Bulk sediment Ba concentrations are between 1000 and 10,000 ppm within the underthrust section at Sites 1039/1253 and 1040/1254, indicating that a significant fraction of the sedimentary Ba subducting at Costa Rica is composed of barite. Barite is soluble in regions where pore fluid SO42– is depleted. The zone of sulfate depletion extends below the décollement at Site 1040 into the uppermost portion of the hemipelagic sediments. From 371 to 401 mbsf at Site 1040, pore fluid Ba2+ concentrations reach 1400 times bottom water value and are ~53 times the concentration of equivalent samples at Site 1039, indicating significant Ba2+ liberation from barite within this interval. Comparison of sediment Ba across sites shows that this process influences the bulk Ba concentrations in lithologic Unit U1 at Site 1040. As the sediment section moves arcward, a greater proportion of the underthrust sediment column will be SO42– depleted, releasing more Ba from barite. This process may thus have a profound impact on the amount of Ba reaching greater depths in the subduction zone.
The pore fluid Ba2+ concentrations measured immediately below the décollement in the uppermost hemipelagic sediments at Site 1040/1254 are ~20 times higher than those measured within the regions of maximum fluid flow in the prism sediments (décollement and upper fault zone). The upper fault zone is situated at ~200 mbsf, whereas the décollement zone extends from ~340 to 371 mbsf at Sites 1040/1254. The décollement and upper fault zone exhibit sharp peaks in lithium, calcium, and C1–C3 hydrocarbon concentrations, as well as low chloride and potassium concentrations, indicating advection of a deeper-sourced fluid within these intervals originating at temperatures of ~150°C (Silver et al., 2000; Chan and Kastner, 2000). Though the décollement and upper fault zone are separated by ~130 m, they have nearly identical dissolved Ba2+ concentrations that are elevated relative to the pore fluid barium concentrations in the prism sediments between them. The sharp discontinuity in Ba2+ concentrations between the décollement and underthrust sediments, but similarity in Ba2+ concentrations between the décollement and upper fault zone, ~130 m apart, precludes any significant contribution of barium from the uppermost hemipelagic sediments to the décollement by advective or diffusive flux, indicating that the underthrust sediment and décollement fluid flow systems are effectively decoupled.
Future work will focus on analyzing additional bulk sediment samples for Ba concentrations in Units U2 and U3. A sequential barite extraction procedure using the methods outlined in Paytan (1995) and Eagle and Paytan (2003) is currently being performed on eight sediment samples from Unit U1 at Sites 1039 and 1040 to quantify the absolute change in barite content between the two sites. Once the data set is complete, a more robust average Ba composition of each of the units at both sites and the percent loss of Ba will be computed. Each of the subunits will be weighted and the bulk composition will be computed and used to compute Ba flux to greater depths in the subduction zone following the equations outlined in Plank and Langmuir (1998). Ba loss due to deepening of the sulfate depletion zone arcward will be modeled, and changes to Ba flux rate will be calculated.