Shi (1986) estimate that about 2700 years would be required to completely fill the Gulf of Bohai, given present sediment discharge rates. However, long before the Gulf is filled, large volumes of Huanghe sediment will be entering the Yellow Sea. If the modern Huanghe delta continues to prograde seaward at present rates (about 0.5 km/yr; Ren and Shi, 1986), the subaerial portions of the delta could conceivably reach the Bohai Straits in about 350 years. At this point, virtually all the Huanghe sediment would be discharged into the Yellow Sea, where it would preferentially accumulate on the foreset beds of

CR ALEXANDER the subaqueous delta as physical energy levels decreased. Conditions would probably be similar at this time to those existing when the subaqueous delta was first formed (~4060-6200 yrs B.P.).

Epicontinental- versus perieont&ental-shelf sedimentation Although the Huanghe discharges into an epicontinental-shelf setting, and the Amazon and Ganges-Brahmaputra discharge into pericontinental-shelf settings, these three major rivers have formed sedimentary deposits that are similar in morphology and in distribution of accumulation rate. These deposits have a clinoform morphology, and, in the Yellow Sea and on the Amazon shelf, are known to exhibit the internal stratification of subaqueous deltas (i.e., subaqueous, gently dipping topset strata, steeply dipping foreset strata, and gently dipping bottomset strata which downlap onto coarse, transgressive deposits; Nittrouer et al., 1986b; Alexander et al., 1987). In all three environments, accumulation rates show a similar distribution: rates are low on the topset deposits, increase to a maximum on the foreset deposits, and rapidly decrease onto the bottomset deposits (Kuehl et al., 1986, 1990). This distribution of accumulation rates leads to aggradation and seaward progradation of subaqueous deltas. Low rates on the topset deposits imply that aggradation is slow relative to the rapid accumulation of the foreset deposits; thus the topset deposits remain subaqueous as the delta progrades. Although the distribution of accumulation rates is similar in all three environments, the highest rates measured (on a 100-yr time scale) are presently an order of magnitude lower on the Shandong subaqueous delta (about 10 mm/yr) than on the Ganges-Brahmaputra and Amazon subaqueous deltas (about 10 cm/yr). The low rates on the Shandong subaqueous delta probably result from a "lag effect" in sediment dispersal; much of the sediment has been accumulating in the modern Huanghe delta since the Huanghe shifted its course back to the Gulf of Bohai in 1855. As increasing amounts of the annual Huanghe discharge bypass the Gulf of Bohai, rates on the Shandong subaqueous delta should increase to be commensurate with

69

SEDIMENT ACCUMULATION IN THE YELLOW SEA

those in the Ganges-Brahmaputra and Amazon systems. The geometry of the clinoform structure and the distribution of the three subaqueous-deltaic environments is strongly influenced by the shelf setting into which the subaqueous delta progrades. The orientation of the dispersal system with respect to the progradation direction of the subaqueous delta and the relative environmental energy (from waves, tides and currents) are critical in the development of the clinoform morphology. Major rivers entering epicontinental-shelf settings may develop dispersal systems parallel or perpendicular to the progradation direction of the subaqueous delta, and shelf-edge processes will be of relatively little importance. The Huanghe dispersal system is oriented parallel to the progradation direction of the subaqueous delta (southward) and both axially and laterally extensive bottomset deposits have developed. These deposits extend the length of the Yellow Sea (about 800 kin), and extend over 300 km across the basin. In contrast, the bottomset deposits of both the Amazon and Ganges-Brahmaputra subaqueous deltas are much more restricted (extending less than 50 km across the shelf). Major rivers which enter pericontinentalshelf environments (e.g., the Ganges-Brahmaputra and Amazon) develop dispersal systems that follow the dominant shelf circulation (along the shelf; Drake, 1976). Such dispersal systems will be oriented perpendicular to the progradation direction of the subaqueous delta, bottomset deposits will be areally restricted, and progradation of these deposits will be slow. The increased energy levels typically encountered on the outer shelf also will act to inhibit sediment accumulation. The Shandong subaqueous delta is similar in general morphology to clinoform structures reported in ancient, fine-grained, epicontinental deposits [e.g., the Mississippian Borden Siltstone (Swann et al., 1965) and Late Cretaceous mudstones of Wyoming (Asquith, 1970, 1974)], and which should be present in most thick, fine-grained sequences (Potter et al., 1980). The distribution of sediment accumulation rates on these ancient clinoform structures was probably similar to that measured on modern subaqueous deltas. Asquith (1970) reports relative volumes of sediment con-

