Holocene Atmospheric Mercury Levels Reconstructed from Peat Bog Mercury Stable Isotopes

Environmental regulations on mercury (Hg) emissions and associated ecosystem restoration are closely linked to what Hg levels we consider natural. It is widely accepted that atmospheric Hg deposition has increased by a factor 3 ± 1 since preindustrial times. However, no long-term historical records of actual atmospheric gaseous elemental Hg (GEM) concentrations exist. In this study we report Hg stable isotope signatures in Pyrenean peat records (southwestern Europe) that are used as tracers of Hg deposition pathway (Δ²⁰⁰Hg, wet vs dry Hg deposition) and atmospheric Hg sources and cycling (δ²⁰²Hg, Δ¹⁹⁹Hg). By anchoring peat-derived GEM dry deposition to modern atmospheric GEM levels we are able to reconstruct the first millennial-scale atmospheric GEM concentration record. Reconstructed GEM levels from 1970 to 2010 agree with monitoring data, and maximum 20^th century GEM levels of 3.9 ± 0.5 ng m^-3 were 15 ± 4 times the natural Holocene background of 0.27 ± 0.11 ng m^-3. We suggest that a -0.7‰ shift in δ²⁰²Hg during the medieval and Renaissance periods is caused by deforestation and associated biomass burning Hg emissions. Our findings suggest therefore that human impacts on the global mercury cycle are subtler and substantially larger than currently thought.