Exploring the lowest levels of environmental 90Sr in carbonates and seawater using a new, highly sensitive Accelerator Mass Spectrometry technique

1 Introduction

Strontium-90 (90Sr) is an anthropogenic radionuclide, which, due to its radiological relevance, has been most intensively monitored in the past. In terms of initial activity, over 630 PBb of this radioisotope have been distributed globally from stratospheric fallout of bomb-testing, and there are more localized contributions from test, accidents, and releases from reprocessing plants.[1] Despite of the massive spike of 90Sr from global fall-out has been very difficult to measure in the ocean, due to the massive dilution in the oceans and the low atom detection efficiency of decay counting measurements. Massive sample sizes (up to 100 l of seawater or 100 g of coral aragonite) were required even right after the peak period of global fall-out from bomb testing. On the other hand, the high amount of strontium dissolved in seawater complicates the use of mass spectrometric methods, as an isotopic abundance sensitivity of at least 1·10-15 is required to detect the estimated main signal. Here we report on the results of successful measurements of such samples using a new Accelerator Mass Spectrometry (AMS) technique [2].

2 Samples and Methods

Our interest in this study lies in the determination of 90Sr as geochemical and ocean current tracer in conjunction with another anthropogenic tracer isotope, 236U. Both the respective elements both get build into marine carbonates such a coral samples, and both are present in significant amounts in seawater. While 236U is a well-established isotope and can be measured at very low levels (<10-13 236U/U) with AMS, only recent advances in isobar separation technique in AMS at the University of Vienna have opened possibility to measure 90Sr at the required level. The new technique uses an ion-cooler and laser-photo-detachment to suppress the stable isobar 90Zr. Besides the isobar suppression in the mass spectrometer system, we also gain a boost in suppression using ion-exchange columns. Since we are interested in measuring 90Sr against 236U we developed a sample preparation technique to extract both efficiently from a calcium carbonate matrix, while suppressing 90Zr, and avoiding pre-concentration steps. In principle, this could also be applied to seawater samples, although the efficiency of uranium co-precipitation in with carbonates still needs investigation. In this study we relied on separate procedures in the case of sea water samples.

3 Results

With initial test samples we could confirm a detection limit for the method corresponding to 0.03 mBq (or better) and a 90Sr/Sr isotopic abundance sensitivity of <8·10-16. We will present results from contemporary coral skeleton material, the methods, requirements, and impact of sample preparation. Further, we explain our sample preparation scheme to extract 236U, another important anthropogenic radionuclide, simultaneously with 90Sr for multi-isotope applications of both. Finally, the sample preparation and blank levels for ocean water samples and the results will be shown.