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AMAP Greenland and the Faroe Islands 1997-2001

1 Atmospheric Mercury and Lead Accumulation Since 5420 14C yr BP at Myrarnar, Faroe Islands

Shotyk, William^1 *, Goodsite, Michael², Roos-Barraclough, Fiona³, Givelet, Nicolas³, Leroux, Gaël¹, Weiss, Dominik⁴, Norton, Stephen⁵, and Knudsen, Kristina⁶

Summary and conclusions

Our findings suggest that the natural background flux of Hg to the Faroe Islands was always elevated, compared to continental bogs. Superimposed on these elevated natural fluxes, however, is a much greater Hg flux in recent samples (past two centuries) which is dominated by Hg from anthropogenic emissions. This interpretation is based on measurements of Hg concentrations, the Hg/Se ratios, Pb and stable Pb isotopes (²⁰⁴Pb, ²⁰⁶Pb,

²⁰⁷Pb, and ²⁰⁸Pb), and ²¹⁰Pb age dating. These conclusions, however, should be verified using at least one more peat core from another site on the Faroe Islands. For example, the peat core which we collected at Klovinmyren is certainly suitable to evaluate in more detail the pre-anthropogenic relationship between Hg, volcanic ash falls, and Se deposition since ca. 9,000 ¹⁴C yr BP. Selenium shows great promise as a reference element for atmospheric Hg deposition in maritime locations, and in this context the geochemistry of Se in blanket peat bogs requires and deserves further study.

The large total mercury peak (498 ng Hg/g dry weight) at Myrarnar occurs between a depth of 5cm and 6cm. We can not explain this concentration by any geochemical mechanism or natural input. It is therefore our opinion that this is an anthropogenic signal. We will better be able to quantify the flux amounting from this signal with more detailed dating. Based on analysis of the Hg/selenium ration (a 17 time increase since the start of the industrial age), we expect that the mercury flux increased by the same amount. We cannot however, draw any conlusions at this time from our study as to whether the source of mercury is local, regional or from long transport.

Our data also suggest that Hg fluxes in other maritime locations such as NW Scotland and the Shetland Islands, warrant detailed investigation.

Finally, the long-term atmospheric deposition of natural Hg combined with the recent addition of Hg from anthropogenic sources may have lasting consequences for local ecosystems. The inventories and dynamics of Hg transformations in local soils and sediments also deserve attention.

Atmospheric Mercury and Lead Accumulation Since 5420 ¹⁴C yr BP at Myrarnar, Faroe Islands

Concern has been expressed about the concentrations and chemical speciation of Hg in the food chain on the Faroe Islands, and the possible implications of these for human health. It is unclear how much of the present day Hg flux is from anthropogenic emissions, and how much from natural sources. The main goal of our study was to reconstruct a long-term record of atmospheric Hg accumulation, and to try to determine how much of the Hg flux is natural, and how much from anthropogenic sources.

A peat profile monolith ca. 15 x 15 x 75 cm was collected from a blanket bog at Myrarnar, on the Island of Streymoy, Faroe Islands. The core was cut into slices of 1 cm and analyzed for total concentrations of Hg and 19 additional major and trace elements, including Pb. Mercury concentrations were measured in solid samples using the Leco 254 Hg analyser which combusts the samples in an oxygen stream, traps the Hg onto gold, and measures Hg after thermal desorption using AAS. Lead and other trace elements, including Se, were measured in solid samples using non-destructive energy dispersive XRF. In addition, the isotopic composition of Pb, often used to fingerprint anthropogenic Pb sources, was measured in acid digests of selected samples using multicollector ICP-MS. A radiocarbon age date of the last sample of the core (ca. 75 cm) dates the profile at 5420 ¹⁴C yr BP.

The vertical distribution of Hg at Myrarnar suggests that the surrounding rocks and soils have not contributed significantly to the Hg inventory of the peat core, but rather that Hg was supplied primarily, if not exclusively, by atmospheric deposition. The peat core contains abundant, visible grains of mineral matter, most likely emitted from Icelandic volcanoes which are often thought to be an important natural source of Hg. While some of the discrete volcanic events found in deeper peat layers appear to have affected the supply of Hg at some times in the pre-historical past, the pronounced peak of highly elevated Hg concentrations (up to 700 ng/g) is difficult to explain by natural emission sources alone. This peak is found at a depth of only 5 cm beneath the top of the peat core which suggests that it must be recent. The maximum Hg concentration in this core exceeds by a large margin the maximum concentrations of Hg found at all of our other study sites (Switzerland, Scotland, Shetland Islands, Denmark, southern and northern Canada, and Greenland). Measurements of Hg in selected peat samples from this core using an independent method (atomic fluorescence spectroscopy of acid digests) at the University of Maine provided identical concentrations. The high Hg concentrations at the surface of the peat bog, however, partly reflect the relatively slow peat accumulation rate (only 10 cm of peat has accumulated during the past two centuries). More important than the Hg concentrations, the rate of atmospheric Hg accumulation to this site requires quantification. This Hg flux, however, requires a detailed reconstruction of the peat accumulation rates, and this will only be obtained once the high resolution age dating (¹⁴C) has been completed.

