Environ. Sci. Technol. 2005, 39, 557-568
Modern and Historic Atmospheric Mercury Fluxes in Northern Alaska: Global Sources and Arctic Depletion W I L L I A M F . F I T Z G E R A L D , * ,† DANIEL R. ENGSTROM,‡ CARL H. LAMBORG,§ CHUN-MAO TSENG,| PRENTISS H. BALCOM,† AND CHAD R. HAMMERSCHMIDT† Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, Connecticut 06340, St. Croix Watershed Research Station, Science Museum of Minnesota, Marine-on-St. Croix, Minnesota 55047, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, and National Center for Ocean Research, Taipei, Taiwan, Republic of China
We reconstruct from lake-sediment archives atmospheric Hg deposition to Arctic Alaska over the last several centuries and constrain a contemporary lake/watershed massbalance with real-time measurement of Hg fluxes in rainfall, runoff, and evasion. Results indicate that (a) anthropogenic Hg impact in the Arctic is of similar magnitude to that at temperate latitudes; (b) whole-lake Hg sedimentation determined from 55 210Pb-dated cores from the five small lakes demonstrates a 3-fold increase in atmospheric Hg deposition since the advent of the Industrial Revolution; (c) because of high soil Hg concentrations and relatively low atmospheric deposition fluxes, erosional inputs to these lakes are more significant than in similar temperate systems; (d) volatilization accounts for about 20% of the Hg losses (evasion and sedimentation); and (e) another source term is needed to balance the evasional and sedimentation sinks. This additional flux (1.21 ( 0.74 µg m-2 yr-1) is comparable to direct atmospheric Hg deposition and may be due to some combination of springtime Arctic depletion and more generalized deposition of reactive gaseous Hg species.
Introduction There is increasing interest and concern about the behavior and fate of Hg in the Arctic and the influence of anthropogenically derived Hg on terrestrial and marine polar ecosystems. Current knowledge concerning Hg cycling and contamination at high latitudes is sparse. Atmospheric mobilization of elemental mercury (Hg0) is the principal route by which Hg is transferred at the Earth’s surface (1, 2). Hg0 has a tropospheric residence time of about 1.3 years and this persistence allows emissions of Hg to the atmosphere from natural and anthropogenic sources to be widely dispersed across the earth’s surface, and remote regions such as the Arctic are not spared (3). Indeed, recent research has * Corresponding author phone: (860)405-9158; fax: (860)405-9153; e-mail:
[email protected]. † University of Connecticut. ‡ Science Museum of Minnesota. § Woods Hole Oceanographic Institution. | National Center for Ocean Research. 10.1021/es049128x CCC: $30.25 Published on Web 12/14/2004
2005 American Chemical Society
suggested that there may be enhanced Hg deposition in polar regions as a consequence of the so-called “springtime depletions” of Hg. This phenomenon, which appears to be recurrent and seasonal, was first observed at Alert (Ellesmere Island, NU, Canada (4)), has been confirmed in the Arctic and reported for the Antarctic (5-9). The variations in total gaseous Hg (TGM, >99% Hg0) are highly correlated with ozone and potential linkages to the chemistry and physics of Arctic haze formation and breakdown (10). The Arctic springtime Hg depletion events are also co-incident with a buildup of reactive Br compounds in the polar atmosphere (6). Interestingly, some have observed a rapid oxidation of Hg near the air-sea interface that may result from mechanistically similar reactions with sea spray halogens (11). Environmentally, and in both instances, production of substantial amounts of oxidized Hg in the gas phase may lead to significant dry depositional fluxes in addition to fluxes associated with precipitation. Schroeder et al. (4) estimated the Arctic Hg depletion flux at about 0.5 Mmol, which is a significant but very modest sink on a global scale (3). Schroeder and co-workers based their estimate of 4.6 µg m-2 yr-1 on a boundary layer of 500 m over an area of 2 × 105 km2 containing ca. 1.8 ng m-3 and emptied of its Hg 5 times. At Barrow, Alaska, fluxes approaching 55 µg m-2 yr-1 have been suggested (6). In either case,this periodic Hg depositional input could still be quite significant for delicate Arctic ecosystems, including the human populations that inhabit them, and requires scrutiny and assessment. Though the removal of Hg from the air during spring at high latitudes is without question occurring, the fate of this material in Arctic ecosystems is less clear. For example, there are suggestions that substantial amounts of the deposited Hg are rapidly returned to the atmosphere as a result of photochemical reduction (6, 12). Therefore, the determination of the effective loadings from atmospheric and snowpack measurements is difficult. We have elected to address the question of the impact of Hg deposition on Arctic ecosystems by developing modern mass balances in Arctic lakes and coupling them with the reconstructions of historical changes in Hg deposition using lake sediments. This work was carried out in the context of a comprehensive examination of Hg cycling within Alaskan tundra lakes (13-15) which has included (a) atmospheric Hg deposition and speciation determinations, (b) sediment core collection, Hg analysis, and geochronologies (210Pb dating), (c) in-lake Hg speciation and cycling studies, and (d) laboratory simulations and incubation experiments regarding Hg methylation and reduction. This research is designed to provide the highquality biogeochemical data needed for a quantitative assessment of the scale of potentially enhanced atmospheric Hg deposition related to increased human-related Hg emissions especially over the past 150 years, and where possible, deciphering current and historic evidence of the impact from this deposition. This report is primarily concerned with the details of the historic reconstruction from lake sediments and summarizes information presented by Tseng et al. (13) regarding aspects of the mass balance studies. Lake sediments have been particularly useful media for the reconstruction of Hg fluxes in a variety of locations, particularly when coupled with geochronology (16-21).
Experimental Methods Field Methods. Study Sites. The studies were conducted in the Arctic environs of Alaska’s North Slope in and around the long-term ecological research (LTER) site at the Toolik VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Bathymetric maps of the five study lakes at the Toolik LTER site in northern Alaska. GPS locations of core sites are indicated by (b); stratigraphically detailed (primary) cores are in red and numbered, and coarse-interval (secondary) cores are in blue. Depth contours are at 1-m intervals.
TABLE 1. Physical and Chemical Properties of Studied Lakes lake
latitude
longitude
surface area (ha)
catchment (ha)
Ac/AL
max depth (m)
mean depth (m)
pH
DOC (ppm)
conductivity (µS cm-1)
Perfect Efficient Relaxing Forgetful Surprise
68°38.9′ N 68°42.1′ N 68°44.4′ N 68°52.9′ N 68°32.7′ N
149°46.6′ W 149°42.2′ W 150°02.3′ W 150°17.0′ W 149°39.0′ W
4.0 4.0 6.0 7.4 2.5
4.8 18.0 17.5 17.4 7.5
1.2 4.5 2.9 2.3 3.0
4.9 13.0 12.4 12.4 6.3
3.1 4.7 4.9 3.6 2.7
8.2 8.3 8.0 8.1 8.0
2.7 3.1 4.3 4.3 3.0
115 194 88 50 144
Field Station (68°38′ N, 149°38′ W; Figure 1). The station lies 125 km south of the Arctic Ocean in the foothills of the Brooks Range (719 m asl); regional vegetation is treeless tundra and permafrost is continuous (22). Most surrounding lakes were formed by glaciation about 10 000-12 000 years ago and lie mainly in a glacial outwash (23). There is a short ice-free season (between mid to late June and late September) with July water temperatures typically
