The importance of volcanic emissions for the global atmospheric mercury cycle

Mercury is a highly volatile, bioaccumulating toxic trace metal with a long ( 1 yr) atmospheric residence time. Hg is strongly enriched in volcanic emanations, and volcanoes are the only natural sources of direct Hg emission to the free troposphere and stratosphere. However, there is considerable uncertainty over the annual emission rate of mercury from volcanoes. Previous estimates, based on limited measurements from volcanic plumes, span three orders of magnitude ( 100–103 Mg Hg/yr), or from <1% to 50% of total natural Hg emissions. Here we critically evaluate published data from volcanic plumes, and combine this with information from natural archives to show unequivocally the significance of volcanoes for the global biogeochemical mercury cycle. ‘Low’ global volcanic flux estimates (< 50 Mg/yr) are based on the inappropriate extrapolation of data from low-temperature fumarolic degassing at non-erupting volcanoes to the high-temperature emissions from active volcanoes. Based on data from active volcanoes, we estimate that the time-averaged volcanic Hg emission is 700 Mg/yr, or 20–40% of total natural emissions. Continuous degassing accounts for only 10% of this flux, while 75% of volcanic Hg is released during ‘smaller’ sporadic eruptions (<10–102 Mg/event). Rare, large (>103 Mg) explosive eruptions overwhelm the total atmospheric burden several times per century, and account for 15% of total volcanic Hg emissions.