Uranium Series Isotopes in Sediments from the Red Sea Hot-Brine Area

Abstract

The distribution of the longer-lived uranium and thorium series isotopes in the iron-rich sediments of the Red Sea geothermal brine area is characteristic as compared to that of normal oceanic deposits. Concentrations of uranium on a salt- and water-free basis range from 6 to 30ppm, whereas those of thorium range from 0.1 to 0.5ppm. The low Th/U ratios are also reflected by the low activity ratios of Th²³⁰/U²³⁴ and Pa²³¹/U²³⁵, the latter ratios being considerably less than the secular equilibrium value of unity. Maximum ages of the deposits may be estimated from the growths of Th²³⁰ and Pa²³¹. These upper-limit ages indicate that the metalliferous sediments accumulated at a rate of at least 40cm/10³ yr and that the sampled four to eight meters of sediment columns in the Atlantis II Deep were deposited in less than 10,000 years. Rates higher than 100cm/10³ yr should not be considered as rare occurrences. While thorium, protactinium and radium in these sediments may originate from the extruding brines, an important fraction of uranium may have been scavenged out of the overlying normal Red Sea water through co-precipitation with the colloidal ferric hydroxide and silica.

The low specific activities of the uranium-unsupported Th²³⁰ (<1dpm/gr) and Pa²³¹ (<0.1dpm/gr) associated with the calcareous lutite sediments impose limitations on the use of the decay of these nuclides for age-dating. They are, however, suggestive of a lutite accumulation rate of the order of 10cm/10³ yr in the Red Sea.

Woods Hole Oceanographic Institution Contribution No. 2201.