ACTINOPTYCHUS SPLENDENS (SHADBOLT) RALFS (BACILLARIOPHYCEAE) - A BIOSTRATIGRAPHIC MARKER FOR THE LATER PART OF THE HOLOCENE COASTAL DEPOSITS ALONG THE SOUTHERN NORTH-SEA

Actinoptychus splendens is a conspicuous and widely distributed marine diatom. At present it is common in the coastal waters of the southern North Sea, but it appears to be ab sent in the older Holocene deposit s along the Dutch and Belgian coasts. Its first postglacial appearance here can be dated at between 4400 and 4100 BP. By 3800 BP it is well established in the region. Although generally not abundant, it can be found in nearly every sample of younger marine or brackish deposits. T he large amount of material investigated from the area, as well as the high preservation potential and easy recognition of the species, prec lude any other explanation for its apparent absence along southern Nor th Sea shores prior to +/- 4100 BP, than a late natural immigration. T his appears to be quite exceptional, all the common species which gene rally occur in association with A. splendens being present from the on set of Holocene sedimentation. A. splendens may become an important ma rker taxon in the Holocene stratigraphy of the coastal plains along th e southern North sea.