The fate of industrial-era mercury in peatlands revealed with micrometeorology, isotopes, paleoecology, genomics, and an ice-age

Since humans began utilizing mercury, pollution has increased atmospheric Hg seven-fold. Mercury (Hg) contamination is the single largest cause of waters in the European Union failing to meet the standards of the EU Water Framework Directive. Peatlands, which have accumulated a legacy of past atmospheric Hg pollution, are major sources of Hg contamination in downstream aquatic ecosystems. Despite peatlands having accumulated Hg for millennia, independent lines of research indicate that some northern peatlands are now returning Hg to the atmosphere. This raises questions about what controls the fate of the pollution legacy Hg stored in peatlands. We hypothesize that legacy Hg accumulated in peat during earlier periods of higher atmospheric Hg pollution is no longer in balance with the lower Hg levels of the contemporary atmosphere, leading to net Hg evasion. Several methodological advances were applied to test this hypothesis on a 2000-year chronosequence of mires created by isostatic uplift along the northern coast of Sweden as well as the nearby Degerö peatland that is even older. Despite uniform climate and atmospheric Hg concentrations across the 15 km extent of the chronosequence, the stock of Hg differs by a factor of two. Novel Hg eddy covariance quantified the Hg exchange between the land and atmosphere. Distributed measurements of dissolve gaseous elemental mercury (GEM) in shallow peat groundwater quantified seasonal variation in a potential source of the evading Hg. Natural abundance of Hg isotopes and community-level expression profiling of microbial metabolisms identified the role of specific processes in the transformation of Hg within peat profiles along the chronosequence. This paper reports on puzzle pieces that have fallen into place, such as isotopic evidence for the role of photoreduction in producing GEM, and the challenges that remain to complete the picture.