SEASONAL COLLECTION OF ARCHAEOLOGICAL COQUINA CLAMS FROM FLORIDA: EVIDENCE FROM SHELL GROWTH AND OXYGEN ISOTOPES

The variable coquina clam, Donax variabilis, is a familiar inhabitant of sandy beach intertidal zones along the Atlantic Coast from New York to south Florida and around the Gulf Coast to Texas. While generally not considered a human food resource in these regions today, archaeological evidence indicates that Pre-Columbian peoples often harvested coquina clams in great numbers. Extensive shell midden deposits along the northeast Florida coast confirm that populations of coquina clams were heavily exploited between the Middle Archaic (ca. 5600 YBP) and St. Johns II Periods (ca. 750 YBP). Some archaeologists postulate that these coastal sites were occupied year-round and that shellfish constituted a major dietary component. Others postulate seasonal site occupation with seasonal migrations between coastal and interior sites. In exploring these questions, we examined modern and archaeological populations of Donax variabilis to determine if there was a seasonal component to clam harvest in northeast Florida throughout the middle to late Holocene. Two approaches were employed. Size-frequency analyses of specimens from archaeological sites were undertaken and compared to seasonal size-frequency diagrams of monthly collections of clams from Matanzas Inlet, FL. No matches were observed as archaeological specimens exhibited much larger size ranges. Furthermore, new recruits into the modern population appeared year-round, creating multi-modal curves that were not diagnostic of any particular season. The second approach involved the analysis of stable oxygen isotopic variation in modern and archaeological shells. The d¹⁸O profiles in two serially sampled modern shells are completely explained by seasonal water temperature variations at the site. Comparison of the isotopic profiles with the annual temperature cycle indicates rapid shell growth in spring and summer. Shell edge isotopic values correspond with temperatures at the time of collection. Similar profiles in four archaeological specimens from different sites indicate shell growth in late spring - summer, with harvest in autumn. Sampling of additional shells from other sites and strata will be necessary to confirm or refute this suggestion of seasonal harvest patterns.