THE INVESTIGATION AND DATING OF TRANSBOUNDARY AIR POLLUTION FOUND IN SOUTHERN GREENLAND

 

Michael Goodsite*(mgo@dmu.dk) (present address: Department of Atmospheric Environment, Danish National Environmental Research Institute, Frederiksborgvej 399, P.O. Box 358, DK-4000 Roskilde, Denmark) Christian Lohse, Torben Stroyer Hansen, (Department of Chemistry, University of Southern Denmark, Odense, Campusvej 55, DK-5230, Denmark), William Shotyk, Fiona Roos (Geological Institute, University of Berne, Berne, CH-3012 Bern, Switzerland), W.O. Van der Knaap, (Geobotanical Institute, University of Berne, Altenbergrain 21, CH-3013 Berne, Switzerland), Jan Heinemeier, AMS 14C Dating Laboratory, Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark), Andriy Cheburkin, (EMMA Analytical, Elmvale, Ontario, Canada), Robert Frei, (Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 København K, Denmark) P.G. Appleby, (Environmental Radioactivity Research Center, Division of Applied Mathematics, Department of Mathematical Sciences, The University of Liverpool, M & O Building, Liverpool, L69 3BX, England, U.K.)

 

ABSTRACT

A direct method for dating transboundary air pollution deposited more recently than 1950 is described. The method takes advantage of the atmospheric bomb-pulse of 14C due to nuclear weapons testing. 14C in peat macro fossil samples was determined with accelerator mass spectrometry (AMS) and directly compared to bomb pulse values for date determination. Peat was sampled from minerotrophic mires in Southern Greenland and an ombrotrophic bog in Denmark. Pb and Hg concentration profiles were determined, providing relevant points to date. Pb isotopes were analyzed to discriminate among possible sources of anthropogenic and natural Pb. Dates obtained were compared to dates obtained with 210Pb dating, and are in good agreement. The method provides selective high resolution dates for the period of 1955 to 1995, with samples weighing less than one milligram, and allows dating of terrestrial profiles without the need to date a continuous sequence of samples.

 

INTRODUCTION

The Arctic environment is clearly being affected by human activity. Since pre-industrial times, the deposition of several trace metals has increased considerably in Greenland and is correlated with the development of anthropogenic activities (Candelone et al., 1995). There are many studies that document the historical records of transboundary air pollution found in the Arctic. Many of these studies analyze heavy metal profiles in ice and snow records (e.g., Boutron et al., 1995), others in marine sediments (e.g., Asmund and Nielsen, 2000) or lacustrine sediments (e.g. Rognerud et al., 1998). Shotyk and colleagues (1998) have shown that peat deposits provide a faithful archive of atmospheric Pb deposition and Cortizas and colleagues (1999) have shown that peat preserves the record of atmospheric Hg deposition, with cold climate promoting Hg accumulation rates. Peat is a terrestrial archive that has been prioritized by the Arctic Monitoring and Assessment Program (AMAP) (AMAP, 1999). Common to all retrospective studies of atmospheric deposition is the need to date the profiles. The dating method used depends on the archive material and the expected age of the material to be dated.  High resolution dating of the last 50 years is especially important, since many anthropogenic activities increased exponentially during this time: earlier activities which released more contaminants to the environment, as well as later activities which have limited their release. The present study investigates the feasibility of using the atmospheric bomb-pulse (ABP) (Figure 1) to date peat from the period of 1955 to the present. The increased amount of radiocarbon Text Box: Figure 1 Lange’s and Donahue’s atmospheric bomb pulse curve for the Northern Hemisphere. Based on measurements of tree rings with known dates, taken from clean air sites and seeds which are from known years. The measurement of 14C is the concentration in the material, not the decay from the material. The fraction of modern carbon on the y-axis is normalized to 1950 (“the present”). Nuclear bomb testing nearly doubles the amount of 14C in the atmosphere. For any given concentration of 14C measured, there is at least two possible corresponding dates, so at least two points must be measured in the profile to establish position relative to AD 1963. in the atmosphere during this period is due to nuclear weapons testing, and achieved its maximum concentration in 1963 when the Limited Test Ban Treaty effectively stopped most atmospheric testing. Levels have been falling since then, primarily due to oceanic uptake. Concentrations of 14C can be directly measured in terrestrial material such as peat (Jungner et al., 1995). By dating two points close in depth to one another, one can see if the level of modern carbon either rises or falls, depending on whether the sample pre-dates or post-dates AD 1963. By dating a point at the surface, one can see if there is a dilution of the curve, due to uptake of older carbon coming from decomposition of organic material (such as observed by Jungner et al., 1995). The method can be very useful for temporal studies of contaminants such as heavy metals during the last 50 years. It provides a means to date terrestrial profiles, or specific points within the profiles, without the need to date an entire column with other radiometric means. This can be useful when investigating short cores (typically taken in contaminant studies of remote areas), that were not dug deep enough to include the entire 210Pb profile. The dating procedure is direct and fast (it can be done in approximately 48 hours if necessary). By selecting appropriate macrofossils, the amount of material used for dating is minimized, with dating possible on samples weighing less than one milligram.

