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.