Formation of Zn and Pb sulfides in a redox-sensitive modern system due to high atmospheric fallout
The study shows that the air-derived metal enrichment (up to 2.3 g Zn kg-1, 1.1 g Pb kg-1, and 62 mg Cd kg-1) is retained in a thin layer (~30 cm) around 10-15 cm below the peat surface. A combination of focused ion beam (FIB) technology and scanning (SEM) and transmission (TEM) electron microscopy reveals that micrometric spheroids are most characteristic for ZnS and (Zn,Cd)S, although the sulfides readily form pseudomorphs after different plant tissues resulting in much larger aggregates. The aggregates have a complex polycrystalline sphalerite structure much more advanced than typically obtained during low-temperature synthesis or observed in other modern occurrences. Platy highly-disordered radially-aggregated submicrometre crystals develop within the time constraints of several decades in the cold (~15{\deg}C) and acid (pH 3.4-4.4) peat. The less abundant Pb sulfides occur as submicron cube-like crystals between ZnS or as flat irregular or square patches on plant root macrofossils. All PbS are crystalline and defect-free. Pb ion complexation with dissolved and solid organic matter is probably responsible for the low number and equilibrium shape of PbS crystals. Iron is absent in the authigenic sulfide mineralization and occurs entirely as organically bound ferric iron (Fe3+), as revealed by Mossbauer spectroscopy. The different affinity of metals to organic matter enhances the precipitation of Zn and Cd as sulfides over Pb and Fe. Our findings demonstrate that human activities lead to the formation of near-surface stratiform metal sulfide accumulations in peat, and the polluted sites can be of use to understand and reconstruct ancient ore deposits' genesis and mechanisms of formation.