Counts of radiolarians at Hole C9001C

The Northwest Pacific Ocean is characterized by a strong mixing of water masses, due to the interplay of three distinct currents, i.e., the Kuroshio, the Tsugaru, and the Oyashio. The C9001C drill core site, located east of the Shimokita Peninsula and directly influenced by the Tsugaru warm current and the Oyashio subarctic current was used here to reconstruct the paleoceanographical history of this region, especially focusing on the Mid-Brunhes Event (MBE) and its consequences. This core provides a continuous record from marine isotope stage (MIS) 18 (740 ka) to the present. Polycystine radiolarian assemblages were analyzed to highlight paleoceanographic and sea surface temperature changes at this site. Based on the radiolarian fauna, seven time periods are defined, which coincide with changes in the dynamics of the Tsugaru and Oyashio currents, respectively. The oldest time interval covered by our analysis (i.e., VII: 740-621 ka) was marked by generally sluggish ocean circulation. The Tsugaru Current influence increased during the following Interval VI (621-478 ka) which encompasses interglacials MIS 15 and 13, while the Oyashio Current strengthened during the Interval V (478-337), i.e. from glacials MIS 12 to 10. These latter intervals (VI to V) constitute a long climatic transitional period where Tsugaru Current and Oyashio Current influences are strengthened. In the time period from Interval IV to Interval I (0-337 ka), the warming intensity of interglacials (MIS 9, MIS 5) appears to be close to the modern one. However, several unusually warm glacials, associated with a relatively strong Tsugaru Current flow, were identified during this interval (e.g., MIS 8 and MIS 6). Radiolarian productivity data suggest that during Intervals VII to V, deep water masses are rich in nutrient content. However, a high vertical mixing event is recorded after Interval IV, when high nutrient concentrations appear to shift to the surface layer.