224Ra: 228Th Disequilibrium in Sediments as a Tracer for Solute Transfer Across the Sediment-Water Interface in Coastal Louisiana

Sediment-water interface represents an important exchange surface that regulates the cycling and transfer of many elements and nutrients between sediments and overlying water. Traditionally, rate of transfer across the sediment–water interface is determined by deploying benthic chambers or modeling the depth profiles of a dissolved species of interest in the sediment. In this study we utilize ²²⁴Ra: ²²⁸Th disequilibrium in the sediment to determine solute transport rates in coastal Louisiana sediments. In marine sediments, ²²⁴Ra (half-life = 3.66 d) is continuously produced by alpha decay of its parent nuclide, ²²⁸Th (half-life = 1.91 y). While ²²⁸Th is strongly bound to sediments, ²²⁴Ra tends to remain in dissolved phase in the interstitial water, and migrate across the sediment–water interface into the overlying water. This results in a deficit of ²²⁴Ra with respect to 228Th in near-surface sediments that can be utilized to calculate solute transport rates. Depth profiles of dissolved and surface-bound ²²⁴Ra and ²²⁸Th in the upper 10 cm sediment column indicate significant deficit of ²²⁴Ra relative to ²²⁸Th. By modeling the ²²⁴Ra depth profiles in the sediment using the general diagenetic equation along with ²³⁴Th bioturbation rates we demonstrated that molecular diffusion and bioturbation together accounts for less than 50% of the measured fluxes of ²²⁴Ra in this region. The solute transport derived from this method are also utilized to understand transport/consumption of other species like O₂ and Fe²⁺ in these sediments.