INCREASED MERCURY CONCENTRATIONS DURING THE YOUNGER DRYAS RECORDED IN AN OMBROTROPHIC PEAT BOG

F. Roos, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne Switzerland.

Tel: +41 (31) 631 87 73 Fax: +41 (31) 631 48 43 email: fiona.roos@geo.unibe.ch

W. Shotyk, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland

Abstract: Mercury concentrations have been determined in the oldest 1.6m of a peat core from Etang de la Gruère in the Swiss Jura mountains which dates back over 12,300 radiocarbon years. A significant Hg peak, more than three times the average “background” value, occurs at a depth corresponding to the Younger Dryas cold period. The depth of the Younger Dryas in the peat is indicated both by radiocarbon age dates and soil dust indicator peaks in a replicate core from the same bog. These results show that climate change is a significant factor in the natural variability of Hg in the atmosphere.

Introduction: During the last three decades there has been increasing interest in Hg as a global pollutant due to its capacity for long range atmospheric transport, and the bioaccumulation of its toxic methylated form. Numerous studies using lake sediments and ice cores suggest that atmospheric Hg deposition has increased significantly since the onset of the industrial revolution, even in remote areas.

However, in order to quantify the anthropogenic component of today’s global Hg budget it is important to understand natural background levels and their variations. Studies have shown ombrotrophic peat bogs to be good archives of net atmospheric Hg deposition (Benoit et al 1998, Shotyk et al, in prep.). Of special importance to the present study is the finding that there exists a relationship between net Hg accumulation in the peat and Holocene climate change, with greater accumulation occurring during cold periods (Martinez-Cortizas et al 1999). Peat formation in the ombrogenic bog “Etang de la Gruère” (EGR) in the Swiss Jura mountains began 12,370 ¹⁴C years BP. This offers an opportunity to reconstruct the complete history of atmospheric Hg deposition throughout the Holocene, extending as far back as ca. 14,500 calendar years, or into the Older Dryas (14ka BP) cold event. Unlike the “Little Ice Age” cold event, the inclusion of the Younger Dryas (ca. 12,000 to 11,000 cal yr. BP) in the profile allows natural variations in net mercury accumulation during an extensive cold climate period in pre-anthropogenic times to be reconstructed. Here we present new measurements of Hg concentrations in the deepest, oldest layers of EGR, to test the hypothesis that Hg accumulation rates were elevated during the Younger Dryas cold event.

Methods: The Etang de la Gruère (EGR, co-ordinates CH 570.525, 232.150) 2A core was used for Hg analysis. EGR lies 1005m above sea level. Annual precipitation at the site is estimated to be 1500mm. The core was collected using a Livingston corer and was 734cm long and 8cm in diameter. There is a sharp transition to clay at 650cm. The core was sliced frozen into 2cm slices and the individual slices were then stored at -18°C until analysis. Mercury was analysed from 656 to 490cm. Tests were performed to assess the effect of air drying on Hg concentration in peat samples compared to the concentration in wet samples and the effects were found to be negligible. These tests are described in detail elsewhere (Roos et al, 2000). Samples measured wet (656-646cm) were allowed to thaw before analysis. Material from these was extracted using plastic tweezers and analysed using the Leco AMA 254. Each slice was analysed in triplicate and the average concentration of each slice is given here. The drying time (s) for the samples was [0.7 x volume of water], with water volume calculated from water content and mass of sample. Water content and bulk density of each slice were determined using peat plugs of known volume. The plugs were obtained from each thawed slice using a stainless steel tube, of diameter 16mm, with a sharpened end. The height of these plugs was measured and the volume of each calculated (v = pr²h) in cm³. The plugs were then dried to constant weight at 105°C and the water content of the slice calculated. The bulk density was expressed in g cm^-3. The dry weight of each sample analysed for Hg was calculated from its mass and water content. Hg concentrations for all samples are expressed as ng g^-1 dry weight.

Slices 644-594cm were air dried overnight in a class 100 clean air cabinet. The wet weight of each slice to be air-dried was recorded. After air drying one peat plug was removed from each slice. Water content and bulk density of these plugs was determined as described above. Material was extracted from the samples as above and analysed for Hg. Each slice was again analysed in triplicate.

Slices 592-490cm were thawed and four plugs were taken from each slice and air dried as above. One plug from each slice was treated as above to determine water content and bulk density. The remaining three plugs from each slice were placed in sealed plastic bags and crushed to a fine powder by hand. This was done to homogenise the samples and to allow as much sample as possible to fit into the sample vessels. These samples were analysed in duplicate. The average relative standard deviation of Hg concentration per slice in wet samples 656-646 was 8.9% (3 samples per slice, 6 slices) and in air-died samples 592-490 was 3% (2 samples per slice, 53 slices).

