Mercury distribution in tephra soil layers in Hokkaido, Japan, with reference to 34,000-year stratification

Abstract

Tephra from volcanic eruptions contains only small amounts of mercury (Hg) right after the eruption because the high temperature at eruption evaporates Hg in volcanic ash. Thus, accumulation of Hg in tephra soil layers during the dormant periods of the volcano may reflect Hg deposition while the layer was exposed to the atmosphere. To estimate sequential changes in Hg deposition, we measured the Hg content and accumulation in tephra layers from 6 sites in Hokkaido known to have many tephra layers derived from volcanic eruptions over a 34,000‐year period. Mercury content and accumulation rate in the soil profiles varied widely depending on the tephra. In each tephra layer, the Hg content and accumulation rates increased principally at the upper soil horizons and decreased at the lower depths. The Hg deposition rates calculated from the amount of Hg accumulated in the tephra layers were similar within the same tephra. These characteristics of Hg distribution indicate that Hg deposition accumulated on the surface of each tephra layer during the period the tephra layer was exposed to the atmosphere. Although physicochemical processes such as leaching out, wind erosion, and volatilization might lead to over‐ and/or underestimation of the deposition rates, our estimated amounts of Hg were markedly higher in the tephra soils after 1,600 year BP than before that time. The results of this study suggest that tephra layers in Hokkaido offer important implications for understanding of the historical changes in atmospheric Hg deposition.

Rapid oxidation of mercury (Hg) at volcanic vents: Insights from high temperature thermodynamic models of Mt Etna's emissions

“…This significant increase in atmospheric Hg is reflected in archives such as lake sediments, peat bogs and ice records which show a 2-to 4-fold increase in the past 150 years (Lindberg et al, 2007;Farmer et al, 2009). These archives also reflect the importance of volcanic explosions (e.g., Shotyk et al, 2005;Hobara et al, 2009;Guevara et al, 2010); ice-core data suggest that large events such as Tambora, 1815, or Krakatau, 1883, can result in deposition rates of Hg similar to those of global anthropogenic processes (Schuster et al, 2002).…”