Bottom scavenging in coastal Lake Michigan and the use of 234Th and 90Y as tracers of particle flux in shallow aquatic systems

James T Waples, University of Wisconsin - Milwaukee, School of Freshwater Sciences, Milwaukee, WI, United States
Abstract:
Measured profiles of 234Th/238U and 90Y/90Sr in coastal Lake Michigan show that observed nuclide scavenging cannot be easily explained by water column removal on sinking particles. Instead, evidence suggests that daughter/parent nuclide disequilibrium in the water column is primarily driven by downward convection and benthic factors of direct nuclide adsorption on bottom or near-bottom sediment (i.e., bottom scavenging).

An estimated vertical 234Th/90Y flux ratio of ~0.31 in the water column agreed with measured 234Th/90Y activity ratios on collected ejecta from bottom dwelling dreissenid mussels (0.26 ± 0.07) and not with water column particles (3.3 ± 1.3). A 238U/90Sr parent nuclide activity ratio of 0.30 ± 0.02 suggests that both 234Th and 90Y are scavenged in toto below the maximum sampling depth and near the sediment/water interface.

Determining the mechanism by which particles are transported to the bottom is important for understanding how benthos are supplied with pelagic material. Moreover, the mechanism of transport has a significant impact on how these nuclide tracer pairs can be used as proxies of particle flux in shallow water.