SCALE OF AGE-MIXING IN PREHISTORIC SHELL MIDDENS IN THE CANARY ISLANDS, SPAIN

The aboriginal population of the Canary Islands, Spain, extensively collected shellfish and discarded the shells into large coastal assemblages known as shell middens throughout the last two millennia. These middens are dominated by several species of the rocky intertidal gastropod Patella sp. and have been interpreted as sites of a one-time, intensive harvesting event. As a result, it is assumed that assigning a date to any single shell from a stratigraphically contiguous midden is sufficient to determine the age of the deposit. However, no study has been conducted to test this hypothesis. In this research we combined two radiocarbon AMS dating techniques (i.e., graphite-target and carbonate-target methods) to measure the scale of time averaging in prehistoric coastal middens in the Canary Islands. Five Patella candei shells per midden were radiocarbon dated across six archaeological sites (n = 30 total). Results indicate that nearly all shells dated from each site were equivalent in age, within the analytical and calibrating error. Thus, these pilot results support the hypothesis that shells accumulated in coastal shell middens likely represent a one-time, intensive harvesting event. This study also shows that carbonate-target ¹⁴C AMS ages are statistically similar to graphite-target ¹⁴C AMS ages (with an error smaller than 3-6%). Therefore, P. candei shells from the Canary Islands can be dated using this new, more affordable and rapid ¹⁴C AMS dating method in future investigations.