Records of atmospheric mercury deposition and post-depositional mobility in peat permafrost archives from central and northern Yukon, Canada

Abstract

Environmental archives provide a feasible means for studying the biogeochemical cycling of heavy metals including mercury (Hg). Although many temperate peat bogs have been successfully used to reconstruct natural and anthropogenic atmospheric Hg deposition fluxes, northern circumpolar permafrost peatlands are largely understudied in similar research. Consequently, substantial gaps remain in our understanding of past atmospheric Hg deposition fluxes and present peatland Hg inventories from northern environments. This thesis presents a critical investigation of using peat permafrost archives for quantifying natural and anthropogenic atmospheric Hg deposition fluxes. The first objective of this research was to develop an effective protocol for collecting and processing peat permafrost samples for inorganic geochemical analyses. Refined techniques were established for accurately measuring bulk density and homogenizing well-preserved fibrous peat. This method was then used to reconstruct historic atmospheric Hg deposition from a permafrost peat plateau near Dawson, Yukon, with the aim of resolving a deposition peak corresponding to extensive Klondike gold mining during the late 19th and early 20th centuries. The results revealed an unexpectedly early increase in anthropogenic Hg deposition, which was ultimately interpreted to result from post-depositional Hg mobility within the seasonally thawed active layer. This discovery implies that peat permafrost archives may not provide reliable high-resolution records of atmospheric Hg deposition. However, a subsequent study involving seven Holocene (10,000-year old) peat plateaus from central and northern Yukon reveal that millennial-scale atmospherically deposited Hg concentrations and fluxes were consistently low; ~20.7 ± 9.8 ngg-1 and ~0.51 ± 0.39 μgm-2y-1, respectively. These results were used to demonstrate how peat permafrost Hg inventories could be estimated at the local scale. Provided additional empirical data from sites spanning the northern circumpolar region, future studies may use the same approach to estimate cryosol Hg stocks at regional to global scales and assess the potential vulnerability of Hg release from thawing permafrost with continued climate warming.