The greatest Hg concentrations in the peat profile are found in the uppermost 10 cm which, according to the ²¹⁰Pb chronology, represents the past two centuries of peat accumulation. In an effort to quantify the enrichment of Hg, we have used Se as an reference element which is also supplied primarily from the atmosphere. Assuming that Se is supplied exclusively by natural atmospheric sources, is effectively retained in the peat column, and is residually enriched as the plant matter is slowly decomposed to peat, we use the Hg/Se ratio to try to distinguish natural from anthropogenic Hg. The Hg/Se ratio was found to be remarkably constant (0.08 ± 0.02, n=54) for nearly six thousand calendar years until it began a pronounced increase in the top 10 cm of the profile, reaching a maximum of 1.32 (17 times the longterm average value). Based on the ²¹⁰Pb chronology, this enrichment dates from the Industrial Period, and reached its maximum extent ca. AD 1935; this is consistent with the unpublished chronologies of Hg enrichment in peat bog profiles from Scotland and the Shetland Islands, and comparable with recently published chronologies from peat bogs in Denmark and southern Greenland.

The subsurface peat layer is also highly enriched in Fe (up to 9 % by weight), but the changes in Hg concentrations clearly preceed and predate the changes in Fe concentration. The porewaters are expected to have a pronounced concentration gradient with respect to Fe(II) which could drive the Fe accumulation (by oxidation to Fe(III) in the oxic surface layers), but the Hg concentration profile would be expected to drive Hg in the opposite direction. Thus, it is difficult to attribute the elevated Hg concentrations to chemical processes operating at the oxic/anoxic boundary. In addition, the vertical distribution of Pb EF (enrichment factor calculated using Ti as a conservative reference element) shows that Pb too, becomes enriched in the peat profile, below, and pre-dating, the changes in Fe concentrations. Thus, shapes of the Hg/Se and Pb EF profiles argue that the Hg and Pb concentration profiles reflect the chronology of atmospheric Hg and Pb deposition, and are not simply artefacts of chemical diagenesis.

The isotopic composition of Pb in the peat profile supports this interpretation. In fact, the isotopic composition of Pb shows pronounced gradients which suggest that mixing by advection or diffusion have been negligible. Moreover, the minimum ²⁰⁶Pb/²⁰⁷Pb ratio (1.1349 ± 0.0029) is found in a peat sample dating from 1985. In fact, the variation in Pb isotope ratios with time (as revealed by the ²¹⁰Pb chronology) is consistent with the chronology of the introduction, phase-out, and gradual elimination of leaded gasoline in Europe.

Taken together, the data obtained thus far suggests that the “natural background” flux of Hg to the Faroe Islands may always have been greater that the “natural background” flux of Hg recorded by continental peat bogs such as the one in Switzerland described by Roos-Barraclough et al (2002). While part of this difference may be due to volcanic Hg emissions from Iceland, there are other possible explanations. Given the correlation between Hg and Br in the continental peat bog profile (Roos-Barraclough et al.,2002), and the recent observations of BrO formation at polar sunrise and its effects on atmospheric Hg fluxes in the Arctic, it may be that the higher Br concentrations in marine aerosols contribute to a more effective oxidation and scavenging of gaseous Hg. This latter hypothesis would help to explain why the long-term rate of atmospheric Hg accumulation recorded by the peat bog profile at Myrarnar (6.1 to 7.8 micrograms/m2/yr) is very similar to a peat bog profile from the Shetland Islands (7.9 to 9.7 micrograms/m2/yr), and why these are much higher than in Switzerland (2.0 to 2.3 micrograms/m2/yr).

A detailed report of the geochemical studies of the peat core collected at Myrarnar is in preparation, and will be submitted to an international scientific journal for publication.

Figure 1.
Gravimetric and volumetric mercury concentrations and the mercury selenium ratio of the Myrarnar (NEO) peat core sampled in May 2000 (by M. Goodsite, and W. Shotyk), and archived at the Institute of Environmental Geochemistry, University of Heidelberg. Solid lines are mercury concentrations measured in whole, air dried bulk peat samples by N. Givelet, University of Berne, using the LECO AMA-254 mercury analyser. Diamonds are mercury concentrations measured in acid digests of sub-samples by Stephen Norton using atomic fluorescence spectroscopy at the Univ. of Maine. Selenium concentrations were measured by Andriy Cheburkin, in solid peat samples using the EMMA x-ray fluorescence analyser. Notice that the Hg/Se ratio in the sample dated 1935 (using 210-Pb)was 1.32. In contrast, the pre-industrial long term average value for Hg/Se in all of the samples below 20 cm, averages 0.08 +/- 0.02.

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2 References

Roos-Barraclough, F., Martinez-Cortizas, A., Garcia-Rodeja, E., and Shotyk, W- (2002). A 14,500 year record of the accumulation of atmospheric mercury in peat: volcanic signals, anthropogenic influences, and a correlation to bromine accumulation. Earth and Planetary Science Letters 202(2):435-451.

2 Department of Atmospheric Environment, National Environmental Research Institute of Denmark, Frederiskborgvej 399, Box 358, DK-4000 Roskilde, DENMARK

3 Institute of Geological Sciences, University of Berne, Baltzerstrasse 1-3, CH-3012 Berne, SWITZERLAND

4 T.H.Huxley School of the Environment, Imperial College of Science and Technology, London, ENGLAND

5 Department of Geological Science, Bryand Global Sciences Center, University of Maine, Orono, Maine 04469-5790 USA

6 Environmental Chemistry Research Group, Department of Chemistry , University of Southern Denmark, Odense University, Campusvej 55, Odense M, DENMARK

* corresponding author

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