TEMPORAL AND SPATIAL TRENDS OF MERCURY IN LAKE SEDIMENTS FROM WEST GREENLAND

R. Bindler^*, I. Renberg

Dept. of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden

^*Corresponding author: e-mail Richard.Bindler@eg.umu.se

N.J. Anderson

Dept. of Geography, University of Copenhagen, DK-1350 Copenhagen, Denmark

ABSTRACT

The Arctic is recognized as an important focus for long-range transport of contaminants, e.g. Hg, from industrial regions at lower latitudes. A large gap in Arctic research is a lack of long-term retrospective time trends, particularly along the ice-free coasts of Greenland, where thousands of lakes are present. Lake sediments along a 150 km transect from the coast inland to the ice sheet margin in the Søndre Strømfjord region were analyzed for Hg. Of 21 lakes, 19 showed enrichments in Hg concentrations, generally 2 to 3-fold, with the highest enrichments in lakes closer to the ice margin, 5 to 10-fold. These increases in Hg exceed the estimated background Arctic contamination rate of 1.3-fold, and are more comparable to contamination increases in remote areas of Sweden and Canada, regions closer to industrial centers. Analyses of sediment profiles suggest that the initial increase in Hg concentration began prior to 1800.

INTRODUCTION

The Arctic is recognized as an important focus for long-range transport of contaminants, particularly from strong south-to-north airflows that carry airborne pollutants from industrial regions at lower latitudes. A diverse range of anthropogenic pollutants has been shown to be present across much of the region (AMAP 1998). Of particular importance are compounds, such as mercury and POPs, which present a risk to native fauna and the inhabitants of these regions. It is hypothesized for some volatile organic compounds, as well as mercury, that there may be a latitudinal fractionation, which contributes to the continued mobilization of these compounds from warmer to colder climates, where they are ultimately deposited and stored (Wania and Mackay 1993). Experimental data and limited field research support this ‘cold-condensation’ hypothesis, at least for some POPs (Blais et al. 1998).

The Arctic represents a huge area with generally few inhabitants and limited infrastructure, which contributes to logistical difficulties for monitoring and research programs. Many areas of the arctic are poorly represented in the scattered sampling programs that have occurred. Time-series data are even more limited, resulting in great uncertainty over the temporal trends of environmental contamination. A specific gap identified in arctic research is the lack of long-term retrospective time trends.

Along the west coast of Greenland is an ice-free region containing thousands of lakes. The limnology of these lakes is not well understood, nor is the impact of long-range atmospheric pollutants. Given the vastness of the region, recent research has been restricted to a generally coarse assessment of pollutants without temporal resolution (Aarkrog et al. 1997). Here, we present a preliminary dataset on temporal trends in Hg pollution in lake sediments and an assessment of the spatial distribution of Hg in the Kangerlussuaq (Søndre Strømfjord) region, West Greenland (67ºN 51ºE). The Kangerlussuaq fjord (Søndre Strømfjord) stretches ca 150 km inland with the ice sheet a further 25 km inland. Inland (Kangerlussuaq airport) mean temperatures are -22 to -15 ºC in January and 6 to 15 ºC in July. Annual precipitation ranges from 400 mm at the coast to <150 mm close to the ice sheet, with the most of the precipitation coming in late summer and early fall.

METHODS

Twenty short sediment cores, ca 20-35 cm in length, were collected in the Kangerlussuaq region (Figure 1) in 1999 using a gravity corer (8.4 cm internal diameter, HTH-Teknik, SE-976 31 Luleå, Sweden). For 18 lakes, top (0-1 cm) and bottom (1 cm interval near the base of the core) samples were collected from each core. For two lakes the complete sediment core was collected and sectioned in 0.5 cm intervals in the field. In addition, a freeze core was collected from a lake situated on a nunatak ca 5 km into the ice sheet (Nunatak lake). The water chemistry of these lakes spans a wide spectrum from oligotrophic to saline (see Anderson et al. 1999).

Freeze-dried samples were digested using H₂SO₄:HNO₃ and analyzed for Hg using CVAFS (IVL, Göteborg, Sweden). QA/QC procedures included replicate digestions and analyses and inclusion of certified reference materials.

Figure 1. The location of the study area in West Greenland and a map of the Kangerlussuaq region. Coring sites are indicated by filled circles (tops/bottoms only, 2-3 lakes per site) and open triangles (profiles).

RESULTS AND DISCUSSION

In all but two lakes Hg concentrations are higher in the surface interval than in the sample from lower in the core (‘reference’ sample). Dividing the surface concentration by the concentration of the lower sample a Hg enrichment factor (EF) can be determined (Figure 2), which permits a simple unit-less comparison among the lakes. The pair of sites inland from Sisimiut, which show no Hg enrichment, are located in a topographically sheltered area. This may contribute to the lack of enrichment. The other lakes have Hg enrichment factors typically about 2 to 3 times above the ‘reference’ sample. However, for lakes nearer to the ice margin, or in the case of Nunatak within the ice margin, the enrichment factors are much higher, with values from 4 to 11 times the ‘reference’ sample.

Analysis of three complete sediment core profiles indicates a rapid increase in Hg concentrations typically beginning at about 5-7 cm depth (Figure 3). For Nunatak lake, ²¹⁰Pb-dating of a parallel core suggests that the initial increase in Hg concentrations precedes the beginning of the 19^th century. The initial Hg increase is also well below the initial appearance of spheroidal flyash particles (SCP) in the sediment (N. Rose, personal communication). In Europe, SCP generally first appear in lake sediment intervals dated to the mid-19^th century (Wik and Renberg 1996; Rose et al. 1999).

In a review of Hg accumulation in the Arctic and other high-latitude lake sediments, (Landers et al. 1998) determined that the minimum contamination rate for remote regions of the Arctic is approximately 1.3-fold. This value is typical for lakes studied in Alaska and Siberia. However, the Hg EF for the Greenland lakes are more comparable to values observed in regions of Canada and Scandinavia that are more directly exposed to long-range atmospheric transport of pollutants from industrial centers to the south (Landers et al. 1998; Bindler et al. 2000). Part of the explanation is atmospheric circulation patterns, which transport pollutants from both Europe and N America to Greenland (Davidson et al. 1993). Additionally, ‘cold condensation’ may be an important factor, particularly to explain the higher EF observed in lakes near to the ice sheet.

	Figure 2. Sediment enrichment factors for Hg concentrations in lake sediments from Kangerlussuaq area, West Greenland. The dashed line indicates no enrichment. Two lakes inland from Sisimiut are indicated by filled squares.
Figure 3. Hg concentrations in a sediment core from Nunatak lake, located ca 5 km into the ice sheet.

Currently, four sediment cores are undergoing ²¹⁰Pb dating (one just completed at the date this paper was submitted), which will allow a more ideal comparison of Hg accumulation trends in these lakes to other studies of Arctic Hg pollution (Landers et al. 1998). Additional Hg analyses, along with organic pollutants, planned in 2000 will contribute to identifying the importance of ‘cold condensation’ on the high EF values observed near to the ice margin, and for West Greenland in general. The project is funded by the Swedish Natural Science Research Council, and is planned to extend through 2001.

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