Geological and oceanographic setting of the Japan Sea and northern East China Sea

Physiographic and tectonic setting

The Japan Sea is a semi-enclosed marginal sea with an area of ~1 × 106 km2 and an average depth of 1350 m. The sea consists of three major basins: the Japan Basin to the north, the Yamato Basin to the southeast, and the Tsushima (Ulleung) Basin to the southwest (Fig. F1). The Japan, Yamato, and Tsushima (Ulleung) Basins are separated by the Yamato Rise. The Japan Sea is connected with other seas by shallow, narrow straits, namely to the East China Sea to the south through the Tsushima Strait (130 m water depth), to the North Pacific to the east through the Tsugaru Strait (130 m water depth), and to the Okhotsk Sea to the north through the Soya (55 m water depth) and Mamiya (20 m water depth) Straits. The shallow depths of these passages figures prominently in the Japan Sea’s sensitivity to glacially driven sea level change.

The Japan Sea and East China Sea are backarc basins in the western margin of the North Pacific. Whereas the Japan Sea has a relatively long geological history, the history of the East China Sea (Okinawa Trough) is relatively short. The Japan Sea opened as a pull-apart basin triggered by the collision of India and Asia (Kimura and Tamaki, 1986; Tamaki et al., 1992) (Fig. F9). The opening started at ~32 Ma and ceased at ~15 Ma (Tamaki et al., 1992; Jolivet et al., 1994). The Japan Basin has oceanic crust especially in its eastern part and opening started at ~32 Ma, whereas the Yamato Basin has thinned continental crust with basaltic sill intrusions and opening started at ~20 Ma (Tamaki et al., 1992; Jolivet et al., 1994; Tada, 1994). The Tsushima (Ulleung) Basin is most likely floored with thinned continental crust as well, although no drilling to the acoustic basement has been conducted. A tectonically quiet “transitional interval” spanned from 10 to 7 Ma, and the east–west compressional stage started at ~7 Ma (Tamaki et al., 1992; Jolivet et al., 1994). Compression became intense, and deformation was focused on the eastern margin of the Japan Sea, where the new plate boundary seems to have formed at ~3 Ma.

The East China Sea is composed of continental shelf in its northwestern part and the Okinawa Trough in its southwestern part. The Okinawa Trough is a backarc basin behind the Ryukyu arc and is one of the rare examples of an incipient continental backarc basin (Gungor et al., 2012; Fournier et al., 2001) (Fig. F10). Whereas the southern Okinawa Trough is formed by focused rifting, the northern Okinawa Trough is characterized by diffuse rifting of thinned continental crust (e.g., Gungor et al., 2012).

The northern part of the Okinawa Trough began to open in the late Miocene (Kimura, 1996; Fournier et al., 2001), whereas the main phase of southern Okinawa Trough opening started at ~2 Ma (Gungor et al., 2012; Sibuet et al., 1998). Fournier et al. (2001) proposed three episodes of opening of the Okinawa Trough. The first episode is late Miocene N40W to N20E extension, the second episode is late Pliocene to early Pleistocene N20E extension, and the third episode is latest Pleistocene to present N20W extension. The northern Ryukyu arc and southern Kyushu Island have rotated ~30° counterclockwise (Kodama and Nakayama, 1993), which probably occurred during the last 3 m.y. and was caused by termination of the opening process toward both extremities of the basin (the tapered extension model of Fournier et al., 2012). From a geometric point of view, it is possible to regard the present stage of opening of the Okinawa Trough as being similar to that of the opening of the Japan Basin 25 m.y. ago, except for the lack of a dextral strike-slip zone along the westernmost margin of the Okinawa Trough (Fournier et al., 2001).

Oceanographic setting

In the East China Sea, the Taiwan Warm Current, a branch of the Kuroshio Current, mixes with Changjiang (also known as the Yangtze) River Diluted Water to form the TWC (Ichikawa and Beardsley, 2002) (Fig. F11). More than 70% of the fresh water discharged from the Yangtze River is carried into the Japan Sea with the TWC (Isobe et al., 2002). As a result, the salinity of the surface water in the Japan Sea is strongly influenced by the freshwater discharge from the Yangtze River. Because the EASM supplies a large amount of fresh water to the catchment area of the Yangtze River, it is likely that the EASM influences the surface waters of the Japan Sea through the TWC. The East China Sea shelf is known as a high-productivity area, sustained by nutrients provided by upwelling of Kuroshio subsurface water (Chen and Wang, 1999) that is probably induced by estuary circulation driven by Yangtze River discharge. Thus, it is possible that productivity and bottom water oxygenation level are also controlled by the Yangtze River discharge.

