Radiolarians under the seasonally sea-ice covered conditions in the Okhotsk Sea: Flux and their implications for paleoceanography

Four time-series sediment traps at two stations and three piston cores from the Okhotsk Sea were quantitatively examined for coarse-sized radiolarian shells (> 63 μm). Traps were deployed at 300 m and 1550 m at Station M4 (53°01′N, 145°30′E) and at 300 m and 700 m at Station M6 (49°30′N, 146°28′E) during August 1998 through May 2000. The chronologies of the piston cores were established applying δ ¹⁸O and paleomagnetic intensity variations; they provide records extending back to marine isotope stage (MIS) 5.51. The modern and past changes in radiolarian assemblages are associated with environmental and productivity changes. Radiolarian fluxes in the sediment traps exhibited significant summer to autumn flux peaks with suppressed values during the winter when sea-ice covered the sea surface at the trap sites. Total radiolarian accumulation rate (RAR) variations in each core tended to correspond to glacial-interglacial cycles and increased during the last deglaciation. In particular, the temporal RAR variation in Core XP98-PC1 (off Kamchatka) showed a similar trend with the climatic changes expressed by δ¹⁸O values during the glacial-interglacial cycles for the last 125 kyr. Regional differences were also apparent. RARs showed extremely low values during 12-40 kyr and 63-70 kyr in Core XP98-PC2 (central Okhotsk Sea), indicating the enhanced sea-ice impact. The sea-ice coverage might have continued for a significant part of the year during the intervals since radiolarians did not appear to increase even during the summer to autumn. RAR variations in Core XP98-PC4 (off Sakhalin) showed higher values than XP98-PC2 during MIS 2 and 4. Thus, in eastern Sakhalin around Site XP98-PC4, sea-ice coverage might have been seasonal, but not perennial even during MIS 2 and 4. Among the radiolarian taxa, Cycladophora davisiana was the most abundant species at the lower traps and in all cores. The fluxes of C. davisiana at lower traps showed much higher values than those of upper traps, and also showed similar temporal patterns with the fluxes of aluminum and terrigenous materials. Therefore, C. davisiana fluxes may be associated with terrigenous organic materials. Increased nutrient supply from the continental shelves, presumably transported by seasonal sea-ice and released by sea-ice melting, might be mainly responsible for the high production of C. davisiana in the Okhotsk Sea, especially during the last deglaciation.