Holocene evolution of depositional processes off southwest Japan: Response to the Tsushima Warm Current and sea-level rise

Abstract An understanding of the characteristics and evolution of shelf sediments is crucial for reconstructing ancient shelves from stratigraphic records. Oceanic currents have a major influence on shelf sediments. However, the evolution of sediments in narrow strait/seaway settings influenced by oceanic currents, especially in response to sea-level change, is still unclear compared with open shelf settings. The Tsushima Strait is the only gateway for the Tsushima Warm Current (TWC) into the semi-enclosed Sea of Japan. The evolution of depositional processes in the strait during the Holocene is not well documented, and this makes it difficult to develop a depositional model for this strait environment. To remedy this, we investigated marine sediments off Fukuoka, southwest Japan, adjacent to the eastern channel of the Tsushima Strait. A vibrocorer allowed us to collect 1.5–4.9 m long cores from 11 sites on the coarse-grained sandy shelf (38–66 m water depth). As a result, 10 lithofacies are recognized, and radiocarbon dating results indicate that these deposits formed mainly at two times, from 11.6 to 8.4 cal kyr BP and since 6.6 cal kyr BP. The main temporal changes recorded in the depositional environment off Fukuoka include (1) coastal and shallow-marine environments before the Last Glacial Maximum, (2) an embayment during the period 11.6–8.4 cal kyr BP, (3) a period of non-deposition from 8.4 to 6.6 cal kyr BP (apart from a few tidal channel deposits), and (4) TWC-influenced shelf deposition after 6.6 cal kyr BP. The formation of the TWC-influenced deposits shifted towards shallower water regions during postglacial sea-level rise. Overall, the evolution of the TWC-influenced deposits suggests that the strait/seaway environment was controlled mainly by water depth, but also by sediment supply from the proximal region in the strait.