The beryllium-10 concentration in deep-sea sediments

Abstract

The cosmic-ray-produced radioactive isotope, ¹⁰Be (half-life 2.7 million years) has been discovered in the upper and lower portions of a 15 m long core from the sediment of the Pacific Ocean. A comparatively simple chemical procedure for its extraction has been developed. If the investigated core represents sedimentation which has remained physically and chemically undisturbed for several million years, the following tentative conclusions can be drawn: The measured concentrations of ¹⁰Be in layers at various depth indicate a sedimentation rate of about 5 mm/1,000 years which is not far from current estimates. The concentration in the upper layers leads to a present-day deposition rate of 2.55 × 10¹⁰¹⁰Be nuclei/cm² year, in excellent agreement with estimates based on the ⁷Be contents of rainwater. The decrease of concentration with depth indicates an intensity of cosmic rays in the past which has changed not at all or very slowly during the last ca. 2.5 million years.