 

METHODS

Mires selected for study are located on the Narsaq peninsula, Southern Greenland (610 10’ N, 450 35’ W). The mires had peat accumulation ranging from 20cm to approximately 100cm deep. Three (15cm x 15cm by approximately 100cm) replicate monoliths of peat from each of two sites were cored using a Ti Wardenaar peat sampler. The replicate cores were removed approximately 1.5m from each other. Further analysis was carried out on cores from one site; based on pH profiles measured in the field of the peat pore water, the site that appeared to have a more acidic upper 20cm was chosen (Tasiusaq: 610 08.31’ N, 450 33.70’ W). The three cores from each site were frozen soon thereafter and shipped to the University of Berne for further processing and analysis. Of the three cores, Core A, was sliced using a stainless steel bread knife into 3-cm slices by hand, and pore water was squeezed out of the slices.  Portions of the slices were then dried overnight at 1050C and milled in a Ti mill. Lead and 19 other elements were then determined using X-ray fluorescence spectrometry (XRF). Core B was cut while frozen into 1-cm slices using a stainless steel band saw, and selected portions of the slices were then dried and milled. Samples were then analyzed as before using XRF. Powders were selected for lead isotope analysis using thermal ionization mass spectrometry (TIMS) based on the Pb concentration profile obtained using XRF. Plant macrofossils were removed from the centers of the slices and processed for 14C dating for AMS using standard procedure for plant material (washed, acid, base, acid treatment).  AMS was run on the samples to reproduce the ABP curve and date peaks in the Pb profile. Core B was also dated using 210Pb.

Samples for Hg analysis were taken near the center of each slice. They were air dried in a Hg free air laminar flow (class 100) cabinet in the Trace Metals Lab, Geological Institute, University of Berne, until they achieved a constant weight (approximately 24 hours). They were then directly analyzed for total Hg using a DMA-80 direct mercury analyzer, following a slightly modified US EPA METHOD 7473. Selected samples were analyzed for Hg concentrations by an independent lab, using an alternative method. Core C remains frozen as an archive. Three cores were similarly sampled and processed from Storelung raised bog, Staaby, Funen, Denmark (550 01.17’N, 080 56.50’E), in order to compare results from an ombrotrophic bog with those from the Greenland minerotrophic mire.

 

RESULTS AND DISCUSSION


Figure 2 is a plot of the AMS measurements versus depth, in the Greenland Core B, and Danish Core B, showing a good reproduction of the ABP curve. 210Pb data (not shown) is in good agreement with the dates obtained by AMS. Figure 3 shows an application of the dating method. In the Hg profile measured in the peat from Greenland the peaks observed in the mid 1960’s and the late 1950’s appear to correspond with maxima observed in the ice record (Boutron et al., 1998), though concentrations in the peat are approximately 10.000 times higher.  Figure 4, shows the dating of the Hg profile for the Danish core. Note the similarity in shape of the two profiles and the similar modern values, and what appear to be higher values in Greenland during prehistoric time. The precision of the atmospheric bomb-pulse method is approximately ± 2 years, due to the atmospheric mixing time of 14C. The method’s precision falls after 1995 as the ABP curve begins to flatten. The ABP method is currently being tested in peat from the Faroe Islands and another site from Denmark. The method supplements current dating methods and combined with environmental radioisotope dating such as 210Pb provides a complete chronology of the last century with high resolution selected points of interest in the last 50 years.

Text Box: Figure 4 Hg profile from a Danish raised bog, dated with the atmospheric bomb pulse method.

 


REFERENCES

AMAP (1999) Report 99:8 Heavy Metals in the Arctic, Proceedings from an International Workshop. Oslo, AMAP.

Asmund G., and Nielsen S.P. (2000), Sci. Tot. Env. 245(1-3): 61-72.

Boutron C.F., Candelone J.-P., Hong S., (1995), Sci. Tot. Env. 160/161: 233-241.

Boutron C.F., Vandal G.M., and Fitzgerald W.F., Ferrari C.P. (1998), Geophys. Res. Let. 25(17): 3315-3318.

Candelone J.-P., Hong S., Boutron C. (1995), J. Geophys. Res. 100:16 605-616.

Cortizas A.M., Ponteedra Pomba X., Garcia-Rodeja E., Novoa Munoz J.C., and Shotyk W., (1999), Science 284:939-942.

Jungner H., Sonninen E., Possnert G., Tolonen K. (1995) Radiocarbon 37(2): 567-573.

Rognerud S., Skotvold, T., Fjeld E., Norton S.A., Hobaek A. (1998), Can. J. Fish. Aquat. Sci. 55(6): 1512-1523.

Shotyk W., Weiss D., Appleby P.G., Cheburkin A.K., Frei R., Gloor M., Kramers J.D., Reese S., and van der Knaap, W.O. (1998), Science 281:1635-1640.

 

ACKNOWLEDGEMENTS

The financial support from the following sources is acknowledged and greatly appreciated: The Danish Cooperation for Environment in the Arctic (DANCEA) and the Danish Ministry of the Environment, The Danish National Natural Science Fund (SNF), The Swiss National Science Foundation (Grant Number 21-55669.98 to W. Shotyk), The GKSS Institute for Hydrophysics, Special thanks to Bent Aaby and Bent Odgaard for their expertise on Danish bogs, Bent Fredskild, Flemming Rune, and Ole Bennike for their expertise on Greenland Mires, Douglas Donahue and Todd Lange for use of their Bomb Pulse Data. G. Asmund, N.J. Anderson, Antonio Martinez Cortizas, Mariza Costa-Cabral, Hans Von Storch, Jesse Ford and Claude Boutron for helpful discussions. Harald Biester for his expertise with Hg measurements. Rikke Brandt and Tommy Nørnberg for field assistance. The people of Tasiusaq, Narsaq Greenland, the Greenland Homerule and the Danish Polar Center.