ABSTRACT

A sediment core obtained from the northeastern Philippine Sea Basin was analyzed for ²³²Th, ²³⁰Th, ²²⁶Ra and ²¹⁰Pb. Three sheets of Ferro-manganese oxide in a matrix of red clay were included in the 73 m core. Although the concentration of ²³²Th of land origin is normal as compared with that of the usual red clay or of the sediment obtained at the neighboring station, the concentration of radiogenic ²³⁰Th is extremely low and does not decrease with increasing depth. The radioactivity of rather soluble ²²⁶Ra at the station is not less than that of ²³⁰Th in the surface sediment, showing a tendency different from that observed in usual cores. Some enrichment in the comparatively short-lived ²¹⁰Pb activity relative to ²²⁶Ra activity was found in the top sediment and in the first ferro-manganese sheet at the station. If the excess ²¹⁰Pb in the ferro-manganese sheet is not due to contamination of the surface sediment, lead should migrate through the sheet. These facts suggest that the core has not been accumulating during the past few hundred thousand years or more.