Sites 1150 and 1151 are located near the confluence of two important currents in the western Pacific: the Kuroshio and the Oyashio Currents (Fig. F1). The Kuroshio Current is part of the North Pacific gyre, flowing northeast along the southeastern shore of Japan and transporting ~40 Sv (1 Sv = 106m3/s). At ~ 35°N, the Kuroshio Current separates from the Japan coast by turning east to form the Kuroshio Extension Current (Fig. F1). The Oyashio Current is part of the North Pacific Current, flowing southward from the Bering Sea. High salinities and relatively cold temperatures characteristic of the subarctic Oyashio Current produce a permanent halocline between 100 and 200 m. Relative to the subtropical Kuroshio ocean current, the Oyashio Current shows characteristically higher rates of biological productivity, with daily production ranging from 0.45 to 1.51 g C/m2 (Taniguchi and Kawamura, 1972). Lower rates of productivity in the oligotrophic Kuroshio Current, ranging from 0.05 to 0.1 g C/m2, result from the presence of a permanent thermocline that prevents the transport of nutrients from the deep ocean to the surface (Ichimura, 1965).
The convergence between the Kuroshio and the Oyashio Currents presently occurs at ~40°N offshore Japan (Fig. F1), and it creates abundant eddies, potentially affecting the climate of Japan. The position where the Kuroshio Current diverges from the Japan coast to form the Kuroshio Extension appears to be persistent (Nishida, 1990). However, occasional swings in the mean position of the current do occur, and they are associated with dramatic changes in the climate of Japan. For example, cooler temperatures in Japan resulted when cold water masses from subarctic waters were able to reach more southerly latitudes as the Kuroshio Current moved south in 1954-1956.
Because of characteristic differences in surface productivity in the Kuroshio and Oyashio Currents, past changes in ocean circulation off eastern Japan can potentially be inferred from variations in sedimentary constituents related to sea-surface productivity. Geochemical proxies for biological accumulation were therefore studied at Leg 186 sites in order to assess changes and trends in primary productivity and to use that information to evaluate changes in ocean circulation in the northwestern Pacific. This research focuses on accumulation of organic matter and carbonate minerals present in the diatomaceous silty clays and claystones extracted during Leg 186. At present, productivity levels at Sites 1150 and 1151 are determined by the Oyashio Current. Thus, a northward shift in the current location of the Kuroshio Current would result in a decrease of organic carbon and carbonate abundances. Conversely, a southward shift of the Oyashio Current would cause increased productivity and would likely be associated with colder climates.