Extreme flood events and their frequency variations during the middle to late-Holocene recorded in the sediment of Lake Suigetsu,central Japan

Abstract

Many studies are reconstructing flood records in the continental margins during the middle to late-Holocene. However, distinguishing the frequency and magnitude of flood events was difficult. Light gray event layers (GELs) in the sediment of Lake Suigetsu in Central Japan can solve this problem because they are recording the occurrence and magnitude of flood events during the last 80 years. Using these GELs, we aimed to reconstruct the frequency and magnitude of flood events during the last 8000 years. First, we verified whether GELs maintained the recording of flood events. We found that thin GELs (less than 40 mg/cm² in mass) were rare, probably because of bioturbation, whereas thick GELs (larger than 40 mg/cm² in mass) were rarely erased. We also found that GELs formed by the same amount of rainfall could have been thinner before 100 years ago. We revealed that the occurrence of extreme flood-origin event layers (EFELs: GELs thicker than 40 mg/cm²) during the last 8000 years may indicate the occurrence of extreme flood events (EFEs). Mass accumulation of EFELs may indicate the minimum estimation of rainfall on EFEs. There are several periods with higher frequency and magnitude of EFEs than those of the present level, agreeing with the periods of high flood activity in western Japan and East China. The frequency and magnitude of EFEs show different temporal variations. Northward and southward shifts in the westerly jet and monsoon fronts can partially explain such patterns.