Well-traveled cockles: Shell transport during the Holocene transgression of the southern North Sea

Radiocarbon dates from 34 shells of the intertidal bivalve Cerastoderma edule demonstrate that shells were transported in both landward and seaward directions during the Holocene transgression of the southern North Sea. Old shells on the beaches of the East Frisian Islands of Germany document landward transport and young shells in the German Bight and Dogger Bank document seaward transport. The area's sea-level curve and the shell ages were used to predict the original depth of each specimen. The difference between a specimen's predicted age and its present depth is a measure of depth displacement. Depth displacements ranged from +35 to -37 m. Eight shells remained within 2.5 m of their original depth and roughly equal proportions of the rest moved landward and seaward. Specimens transported into deeper water are a very small fraction of the shells at that depth, whereas specimens transported into shallow water occur alongside abundant indigenous individuals of the same species. Rare fossils should not be used to estimate paleodepths.