Several sediment cores in the Norwegian Sea reveal, in the early Preboreal, abundance peaks of up to 14% of Rhizoplegma boreale (Cleve). These peaks generally coincide with a transition from cold to warm water radiolarian assemblages and a high number of diatom valves ( Chaetoceros sp.), indicating highly productive surface waters. The R. boreale peak represents an ecological event caused by an influx of North Atlantic Water, and can potentially be used as a chrono-stratigraphic marker within the eastern part of the Norwegian Sea. In core HM 79-6.2 the R. boreale peak reaches 9.2%, and is found between two 14C-dated ash layers. We estimated the age of the R. boreale peak to be 9880±55 yr BP by linear interpolation between datum points. The characteristic stratigraphical distribution of R. boreale was used to establish a better age model for the studied section. Rhizoplegma boreale has been mapped in surface sediment samples from the Iceland Sea, the Norwegian Sea and the Norwegian fjords. An oceanic form with six and a neritic form with eight radial spines were found in these areas. Recent distribution of Rhizoplegma boreale is mainly confined to the Nordic Seas (Greenland, Iceland, and Norwegian Seas), the North Pacific (including the Bering Sea and Sea of Okhotsk) and in the Southern ocean. The highest occurrence of R. boreale is found in areas with high primary production and mixing of water masses.