Middle to late Holocene delta plain evolution of the Kimotsuki lowland, Kyushu (southern Japan)

摘要

The evolution of delta plains and coastal floodplains is commonly believed to be influenced by sea-level and climate changes. However, regional climate change influences on the evolution of delta plains and floodplains are poorly understood compared with responses to rapid sea-level rise since the last deglaciation. This study investigated the Kimotsuki lowland delta plain evolution (Kyusyu, southern Japan), including allogenic controls, based on 28 auger cores and 28 radiocarbon ages. Large-scale crevasse splay deposition occurred at 7000-6000 cal yr BP in response to sea-level rise or increased sediment supply following a volcanic eruption. Afterwards, widespread peat was formed in the absence of substantial fluvial sedimentation. Decreased precipitation from a weakened East Asian summer monsoon may have dried the peatlands around 4000 cal yr BP. With increasing precipitation and storminess around 2000 cal yr BP, peat formation resumed at areas further from the Kimotsuki River, while increased overbank flows deposited organic-rich mud near the river. These changes may reflect an increased frequency of La Nina events and higher ocean surface temperatures of the Kuroshio extension. The increased storminess around 2000 cal yr BP may have produced similar influences on delta plain and coastal floodplain evolution across the East Asia region. The results of this study suggest that regional climate change in the late Holocene has a potential to control the evolution of coastal floodplains and delta plains.