Environ. Sci. Technol. 2005, 39, 910-912
Response to Comment on “Atmospheric Mercury Accumulation Rates between 5900 and 800 Calibrated Years BP in the High Arctic of Canada Recorded by Peat Hummocks” The main points raised by Bindler et al. (1) are with respect to (i) the coring sites selected, (ii) the peat accumulation rates inferred by the radiocarbon ages, (iii) the possible importance of solifluction, (iv) the preferential decay of specific organic matter fractions and the effects of this on mass movement and Hg retention, and (v) age-depth modeling and the estimated rates of pre-anthropogenic, atmospheric Hg accumulation. With respect to scientific study of Arctic peat deposits, finding suitable sites for study is one of the greatest challenges (2). Our field work was planned with the help of Dr. Wes Blake Jr. of the Geological Survey of Canada, who has been studying the Quaternary geology, including peat formation, on Bathurst Island for more than 40 years. Dr. Blake was responsible for the first systematic studies of the glacial geology of Bathurst Island (3). Moreover, Dr. Blake kindly provided us with his original peat core collected in 1963, which allowed us to undertake preliminary geochemical studies of a complete peat profile prior to our own field campaign. Our field studies (July 2000) would not have been possible without the pioneering work of Dr. Blake and his generous help. Our plan had been to collect fresh peat cores at the same sites where Dr. Blake had collected his cores in 1963. Unfortunately, a significant accumulation of snow prevented us from coring the same sites (snow banks that persist on the island for many years are common), but we cored peat hummocks as close as possible to his sampling locations at the Museum Station and Bracebridge Inlet. Our field work is described in a field report (4), which is available as a PDF from the web page of this Institute. With respect to peat accumulation, the 2.75 m long core of frozen peat described by Blake (3) yielded a radiocarbon age of 9210 ( 170 yr BP at the base (2.61-2.64 m, GSC-180), yet the surface layer (17-21 cm) immediately above the level at which the peat was frozen was a remarkable 7100 ( 140 yr BP (GSC-233). Considering the radiocarbon ages (inferring a net peat accumulation rate of ca. 250 cm in just 2000 years), Blake (3) concluded that “conditions on Bathurst Island may have been more favorable to plant growth and peat formation in early postglacial time than they are today”. As an independent check on the radiocarbon ages published by Blake (3, 5), we selected samples from the 1963 core and dated them using 14C. Our set of radiocarbon ages are consistent with the original values published by Blake (3, 5) and indicate rapid peat accumulation during the early Holocene, followed by a marked cessation (Table 1). After the pioneering studies by Blake, other investigations of peatlands in the Canadian Arctic came to the same conclusion regarding the initial rapidity and subsequent hiatus in peat accumulation rates. The early Holocene was a major period of peat accumulation in the western Arctic (6). Peat accumulation rates in boreal and subarctic peatlands of Canada have been reviewed by Ovenden (6, 7). She concluded that climate change is responsible for the “late Holocene decline in growth of many subarctic peat deposits”. Heaving up into mounds caused by the growth of ice lenses probably accelerates the drying and desiccation of surface 910
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TABLE 1. Radiocarbon Ages of Selected Peat Samples from the Blake (1963) Core, Measured at the Heidelberg Academy Sciences during 2000-2002 depth (cm)
laboratory no.
date (14C yr BP)
25-27 41-43 55-57 59-61 65-67 80-82 96-98 109-111 167-169 209-211
Hd-21489 Hd-21501 Hd-21895 Hd-21602 Hd-21889 Hd-21863 Hd-21864 Hd-21874 Hd-21655 Hd-21865
8000 ( 50 7885 ( 95 8061 ( 36 8109 ( 60 8114 ( 42 8276 ( 43 8349 ( 62 8322 ( 60 8526 ( 48 8954 ( 47
peat layers (2). Details of these processes are presently unclear, and it remains to be explained how such vertically extensive accumulations of organic matter may have formed in the past, in areas where today so little peat is forming (2, 8). Our Field Report (4) indicates that an ice lense was found under the peat at Museum Station and extended from 90 cm down to 235 cm, and this might have contributed to heaving, drying, and desiccation of the surface layers. Selected samples from the 1963 core (3) were also measured for trace elements (Table 2). The chemical composition of the peats from the 1963 cores and the values reported for the cores collected during 2000 provide additional confidence that the peat cores collected by Wes Blake, and those collected by our team, really are comparable. Bindler et al. (1) suggested that the excursions in δ13C seen in our cores (9) may reflect variable changes in the extent of humification of the organic fractions and that this could have important ramifications for the rheological properties of the peat giving rise to mass flow; these in turn