Spatial and seasonal distribution of planktonik foraminifera Neogloboquadrina pachyderma in the northwestern Pacific and reconstruction of paleoceanography off Shimokita

Planktonic foraminifera can record the upper ocean environments. For the purpose of the better understanding of the subpolar species Neogloboquadrina pachyderma, plankton tows, IMAGES core and sediment trap samples were analyzed. Plankton tow results indicate that N. pachyderma mainly inhabits below the pycnocline (>20 m). On the other hand, N. incompla prefers shallower and warmer waters than N. pachyderma, and its abundance is correlated with the chlorophyll-a concentration around the Japanese Islands. Based on these spatial distributions, the changes in mass accumulation rates (MAR), and coiling ratio of N. pachyderma, the palaeo-environment and water mass structure off Shimokita area (41°33.9'N, 141°52.1'E) were reconstructed during the last 26,900 years. The only Oyashio Current affected the surface and subsurface waters in 26.9-10.6 thousand calendar years before present (cal. kyr BP) with relatively subsurface warming at ∼14 cal. kyr BP. The Tsugaru Current started to enter the Pacific about 11.2-10.6 cal. kyr BP and led to the baroclinic conditions that surface and subsurface layers were under the influence of the Tsugaru and Oyashio currents, respectively. Stratification of water column and surface layer warming developed with increasing of inflow of the Tsushima Current and finally the subsurface layer started to warm at about 6 cal. kyr BP. Time-series sediment trap studies were also conducted in the northwestern North Pacific in 1998-2000. Seasonal flux profile of right-coiling N. pachyderma was consistent with that of the left-coiling one at the relatively northern area (50°N), while they were different at the southern area (40°N). It would be caused by the genetic variation of right-coiling N. pachyderma observed in the North Atlantic, and this boundary seemed to be 5% of right-coiling ratio of N. pachyderma.