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doi:10.2204/iodp.proc.346.104.2015

Site U1423¹

R. 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. Ziegler²

Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1423 is in the northeastern part of the marginal sea surrounded by the Japanese Islands, the Korean Peninsula, and the Eurasian continent at 41°41.95′N, 139°4.98′E and 1785 meters below sea level. The site is ~130 km south of Ocean Drilling Program (ODP) Site 796 and ~100 km northwest of the entrance of the Tsugaru Strait (Fig. F1). Site U1423 is situated on a terrace on the middle of the slope from Oshima Island, a small volcanic island 30 km to the southeast. The site is under the direct influence of the Tsushima Warm Current (TWC) that flows further north beyond the Tsugaru Strait toward the Soya Strait (Yoon and Kim, 2009). Because the sill depth of the Soya Strait is only 55 m, the influence of the TWC on the site should have been significantly affected by glacioeustatic sea level changes during the Quaternary. Although Site U1423 is relatively close to Site 796, the tectonic setting of the two sites seems different. Site 796 has been directly influenced by west–east compression caused by incipient subduction along the nearby plate boundary between the North American and Eurasian plates (Tada, 1994). In contrast, Site U1423 seems less influenced by this compression because seismic profiles suggest conformable deposition for at least for the last ~5 m.y. (upper 300 m of sediment). Relatively low linear sedimentation rates (LSRs) are anticipated based on results from the site survey. The rates are likely to be low enough to detect the contribution of eolian dust from the Asian continent. Analyses of a site survey core confirm occasional dropstones in the upper 150 m of the sequence, suggesting its appropriateness for studies of ice-rafted debris (IRD).

Site U1423 is the middle site of the northern half of the latitudinal transect targeted by IODP Expedition 346 and is also the middle depth site of the depth transect. The location of Site U1423 in the northern part of the marginal sea was selected to identify the spatial extent of IRD events and their temporal variations. Because sea ice formation in this marginal sea occurred along its northwestern margin as a result of strong winter cooling by the East Asian winter monsoon (EAWM) wind (Talley et al., 2003), we expected the intensity of the IRD events to reflect the strength of the EAWM. At Site U1423, we hoped to reconstruct the EAWM intensity through examination of IRD abundance and distribution along the northern latitudinal transect in the marginal sea. Because stronger EAWM wind produces deep water, called Japan Sea Proper Water (JSPW), through sea ice formation in the northwestern part of the sea (Talley et al., 2003), sea ice formation and deepwater ventilation could also reflect EAWM intensity.

Because of the relatively shallow water depth of Site U1423, calcareous microfossils were expected to be better preserved than at the other sites in the Japan Basin. Planktonic microfossils may allow us to study the nature and strength of the influx of the TWC through the Tsushima Strait and/or the intensity of winter cooling, whereas study of benthic microfossils may allow us to discern the nature of the deep water (e.g., oxygenation, saturation level with respect to CaCO₃, temperature, and salinity). Examination of the relation between surface water and deepwater characteristics may allow us to explore the linkage between the nature of the TWC and deepwater ventilation. Furthermore, comparison of CaCO₃ burial flux and its temporal changes at this site to those at IODP Site U1422 will allow us to reconstruct behavior of the calcium carbonate compensation depth.

Site U1423 is also appropriate for reconstruction of eolian dust flux, sediment grain size, and provenance changes since 5 Ma, considering the relatively low expected LSR. Although only a slight contribution of IRD to the total terrigenous flux may be expected, the specific grain size range (4–32 µm) may be used to differentiate the eolian dust component from other terrigenous components, including IRD.

¹ Tada, R., Murray, R.W., Alvarez Zarikian, C.A., Anderson, W.T., Jr., Bassetti, M.-A., Brace, B.J., Clemens, S.C., da Costa Gurgel, M.H., Dickens, G.R., Dunlea, A.G., Gallagher, S.J., Giosan, L., Henderson, A.C.G., Holbourn, A.E., Ikehara, K., Irino, T., Itaki, T., Karasuda, A., Kinsley, C.W., Kubota, Y., Lee, G.S., Lee, K.E., Lofi, J., Lopes, C.I.C.D., Peterson, L.C., Saavedra-Pellitero, M., Sagawa, T., Singh, R.K., Sugisaki, S., Toucanne, S., Wan, S., Xuan, C., Zheng, H., and Ziegler, M., 2015. Site U1423. In Tada, R., Murray, R.W., Alvarez Zarikian, C.A., and the Expedition 346 Scientists, Proc. IODP, 346: College Station, TX (Integrated Ocean Drilling Program). doi:10.2204/iodp.proc.346.104.2015

² Expedition 346 Scientists’ addresses.

Publication: 28 March 2015
MS 346-104

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