Elevated Mercury Concentrations in Southern Patagonian Peat Bogs – An Anthropogenic Signal ?

*Harald Biester¹, Rolf Kilian², Carola Hertel¹ and Heinz Friedrich Schöler¹

Institute of Environmental Geochemistry, INF 236, 69120 Heidelberg, Germany

Dept. of Geology, University of Freiburg, Albertstr. 1b, 79104 Freiburg, Germany

*corresponding author: hbiester@ugc.uni-heidelberg.de

Abstract

Mercury was analysed in two peat cores from an ombrogenic peat bog in the southernmost Andes/Chile. Mercury concentrations range between 40 and 80 µg/kg in pre-industrial times and show a sharp increase to 160 µg/kg in modern times. The high Hg levels in the upper part of the cores were attributed to the strong increase of Hg emissions from anthropogenic sources in the past 100 yr. Sources of Hg others than atmospheric deposition could be excluded in the study area. It is assumed that the comparatively high accumulation of Hg in this remote area is mainly due to the high precipitation rates reaching up to 10.000 mmm per year. This suggests that Hg emitted from anthropogenic sources is dispersed all over the atmosphere and even accumulated in remote sub-antarctic areas.

Introduction

Numerous studies in the past two decades confirm an increase of Hg concentrations in the atmosphere due to modern anthropogenic emissions. Fluxes of Hg from industrial sources such as coal fired plants, ore smelters or chlor alkali plants are comparable in magnitude to natural fluxes (Nriagu, 1989). The biogeochemical cycle of Hg is characterized by distribution of the metal all over the planet due to its unique property of being volatile. Therefore, the atmosphere is the main pathway for the distribution of mercury at the Earth‘s surface. The residence time of Hg in the atmosphere is estimated to be about one year. The dominant atmospheric Hg species is elemental Hg which could be partly oxidized. Both Hg species reach the Earth‘s surface through wet or dry deposition. In recent years increasing attention has been given to atmospheric deposition of Hg in remote areas especially in arctic regions. Increased Hg levels found in several natural archives such as snow and ice, lake sediments and ombothrophic peat bogs confirm increased fluxes of anthropogenic Hg from mid lattitudes to the arctic. Here, deposition of Hg, derived from mid latitudes was found to be mainly due to a gernerally increased deposition of gaseous compounds at low temperatures (cold condensation). The impact becomes clear regarding the high Hg levels in arctic mammals. However, up to now little is known about the fate of Hg in the southern hemisphere especially in higher latidues. While ice core records from Antarctica have provided excellent chronological records of atmospheric lead, little is reported about past and present deposition of Hg. Vandal et al. (1993) investigated mercury deposition in Antarctica ice cores and found that variability of pre-anthropogenic Hg levels is due to variations of productivity in the oceans. It is still unknown, if anthropogenic Hg emission on the northern hemisphere also cause increased Hg deposition in high latitudes of the southern hemisphere.

In our study we have investigated the record of atmospheric Hg deposition in peat bogs located in the southernmost Andes/Chile. Several studies have shown that ombothrophic peat bogs are excellent archives for records of atmospheric Hg deposition (e.g. Pheiffer-Madsen, 1981; Benoit et al., 1994). The southernmost Andes are characterized by narrow succession of climatic zones which are characterized by heavy rains and strong westerly winds driven by the antarctic circum polar vortex on the west side and dryness on the east side. The sampling area is located in the area of maximum anual precipitation reaching up to 10.000 mm . Vegetation in the area is characterized by a mixture of magellanic moorland and subarctic forests.

Methods and materials

Two peat cores were taken from an ombrogenic raised bog (GC1) located on the alluvial plain of the Gran Campo Nevado glacier (52° S). The cores were taken by means of a 2 m stainless steel Wardenaar corer. Length of the both peat monolith was about 150 cm, respectively. The cores were immediately cut into 2 cm slices (10 x 10 cm) using a serrated stainless steel bread knife and packed into polyethelen bags. All samples were stored deep frozen. Bulk density was determined from 3 subsamples, which were cut in the field from the fresh peat slices using an apple corer. Before analysis approximately half of each slice was freeze dried and milled. Mercury concentrations were determined by atomic absorption spectroscopy after thermal combustion of the freeze dried samples and Hg preconcentration on a gold trap. SRM (apple leaves, rye grass and olive leaves) were used for quality control of Hg analysis. RSD was in the range of 1.5 - 2.1 % (n = 4). About 3 g of the dried and milled peat of the uppermost 15 cm were dated using ²¹⁰Pb age. Trace-metal free protocols were employed during all stages of collection and analysis. The ash content was determined after combustion of approximately 1 g of dried peat at 550 °C.