The East China Sea and the Japan Sea are inextricably linked to each other. At present, the TWC, a branch of the Kuroshio Current, is the only current flowing into the Japan Sea (Fig. F1). After entering the Japan Sea, the TWC is divided into three branches. The first branch of the TWC flows into the Japan Sea through the eastern channel of the Tsushima Strait (Katoh, 1993; Hase et al., 1999) and moves northeastward along the Japanese coast. Its flow path is trapped within a narrow zone on the inner shelf (Hase et al., 1999; Watanabe et al., 2006) and exists throughout the year (Hase et al., 1999). This first branch is characterized by a relatively stable flow of 1–2 Sv (1 Sv = 106 m3/s) (e.g., Isobe, 1994; Watanabe et al., 2006). The second branch of the TWC flows into the Japan Sea through the western channel of the Tsushima Strait (Kawabe, 1982; Hase et al., 1999) and flows northward along the continental shelf and slope with high variability (Hase et al., 1999). Its flow path is located to the west of the first branch of the TWC and is characterized by eddies associated with its own meander (Kim and Yoon, 1999; Hase et al., 1999). This second branch of the TWC becomes distinct during late spring to early fall when the total volume transport of the TWC increases (Kawabe, 1982; Kim and Yoon, 1999; Hase et al., 1999), and the seasonal variation of its flux is relatively large (between 1 and 4 Sv) (Takikawa et al., 2005). The third branch of the TWC, called the East Korea Warm Current, flows into the Japan Sea through the western channel of the Tsushima Strait from late spring to fall and flows northward along the eastern margin of the Korean Peninsula to ~38°N. This third branch then deflects eastward to cross the central part of the Japan Sea, forming the Subpolar Front. It then merges with the first and second branches at ~40°N along the western margin of Honshu Island.

The majority of the TWC exits the Japan Sea through the Tsugaru Strait, whereas the rest flows further north along the western margin of Hokkaido Island and subsequently flows out through the Soya Strait into the Okhotsk Sea. As a result, the TWC carries heat as far north as 45°N, compared with the Kuroshio Current that penetrates only to 38°N, and gives a significant influence on the climate not only in Honshu and Hokkaido but further north in the southern part of Sakhalin.

The Japan Sea has its own deep water, JSPW. JSPW is present below ~300 m water depth and is characterized by a nearly constant salinity of 34.06‰–34.08‰, rather cold temperatures of 0.0°–0.6°C, and high dissolved oxygen concentrations of 5–7 mL/L (Sudo, 1986) (Fig. F12). This high oxygenation level reflects vigorous ventilation of the deep water with a residence time of a few hundred years (e.g., Gamo and Horibe, 1983; Harada and Tsunogai, 1986; Watanabe et al., 1991; Tsunogai et al., 1993).

JSPW is formed in the northwestern part of the Japan Sea as a result of severe winter cooling and consequent formation of sea ice (Talley et al., 2003) (Fig. F13). Consequently, deepwater ventilation could be influenced by winter monsoon intensity (Gamo, 1999). Production of JSPW is roughly balanced with inflow of the TWC during winter (Yanagi, 2002), suggesting a possible link between the influx of the TWC and ventilation in the deeper part of the Japan Sea. The concentration of dissolved oxygen in Japan Sea deep water has been significantly reduced below 1500 m water depth during the last 70 y. If this trend continues, dissolved oxygen may be eliminated within 300 y, and it has been suggested that global warming and a subsequent weakening of the winter monsoon is responsible (Gamo, 1999).

The modern Japan Sea is rather poor in nutrients partly because of the high ventilation rate and consequent short residence time of the deep water. The concentration of dissolved phosphorous in JSPW is 2.3 mM (Yanagi, 2002). The major source of phosphorous to the Japan Sea is the TWC through the Tsushima Strait, with a flux estimated at 2.1 × 1010 mol/y (Fig. F6). This is more than an order of magnitude greater than the influxes from surrounding rivers and eolian input. In the modern Japan Sea, ~90% of the phosphorous influx exits through the Tsugaru and Soya Straits. This is because the sill depth of the Tsugaru Strait is ~130 m, which is deeper than the thermocline depth and thus allowing nutrient-rich subsurface water to leave the Japan Sea. The remaining ~10% of the phosphorous influx is buried in the form of organic phosphorous and inorganic phosphorous adsorbed on iron oxides.

At present, significant amounts of eolian dust falls over the Japan Sea, especially during early spring. Sediment trap studies estimate annual eolian dust fluxes of 45 g/m2/y in the western Japan Basin and 23 g/m2/y in the Yamato Basin (Otosaka et al., 2004). These values are comparable to average mass accumulation rates of Quaternary sediments of 41 g/m2/y at ODP Site 797 (Irino and Tada, 2000), suggesting a significant contribution of eolian dust to the bulk Quaternary hemipelagic sediments of the Japan Sea. Nagashima et al. (2007) demonstrated that quartz in the silt fraction (>4 µm) of hemipelagic sediments of the Japan Sea is dominantly eolian, whereas quartz in the clay fraction (<4 µm) is a mixture of eolian and detrital grains derived from the Japanese islands.