Mass-independent fractionation of mercury isotopes in Arctic snow driven by sunlight

After polar sunrise in the Arctic, sunlight-induced reactions convert gaseous elemental mercury into compounds that are rapidly deposited to the snowpack. These atmospheric mercury depletion events occur repeatedly until snowmelt(1,2). Following deposition, the mercury can be reduced by sunlight-induced reactions and emitted as a gas(3-6), or can be retained in the snowpack(7,8), where it may affect Arctic ecosystems following snowmelt. However, the proportion of mercury that remains in the snowpack is uncertain. Here, we measured the mercury isotopic composition of snow samples collected during an atmospheric mercury depletion event in Barrow, Alaska. We report large negative mass-independent fractionation of mercury isotopes in the Arctic snow. Results from a flux chamber experiment suggest that mass-independent fractionation is coupled to the re-emission of elemental mercury to the atmosphere, and is triggered by sunlight-induced reactions. On the basis of the above, we estimate that photochemical reactions triggered the release of a significant portion of the mercury deposited during this atmospheric mercury depletion event.