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

Wokil Bam1, Kanchan Maiti2 and Neha A Ghaisas1, (1)Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States, (2)Louisiana State University, Oceanography and Coastal Sciences, Baton Rouge, LA, United States
Abstract:
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.