Holocene temporal and spatial variation in the radiocarbon reservoir age of three Danish fjords

Holocene temporal and spatial reservoir age variations have been investigated in three Danish fjords (Horsens Fjord, Tempelkrog in Isefjord and Skælskør Nor) by 14 C dating paired samples of terrestrial plant material and marine molluscs in sediment cores. The local reservoir age offsets at all three sites show significant temporal and local variations, with ΔR(t) values ranging from −425 to 620 yr. The ΔR(t) values for Horsens Fjord range from −425 to 0 yr, for Tempelkrog from −230 to 360 yr and for Skælskør Nor from 30 to 620 yr. The differences are best explained by differences in local hydrographic configuration, catchment characteristics and connection with the inner Danish seas. The observed temporal ΔR(t) variations are not synchronous, which is probably due to the overriding role played by numerous local factors affecting the ΔR(t) values. Based on the reservoir data from Horsens Fjord and Tempelkrog, it is suggested as a working hypothesis that the ΔR value for Kattegat prior to 4500 cal. yr BP was negative, i.e. between −130 and −200 yr. The temporal and local variations in the reservoir age underline the problems and uncertainties connected with radiocarbon‐dating fossil human skeletons with marine diets. Overall, our data show that reservoir ages in Danish fjords and estuaries are both site‐specific and time‐specific, implying that modern reservoir ages cannot simply be applied back in time. This conclusion may also be valid in the case of fjords and estuaries in large parts of the Baltic too.