From oysters to cockles at Hjarnø: chronology and economy on a Mesolithic kitchenmidden site

At Hjarnø, an island in Horsens fjord (Denmark), a submerged Mesolithic settlement has been known since at least 1936. In 2010 and 2011, a refuse layer of the former settlement was excavated. Despite the threat of erosion, organic preservation was generally good. For example, fragments of paddles with remains of painted decorations were found. In 2015, a shell layer at the site, facing imminent destruction by waves, was sampled. In total, 8 large boxes of shells were collected, one from each of the 1m² squares.A sample of ca. 18.000 shells NISP (ca. 3000 MNI) from the eight squares was identified to species. Of these, a subsample of 780 cockles and 115 oysters were measured. Preliminary results show that oysters became on average larger and cockles smaller moving up the sequence, as the species dominating the assemblages shifted from oysters to cockles. Overall, several lines of evidence, including the taphonomy of the shells, suggest that the shell accumulation is anthropogenic.Radiocarbon dates of associated shell and charcoal samples were used to calculate the local reservoir effect and to establish the chronology of the shell midden. Interestingly, our results indicate that the change in dominant species, from oysters to cockles, happened at about 5300 cal BC. The orthodoxy is that this shift in mollusc species occurred at 4000 cal BC and a hypothesis for the introduction of the Neolithic economy was partly driven by ‘the oyster decline’. We will also present δ¹³C and δ¹⁸O measurements on CO₂ subsamples from the shells, which will enable us to study the marine environment in more detail.