Radiocarbon dating of coastal sites and new determinations of ΔR.

Radiocarbon dates for marine-derived sample materials usually appear ‘older’ than dates for terrestrial samples from the same archaeological contexts. This phenomenon is due to uneven rates of mixing of carbon in terrestrial and marine systems, and to slow turn-over of water from the deep ocean. The effect is well-known and can be corrected for if one knows the marine offset in the region of interest. Some data on marine offsets along African coastlines already exists, but we would like much more information. Recent analyses of early 20th century shells from the west coast of South Africa show that the marine offset there is about 600 years – substantially more than the figure of c. 200 years for the south-eastern coast. This is consistent with the greater degree of deep-water upwelling along the west coast. We would now like to extend this study, and are seeking suitable shells from museum collections elsewhere in Africa.