From oysters to cockles at Hjarnø Sund:Environmental and subsistence changes at a Danish Mesolithic site

Recent archaeological fieldwork at Hjarnø Sund, on the present-day island of Hjarnø in Horsens Fjord (eastern Jutland, Denmark), have explored a large eroding Mesolithic site. Two large shell middens have been identified at this locality. Emergency underwater excavations have targeted the one that is currently being eroded by sea currents and wave action. The stratigraphy of this deposit is characterized by two layers extending over tens of metres and containing marine mollusc taxa typically collected by Mesolithic hunter-gatherers for food. In the field, the lower layer appeared to be dominated by the European flat oyster (Ostrea edulis), whilst the upper one to be dominated by common cockle (Cerastoderma edule). Our zooarchaeological study of midden samples confirmed the field observations on the taxonomic representation of marine shells in the two layers. AMS radiocarbon dating on shells and paired charcoal samples from the two layers indicates that these are chronologically consecutive (separated by as little as 0 to 157 years (95.4%) and that the oyster-to-cockle shift dated between ~5500-5300 cal BC and ~5300-5200 cal BC (around or just after the Kongemose / Ertebølle transition). The shell midden at Hjarnø Sund is, thus, one of the oldest-known in Denmark, demonstrating that intensive shellfish exploitation was a hallmark of the Ertebølle culture from its inception. Our study suggests that oyster-to-cockle shifts also occurred at times other than the Mesolithic-Neolithic Transition and that they may have been ultimately caused by the effects of local shoreline displacements, and consequent increases in sedimentation, resulting from drops in relative sea level.