Surface water changes in the Ryukyu Trench slope region,western margin of the North Pacific during the last 320,000 years

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  • 988 Surface water changes in the Ryukyu Trench slope region, western margin of the North Pacific during the last 320, 000 years
  • Surface water changes in the Ryukyu Tre

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Two piston cores taken from the Ryukyu Trench slope, western margin of the North Pacific, contain a micropaleontologic record of the last 105, 000 years (covering the period from isotope Stage 5 to Stage 1) and 320, 000 years (covering the period from Stage 8 to Stage 1), respectively. Planktic foraminiferal assemblages are examined to reconstruct the past surface water conditions. The surface water during interglacial stages 5 and 7 was similar to the postglacial situation, when the studied region was dominated by warmer waters associated with the Kuroshio Current. During glacial stages 2, 3, 4, 6 and 8, the Kuroshio-related warm water was weaker; alternatively the influence of the temperate water which currently occupies the northern part of the Western North Pacific Central Water was stronger. In these glacial stages, the temperate water affected this region more intensely than it does today. However, the polar front characterized by the Oyashio Current has never attained this area during the last 320, 000 years. During stages 2, 4 and 6, the surface water was eutrophic; the primary productivity in DCM (deep chlorophyll maximum) layer was probably enhanced during stages 2 and 8. The sea surface temperature is estimated based on percentage frequency data of planktic foraminifera, using transfer function FP12-E. The estimated paleo-SST are 27-29°C for summer and 18-26°C for winter. As a result, glacial-interglacial fluctuations of the estimated SST are 1-2°C for summer and 4-5°C for winter; seasonal contrast during interglacials is similar to that of today and slightly greater during glacials. The decline of winter SST during glacial periods is believed to have been caused not only by the southward displacement of the temperate water mass, but also by the upward transporting of the cooler water from the underlying water column.

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