POLYACRYLAMIDE GEL PROBES FOR REDOX-SENSITIVE TRACE METAL PORE WATER SAMPLING

Pore water metal profiles are important for identifying redox horizons, calculating fluxes, and understanding trace metal geochemical cycling. One difficulty of pore water sampling across oxic/anoxic transitions is the potential for sample oxidation. Typically pore waters, which can have low concentrations, are in contact with sediments with much higher concentrations. We are investigating DET (diffusional equilibration in thin films) polyacrylamide gel probes for obtaining pore water trace metal profiles. A gel probe is inserted into a recovered sediment core, where water within the gel can equilibrate with pore waters. The obtainable vertical profile is dictated by the geometry of the probe design, with higher resolution at the top (0.3 cm) and lower resolution at the bottom (1 cm). We are developing gel probes for redox-sensitive trace metals, such as U, Re, and Mo. Initial solution equilibration tests and laboratory core incubation tests suggest that equilibration times for gel probes in sediments are approximately 24-48 hours. Field work done in Buzzards Bay, Massachusetts, show that the redox depth horizons determined from cores sectioned by slicing and centrifuging are similar to gel probe results. Lower Mo concentrations were measured in gel probe samples below the depth of Mo removal compared to measurements from samples from the sliced/centrifuged core. We believe this is either due to Mo being complexed in the pore waters to species that are not able to cross the membrane covering the gel probe, or additional Mo is released from sediments during the slice/centrifuge method. Elevated Re pore water measurements at the sediment-water interface as determined in sliced/centrifuged samples compared to concentrations determined using gel probes suggest potential oxidation of the sliced and centrifuged sample during pore water separation.