²³⁴Th/²³⁸U disequilibrium in near-shore sediment: Particle reworking and diagenetic time scales

The distribution of ²³⁴Th (t _{1 2} = 24.1 days) in excess of its parent ²³⁸U in the upper layers of near-shore sediment makes possible the evaluation of short-term sediment reworking and diagenetic rates. ²³⁴Th has a maximum residence time in Long Island Sound water of 1.4 days. Seasonal measurement of ²³⁴Th/²³⁸U disequilibrium in sediment at a single station in central Long Island Sound demonstrates rapid particle reworking and high ²³⁴Th_xs (>1 dpm/g) in the upper 4 cm of sediment with slower, irregular reworking and low ²³⁴Th_xs to at least 12 cm. The rate of rapid particle reworking varies seasonally and is highest in the fall. The rapidly mixed zone is characterized by steep gradients in sediment chemistry implying fast reactions spanned by ²³⁴Th decay time scales. ²³⁸U is depleted in the upper mixed zone and shows addition in reducing sediment at depth.