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

[1] Measurements of 234Th/238U 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 km2). The mean activity of 238U in Lake Michigan measured 230 ± 20 dpm m−3, approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (<0.45 μm) and excess particle-bound 234Th were 8.0 and 60.6 dpm m−3, respectively. Using a simple one-dimensional model, median residence times for dissolved and particle-bound 234Th were ∼1 and ∼14 days, respectively. 234Th-based particle settling velocities had a median value of 0.4 m d−1 (range: ∼0.0–1.4 m d−1), and instantaneous 234Th-based estimates of the net vertical mass flux had a median value of 0.4 g m−2 d−1 (range: ∼0.0–5.2 g m−2 d−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 234Th-based cross-margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 × 109 kg yr−1, which is in excellent agreement with a 210Pb − 137Cs based, basin-wide mass sedimentation rate of 1.28 × 109 kg yr−1.