The Distributions of Calcareous Nannofossils and Grain Size in the Surface Sedime,nts of the East China Sea and Their Relationship to the Current Pattern

Nannof ossil assemblages, carbonate content and grain size distributions were ana­ lyzed in 46 samples of surface sediments collected in the middle and outer shelf of the East China Sea. Thirty three indigenous taxa were identified. Emiliania huxleyi and Gephyrocapsa oceanica were the two dominant calcareous nannoplankton species. The f or1ner w as derived from both the China Coastal Current and the Kuroshio Current. The latter originated mainly from the Japan longshore current. Sediments containing high carbonate content (>30%) were deposited along the shelf break of the East China Sea. About 80% of the total carbonate material was composed of shell fragments and foraminiferal tests. Nannofossils were found mainly in the south of the loop current off the Chagjiang River, the western tip of the Japan longshore current and in the western flank of the Okinawa Trough. The grain size distribution pattern can be divided into four types: a unimodal pattern in the fine grain material (about 10 µm) which is composed mainly of loess; a unimodal pattern occurrring in the coarse grain size (about 200-300 µm) which is com­ posed mainly of f oraminifera and shell fragments; a bimodal pattern with a higher con­ centration of fine sediment, and a bimodal pattern with less fine-grained sediment. The bimodal sediments are distributed between the two single modes. The finer carbonate­ free sediment with bimodal size distribution is derived mainly from the Changjiang River. The coarse fractions are composed of terrigenous materials derived from either mainland China or from Taiwan. From the factor analysis of ten selected nannofossil species and groups, the sed­ iment transportation path of each current can be deduced ·from the factor loading contours. In the 1niddle the are


INTRODUCITON
The distribution of calcareous nannofossils in surf ace sediments has been a subject of intense interest in the Pacific Ocean (Geitzenauer et al. , 1976;Mcintyre et al., 1970;Roth and Berger, 1975;Roth and Coulboum, 1982;Tanaka, 1991) since it may record the environmental conditions under which these nannof ossils were produced in the overlying surface waters (Roth and Berger, 1975;Geitzenauer et al., 1976;Loubere, 1982). However, the distribution may be modified by other processes thereby causing the relative abundance of the species in sediments not to always reflect a unique paleoceanographic variable such as temperature. One of these modifying processes is the current pattern (Fincham and Winter, 1989;Winter, 1985;Tanaka, 1991;Zhang and Siesser, 1986).

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In the East China Sea, the distribution of the calcareous nannoplankton thanatocoenoses in the surface sediments has been studied by Wang and Min (1981), Wang and Samtleben (1983), Zhang and Siesser (1986). These studies were restricted to the inner shelf and to the estuary of the Changjiang (Yangtze River), and the conclusions were mostly qualitative. In fact, neither the critical factors controlling the distribution of the coccoliths nor the grain-size distribution in the surface sediments was presented in any detail. In this study, the authors try to : (1) deter1nine the distribution of calcareous nannofossils in the surface sediments in the middle to outer shelf of the East China Sea, (2) relate the distribution of selected coccolith assemblages to the current pattern in the East China Sea, and (3) determine the grain-size distribution of the surface sediments in order to understand the sources of the surface sediments in the middle and outer shelf of the East China Sea.
2. STUDY AREA . The East China Sea is a marginal sea situated between approximately 24°N, 121°E and 33° N, 130°E. It may be subdivided into three regimes: the inner, middle and outer shelves, with boundaries conventionally located at the 50-, 100-and 150-m isobaths, respectively (Zhang and Siesser, 1986). A branch of the Kuroshio Current intrudes into the southeastern boundary of the East China Sea. The warm Kuroshio Current dominates the outer shelf of the East China Sea, the shelf break, the continenal slope and the deep waters of the Okinawa Trough (Chem et al., 1990). Another warm current which comes through the Taiwan Strait infl uences the middle shelf. The China Coastal Cu11ent, which is hyposaline (less than 30° / 00 ) and 6° to 10°C colder than the Kuroshio Current, flows southward over the inner and middle parts of the shelf and mixes with the highly saline (more than 34° / 00 ) Kuroshio Current at the edge of the East China Sea (Chu, 197 1;Fan, 1985). The Changjiang River transports a large amount of sediment (over 2000 tons per year) into the East China Sea (Zhang and Siesser, 1986). The concentration of suspended matter can reach as much as 1000 mg/I at the mouth of the Changjiang River and 5 mg/I in a belt which is almost parallel to the 50-m isobath (Yang and Milliman, 1983;Wang and Samtleben, 1983).
Forty-six surface sediments were collected at 49 stations by a grab sampler during the Russia-ROC collaborative cruise (Figure 1), ' ' KEEPMASS EXPEDmON'', in July, 1992. The locations and the water depths of these stations are listed in Table 1 and shown in Figure  2. Samples were not collected at Stations E19, E23, E34.