Results and discussion

Both profiles are marked by an about 12 cm thick, coarse grained tephra layer at about 100 cm depth, which was identified as a big Mt. Burney eruption dated to 8500 B.P. (Stern, 1992). The cores are assumed to cover about 13.000 yr. The Hg records in both profiles are very similar in shape indicating that the variation of Hg concentrations in corresponding sections within the bog is low. Both cores show high Hg levels of about 100 µg/kg in the deepest
20 cm which probably indicate Hg fluxes from the basement. At about 120 – 130 cm Hg levels drop to about 40 µg/kg. Above the tephra layer Hg concentrations increase from about 40 µg/kg to values between 80 and 100 µg/kg in the section from 50 to 15 cm. In the uppermost 15 cm of the profiles Hg concentrations increase drastically to maximum values of 160 µg/kg.

A. B.

Fig. 1: Records of Hg in two cores from a peat bog taken in southernmost Patagonia/Chile

A: Record of Hg and ash content in core 1 B: Hg record and Hg record corrected for density in core 2.

As the upper 15 cm are within the ²¹⁰Pb excess activity it is most reasonable that this peak indicates increased anthropogenic emissions of Hg in the last 150 yr. Those Hg concentrations are at the same levels as those found in Greenland peat for the same period of time (Shotyk et al. 2000). The enrichment factor of Hg in modern times is about 2 - 4 compared to the variability of pre-industrial levels (40 – 100 µg/kg). The reasons for the generally increase of Hg from about 40 to 100 µg/kg in pre-industrial times is unknown and might be related to climatic factors as indicated in other studies (Martinez-Cortizas et al., 1999).

Figure 1a shows the Hg record of one of the peat cores and records of ash content. It seems that in the ombrogenic part Hg concentration are independend from the ash content which is different for other trace metals (analysed in ongoing studies). Figure 2b shows the record of Hg in the second core and the shape of the curve if Hg is corrected for density. It was found that the variation of the density of the peat is low and the shape of the Hg record changed only little if corrected for density.

Conclusions

The Hg record in ombothrophic peat from the south Patagonian bog shows a clear signal of increased anthropogenic Hg emission within the last 150 yr. The signal found in peat of which has not yet been reported from Antarctic ice cores is concluded to be related to the high precipitation rates in the study area which cause a strong washout of Hg from the atomsphere.

These results confirm the long term transport of Hg and its accumulation even in sub-antarctic areas.

References

Nriagu, J.O. (1989), Nature, 338: 47-49.

Vandal, G.M., Fitzgerald, W.F.Boutron, C.F. Candelone (1993), Nature 362 :621-623.

Pheiffer-Madsen, (1981), Science, 293:127

Benoit, J.M., Fitzgerald, W.F., Damman, A.W.H. (1994). In: Mercury Pollution Integration and Synthesis C.J. Watras and J.W. Huckabee, Editors), Lewis Publishers, pp. 187202

Martinez-Cortizas A, Pontvedra-Pombal X, Garcia-Rodeja E, Novoa-Munoz JC, Shotyk, W (1999) Science, 284: 939-942.

Shotyk W, Goodsite ME, Lohse C, Hansen TS, Roos F, Biester H, Cheburkin AK, Frei R, Heinemeier J, Appleby PG, Reese S, Asmund G (2000) Continuous 3000 year record of atmospheric Hg in Southern Greenland and Denmark, submitted.

Stern, C.R. (1992) Tefrochronologia de Magallanes: Nuevos datos e implicaciones. Ans. Inst. Pat. Ser. Cs., Punta Arenas (Chile), 21: 129-141.