Use of Br and Se in peat to reconstruct the natural and anthropogenic fluxes of atmospheric Hg: A 10000-year record from Caribou Bog, Maine.

Using Br and Se as reference elements, the natural and anthropogenic fluxes of atmospheric Hg were reconstructed for the past 10,000 years using peat cores from Caribou Bog, ME. In the ombrotrophic peat layers, the average background Hg accumulation rate (AR) was 1.7 +/- 1.3 microg m(-2) year(-1) which is comparable with the natural rate of atmospheric Hg accumulation reported in other retrospective studies. The average Hg AR determined using all peat samples dating from preindustrial times, including minerotrophic peat, was slightly greater (3.1 +/- 2.3 microg m(-2) year(-1)) which may reflect differences in canopy interception due to the changes in plant communities, aquatic inputs, or possibly climatic factors. The maximum Hg AR (32 microg m(-2) year(-1)) occurred ca. 1961 A.D. In samples predating the settlement by Europeans, there is a linear correlation between the AR of Hg and those of Br and Se; this relationship allows both Br and Se to be used to calculate the natural AR of Hg (Hgnat). The difference between Hg AR and Hg(nat) is the Hg AR in excess of background (Hg(ex)). Because Hg(ex) was positive only after ca. 1840 A.D., it is assumed to represent the anthropogenic Hg component. By the late 19th century, Hg(ex) deposition was equal to the natural flux. At the peak in Hg deposition in 1961 A.D., Hgex made up >90% of total atmospheric Hg deposition. The AR in the uppermost peat decreased to 25% of peak values by 2000 A.D.

An analytical protocol for the determination of total mercury concentrations in solid peat samples.

Traditional peat sample preparation methods such as drying at high temperatures and milling may be unsuitable for Hg concentration determination in peats due to the possible presence of volatile Hg species, which could be lost during drying. Here, the effects of sample preparation and natural variation on measured Hg concentrations are investigated. Slight increases in mercury concentrations were observed in samples dried at room temperature and at 30 degrees C (6.7 and 2.48 ng kg(-1) h(-1), respectively), and slight decreases were observed in samples dried at 60, 90 and 105 degrees C (2.36, 3.12 and 8.52 ng kg(-1) h(-1), respectively). Fertilising the peat slightly increased Hg loss (3.08 ng kg(-1) h(-1) in NPK-fertilised peat compared to 0.28 ng kg(-1) h(-1) in unfertilised peat, when averaged over all temperatures used). Homogenising samples by grinding in a machine also caused a loss of Hg. A comparison of two Hg profiles from an Arctic peat core, measured in frozen samples and in air-dried samples, revealed that no Hg losses occurred upon air-drying. A comparison of Hg concentrations in several plant species that make up peat, showed that some species (Pinus mugo, Sphagnum recurvum and Pseudevernia furfuracea) are particularly efficient Hg retainers. The disproportionally high Hg concentrations in these species can cause considerable variation in Hg concentrations within a peat slice. The variation of water content (1.6% throughout 17-cm core, 0.97% in a 10 x 10 cm slice), bulk density (40% throughout 17-cm core, 15.6% in a 10 x 10 cm slice) and Hg concentration (20% in a 10 x 10 cm slice) in ombrotrophic peat were quantified in order to determine their relative importance as sources of analytical error. Experiments were carried out to determine a suitable peat analysis program using the Leco AMA 254, capable of determining mercury concentrations in solid samples. Finally, an analytical protocol for the determination of Hg concentrations in solid peat samples is proposed. This method allows correction for variation in factors such as vegetation type, bulk density, water content and Hg concentration in individual peat slices. Several subsamples from each peat slice are air dried, combined and measured for Hg using the AMA254, using a program of 30 s (drying), 125 s (decomposition) and 45 s (waiting). Bulk density and water content measurements are performed on every slice using separate subsamples.