Reconstruction of Holocene environmental changes in the Kiso-Ibi-Nagara compound river delta, Nobi Plain, central Japan, by diatom analyses of drilling cores

Holocene environmental changes were reconstructed by analyzing diatom assemblages from sediment cores YM, KZN, NK, and MC from the Nobi Plain, central Japan. Five diatom assemblage zones were identified: (1) at the beginning of the Holocene, freshwater species were dominant; (2) then, marine and brackish-marine species increased, indicating transgression; (3) in the middle Holocene, proportions of marine and brackish-marine species became almost constant, with marine species dominant; (4) marine species began to be replaced by freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species again became dominant. These diatom assemblages correlate with previously defined lithological units: zones 1 and 2 with unit B (fluvial to coastal plain), zone 3 with unit C (inner bay or prodelta), zone 4 mainly with unit D1 (delta front slope), and zone 5 with units D2 (delta front platform) and E (delta plain and flood plain). Although the shoreline migrated landward (transgression) faster than it migrated seaward (regression), transgressive diatom assemblage changes (decrease in marine-brackish water species) took up to 1000 years, whereas regressive changes required only a few hundred years. Diatom analysis is useful for reconstructing not only Holocene sea-level changes and sedimentary environments but also local geographic effects.