DECIPHERING SEASONALITY IN MODERN AND ARCHAEOLOGICAL SHELLS OF THE EUROPEAN LIMPET (PATELLA VULGATA): CAN WE DETERMINE SEASON OF HARVEST?

Shells of the European limpet, Patella vulgata, are common constituents in late Holocene archaeological middens along eastern North Atlantic coastlines. Thus, they potentially serve as valuable records of paleoclimate information and human activity. Oxygen isotope sclerochronology is a powerful tool for deciphering seasonal changes in growth temperature (a proxy for seawater temperature) and season of death. Previous calibration work on modern P. vulgata demonstrated its utility for reconstructing seawater temperature after correcting for a predictable offset from oxygen isotope equilibrium. Ultra high-resolution microsampling of P. vulgata shells at submonthly scale can potentially capture the entire range of seasonal variability assuming growth rates are greater than 1 mm per year. Using isotope sclerochronology, we evaluate the potential of archaeological P. vulgata shells to accurately record information on season of harvest. Modern shells were collected alive from Rack Wick Bay off Westray in the archipelago of Orkney, Scotland on 16 August 2006, the warmest month of the year, and three were selected for isotopic analysis. One shell clearly recorded summer temperature at the growth margin. Estimated temperature along the growth margins of the other two shells were more similar to mid-spring temperature, likely reflecting a time-averaging bias due to slowed growth with increasing ontogenetic age. Therefore, it is critical to take growth rate into account when estimating season of harvest. A preliminary suite of archaeological shells from a nearby midden dating to the Medieval Climate Anomaly (n=8) and Little Ice Age (n=7) were analyzed isotopically and suggest harvesting occurred year round. We will more rigorously test this hypothesis with a statistically significant sample size, using shells with sufficient annual growth rates that are least likely to suffer from time-averaging biases, and selecting shells with no abraded margins.