Middle to late Holocene flood activity estimated from loss on ignition of peat in the Ishikari lowland, northern Japan

摘要

Reconstruction of the magnitude and frequency of past floods is important for assessments of flood risk. However, there have been few reconstructions of past flood activity based on long-term continuous records from a single fluvial system and thus suitable for comparison with other paleoclimate proxies. Consequently, the influence of climate change on flood activity remains unclear. Floodplain peat beds can be preserved for long periods of geologic time; they also contain both elastic sediments from overbank flooding and organic material produced in situ, both of which are important inputs for geologic reconstructions. In this study, we examined 24 cores from the Ishikari lowland, northern Japan, to find out if temporal changes of loss on ignition (LOI) of peat can provide a record of flood magnitude and frequency and to investigate the influence of regional climate change on flood activity. Our plant macrofossil analyses, the distribution of characteristic LOI depth profiles, and sedimentary facies suggest that the LOI of peat increases when the magnitude and frequency of floods decreases, which in turn suggests that temporal changes of LOI in peat might be useful as a proxy for past flood activity. Our results indicate that increases in the LOI of peat deposited near the Ishikari River at ca. 5500-5000 and 4000-3500 cal BP may correspond to weakened activities of the East Asian summer monsoon (EASM) and El Nino-Southern Oscillation (ENSO). Furthermore, our analyses show that an increase in flood activity of the Ishikari River during 1400-1300 cal BP may be related to a decrease in ENSO activity. However, temporal changes of the LOI of peat formed near a tributary of the Ishikari River did not correlate with these climate changes. Our results suggest that the past flood activity of rivers with large watersheds reflects regional climate change, because the extensive precipitation that induces flooding of such rivers is generally associated with regional climate systems such as the EASM and ENSO, whereas this association is not necessarily the case for rivers with small watersheds.