Mercury Transport, Transformation and Accumulation Recorded by Stable Isotopes during Retreated Glacier Chronosequence of 250 Years

Abstract

Vegetative development in regions where glaciers retreated due to global warming forces the mercury (Hg) cycle in the cryosphere. This study depicts the fate of Hg in a glacier-retreated chronosequence over the last 250 years recorded by signals of stable Hg isotopes. Results show that the Hg storage in surface soil increases by 3.2 times over 250 years after the glacier retreated. 53 ± 11% of Hg in grass shoots is from the uptake of atmospheric Hg⁰ and 47 ± 11%, from the uptake of soil Hg. Atmospheric Hg²⁺ is the primary source of surface soil Hg (54 ± 13%), followed by atmospheric Hg⁰ (40 ± 10%) and geogenic Hg. The Hg accumulation in soils increased by a factor of 5 at an accelerating rate from the 1870s to 2010s. The Hg release flux from melting glaciers is 3.51 ± 0.01 μg m^–2 yr^–1. The highly positive Δ¹⁹⁹Hg (1.03 ± 0.49‰) in precipitation due to photoreduction of Hg²⁺ in water droplets causes all samples in ecosystems to have positive Δ¹⁹⁹Hg values. Isotopic evidence suggests that photolytic and abiotic dark reduction processes have driven Hg⁰ re-emission from glacier and underlying soil after melting. The accelerated Hg release from melting glaciers and soil Hg accumulation caused by global warming alter Hg cycling in the cryosphere.