Site 1150 (39°11´N, 143°20´E; water depth = 2681 m) is located in the deep-sea terrace on the landward side of the Japan Trench. The upper 200 meters below seafloor (mbsf) of sediment from Hole 1150A consists of interbedded diatomaceous ooze and clay (Sacks, Suyehiro, Acton, et al., 2000). Maruyama and Shiono (this volume) suggest the sedimentation rate of 170.1 m/m.y. for the upper 51 mbsf, based on the last occurrence at 0.30 Ma of Proboscia curvirostris at 51.03 mbsf. The upper 27 mbsf of Hole 1150A was estimated to be late Quaternary in age and was utilized for this study (Table T1).
Site 1151 (38°45´N, 143°20´E; water depth = 2178 m) is located 48 km south of Site 1150, in the deep-sea terrace of the Japan Trench. The upper 100 mbsf of Hole 1151C is diatomaceous silty clay. The sedimentation rate is 38.3 m/m.y. based on the last occurrence of P. curvirostris at 11.50 meters composite depth (mcd). The upper 22 mbsf covers the latest Quaternary used in this study (Table T2).
Dried sample (0.10 g) was placed into a 100-mL beaker with ~10 mL of hydrogen peroxide solution (15%) for several seconds and then left to stand for 24 hr after diluting with distilled water. After pouring off the suspension, the residue was diluted again with 75 mL of distilled water and homogenized for ~3 s in an ultrasonic washer (Clean Matic; 20 W, 40 kHz). Using a micropipette (Justor-Jv 500 µL), 0.25 mL of the solution was placed on a cover glass (18 mm x 18 mm), dried on a hot plate at 50°C, and then mounted on a glass slide using Pleurax.
All diatoms were identified and counted until the number of individual specimens reached 200, excluding Bacteriastrum spp. and Chaetoceros spp. But only Chaetoceros furcelatus was counted.
Sediment accumulation rates were calculated on the basis of the diatom events.