  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.

Materials Science