Exploring the lowest levels of environmental Strontium-90 compared to Uranium-236 in marine carbonates and seawater

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 PBq of this radionuclide have been distributed globally from stratospheric fallout of bomb-testing, and there are more localized contributions from tests, accidents, and releases from reprocessing plants which will be superimposed on this background. In the past, massive sample sizes (up to 100 L of seawater or 100 g of coral aragonite) were required to quantify ⁹⁰Sr, even immediately after the peak period of global fall-out from bomb testing. With mass spectrometry it would be possible to reduce the sample size requirements at least by a factor of 100, i.e. sample sizes of 1 L of seawater or 1 g of aragonite. 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 ⁹⁰Sr signal. With recent advances in isobar separation techniques in accelerator mass spectrometry (AMS) at the University of Vienna, this has come within reach, offering new research possibilities. The new technique uses an ion-cooler and laser-photo-detachment to suppress the stable isobar ⁹⁰Zr, which interferes with measurements of ⁹⁰Sr, almost completely. With initial test samples, we could confirm an isotopic abundance sensitivity of 8·10⁻¹⁶ (⁹⁰Sr/Sr), sufficient for application to ocean water samples. In this presentation, we will show a comparison of ⁹⁰Sr to ²³⁶U (Uranium-236), another radioactive ocean tracer that has been studied intensively recently. Using this approach, we studied contemporary coral skeleton material in the Indian Ocean (Pemba Bay, Mozambique) and the Pacific Ocean (Tarawa, Kiribati), and further the methods, requirements, and impact of variations in sample preparation. We also present the first results from ocean water samples from two depth profiles in the south Atlantic (GEOTRACES cruise GA10/JC068), and the associated sample preparation and blank levels for these types of samples. Finally, we will present the implications for the overall abundance of both tracers from global fall-out, compare with historic data, and discuss the potential for multi-isotope applications of both tracers.