An early Holocene peak occurrence and recent distribution of Rhizoplegma boreale (Radiolaria): a biomarker in the Norwegian Sea

Several sediment cores in the Norwegian Sea reveal, in the early Preboreal, abundance peaks of up to 14% of Rhizoplegma boreale (Cleve). These peaks g enerally coincide with a transition from cold to warm water radiolarian ass emblages and a high number of diatom valves (Chaetoceros sp.), indicating h ighly productive surface waters. The R. boreale peak represents an ecologic al event caused by an influx of North Atlantic Water, and can potentially b e used as a chrono-stratigraphic marker within the eastern part of the Norw egian Sea. In core HM 79-6.2 the R. boreale peak reaches 9.2%, and is found between two C-14-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 e stablish a better age model for the studied section. Rhizoplegma boreale has been mapped in surface sediment samples from the Ic eland 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 (includin g 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 an d mixing of water masses. (C) 2001 Elsevier Science B.V. All rights reserve d.