Global mercury flux from volcanic and geothermal sources

The global Hg flux from natural sources is poorly known. We present data on Hg levels in volcanic fumaroles that, in combination with sulfur analyses and published SO₂ flux data, lead to new global Hg flux estimates. The volcanogenic Hg flux from passively degassing volcanoes is small (30 Mg Hg/yr). The flux from erupting volcanoes is much larger but less well constrained; we estimate it at about 800 Mg/yr. These data are interpreted as evidence for early Hg degassing from magmas. Geothermal sources contribute roughly 60 Mg/yr Hg to the atmosphere, an estimate based on average Hg contents of hot springs and global convective heat transport. Total natural Hg emissions from land amount to 1290 Mg/yr, including a revised soil vapor flux of 400 Mg/yr. Estimates of anthropogenic contributions to the atmospheric Hg budget vary between 3000 and 11 500 Mg/yr and are substantial compared to our natural Hg flux estimate.