Time-series radiolarian fluxes were observed employing sediment traps at Station K2 (47°N, 160°E, water depth: 5280 m) located in the Western Subarctic Gyre in the northwestern Pacific Ocean during March 2005 through May 2006. It is suggested that the northwestern North Pacific around Station K2 is the area influenced by Okhotsk Sea water masses rich in iron and nutrients, which were originated from the continental shelf and coastal seawater masses from the Kuril Islands and eastern Kamchatka. Based on available plankton tow data with four depth intervals, the flux data at specific water depth intervals were evaluated. We also characterized the relationships between radiolarian seasonal flux changes and seasonal variations of water masses at their dwelling depths.

Biogenic silica was investigated in the abyssal surficial sediment from the western North Pacific over a wide area, high biogenic silica sediment which composed of Ethmodiscus fragments was observed at a station in the oligotrophic subtropical region. Additional surface sediment samples were analyzed to survey whether it was distributed around this station or not, and several high biogenic silica sediments were found in the Philippine Sea. It seems to occupy only relatively small separate areas which are scattered randomly on the sea floor. Some hypotheses were proposed to explain the occurrence of Ethmodiscus ooze, and it is thought that the geophysical factors have a considerable impact on the process. In this paper, a review of the reason for the high biogenic silica sediments is discussed. The high concentration of chlorophyll a was observed after a typhoon passage in the subtropical western North Pacific by SeaWiFS ocean color data where the high biogenic silica sediments were observed frequently. The relevance of these two phenomena is examined with existing data and reports from other open ocean， and supports a bloom of Ethmodiscus hypothesis.