Development of a Plutonium Method for Radioisotope Geochronology


Funding Agency: ONR

Summary: Knowledge of seabed dynamics in nearshore environments is needed to address problems ranging from to the evolution of coastal geomorphology to the changing nature of seabed acoustical properties.  One successful approach, used primarily in fine-grained sedimentary environments, has been the application short-lived particle-reactive radioisotopes to quantify sediment deposition and accumulation rates, and to measure the depth and intensity of physical and biological mixing. Unfortunately, the utility of this approach in coarser sediments typical of coastal environments has been hindered by the low inherent concentration of these isotopes in sandy sediments, and the relatively high lower limit of detection using traditional decay counting techniques. 

Collaborator Michael Ketterer's ICPMS

The proposed study would attempt to extend this approach into sandy sedimentary environments by taking advantage of a relatively new measurement technique, high-resolution inductively coupled plasma-mass spectroscopy (ICP-MS).  Recent studies by Kenna (2002) have successfully measured Pu concentrations as low as 0.5 fg g-1, a value roughly two orders of magnitude lower than possible by traditional decay counting techniques.  239/240Pu is one of the major byproducts of thermonuclear explosions, and hence is a key time marker for the period since 1954, the onset of intensive atmospheric weapons testing.  This study would attempt to measure 239/240Pu concentrations in sediment cores collected from Duck, NC.  A transect of core samples with a range of textural characteristics will be obtained from other ongoing studies of this area and analyzed for 239/240Pu.  The major goal will be to determine the limits of this new technology in its application to sediment geochronology of sandy sediments.   


Northern Arizona University: Michael Ketterer

Laboratory and Isotope Methodology: Recent advances in high-resolution inductively coupled plasma-mass spectroscopy (HR ICPMS) present an unparalleled opportunity to advance the measurement of environmental levels of bomb Pu isotopes in sediment geochronology. A combination of advances in instrumentation and separation techniques has increased the sensitivity of bomb Pu measurement by two orders of magnitude over that typically achieved by traditional decay counting techniques. In fact, the present technology allows the measurement of Pu levels as low as 0.5 fg g-1 (Kenna, 2002). For perspective, a rough equivalent in terms of radioactivity is only a single disintegration per day, a quantity that could never be measured using decay counting. HR ICPMS measurement of bomb Pu, therefore, is likely to replace traditional decay counting techniques in the near future for application to problems in marine sediment geochronology. The dramatically lower limit of detectability achieved using this technique should allow us to extend studies heretofore limited to fine-grained sedimentary environments to the sandy environments of the nearshore and inner continental shelf.