Article

A 270-year Ice Core Record of Atmospheric Mercury Deposition to Western North America

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Abstract

The Upper Fremont Glacier (UFG), a mid-latitude glacier in the Wind River Range, Wyoming, U.S.A., contains a record of atmospheric mercury deposition. Although some polar ice-core studies have provided a limited record of past mercury deposition, polar cores are, at best, proxy indicators of historic mercury deposition in the mid-latitudes. Two ice cores removed from the UFG in 1991 and 1998 (totaling 160 meters in length) provided a chronology and paleoenvironmental framework. This aids in the interpretation of the mercury deposition record. For the first time reported from a mid-latitude ice core, using low-level procedures, 97 ice core samples were analyzed to reconstruct a 270-year atmospheric mercury deposition record based in the western United States. Trends in mercury concentration from the UFG record major releases to the atmosphere of both natural and anthropogenic mercury from regional and global sources. We find that mercury concentrations are significantly, but for relatively short time intervals, elevated during periods corresponding to volcanic eruptions with global impact. This indicates that these natural events "punctuate" the record. Anthropogenic activities such as industrialization (global scale), gold mining and war-time manufacturing (regional scale), indicate that chronic levels of elevated mercury emissions have a greater influence on the historical atmospheric deposition record from the UFG. In terms of total mercury deposition recorded by the UFG during approximately the past 270 years: anthropogenic inputs contributed 52 percent; volcanic events contributed 6 percent; and pre-industrialization or background accounted for 42 percent of the total input. More significantly, during the last 100 years, anthropogenic sources contributed 70 percent of the total mercury input. A declining trend in mercury concentrations is obvious during the past 20 years. Declining mercury concentrations in the upper section of the ice core are corroborated by recent declining trends observed in sediment cores. This is also verified by similar concentrations in UFG snow samples collected in 1999. This decline may be in response to the United States Clean Air Act of 1970.

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Article
Mercury (Hg) deposition through litterfall has been regarded as the main input of gaseous elemental mercury (Hg0) into forest ecosystems. We hypothesize that earlier studies largely underestimated this sink because the contribution of Hg0 uptake by moss and the downward transport to wood and throughfall is overlooked. To test the hypothesis, we investigated the Hg fluxes contributed via litterfall and throughfall, Hg pool sizes in moss covers and woody biomass as well as their isotopic signatures in a glacier-to-forest succession ecosystem of the Southeast Tibetan Plateau. Results show that Hg0 depositional uptake by and pool sizes stored in moss and woody biomass increase rapidly with the time after glacier retreat. Using the flux data as input to a Hg isotopic mixing model, Hg deposition through litterfall accounts for 27–85% of the total accumulation rate of Hg0 in organic soils of sites glacial retreat 20 to 90 years, revealing the presence of additional sources of Hg0 input. Atmospheric Hg0 accounts for 76±24% in ground moss, and 86±15% in tree moss, and 62–92% in above ground woody biomass (branch-bark-stem), and 44–83% in roots. The downward decreasing gradient of atmospheric Hg0 fractions from the aboveground woody biomass to roots suggests a foliage-to-root Hg transport in vegetation after uptake. Additionally, 34–82% of atmospheric Hg0 in throughfall further amplifies the accumulation of Hg0 from atmospheric sources. We conclude that woody biomass, moss and throughfall represent important Hg0 sinks in forest ecosystems. These previously unaccounted-for sink terms significantly increase the previously estimated atmospheric Hg0 sink via litterfall.
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