Determination of low level of actinium 227 in seawater and freshwater by isotope dilution and mass spectrometry

Abstract

By diffusing from the sediments into the ocean, 227Ac (half-life = 21.8 y) is a powerful tracer of vertical mixing in the deep ocean on decadal time scales. However, its use is limited by its very low concentration resulting in large volumes (hundreds of L) of water required for its analysis. We have developed a new method of 227Ac analysis by isotope dilution and MC-ICPMS that significantly improves the measurement accuracy and reduces the sample size (10−30L). After spiking water samples with 225Ac milked from a 229Th solution, actinium isotopes are preconcentrated by manganese co-precipitation, purified by chromatographic methods and then measured by MC-ICPMS. The performance of the analytical method (accuracy, precision) was estimated with a homemade actinium standard solution. An internal quality control was carried out to validate the method by repeated measurements of 2 L of surface seawater doped with 227Ac (1000 ag/kg) and duplicates of the Vienne river water (6.1 ± 1.7 ag/kg and 4.1 ± 1.3 ag/kg). 231Pa was also co-precipitated, purified during the chromatography and analysed by MC-ICPMS. The combined measurement of 227Ac and 231Pa from the same sample allows discriminating 227Ac supported by 231Pa decay from the 227Ac released by remobilization from the sediments. The 227Ac concentrations measured on the first seawater samples of 29 L from the South China Sea water range from below the detection limit in surface water (~ 0.5 ag/kg for 30 L) to 3.4 ± 0.5 ag/kg at 2760 m depth (uncertainties are given in 2σn). The 227Ac measured in the deep South China Sea waters entering through the Luzon strait are consistent with previous data obtained in the same water mass in the Pacific Ocean (PDW). Seawater from the southernmost station of Bonus GoodHope, in the Weddell Gyre, were also analysed, with 227Ac concentration ranging from 4.2 ± 0.4 ag/kg to 10.9 ± 1.0 ag/kg in good agreement with previous measurement in the Weddell Gyre by Geibert et al. (2002, 2008).