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

Abstract

The distribution of 234Th(t 1/2 = 24.1days ) 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.