Seasonal and spatial dynamics of super(234)Th/ super(238)U disequilibria in southern Lake Michigan

• Published in: Journal of Geophysical Research. C. Oceans Vol. 109; no. C10
• Main Authors: Waples, James T, Orlandini, Kent A, Edgington, David N, Klump, JVal
• Format: Journal Article
• Language: English
• Published: 01-09-2004
• Subjects: Freshwater; Marine; USA, Michigan L
• ISSN: 0148-0227; 2169-9275; 2169-9291
• DOI: 10.1029/2003JC002204

Measurements of super(234)Th/ super(238)U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10-40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km super(2)). The mean activity of super(238)U in Lake Michigan measured 230 +/- 20 dpm m super(-3), approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (<0.45 mu m) and excess particle-bound super(234)Th were 8.0 and 60.6 dpm m super(-3), respectively. Using a simple one-dimensional model, median residence times for dissolved and particle-bound super(234)Th were similar to 1 and similar to 14 days, respectively. super(234)Th-based particle settling velocities had a median value of 0.4 m d super(-1) (range: similar to 0.0-1.4 m d super(-1)), and instantaneous super(234)Th-based estimates of the net vertical mass flux had a median value of 0.4 g m super(-2) d super(-1) (range: similar to 0.0-5.2 g m super(-2) d super(-1)). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind-induced wave height. Using a simple two-box model, the super(234)Th-based cross-margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 x 10 super(9) kg yr super(-1), which is in excellent agreement with a super(210)Pb - super(137)Cs based, basin-wide mass sedimentation rate of 1.28 x 10 super(9) kg yr super(-1).