Abstract
232Th, 230Th and 226Ra were determined for three pelagic cores collected in the northern North Pacific and the western and eastern basins of the Bering Sea. The surface sediment collected in the Pacific contains much 232Th and 230Th decreasing in concentration with depth indicating a sedimentation rate of 2.2 × 10-3 cm/yr. The Bering Sea sediments do not show vertical profiles of decreasing concentration with depth. This is due to a fast sedimentation rate in the Bering Sea. The excess radioactivity of 226Ra relative to that of 230Th was observed in the surface sediment collected in the eastern basin of the Bering Sea where the sediment was of siliceous ooze, showing a vertical profile decreasing in concentration with depth. The excess 226Ra is caused by the biogenic silicate material and has been used as a geochronometer. The sedimentation rate in the Bering Sea basin turns out to be 5 × 10-2cm/yr, which shows good agreement with that obtained by the 14C method. Therefore, 226Ra chronology is applicable to pelagic or hemi-pelagic sediment of siliceous ooze.
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