tained within Late Cretaceous Wyoming clinoform deposits to be topset, 30-40%, foreset, 50-60%, and bottomset, 10%. This observation suggests that accumulation rates were highest in the foreset deposits of these clinoforms. These ancient features are thicker (average thickness 150m) than the Shandong subaqueous delta, probably because they formed in foreland basins where active subsidence was taking place and over much longer time scales. If the Yellow Sea basin were to begin to subside rapidly, the Shandong clinoform structure would not only become thicker, but also would more likely be preserved during the next lowstand of sea level.

Summary 21°Pb sediment accumulation rates in the Yellow Sea show that the major area of modern sediment accumulation is the Shandong subaqueous delta. Highest rates are observed on the foreset deposits (4-9mm/yr), with lower rates on the topset (1-2 mm/yr) and proximal bottomset (2-4 mm/yr) deposits. This distribution of rates leads to aggradation and seaward progradation of the subaqueous delta. Although these rates are rapid enough to have constructed the subaqueous delta within the past 7000 yrs, 14C data on the age of sediments within the subaqueous delta and stratigraphic information indicate that it probably formed between 4060-6200 yrs B.P. (at rates approaching 20 mm/yr). Following this period of rapid accumulation associated with the progradation of the foreset deposits of the subaqueous delta, rates decreased to those measured today as topset deposits were formed. The shift of the river to south of the Shandong Peninsula between 861-134 yr B.P. (1128-1855 A.D.) allowed strong currents to erode the topset deposits until about 130 years ago, when the river shifted back into the Gulf of Bohai. Thus, the Shandong subaqueous delta formed relatively early in the Holocene history of the Yellow Sea, but continues to aggrade and prograde when conditions are favorable. Presently, 9-15% of the annual Huanghe discharge is escaping the Gulf of Bohai and is accumulating in the Yellow Sea. Most of this sediment (62%) is accumulating in the topset, foreset and

70

proximal bottomset deposits of the Shandong subaqueous delta. The lower estimate reflects the observation that bottomset accumulation rates are maximum estimates of accumulation rate; the actual contribution from the bottomset region is difficult to quantify. The fraction of Huanghe sediment escaping the Gulf of Bohai into the Yellow Sea probably will increase with time, as a result of stabilization of the river course and infilling of the Gulf of Bohai. The morphology of the Shandong subaqueous delta illustrates that subaqueous deltas are the general rule when major rivers enter energetic shelves, whether epicontinental or pericontinental. However, the distribution of the topset, foreset, and bottomset sub-environments may be different in these two shelf settings. Relatively thin, but both laterally and axially extensive (covering thousands of square kilometers) bottomset deposits may be developed in epicontinental settings. The preferential development of extensive bottomset deposits in epicontinental settings, in contrast to pericontinental settings, results from fundamental differences in possible orientations between the dispersal system and the progradation direction of the subaqueous delta, in addition to differences in environmental energy.

Acknowledgements This research was supported by a grant from the National Science Foundation (INT-8501366). J. Milliman and Y. Qin undertook the complex scheduling of the cooperative research cruises. The cooperation of both Korean and Chinese colleagues, as well as that of the crews of the three research vessels involved in this project, was instrumental in the successful completion of the field program, and is gratefully acknowledged. Y.A. Park, S.C. Park, and R. Elliott assisted in core collection and sub-sampling. S. Hardin and R. Elliott provided some radiochemical data. S. McIntosh drafted the figures and L. Land typed the manuscript. D. Gorsline and an anonymous reviewer critically reviewed the manuscript.

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