MEmoos
Min-Pen Chen and Chao-Kai Huan g Samples were prepared for the examination of calcareous nannoplankton using the standard settling technique;-The slide was mounted with the Entellan for examination by light microscopy. Nannofossils were examined at a magnifi cation of x 1,500. More than 300 specimens were identified and counted for each slide at random. Some samples were also studied by a scanning electron microscope (SEM) to identify and study the moiphologies of the coccoliths in detail.
A description of the surface sediment is listed in Table 2. The surf ace sediments vary in lithology from clay to sand, and the grain size is independent of the water depth. Calcareous nannofossil species found in the studied samples . are listed in Table 3 in which the species are listed according to the alphabetj�al order of genus epithets.
A laser particle size analyzer, the ''ANALYSE'I*fE 22 '', was used to measure the grain size distribution in the samples within the grain-.size range of 0.16 to 1000 µm. For coarser particles (larger than '000 µm ), the grain size was measured by sieving.
The carbonate content was deter1nined by the weight loss method outlined by Molnia (1974). Before the carbonate material was dissolved with IN hydrochloric acid, the salt was removed by washing the sample in distilled water . The laser particle size analyzer, the ''ANALYSETI.E 22 '', was also used to deter1nine the size distribution of the carbonate-free residual.

RELATIVE ABUNDANCE AND DISTRIBUTION OF COCCOLITHIN SURFACE SEDIMENTS
The distribution of the carbonate content in the surface sediments of the study area is shown in Figure 3. The coarser carbonate material (>50 µm) is composed mainly of foraminiferal and shell fragments. The finer size (<50 µm) can be divided into fine bioclasts and coccoliths. The distribution of these finer particles is indicative of the distribution of the coccoliths (Figure 3). Concentrations exceeding 10% of the total carbonate content were found in the northern comer of the study area, and they may represent material from Huanghe, which transports eroded loess with carbonate contents of up to 15% (Qin and Li, 1983). Since marine biogenic carbonate may be added to the sediment during or after the transport of the particles, a high carbonate content jn the sediment can only be used as a very general indicator (Li, et al., 1991;Zhang, 1988;Zhu and Wang, 1988). Higher concentrations of coccoliths ai'e found in two areas: in the lower middle portion of the studied area, and in the Okinawa Trough at the northeastern comer of the area (Figure 3). Zhang and Siesser (1986) also reported the occunence of fewer coccoliths in the inner-shelf sediments and the progressive 134 • TAO, Vol.6, No.1, March 1995

Medium-E7
Coarse Sand   increase in the concentration with increasing water depth. The water in the middle of the study area contains high nitrite (more than 0.25 µmol), nitrate (more than 8 µmol) and phosphate (more than 0.6 µmol) near the 50-m mark (Chen and Bychkov, 1992). This high nutrient area mostly coincides with the high coccolith content area ..  Okada and Honjo, 1973 • Gephyrocapsa oceanica Kamptner 1943 (1 -width larger than 3µm; m-width between 1-3µm ) small Geophyrocapsa (see Gartner, 1977) Helicosphaera carteri (Wallich) Kamptner, 1954Helicosphaera hyalina Gaarder, 1970 Helicosphaera neogranulata (Gartner) Chen, 1978 Helicosphaera pavimentum Okada and Mcintyre, 1977 Helicosphaera wallichi (Lohmann) Okada and Mcintyre, 1977 Oolithus fragilis (Lohmann) Okada and Mcintyre, 1977 Pontosphaera messinae Bartolini, 1970 Pontoshpaera multipora (Kamptner) Roth, 1970 Chen, 1978 Umbi�icospha�ra sibogae (Weber-van Bosse} Gaarder, 1970 Wang andSamtleben (1983) recorded 40 species of calcareous nannoplankton, while Zhang and Siesser (1986) identified 36 taxa in the continental shelf sediments. In this study, 33 indigenous species and groups were found (Table 3). In the actual counting, reworked specimens and unidentifi ed fragments were also counted as separate entities. E. ht,xleyi is the most dominant species, and its relative abundance varies from 26% at Station E46 to 72.3% at Station El 7 (Figure 4). G. oceanica is also abundant in the surface sediments of the East China Sea Shelf, with its abundance varying from 14.7% at Station E17 to 44.7% at Station E46 (Figure 4). These two species account for more than 70% of the total fl ora observed in the surface sediments. The distribution of the two taxa, however, is markedly different. E. huzleyi concentrates in the central area, whereas G. oceanic spreads around ' • Min-Pen Chen and Chao-Kai Huang 137

