Seasonality of Shell Collecting Activity and Accumulation Speed of Shell-Midden Sites in Kanto, Japan

Abstract

Seasonal dating by counting the daily growth lines of the shells of the bivalves Meretrix lusoria, Tapes japonicus and Corbicula japonica was applied to the materials excavated from 12 archaeological sites ranging in age from Earliest through Late Jomon, the Kofun and the Edo periods (4 shell layers from shell-midden deposits, 7 shell deposits in abandoned dwelling pits, and 8 shell deposits in other pits).
1) The stratigraphic distribution of collection seasons of the bivalves in shell deposits can be used to distinguish primary deposits from secondary deposits. These latter deposits have several seasons within one block and the seasons are scattered homogeneously throughout the shell layer. Shell-midden layers on sloping ground were found to be secondary deposits. Most of the shell deposits in dwelling pits and storage pits seemed to be primary deposits in which the particular collection seasons clustered stratigraphically.
2) The seasonal sequences of the primary deposits are illustrated in Fig. 5. In all the deposits only 5 layers were determined to contain shells from only one 1/8-year collection season. The remaining layers contained shells collected during several 1/8-year seasons, and some of them showed successive accumulation over several seasons. In a few of these layers the time span of accumulation exceeded a one-year cycle (Fig. 6).
3) The accumulation speed of the primary deposits was then calculated from the volume of the shell layers accumulated within the largest 1/8-year layer in each deposit (Fig. 7). In general, the shell deposits of the Jomon period showed rapid accumulation, for example, 1000×10³cm³ in the 1/8-year shell deposit of the Kode dwelling pit 407. Those of the Kofun and Edo periods showed slower rates.
4) Each assemblage usually contained shells collected throughout the year, suggesting that these sites were used for full one-year cycles as permanent settlements. The general pattern of the seasonal assemblages of shell collecting activity examined in shell-midden layers shows about 70% of the total shells dated to spring and early summer, then this percentage decreasing gradually in late summer and autumn, reaching almost zero in winter.