Overall abundance and species composition of radiolarian faunas were analyzed in surface sediment samples from representative areas of South China Sea, East China Sea, Sea of Japan, Sea of Okhotsk, Bering Sea, Philippine Sea, and the western boundary current regions of the North Pacific, in order to understand the biogeographic distribution ofradiolarians in the Northwest Pacific and explore its relationship with the main environmental factors and the North Pacific circulation. The results showed that radiolarians in the Northwest Pacific surface sediments can be divided into two large biogeographic provinces---cluster A and cluster B. Cluster A is characterized by the dominance of warm-water species and distributed primarily in tropical and subtropical seas with high radiolarian abundance and diversity; whereas cluster B is predominated by cold water species and distributed mainly in the Arctic and subarctic seas with comparably low abundance and diversity. Cluster A is further divided into five subclusters, AI to A5, which correspond to East China Sea, Philippine Sea, South China Sea, Sea of Japan, and Kuroshio Current, respectively; cluster B is divided into three subclusters, B1 to B3, which correspond to Sea of Okhotsk, Bering Sea, and subarctic gyre area, respectively. Based on the relationships between radiolarian faunas and major environment parameters in different biogeographic provinces, we suggest that the sea surface temperature （SST） and sea surface salinity （SSS） are primary factors that influence productivity, composition, and distribution pattern of the radiolarian fauna in the Northwest Pacific regions, while water depth is likely responsible for regional differences in the radiolarian fauna in each marginal sea. In addition, according to the distribution and abundance patterns of common radiolarian species in different areas, we identified five special radiolarian assemblages, which may be used as indicators for main Kuroshio Current, Kuroshio-East China Sea Branch, Kuroshio-South China
LIU LingZHANG QiangCHEN MuHongZHANG LanLanXIANG Rong
Using a planktonic net(62-?m mesh) and a Rose-Bengal staining method, we studied the spatial distribution of living radiolarians in spring along two sections of the South China Sea(SCS) in spring and discussed the responses of living radiolarian distribution to tropical environmental factors. Generally, the highest abundance of living radiolarians occurred at the depth range of 25–75 m, where the chlorophyll-a maximum and the highest primary productivity were. In contrast, the maximum living abundance occurred in the top 25 m in cold eddies in the open seas and the abundance decreased with depth. We found that the inhibition effect of changing salinity(due to runoffs) on living radiolarians was much stronger than the promotion effect of mesoscale cold eddies. We observed that large variation of temperature was unfavorable for living radiolarians. The dominant species composition consisted of tropical-subtropical warm species. We identified some indicator species for tropical environments. Living Didymocyrtis tetrathalamus tetrathalamus could be an indicator for tropical surface water or mixed-layer water, and even for tropical oligotrophic water. Living Tetrapyle octacantha could be used to indicate tropical thermocline and eutrophic environment. Living Acanthodesmia vinculata could indicate tropical surface and subsurface waters. T. octacantha and A. vinculata should only be used as indicators for upwelling in the open seas, i.e., far away from river mouths. Living Siphonosphaera polysiphonia preferred to form colonies, which might be related to the effect of warm eddies. Living Cyrtopera laguncula and living Cornutella profunda occurred in the tropical upper layer, even in the surface layer, which suggests that they should not be used as indicators for intermediate and deep waters.
HU WeiFenZHANG LanLanCHEN MuHongZENG LiLiZHOU WeiHuaXIANG RongZHANG QiangLIU ShiHao
Detailed radiolarian biostratigraphy in the Plio-Pleistocene was analyzed by using samples from IODP Site U1340 that was drilled to a core depth of 604 m in the southern Bering Sea.A total of 227 species belonging to 102 genera were identified.Based on the distributions of the radiolarian index species at Site U1340,five radiolarian zones since the Pliocene were established in the southern Bering Sea for the first time,and 25 radiolarian bioevents were recognized.Their ages were estimated on the basis of the age-depth plot that was constructed by the synthetical datum of the effective biostratigraphic and magnetostratigraphic events.The radiolarian zones at Site U1340 were systematically compared with those in its adjacent regions since the late Early Pliocene,which further improved and interpreted the biostratigraphic datum as well as their correlations in the middle-high latitude of the North Pacific.In addition,the comparative results of radiolarian zones show that Botryostrobus aquilonaris Zone emended in this paper is equivalent to the upper part of the same zone defined by Hays,1970,and Druppatractus irregularis-Dorydruppa bensoni Zone as well as Spongodiscus sp.Zone,newly proposed in this paper,are well correlated with Cycladophora sakaii Zone and Stylatractus universus Zone in the subarctic North Pacific,respectively.
Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Pliocene （the last -4.3 Ma）. The re- sults show that clay minerals at Site U1340 are dominated by illite, with a moderate amount of smectite and chlorite, and minor kaolinite. Sediment source studies suggest that the clay mineral assemblages and their sources in the studied core are controlled primarily by the climate conditions. During the warm periods, clay minerals originated mainly from the adjacent Aleutian Is- lands, and smectite/（illite＋chlorite） ratios increased. During the cold periods, clay minerals from the Alaskan region distinctly increased, and smectite/（illite＋chlorite） ratios declined. Based on smectite/（illite＋chlorite） ratios and clay mineral crystallinities the evolutionary history of the paleoclimate was revealed in the Bering Sea. In general, the Bering Sea was characterized by warm and wet climate condition from 4.3 to 3.94 Ma, and then cold and dry condition associated with the enhanced volcanism from 3.94 to 3.6 Ma. Thereafter, the climate gradually became cold and wet, and then was dominated by a cold and dry condi- tion since 2.74 Ma, probably induced by the intensification of the Northern Hemisphere Glaciation. The interval from 1.95 to 1.07 Ma was a transitional period of the climate gradually becoming cold and wet. After the middle Pleistocene transition （1.07 to 0.8 Ma）, the Bering Sea was governed mainly by cold and wet climate with several intervals of warm climate at -0.42 Ma （MIS 11）, -0.33 Ma （MIS 9） and ~0.12 Ma （MIS 5）, respectively. During the last 9.21 ka （the Holocene）, the Bering Sea was characterized primarily by relatively warm and wet climatic conditions.
ZHANG QiangCHEN MuHongLIU JianGuoYU ZhaoJieZHANG LanLanXIANG Rong
