Sclerochronology of Hard Clams (Mercenaria spp.) as a Model for Season of Exploitation and Anthropogenic Change in Shell Middens from the Southeastern USA

Shell middens represent a world-wide record of human-environmental interaction. Sclerochronological analysis of shell midden taxa is a powerful tool that can establish baseline natural history data that extend back over 140,000 years. This record of geological and environmental change highlights the consequences of the human transition from mobile hunter-fisher-gatherers to sedentary, state-level societies. Two enduring questions emerge when examining the zooarchaeological record of shell middens. First, when in the seasonal round of subsistence did people occupy these sites? Were maritime people seasonally mobile or did they practice a sedentary lifestyle? Second, is there evidence for anthropogenic change? Emerging data indicate that biotic anthropogenesis is a phenomenon not isolated to the Industrial Revolution, but appeared early in human societal development.

Each of these questions can be addressed using sclerochronological analysis of shell midden taxa. As an example, we characterize the temporal pattern of annual growth increment formation in modern hard clam (Mercenaria spp.) shells from the southeastern United States. Year-round collections from six modern hard clam populations at monthly intervals from Litchfield Beach, SC (N = 505), St. Catherines Island, GA (N = ~720), Kings Bay, GA (N = 451), Indian River, FL (N = 1100), Cedar Key, FL (N= 259), and Charlotte Harbor, FL (N = 399) were used to track seasonal patterns of annual shell increment formation. Oxygen isotopic analyses of the growth increments were used to validate the seasonal formation of these structures and show that a couplet of light and dark shell increments reflects one year of life. When applied to archaeological (ca. 5000 YBP to AD 1513) hard clams (N = ~5200), a year-round pattern of procurement was identified. Further, the mean ontogenetic age of hard clams declined with human population growth and/or intensification of harvest.