Min-Pen Chen and Chao-Kai Huang
. the four comers of the studied area ( Figure 4). Small Gephyrocapsa is more common in the southern part, while the larger fomt of G. oceanica is more abundant in the northeastern part of the studied area ( Figure 4).

S. NANNOFOSSILS FACTOR ANALYSIS
In order to gain an overview of cocolith distribution based on their relative abundances, the Q-mode factor analysis was applied. Ten species and groups (Table 4 ), with maxium abundances greater than 2% were studied. Three factors were chosen for this study. The distribution and characteristic sp�cies of these three factor loadings are shown in Figure 5 and are listed in Table 5.
Factor 1 is characterized by E. huxleyi and small Gephyrocapsa. The forrner exists in both warm-water and cold-water ecophenotypes (Mcintyre and Be, 1967;Okada and Honjo, ·t973, 1975;Roth and Berger, 1975;Roth and Coulbourne, 1982). Both types occur on the East China Sea continental shelf. The ratio of the warm-to cold-water variety is abou� I: 1 in the inner-shelf and 10: 1 in the outer-shelf sediments, reflecting the influence of the China Coastal Current and Kuroshio Current, respectively (Zhang and Siesser, 1986). Gartner (1988) indicated that the dominant small Gephyrocapsa assemblage �ay imply an abundant nutrient content and a low temperature in the photic water column. Therefore, Factor 1 may represent a water mass admixture which was brought by the Kuroshio Current, China Coastal Current and the Taiwan Current. The high loadings of Factor I are located in the northwest, middle and the southeast of the studied area ( Figure 5). In these areas, the coccoliths were transported and deposited by the currents .

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Factor 2 is characterized by E. huxleyi and F. profunda. F. profunda is distributed from the shelf to the basin in the Pacific Ocean (Okada and Honjo, 1973). Its relative abundance increases gradually offshore and becomes very abundant at latitudes lower than 29°N (Tanaka, 1991). Both E. huxleyi and F. profunda are considered to be derived from the Pacific Ocean by the Kuroshio Current and its branch from the western part of the studied . area. This trend may be explained by the trapping of these two species by the loop current off the Changjiang River. But, the Factor 2 loading distribution shows a high value near the estuary and the central part of the studied area ( Figure 5). It is reasonable to believe that Factor 2 represents an admixture of a branch of the Kuroshio Current and a loop current off the Changjiang River.
Factor 3 is characterized by G. Oceanica and H. carteri, and its loading distribution shows a very high value near the northeast comer ( Figure 5). G. Oceanica is most abundant in the coastal areas of Japan, particularly off the Choshi-Hachinohe area and along the east coast of Kyushun, where its relative abundance is higher than 60%, and it has a tendency to decrease with increasing distance from the coast (Tanaka, 1991). Although this species has a wide distributional range throughout the studied area, it is more concentrated at the NE comer (more than 40%) ( Figure 5). This high concentration of G. oceanica implies that it may have been discharged by the longshore current from Japan. Factor 3 represents an influence of current from Japan.

