Mid-late Holocene paleoenvironment in Karako lowland, western Japan, inferred from diatom analysis

Abstract

12-m and 10-m sediment cores were taken from Karako Lowland in western Kyushu Island, Japan. Diatom assemblages of the sediments were analyzed for clarifying the mid-late Holocene development of water environment in Karako Lowland in relation to tectonic activities. Diatom records showed a variety of water environments from an inner bay environment in the mid Holocene to the present marsh environment. Before 6000 cal BP, diatom-inferred water salinity was relatively high with a transition from an inner bay environment to an unstable estuary environment. The salinity decreased thereafter, and a fresh water lake environment appeared after 5600 cal BP and persisted until 4500 cal BP. There was, however, evidence of a temporary low water depth period at roughly 5500–5000 cal BP. At 4500 cal BP, water depth decreased enough to allow benthic diatoms to dominate and a peat bog was formed, but a short succession of water salinity increase was observed from 2500 cal BP. Although more than 12 m of regional tectonic subsidence during the Holocene was deduced from diatom records, tectonically induced marine incursion events were not prominent. This is probably due to the rapid development of a barrier structure. The existence of a fault and its possible activation, probably as one constituent of the inferred large subsidence, at around 5980 cal BP were proposed from the comparison of the two cores.