Influence of tidal-flat dynamics on shell-midden distribution during the Jomon transgression in Okutokyo Bay, Japan

The Jomon Transgression was a relative sea-level rise that reached a highstand at 7 ka, during which Tokyo Bay formed a semi-enclosed embayment known as Okutokyo Bay (2200 km 2 ). More than 600 Jomon shell middens occur on the surrounding uplands, making this region one of the most densely concentrated shell-midden areas worldwide. Reconstructing shoreline evolution and shell-midden distribution during this transgression provides a representative framework for understanding shell-midden formation processes. We obtained 36 new radiocarbon ages from the Shinoyama Shell Midden, the largest midden in the innermost part of Okutokyo Bay, and we compiled 63 previously published ages from five sediment cores and two shell-midden sites. These data were integrated with sedimentary facies, paleowater-depth reconstructions, relative sea-level changes, Holocene basal topography, and the spatial and temporal distributions of 28 shell middens. We found that (1) river-mouth flats developed in the innermost part of Okutokyo Bay during the peak of the Jomon Transgression (highstand of at least +4 m); (2) shell middens formed between 8 and 6 ka were more closely associated with the spatial dynamics of river-mouth flats than with contemporaneous shoreline positions; and (3) shell-midden formation was largely restricted to within ∼2 km of river-mouth flats.