Post-depositional Hg transformation processes in an open boreal peatland constrained by Hg stable isotopes

Peatland vegetation constantly takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic Methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down Methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remain, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations above the water table, and that supersaturated gaseous Hg below the water table was likely created more by photoreduction of rainfall rather than release from peat soil. These findings constrain the possibilities for mobilizing previously deposited Hg. Identification and quantification of these processes further advance our understanding of the fate of Hg in peatlands and their export of Methyl-Hg to downstream ecosystems.