Mid-Holocene forcing of the Tsushima Warm Current to the coastal environments in southwestern Japan with a view to foraminiferal faunas

摘要

We investigated fossil benthic and planktonic foraminifera in sediment cores FV10-05 and FV10-06-2, off Fukuoka, southwestern Japan to evaluate the impact of the Tsushima Warm Current on coastal environments around the strait between southeastern Korea and southwestern Japan during the early to middle Holocene. A cluster analysis of benthic foraminifera in these cores established four sample clusters (A, B, C, and D), which are interpreted as indicators of relatively high energy condition due to wave/current processes. Clusters A and C, characterized by attached taxa, became dominant at similar to 6.6 ka. Such faunal changes are consistent with the current-influenced sedimentation, confirming that the Tsushima Warm Current reached its present-day distribution at that time. A hiatus at similar to 8-7 ka has been reported in cores from deeper water off Fukuoka, whereas our shallow core shows deposition under high energy conditions was continuous even at similar to 8-7 ka near the coast off river mouth. Across the strait, the maximum landward extent of benthic foraminifera occurred in the Nakdong River delta (southeastern Korea) in similar to 8-7 ka, and an outer bay fauna of benthic foraminifera with seagrass beds occurred transiently in the restricted bay of the San-in district (southwestern Japan) at that time. The evolution of the Tsushima Warm Current during the early to middle Holocene thus appears to play an important role in coastal environments not only in the strait areas between southeastern Korea and southwestern Japan, but also in the San-in district.