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SEDIMENT TRAP RESULTS FROM THE JAPAN TRENCH IN THE KUROSHIO DOMAIN: SEASONAL VARIATIONS IN THE PLANKTIC FORAMINIFERAL FLUX

¹ Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. E-mail: address: odam{at}mail.tains.tohoku.ac.jp
² Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku 783-8502, Japan.

Time-series sediment traps were deployed at two depths at Station JT (Japan Trench) along the northern margin of the Kuroshio current system domain from March 1991 to May 1992. Total (>125 µm) foraminiferal fluxes were 334–1028 shells/m²/day in the shallow trap (1200 m) and 657–2130 shells/m²/day in the deep trap (3700 m). A total of 35 planktic foraminiferal species belonging to 13 genera were identified. The assemblage was dominated by Globigerina bulloides, Globigerinita glutinata, Globigerinoides ruber and Neogloboquadrina dutertrei, whose fluxes varied seasonally. Globigerina bulloides and Globigerina quinqueloba occurred during the early spring phytoplankton bloom. These taxa were replaced by Globigerinoides ruber, Globigerinoides sacculifer, N. dutertrei and Pulleniatina obliquiloculata during the late spring to early autumn, with the development of a sharp thermocline and a deep chlorophyll a maximum just below the thermocline. The maximum shell flux of Globigerinita glutinata, Globigerina falconensis, Globorotalia inflata and Globorotalia truncatulinoides occurred during the winter near the 18 °C isotherm in well-mixed surface water that resulted from the extensive vertical mixing induced by the winter monsoon.

Temporal changes in the path of the Kuroshio current system caused by its meandering appeared to be the dominant flux control factor, based on the correlation between the meander types of the Kuroshio and the timing of large fluxes in the deep trap. During the trap experiments, major meanderings were observed from September to October 1991 and from mid-April to May 1992. During these two periods, prominent flux peaks of Globigerina bulloides and Neogloboquadrina incompta were observed in both shallow and deep traps. The major meander of the Kuroshio current system was near Station JT during these periods, and the nearshore side of the Kuroshio Front, which is characterized by high productivity in the euphotic zone, was very close to Station JT. These results suggest that these two species were produced on the nearshore side of the Kuroshio Front and collected in the sediment trap. The temporal occurrences of N. incompta at Station JT associated with the meanders of the Kuroshio are consistent with the high abundance of N. incompta in the surface sediment record from beneath the modern cold water mass off Kishu and Enshu-nada, between the Japanese coast and the axis of the Kuroshio current system.

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