COI Funded Project: An Investigation of the Seasonal Variability in Trace Metal Cycling in Pristine and Contaminated Buzzards Bay Sediments

Roger François, Marine Chemistry and Geochemistry

Project Duration: 6/1/99-12/31/00
Key Words: Buzzards Bay, trace metal cycling, sediment-water interface, benthic flux, seasonal oscillation

The goals of this proposal were to develop two new tools, polyacrylamide gel probes and in situ oxygen-controlled benthic flux chambers, to quantify the diagenetic reactions and benthic fluxes of trace metals in coastal sediments, and to establish the effect of seasonal sediment redox changes and bioturbation. The polyacrylamide gel probes are used to measure pore water solute concentration profiles at high resolution with minimal disruption of the sediment-water interface. The in situ oxygen-controlled benthic flux chambers directly measure metal fluxes across the sediment-water interface, while maintaining bottom water oxygen concentrations near the ambient oxygen level during the five-day deployment.

Initial research involved the optimization of the gel probes for our metals of interest (Mn, Fe, Mo, Cu, Re, Pb, U, Ag, Ba and Cd). Experiments involved placing seven probes in an electrolyte solution (NaCl/MgSO4) spiked with our suite of metals. Probes were removed over time and analyzed for trace metals. Our results suggest that the probes will equilibrate to a known solution concentration after 12 hours. Subsequent experiments determined the equilibration times in sediment cores. We retrieved six sediment cores from an inshore Massachusetts Bay site (Boston Harbor). Two cores were immediately sectioned using traditional pore water separation techniques (slicing and centrifuging under a nitrogen atmosphere). The other four cores were transported to a WHOI cold lab (4°C) where they were continuously bubbled with air and the overlying water was periodically exchanged with recovered bottom waters from the site. Two probes were inserted into three cores, while the fourth core was incubated without probes to test the incubation method. The probes were removed after different equilibration times, analyzed and compared with the two traditionally sectioned cores. Our data suggests that equilibration of the DET probes occurs between 24-48 hours.

We deployed the benthic chambers and retrieved sediment cores from Buzzards Bay, Boston Harbor and Massachusetts Bay. Buzzards Bay deployments were made in August 2000 and May 2001. Vigorous bioturbation and irrigation characterize both time periods. The benthic chamber data for August 2000 shows a large flux of Mn to the overlying water (Fig. 3), consistent with the pore water Mn profile. A comparison of the sliced/centrifuged pore water samples shows similar redox horizons for both time periods, and more intense Fe reduction occurring during May 2001. U and Mo profiles clearly document the removal of these metals from pore water in the Fe and sulfate reduction zones, while Ba is added to pore waters, presumably as a result of barite dissolution. The Cu profiles suggest that Cu cycles with Fe. The depth of Fe reduction, which is expected to vary seasonally, could then affect the ultimate fate of Cu, U and Mo in the sediments. Analysis of some of the other metals has been hampered by analytical difficulties. For most metals, these problems are tractable and soon to be resolved. The benthic chamber and gel probe samples for May 2001 are currently being analyzed. Publications in peer-reviewed journals are in preparation. Based on these very encouraging initial results, we have submitted an NSF proposal to pursue and expand this type of research in the coastal environment.