U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing oil the bank tops. These waters have significantly lower Th-232/Th-230 (similar to10,000) than those immediately adjacent to the banks (24,000-31,000) and a ((231)pa/Th-230) near the production ratio (similar to0.1). The change in Th-232/Th-230 and ((231)pa/Th-230) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a Mixture of Th-231 and Pa-231 alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both Th-232/Th-230 (similar to13,000) and ((231)pa/Th-230) (similar to0.5) is observed at similar to430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa call be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).

The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater delta(234)U averages 146.6 and does not vary by more than 2.5parts per thousand, and that carbonates capture this value. Additional high precision measurements (approximate to+/-1parts per thousand) on modern carbonates confirm that all oceans have identical delta(234)U. Modern marine carbonates are shown to have Th-232/Th-230 ratios that reflect the local seawater in which they formed. Copyright (C) 2004 Elsevier Ltd.