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

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.