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doi:10.2204/iodp.proc.346.109.2015 Sites U1428 and U14291R. Tada, R.W. Murray, C.A. Alvarez Zarikian, W.T. Anderson Jr., M.-A. Bassetti, B.J. Brace, S.C. Clemens, M.H. da Costa Gurgel, G.R. Dickens, A.G. Dunlea, S.J. Gallagher, L. Giosan, A.C.G. Henderson, A.E. Holbourn, K. Ikehara, T. Irino, T. Itaki, A. Karasuda, C.W. Kinsley, Y. Kubota, G.S. Lee, K.E. Lee, J. Lofi, C.I.C.D. Lopes, L.C. Peterson, M. Saavedra-Pellitero, T. Sagawa, R.K. Singh, S. Sugisaki, S. Toucanne, S. Wan, C. Xuan, H. Zheng, and M. Ziegler2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1428 is in the northernmost part of the East China Sea at 31°40.64′N, 129°02.00′E and 724 meters below sea level (mbsl), whereas IODP Site U1429 is only 7.4 km away at 31°37.04′N, 128°59.85′E and 732 mbsl (Fig. F1). The sites are in the southern part of the Danjo Basin, which is in the northern tip of the Okinawa Trough. The Danjo Basin is a depression ~80 km wide and ~800 m deep with a sill depth at ~700 m and is surrounded by continental shelves to its west, north, and east. Only its south side is open to the Okinawa Trough (Fig. F2). Currently, the >500 km wide continental shelf of the East China Sea spreads to the west of the Danjo Basin and also extends further to the northwest to the Yellow Sea. Because the two large rivers, the Yangtze and Yellow Rivers, drain into the Yellow Sea, a significant contribution of fine detrital material from the west is expected. During glacial lowstands, about one-half of the shelf was subaerially exposed and the mouth of the Yellow River advanced southeastward toward the Danjo Basin. Thus, the mouth of the Yellow River was perhaps located only ~150 km northwest of Sites U1428 and U1429 during glacial maxima. During glacial periods, the mouth of the Yangtze River instead advanced to the southeast and was located ~400 km to the southwest. Sites U1428 and U1429 lie beneath the Tsushima Warm Current, which branches from the Kuroshio Current ~250 km to the south. The sites are also under the influence of East China Sea Coastal Water, which expands eastward from the continental shelf of the northern East China Sea because of the larger discharge of the Yangtze River during summer. Studies on piston cores retrieved from nearby locations suggest fast sedimentation rates of ~300–800 m/m.y. (Kubota et al., 2010; Kubota, 2013), which should allow for high-resolution paleoceanographic reconstruction of the northern East China Sea. One of the major objectives of IODP Expedition 346 is to reconstruct high-resolution changes in East Asian summer monsoon (EASM) intensity since the Pliocene. The locations of Sites U1428 and U1429 were selected to explore high-resolution changes in Yangtze River discharge through reconstruction of sea-surface salinity (Kubota et al., 2010). Because the Yangtze River drainage basin occupies the portion of southern China where EASM precipitation is most intense, it is reasonable to consider that Yangtze River discharge reflects the intensity of EASM precipitation. The influx of water through the Tsushima Strait is the major source of nutrients as well as freshwater to the marginal sea between the Eurasian continent and the Japanese Islands. Therefore, data from Sites U1428 and U1429 will constrain the history of surface water salinity and nutrient concentration of water that flows into the marginal sea. The difference in salinity of surface water relative to that of deep water is one of the major controls of deepwater ventilation in the marginal sea west of Japan. The nutrient influx together with the deepwater ventilation rate is the major control of biological productivity in the surface as well as of bottom water oxygenation. Therefore, it is important to document the freshwater and nutrient budgets to best understand the origin of the dark and light layers in the sedimentary record as well as the overall paleoceanographic evolution of this marginal sea.
Publication: 28 March 2015 |
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