The biogeochemistry of an ombrotrophic bog: evaluation of use as an archive of atmosphere mercury deposition.

The utility of ombrotrophic bogs as archives of atmospheric mercury deposition was assessed with an investigation in Arlberg Bog, Minnesota, U.S.A. Since the use of ombrotrophic bogs as archives depends on the immobility of deposited trace metals, we examined the postdepositional transport processes revealed by the solid-phase distributions of mercury and ancillary metals (Fe, Al, Mn, and Pb) in this bog. We modeled metal speciation in bog porewaters as a function of pe in order to understand metal behavior in ombrotrophic peat. Specifically, we considered the effect of water movement and resultant shifts in redox potential gradients on metal retention. Our results indicate that Hg and Pb are immobile in ombrotrophic peat, so their distribution can be used to determine temporal changes in deposition. To substantiate the deposition estimates determined in this study, we emphasized the importance of confirming the validity of the dating scheme, assessing the degree of horizontal homogeneity in the accumulation record, and providing evidence for retention of Hg based on geochemical modeling. As recorded in Arlberg Bog, historic atmospheric Hg deposition increased gradually after the mid-1800s, peaked between 1950 and 1960, and may have declined thereafter. Preindustrial deposition was about 4 micrograms/m2 year and recent deposition about 19 micrograms/m2 year. The results of this study indicate that deposition at Arlberg Bog has been influenced by a regional and/or local-scale source.