GRAIN SIZE DISTRIBUTION OF SURFACE SEDIMENTS
From the distribution pattern of grain-size frequency (Figure 6), these sediments may be subdivided into four groups. Patterns A and D show unimodal distributions, whereas Patterns B and C demonstrate bimodel distributions (Figure 7). The mode peak of Pattern A is located at 6</> (16 µm) and 7</> (8 µm) and is mainly distributed in the northern comer of the studied area. · A large patch of muddy sediment extends through the Bohai Sea into the Yellow Sea and reaches the East China Sea (Lee and Chough, 1989). Smaller mud areas are present off the estuary of the Changjiang River extending southward along the Chinese coast, and on the central shelf of the East China Sea at the south of Cheju Island. The fine particles deposited within the upper comer of the study area with a common carbonate content ( Figure   3) represent the loess derived from the Yellow Sea.
The bimodal patterns B and C (Figure 7) are different from each other in two aspects.
The coarse fraction in Pattern B is finer than that in Pattern C, ·and the finer fraction (finer than 63 µm) contains more abundant portions than Pattern C. These two patterns are spread between Patterns A and D. The grain size of the surface sediment decreases from the northern part toward the southern part.

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Although Patterns A and D show unimodal distributions, the size distribution is concen trated within 1.5¢ (350 µm) and 2.5</> (177 µm) which is markedly different from Pattern A showing the higher modes in fine to medium sand classes (Figure 7). This coarse material in Pattern D is composed of either terrigenous fine to medium sand or planktonic f oraminifera.
The size distribution of the carbonate sediment shows three bimodal patterns ( Figure   8). The size of loess ranges from 4</> (63 µm) to 12</> (0.24 µm) within the northern comer in Figure 8. In the southern comer, these three parts of biogenic sediments occur together (Figure 8). The size fraction larger than -0.5</> (1.41 mm) consists of shell fragments which are believed· to be relict sediments in general. The size fraction between O</J (1 mm) and 4</> (63 µm) is mainly composed of foraminifera, and the size smaller than 4</> (63 µm) consists mainly of coccoliths. ; . ..

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• is generally higher than 60%. The size distribution of this material also shows single and double modes ( Figure 10). The single mode has its peak at the size class of 6</> (16 µm) to 7</> (8 µm), which may have been derived from the Yellow. Sea. The coarse portion of the double mode pattern is quite abundant within two thirds of the study area. The sand size from 1¢ (500 µm) to 3¢ (125 µm) may have been not only transported from the    10). Although most of the suspended sediment from the Changjiang River is deposited at the vicinity of the river mouth or was transported to the south, the particles dispersed eastward are taken up in the large gyre on the middle and outer shelves. The eastward transport occurs during high river flooding in the summer season (Yun et al.,198 1 ). I ,an dsat images show a marked turbid flow to the north northeast of the river mouth during that period (Yun et al.,198 1 ). The gyre on the shelf consists of a branch of the Taiwan Current from the south and a branch of the Kuroshio Current from the southeast. These branches supply sediment from the west and north coast of Taiwan . The sediment from the Pacific Ocean is entrapped in the center of the gyre.
The main branch of the Kuroshio Cu11ent passes the east side of the shelf break and flows into the Okinawa Trough. Another branch of the Kuroshio Current becomes the Tsushima Cu11ent of the Japan Sea. They separate at about 30.7°N between the Kuroshio and Tsushima Currents, the sediment is dispersed by the longshore current from the offshore area of the Japanese east coast (Figure 11 ). Figure 11 gives the general circulation pattern with some seasonal variations. Dispersal along the bottom may differ from dispersal through the surf ace water. On the basis of the tides alone, Dong et al. (1989) calculated that a pattern of areas of divergence and convergence exists in the Yellow Sea with currents directed mainly southward. Cai (1982) in. dicated that the amount of Changjiang River suspended matter going eastward is very small because the concentration in the eastward flow is on the order of 6 mg/I while that of the southward ftow is about 500 mg/I.

CONCLUSIONS
Analysis of the calcareous nannof ossil assemblage, carbonate content and grain size distributions on the middle to outer shelves of the East China Sea middle indicates seven cu1·rent paths: the Kuroshio Current, a branch of the Kuroshio, the Taiwan Current, the loop cu11ent off the Changjiang River, the China Coastal Current, the Tsushima Current and the longshore current from Japan. The grain-size distribution of sediment discharged from the Yellow Sea is characterized by a higher portion of fine sediment. The sediment along the shelf break is composed of coarse mollusca fragmen.ts and f oraminifera. A bimodal distribution pattern occurs between those single mode areas indicating the admixture of loess, terrigenous material and biogenic matters.