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

We utilized Ra-224/Th-228 disequilibrium in the sediment to investigate processes that regulate solute transfer across the sediment-water interface. Depth profiles of dissolved and surface-bound Ra-224 and Th-228 in the upper 0-20 cm sediment column were measured using a delayed coincidence counter during a cruise to the Yangtze estuary from 15 to 24 August 2011. Along with Ra-224 and Th-228, depth profiles of Th-234 were collected to determine the bioturbation rate in the sediment. At most study sites, a significant deficit of Ra-224 relative to Th-228 was observed in the upper 0-10 cm. In contrast, Ra-224 was in excess with respect to Th-228 in the upper 0-5 cm at the river mouth, possibly due to redistribution of Ra-224 from the mid-salinity region. By modeling the Ra-224 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 similar to 20-30% of the measured flux of Ra-224. We concluded that other mechanisms, especially irrigation, must be invoked to explain the remnant 70% of the observed deviation of Ra-224 relative to Th-228. On the basis of the Ra-224/Th-228 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 Ra-224/Th-228 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 O-2. The result reveals that benthic consumption is an important loss term of dissolved O-2 in the Yangtze estuary and must be considered as one of the mechanisms that lead to hypoxia in this area. (C) 2013 Elsevier Ltd. All rights reserved.