We analyzed core samples from two sites in the Tokyo lowland to clarify paleoenvironmental changes that occurred during the Holocene in the northern Tokyo Bay area. The samples are from (1) Core HK, which was bored at Hibiya Park, Chiyoda Ward, Tokyo, and from (2) Core DK, which was bored at Civil Engineering Center Tokyo Metropolitan Government in Koto Ward, Tokyo. Both cores belong to Civil Engineering Center Tokyo Metropolitan Government. Analyses of both lithofacies and diatom assemblages of the cored samples found the following paleoenvironmental changes.
The first distinct regression occurred in the early Holocene (11,450-10,600 cal. yrBP) and was followed by one in the high-stand relative sea-level stage at 7,300-6,250 cal. yrBP. Subsequently, two small-scale regressions occurred at 3,600-1,200 cal. yrBP and 1,000 cal. yrBP, respectively. These two regressions were documented by temporal variations in diatom assemblages from marine to brackish-water diatoms to brackish and fresh-water diatoms. During the high-stand stage (7,300-6,250 cal. yrBP), the Hibiya embayment area was inundated with seawater and became part of Paleo-Okutokyo Bay. The marine transgressions and regressions inferred from diatom assemblages in both cores might be correlated with small-scale sea-level fluctuations at one- or two-thousand-year intervals during the Holocene, such as a minor regression during the middle Jomon period, Yayoi regression, and Heian transgression.