Results and Discussion:

The results of the Hg analyses are shown (Figure 1).

Correspondence of Hg Peak to Younger Dryas Cold Period

A pronounced peak in Hg concentration appears at 550cm. In a corresponding core from the same bog (core EGR 2P, collected 22.6.1993) (Shotyk et al, in review), the slice at 555cm depth was dated at 10,590±60 radiocarbon years BP, which corresponds to 10,572 calendar years BC. The Younger Dryas occurred from 11.5-12.8 calendar years BP (Alley et al, 1993), which corresponds to 9,550-10,850 calendar years BC. Thus, the Hg peak in the core EGR 2A corresponds to the Younger Dryas period. The EGR 2P core has also been analysed for soil dust indicators such as Zr, Ti and Hf by XRF analysis. These elements show pronounced concentration peaks (ten times background values) at 550cm depth in the 2P core (Figure 2). During cold periods precipitation decreases, wind strength increases and vegetation recedes (Holmgren et al, 1998). These factors all lead to an increase in soil dust in the atmosphere. Therefore increases in soil dust indicator values in the peat profile indicate cold, dry periods such as the Younger Dryas. The Hg peak and the soil dust peak in the two profiles occur at the same depth (550cm), indicating that Hg concentrations were clearly elevated during the YD compared to deeper, older peat layers.

Possible Causes of Hg Peak During theYounger Dryas

The Hg peak corresponds to an increase in soil dust and it is possible that the Hg increase is directly due to an increase in weathering and atmospheric transport of Hg-containing minerals; this would have caused an increase in particulate mercury in the atmosphere. However, particulate mercury (Hg_p) in non-industrial regions makes up only 0.3-0.9% of total atmospheric mercury (Slemr et al, 1985), the major component being gaseous Hg(0). It is unlikely, therefore, that enhanced rates of soil dust deposition caused the elevated Hg concentrations. Also, the bog is unaffected by any major regional natural Hg-containing mineral sources. It is assumed therefore that the major sources of Hg to the bog are wet deposition of Hg (II), oxidised from gaseous atmospheric Hg(0) in water droplets in the atmosphere (Lin et al 1999) and direct dry deposition of Hg(0) to the bog surface. An increase in Hg deposition would therefore require either greater atmospheric Hg concentrations, greater dry deposition rates and/or a decrease in revolatilisation once deposition has occurred. A decrease in temperature would reduce the volatility of Hg(0) and therefore could increase the net rate of accumulation, both by increasing the rate of dry deposition and also by decreasing the likelihood of revolatilisation.

Implications

It is clear from the results shown here that climate dependency makes natural net Hg accumulation rates very variable. Also, climate dependency of net Hg deposition could mean that regions of colder climate today such as polar and Alpine environments could act as natural sinks of Hg in the environment, with more condensation and less revolatilisation of Hg(0). Further studies of long-term records of Hg accumulation in colder regions and studies to determine the relative importance of wet and dry deposition are required.

Figure 1: Large squares represent sediment samples, diamonds represent peat samples.

Figure 2 (Shotyk et al, in review)

References:

Benoit JM, Fitzgerald WF, Damman AWH (1998), Environmental Research Sect A 78:118-133

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. Continuous 3000 year record of atmospheric Hg in Southern Greenland and Denmark, in prep.

Roos F, Biester H, Goodsite ME, Martinez-Cortizas A, Shotyk W (2000) Development of an analytical protocol for the determination of mercury concentrations in solid peat samples: effects of sample preparation. Heavy Metals in the Environment 25^th Anniversary Meeting, Workshop on Peat Bog Archives

Alley RB , Meese DA, Shuman CA, Gow AJ, Taylor KC, Grootes PM, White JWC, Ram M, Waddington ED, Mayewski PA, Zielinski GA (1993) Nature 362:527-529

Holmgren K, Karlen W, (1998) Late Quaternary Changes in Climate, technical report TR-98-13, Swedish Nuclear Fuel and Waste Management Company

Slemr F, Schuster G, Seiler W (1985) J. Atmos. Chem. 3:407-434

Lin CJ, Pehkonen SO (1999) Atmos. Environ. 33 2067-2079

Shotyk W, Weiss DJ, Kramers JD, Frei R, Cheburkin AK, Gloor M, and

Reese S: Geochemistry of the peat bog at Etang de la Gruère, Jura

Mountains, Switzerland, and its record of atmospheric Pb and lithogenic

trace elements (Sc, Ti, Y, Zr, Hf and REE) since 12,370 14C yr BP.

Geochimica et Cosmochimica Acta (in review).