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 absent in the older Holocene deposits 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. The large amount of material investigated from the area, as well as the high preservation potential and easy recognition of the species, preclude any other explanation for its apparent absence along southern North Sea shores prior to ±4100 BP, than a late natural immigration. This appears to be quite exceptional, all the common species which generally occur in association with A. splendens being present from the onset of Holocene sedimentation. A. splendens may become an important marker taxon in the Holocene stratigraphy of the coastal plains along the southern North sea.