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.
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