Changes in radiolarian assemblages from surface sediments of the Northwestern Pacific

Polycystine radiolarians, are one of the planktonic microorganisms group, which bear siliceous skeletons. Because their fossils are well preserved in the deep-sea sediments of the North Pacific, numerous studies have been conducted for better understand radiolarian species geographic distributions in this area. It's appear that radiolarians are very sensitive to sea water ecological changes forced by the climate. On the other hand, the northwestern Pacific is characterized by the flows of the Kuroshio Current, transporting warm and saline water from the tropics and by the Oyashio Current, transporting cold water from the subarctic regions. In this context, we propose in this study to clarify and understand how these currents influence the radiolarian assemblages. We have analyzed 80 surface sediment samples collected by the Geological Survey of Japan, from the Okinawa regions to the Sea of Okhotsk. The samples were treated with H2O2 and HCl and sieved using a 45 μm screen before to be mounted on microscopic slides. We have counted 300 to 600 specimens per samples. Because warm water characterized the Kuroshio Current and cold water characterized the Oyashio Current, in this study we have compared all the relative abundances of the surface dwellers with sea surface temperature (SST) data provided by the World Atlas Ocean 2013. As a major results, the area from the coast off Okinawa Islands to off Bôsô Peninsula, fed by the Kuroshio Current, is characterized by high abundance of Tetrapyle ocatacantha group. This species group high abundances seems to correspond to SST between 20-30℃. Species as Didymocyrtis tetrathalamus present its highest abundances in the Okinawa region to south of Kyushu Island where the SST is between 25-30℃. On the other hand, species as Ceratospyris borealis is abundant in the northern Japan, where the Oyashio Current flows. The high abundance of Ceratospyris borealis seems to correspond to SST between 5-10℃. Based on these new results, an updated transfer function has been developed, in order estimate accurately the past sea surface temperature in the Northwest Pacific, based on radiolarian species.