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

Saegusa, Yoshie*; Sugai, Toshihiko*; Ogami, Takashi*; Kashima, Kaoru*; Sasao, Eiji   

We reconstructed Holocene environmental changes by analyzing diatom assemblages of four drilling cores from the Nobi Plain, central Japan. We identified five diatom assemblage zones: (1) freshwater species dominant; (2) increase of marine and brackish-marine species, indicating transgression; (3) marine species dominant; (4) increase of freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species dominant, in ascending order. It became clear that the shoreline migrated landward (transgression) faster than it migrated seaward (regression) by the comparison the diatom assemblages and lithological units. The 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.