Y-90/Sr-90 and Th-234/U-238 Disequilibria: The Utility of Dual fast Process Chronometers as Tracers of Particle Dynamics in Shallow Coastal, Estuarine and Freshwater Systems
ABSTRACTOCE-0351824In marine environments, particle-reactive scavenging rates, sediment trap efficiencies (e.g., Buesseler et al., 1994), surface sediment mixing, and vertical and horizontal particle transport rates have all been calculated in part by measuring the disequilibrium between naturally occurring, particle-reactive 234Th and its generally soluble parent, 238U. While measurements of 234Th/238U disequilibria have proven very useful in understanding particle dynamics in the open ocean, the utility and facility of using this radio chronometer diminishes in nearshore, estuarine and freshwater systems. Interestingly, 90Y is similar to 234Th in that it is particle reactive while its parent is generally non-reactive and a conservative constituent in aquatic systems. However, 90Y has a half-life of only 64 hours while 234Th has a half-life of 24.1 days.In this study, researchers at the University of Wisconsin-Milwaukee intend to explore the utility of simultaneously measuring 90Y /90Sr and 234Th/238U disequilibria in both nearshore and offshore Lake Michigan as a novel means of quantifying short term temporal dynamics of particles in aquatic systems. Under steady state and non-steady state conditions both dissolved (< 0.45 um) and particle bound nuclide activities of 90Y, 90Sr, 234Th and 238U will be measured. Over the four-year project, these nuclides will be measured simultaneously in both shallow (<20 m) and deep (> 100 m) water stations with special emphasis on collecting samples during periods of calm, intense bottom sediment resuspension, intense primary production, and a whiting event. During the third and fourth years of this project, daily variations in 90Y /90Sr and 234Th/238U disequilibria in the water column will be measured in conjunction with daily measurements of excess particle-bound inventories of 90Y and 234Th on the lakebed during a series of 5-day cruises over different physical/seasonal regimes. Mathematical expressions used to relate 234Th/238U disequilibria to the physical dynamics of particles and particle-reactive species also apply to the 90Y /90Sr pair so that separate estimates of both particle scavenging and removal rates will be made.