International Workshop on Tracer Applications of Noble Gas Radionuclides in Geoscience Research Neil Sturchio, University of Illinois-Chicago
This proposal is for support of an International Workshop on Tracer Applications of Noble Gas Radionuclides in Geoscience Research, which will be held at Argonne National Laboratory near Chicago on June 21-22, 2012. This workshop is in response to the recent technical advances in atom-trap trace analysis (ATTA), a novel analytical method for measuring isotopic abundances of the ultratrace noble gas radionuclides 81Kr, 85Kr and 39Ar in natural samples of air, water, and ice, using a laser-based magneto-optical atom trap apparatus (www.phy.anl.gov/mep/atta/research/atta.html). The leading facility in terms of ATTA development thus far has been Argonne National Laboratory, but several other laboratories in Asia, Europe, and the US have been pursuing ATTA instrumentation and/or methods for sample collection and sample preparation for ATTA.
The potential tracer applications of noble gas radionuclides provide a unique and powerful set of isotopic tools that can be used for a broad spectrum of geosciences research, including hydrology, oceanography, glaciology, paleoclimatology, and volcanology. For example, key applications of ATTA include determining residence times of very old groundwater, brines, and glacial ice using 81Kr; determining circulation and mixing time scales in the deep tracing circulation patterns of the ocean using 39Ar; and tracing young groundwater using 85Kr and 39Ar. The new technology of ATTA opens the door to these applications, as well as many other scientific opportunities that are less obvious but equally transformative.
The major objectives of this workshop are (1) to identify the salient scientific opportunities provided by ATTA, (2) to increase awareness and to catalyze more widespread application of ATTA in a broad range of transformative geosciences research, and (3) to map out the optimal path for rapid deployment of ATTA in the relevant international scientific communities. ATTA is likely to have broad impact in a wide range of geoscience applications.
The noble gas radionuclides 81Kr (half-life = 229,000 yr), 85Kr (10.8 yr), and 39Ar (269 yr) have ideal properties for tracing fluid transport in Earth systems. Their principal applications include determination of groundwater residence times (81Kr, 85Kr, and 39Ar) and seawater ventilation ages (39Ar). In practice, however, few studies have employed noble gas radionuclides because of the large sample sizes required and the complex analytical systems required for measuring their extremely low isotopic abundances (10-16 to 10-11). Most published studies have used low-level radioactive decay counting for 39Ar and 85Kr, but the low activity of 81Kr has necessitated the development of various atom-counting methods over the past four decades. A workshop convened with support from the National Science Foundation, Argonne National Laboratory, and the University of Illinois at Chicago explored applications of noble gas radionuclides to scientific problems in the fields of hydrology, oceanography, glaciology, and other areas of the earth sciences. It was organized in response to recent developments in atom-trap trace analysis (ATTA) that have enabled routine measurements of the radiokrypton isotopes, as well as the demonstration of the ability to measure 39Ar in environmental samples. With the advent of ATTA as a new tool for routine measurements, the attention of the relevant geoscientific communities is focused on how to best apply it to compelling scientific problems. Workshop attendees considered this topic for two days and reached consensus on some important issues. There was unanimous agreement that it will better serve the community to establish a dedicated ATTA facility, with separate instrumentation for routine measurements and research dedicated to the development of ATTA methods, respectively. The size and shape of such a facility will be dictated by the projected evolution of the ATTA technology, which currently allows 120 measurements of 81Kr and 85Kr per atom-trap per year. Additional work is needed to optimize sample collection and purification methods. An ATTA facility can be designed with multiple atom-traps to optimize its efficiency and versatility. Principal scientific thrusts of an ATTA facility are likely to include surveys of groundwater residence times using 81Kr, 85Kr, and 39Ar, age distributions of glacial ice using 81Kr, and a 39Ar survey of the main water masses of the oceans, to estimate circulation pathways and mean residence times and better constrain circulation models and climate history. Other scientific questions involving deeper circulation of fluids in the Earth’s crust and mantle also are within the scope of future applications. Presentations from the workshop may be viewed at the following website: www.phy.anl.gov/events/tangr2012/index.html.
