We describe a nuclear detector system for measuring low activities of 223Ra and 224Ra in natural waters based on an original design of Giffin et al. (1963). Samples are obtained by adsorbing 223Ra and 224Ra onto a column of MnO2 coated fiber (Mn fiber). The short-lived Rn daughters of 223Ra and 224Ra which recoil from the Mn fiber are swept into a scintillation detector where alpha decays of Rn and Po occur. Signals from the detector are sent to a delayed coincidence circuit, which discriminates decays of the 224Ra daughters, 220Rn and 216Po, from decays of the 223Ra daughters, 219Rn and 215Po. The system is calibrated using 232Th and 227Ac standards with daughters in equilibrium adsorbed on Mn fiber. Results of samples from Tampa Bay, Florida, and the Atchafalaya and Mississippi Rivers mixing zones are reported. The method is extendible to measurements of 227Ac, 231Pa, 228Th, and 228Ra.

Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter

Of the possible uranium-series dating schemes, the most important and most widely applied to marine carbonates is 230Th dating, with 231Pa dating playing an increasingly important role. For this reason, this review will focus on these two methods. 230Th dating, also referred to as U/Th dating or 238U-234U-230Th dating, involves calculating ages from radioactive decay and ingrowth relationships among 238U, 234U, and 230Th. 232Th is also typically measured as a long-lived, essentially stable index isotope (over the time scales relevant to 230Th dating). At present 230Th dating can, in principle, be used to date materials as young as 3 years and in excess of 600,000 years (Edwards et al. 1987a, 1993; Edwards 1988; see Stirling et al. 2001 for an example of dating corals in excess of 600,000 years old). 231Pa dating, also referred to as U/Pa dating, involves calculating ages from the ingrowth of 231Pa from its grandparent 235U. At present 231Pa dating can be used to date materials as young as 10 years and as old 250,000 years (Edwards et al. 1997). 230Th dating covers all of the 231Pa time range and more, with somewhat higher precision, and is therefore the method of choice if a single method is applied. However, the combination of 231Pa and 230Th dating is of great importance in assessing possible diagenetic mobilization of the pertinent nuclides, and thereby, the accuracy of the ages (Allegre 1964; Ku 1968). Even if the primary age exceeds the 250,000 year limit of 231Pa dating, the combined methods can be used to assess the degree to which the samples have remained closed over the past 250,000 years (e.g., Edwards et al. 1997). Thus …

Uranium-series Dating of Marine and Lacustrine Carbonates

The geochemistry of thorium and uranium

Sea water samples of 130 to 190 liters were collected to measure the concentration of thorium and protactinium in the ocean After concentration at sea and chemical isolation of the elements in the laboratory, the various isotopes were resolved with an α-particle spectrometer The precipitation of Th230 and Pa231 as they are formed from uranium was found to be essentially quantitative, less than 005% of the Th230 and less than 02% of the Pa231 remaining in solution All thorium isotopes seem to be more concentrated in the one sample of North Atlantic Bottom Water than in several samples of surface or intermediate waters An activity ratio of 15 was observed for Th228/Th232 at all depths This ratio is probably due to a partial solution of Ra228 as it is formed from Th232 in the sediment and/or river contribution of Ra228 The presence of excess Ra228 in coastal waters was confirmed by measurements made on samples of shell material

Friedrich F.F. Koczy

Papers

Mesure des isotopes du thorium dans l'eau de mer