Ra-226 and Ba concentrations in the Ross Sea measured with multicollector ICP mass spectrometry

• Published in: Marine Chemistry
• Main Authors: Foster, D, Staubwasser, M, Henderson, G
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Antarctic; Ross Sea; Southern Ocean; The Antarctic; The Spike; Antarc*
• Online Access: https://doi.org/10.1016/j.marchem.2004.02.003; https://ora.ox.ac.uk/objects/uuid:be216a94-68ac-4a29-a7bc-5149c10fbe03

Analysis of 226Ra concentrations in natural samples has been limited by the low efficiency of counting techniques, or by the time-consuming nature of thermal ionization mass spectrometry (TIMS). In this study, we have developed and tested the use of multiple ion-counting plasma source mass spectrometry to provide a fast and effective analytical technique for 226Ra. We use a Nu Instruments mass spectrometer to collect 226Ra and the spike isotope, 228Ra, synchronously, thereby maximising ion yields. This approach requires assessment of the gain of the ion counters and of mass discrimination, both of which are achieved using a U standard before and after analysis. Molecular interferences in the Ra spectrum must also be assessed. It is shown that, when the torch region is contaminated with Ba, significant interferences (∼100 cps) at +225.8 and 227.8 amu are observed. When clean of this Ba interference, however, the Ra spectrum has a very low background so that rapid and accurate measurement of 226Ra concentration is possible. We use this new technique to measure 226Ra concentrations in three water column profiles from the Ross Sea area for which Ba concentrations were also measured. 226Ra concentrations range from 0.36 to 0.42 fmol/kg, and Ba concentrations from 78.2 to 90.3 nmol/kg. The 226Ra/Ba ratio is constant at 4.7×10-9 mol/mol, within error of surface water values measured by TIMS. The constancy of the 226Ra/Ba in the Ross Sea, at a value typical for the open ocean, argues against addition of significant Ra or Ba by meltwater or groundwater from the Antarctic continent. This result makes the dating of carbonates from the area using 226Ra/Ba possible. The lack of surface depletion in either Ba or 226Ra differs from the pattern observed in the open Southern Ocean where concentrations increase with depth, suggesting that either the calcite-dominated productivity in the Ross Sea area does not remove 226Ra and Ba as effectively as opal-dominated productivity in the Southern Ocean, or that water column mixing in ...