Variation of the Path and Intensity of the Kuroshio During the Past 30 Kyrs Based on Alkenone Sea Surface Temperatures

The Kuroshio Current is a greatest and important warm surface current in the North Pacific. The Kuroshio plays a role of main heat transport from the tropical ocean to the North Pacific. Therefore, it is seemed that the historical changes of the path and intensity of the Kuroshio influenced the climate changes in the Northwest Pacific and the surrounding land area. Three sediment cores, MD01-2422, TSK-1PC, and OTK-2PC were collected from off Shikoku, Tokara Strait, and southern Okinawa Trough to reveal the paleoceanographic changes in the Kuroshio flow region. Age models of the cores are established by AMS 14C dates (calendar ages) of planktonic foraminifera. At the site of off Shikoku (MD01-2422), alkenone sea surface temperatures (SSTs) fluctuated between 21 and 25 degrees C with amplitude of about 4 degrees C during the past 30 kyrs. Alkenone SSTs were abruptly increased by about 3.5 degrees C at 17 kyrs B.P. The rapid warming corresponded to a sharp decrease of alkenone production. These results indicate that the surface water at core site was significantly influenced by the warm and nutrient-poor Kuroshio at 17 kyrs B.P. The SST differences between the off Shikoku and the Nishishichitou Ridge (Sawada and Handa, 1998) were very small during the last glacial maximum (LGM) and middle Holocene (2-4 kyrs). Lowered SSTs for these periods in off central Japan and off SW Japan may have been influenced by a northern-origin cold water mass. Middle Holocene cooling at off Shikoku may correspond to the so-called Pulleniatina minimum event (PME) in the East China Sea. On the other hand, SSTs in off Shikoku were much warmer than eastern Kuroshio region (Nishishichitou Ridge) during the last deglaciation, early Holocene, and latest Holocene. The path of the Kuroshio main flow at the LGM was shifted southward due to the global cooling based on the distribution patterns in alkenone SSTs. The intensity of the Kuroshio was significantly enhanced at the early Holocene (7-9 kyrs) because the warm surface water was distributed in the entire area of off central and SW Japan.