Source and selectivity in the accumulation of mercury and other metals by the plants of Mt. Etna

The air Hg content at and near the summit of Mt. Etna is approximately 500- to 2000-fold lower than it is in the atmospheres around Antarctic, Hawaiian and Icelandic volcanoes. In contrast, the soils and plants on Mt. Etna show only 10- to 30-fold reductions in Hg content; in other words, there is at least a ten-fold enrichment relative to air. This disparity called into question the source of Hg in the vegetation and upper soil layers. Soils and a variety of plant species were analyzed for Hg, Fe, Cu, and Mn content at a number of stations on Mt. Etna including several also sampled for air Hg, and compared with the data for plants and soils from other volcanic and non-volcanic locations, especially Hawaii, Africa and Iceland. Etna vascular plants do not accumulate Hg, and lichens do so only to a moderate extent. Relative to their Fe content, however, all the Etna soils are enriched in Hg, but the reverse is true for Cu relative to Hg. The plants, on the other hand, when compared with their soils, are enriched in Hg relative to Cu. By comparing Fe/Hg atomic ratios for plants and soil, we calculated an Enrichment Factor (EF) for Hg. This value ranged from 19 to 102 for Etna, and 19 to 184 for all subtropical plants discussed here. The Hg EF for Icelandic samples was ca. 823, reflecting other environmental/geochemical determinants. No net surface deposition of Hg takes place on Etna from plant or atmospheric sources, and the relative Hg contents of soils and plants do not show a consistent relation to air Hg concenctration. Nevertheless, the plant/soil Cu and Hg ratios (CR) vary similarly as do the atomic ratio (AR) values for Fe and Hg. We conclude from these relationships that the atmosphere is not a major source of plant (or soil) Hg and that the likely alternatives are: release at some relatively remote point in time, but not to any significant degree since; release into the atmosphere as Hg = species other than Hg⁰; or movement from very deep subsurface compartments. These alternatives are not mutually exclusive. It is highly improbable that summit emissions constitute a significant source of Hg in the Mediterrean Basin.