Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition

Gaseous elemental mercury (GEM) is the dominant form of mercury in the atmosphere. Its conversion into oxidized gaseous and particulate forms is thought to drive atmospheric mercury wet deposition to terrestrial and aquatic ecosystems, where it can be subsequently transformed into toxic methylmercury. The contribution of mercury dry deposition is however largely unconstrained. Here we examine mercury mass balance and mercury stable isotope composition in a peat bog ecosystem. We find that isotope signatures of living sphagnum moss (Delta Hg-199 = -0.11 +/- 0.09 parts per thousand, Delta Hg-200 = 0.03 +/- 0.02 parts per thousand, 1 sigma) and recently accumulated peat (Delta Hg-199 = -0.22 +/- 0.06 parts per thousand, Delta Hg-200 = 0.00 +/- 0.04 parts per thousand, 1 sigma) are characteristic of GEM (Delta Hg-199 = -0.17 +/- 0.07 parts per thousand, Delta Hg-200 = -0.05 +/- 0.02 parts per thousand, 1 sigma), and differs from wet deposition (Delta Hg-199 = 0.73 +/- 0.15 parts per thousand, Delta Hg-200 = 0.21 +/- 0.04 parts per thousand, 1 sigma). Sphagnum covered during three years by transparent and opaque surfaces, which eliminate wet deposition, continue to accumulate Hg. Sphagnum Hg isotope signatures indicate accumulation to take place by GEM dry deposition, and indicate little photochemical re -emission. We estimate that atmospheric mercury deposition to the peat bog surface is dominated by GEM dry deposition (79%) rather than wet deposition (21%). Consequently, peat deposits are potential records of past atmospheric GEM concentrations and isotopic composition.