224Ra:228Th disequilibrium in coastal sediments: Implications for solute transfer across the sediment–water interface

Abstract

We utilized 224Ra/228Th disequilibrium in the sediment to investigate processes that regulate solute transfer across the sediment–water interface. Depth profiles of dissolved and surface-bound 224Ra and 228Th in the upper 0–20cm sediment column were measured using a delayed coincidence counter during a cruise to the Yangtze estuary from 15 to 24 August 2011. Along with 224Ra and 228Th, depth profiles of 234Th were collected to determine the bioturbation rate in the sediment. At most study sites, a significant deficit of 224Ra relative to 228Th was observed in the upper 0–10cm. In contrast, 224Ra was in excess with respect to 228Th in the upper 0–5cm at the river mouth, possibly due to redistribution of 224Ra from the mid-salinity region. By modeling the 224Ra depth profiles in the sediment using the general diagenetic equation, we demonstrated that in most cases molecular diffusion and bioturbation together can account for only ∼20–30% of the measured flux of 224Ra. We concluded that other mechanisms, especially irrigation, must be invoked to explain the remnant 70% of the observed deviation of 224Ra relative to 228Th. On the basis of the 224Ra/228Th disequilibrium in the sediment and a concept of increased surface area for exchange by irrigation as developed by early investigators, we proposed a new approach – the 224Ra/228Th disequilibrium approach to quantify the transfer rate of other dissolved species across the sediment–water interface. We have utilized this new approach to determine the benthic consumption rate of dissolved O2. The result reveals that benthic consumption is an important loss term of dissolved O2 in the Yangtze estuary and must be considered as one of the mechanisms that lead to hypoxia in this area.