might have affected the supply of organic matter to the peat mound and have influenced the retention of Hg. The isotopic compositions of C in modern plants from the Canadian Arctic published by Blake (10) show δ13C values ranging from -21.0 to -30.1 for mosses and from -25.8 to -31.4 for vascular plants. Plant parts themselves show considerable variation, as the wood of Empetrum nigrum yielded -25.8 versus -30.6 for the leaves. For comparison, the δ13C values reported in our paper for bulk peat samples range from -24.0 to -32.0. While the variation in δ13C in bulk peat samples from welldecomposed peat cores (such as the ones described by Givelet et al.; 9) might reflect changes in degree of humification of specific organic matter fractions, a simpler explanation is that they merely reflect variations in the botanical composition of the peat. The suggestion by Bindler et al. (1) that the δ13C may reflect changes in humification and that this process might have modified the rheological properties of the peat such that it would be more susceptible to solifluction is not only speculative but also unlikely (see below). The development and genesis of soils on Bathurst Island was studied by Walton and Tedrow (11). Solifluction is found extensively on Bathurst Island, but it is restricted to welldrained and moderately well-drained sloping areas: solifluction does not occur in the flat, waterlogged sites where we collected our cores (Photo 1, Supporting Information). The relationship between soil varieties and landscape on Bathurst Island can best be illustrated by looking at Figure 4 of Walton and Tedrow (11) (Figure 1). In fact, solifluction can be so intense on the slopes of Bathurst Island (Figure 1, position S) that lobes of debris may have an accumulated 10.1021/es040107c CCC: $30.25
2005 American Chemical Society Published on Web 12/30/2004
TABLE 2. EMMA XRF Analyses of Selected Samples from the Blake (1963) Core sample depth (cm)
Ca (%)
Ti (ppm)
Fe (ppm)
Ni (ppm)
Cm (ppm)
Zn (ppm)
Ga (ppm)
Se (ppm)
Br (ppm)
Rb (ppm)
Sr (ppm)
Y (ppm)
Zr (ppm)
Pb (ppm)
U (ppm)
LLD 1 2 75-76 108-109 146-147 184-185 229-230 260-261
0.05 2.07 1.86 2.72 2.16 1.98 2.32 6.04 3.17
30 814 717 841 755 1300 571 884 1402
10 8083 5605 13575 12200 13866 8091 15151 10287
2.5 31.3 15.1 41.9 29.1 35.7 23.2 54.7 18.8
1.5 17.2 9.0 27.0 19.2 20.6 15.1 43.3 9.5
1.0 86.4 39.3 200.4 188.8 132.9 109.6 336.3 85.0
0.7 6.2 4.7 6.3 4.6 9.0 3.8 3.1 5.3
0.6 1.0 0.8 2.5 2.8 3.5 2.4 5.6 1.3
0.7 82.7 85.6 106.5 94.5 77.0 71.8 121.7 26.9
0.7 56.6 53.6 48.1 43.1 68.7 25.3 11.6 55.0
0.8 94.7 96.5 112.6 108.5 120.5 82.6 82.3 106.5
1.0 14.2 1.43 16.7 14.2 15.4 7.9 10.3 21.4
1.0 126.2 145.9 61.3 66.8 70.7 69.3 32.1 254.3
0.6 6.4 4.4 4.3 3.5 6.2 2.5 5.2 6.4
2.6 56.9 51.7 69.5 64.2 57.9 45.5 31.0 4.8
FIGURE 1. Development of soils along a catena on Bathurst Island (data from Walton and Tedrow; 11). thickness of more than 1 m (11). The sites that we cored, however, correspond to position B of Figure 1 and have been described by Walton and Tedrow as “bog”: they are low, flat, wet, and prone to accumulation of fossil plant matter as peat: “on the floor of Polar Bear Pass, the soils consist of a mosaic of bog ... much of this area remains water saturated throughout the summer months ... a few peat mounds are elevated up to a meter above surrounding areas”. The photo of this area (Pl. 6) provided by Walton and Tedrow (11) is very similar to Photo 64 of our Field Report because both of us worked in the same area of Polar Bear Pass as Blake (3, 5). We did not use the appellation “bog” for our coring site because it is very different from the typical ombrotrophic bogs of the temperate zone, which are increasingly being used by ourselves and by others as archives of atmospheric trace elements (12). In summary, solifluction does indeed occur on Bathurst Island but not where we collected our cores. Moreover, the macrofossils that were found in the peat core studied by Blake (3) include Drepanocladus revolvens, Scorpidium scorpiodes, and Campyllium stellatum (5); these species, found both at the base of the core (2.61-2.76 m) and the top of the frozen layer (15 cm), are characteristic of graminoid fens (13) as well as the shallow pools that occur commonly in the Arctic (2). The presence of these species at the bottom and at the top end of the peat core studied by Blake (3) suggests that these peat mounds were always very wet and that this is a sedentary peat formation (14) having formed in-situ. The value of careful age-depth modeling is not unknown to us, as we have described its importance in some detail in a recent paper (15). In our studies of the peat cores from Bathurst Island, a great effort was made by our botanical colleagues in Switzerland and in the United States to identify plant macrofossils in the peat core so that we could age date these materials using AMS. As we already noted in our paper because of the extent of decay, no macrofossils could be
found. As a consequence, there was no alternative but to date bulk peat samples. The radiocarbon ages we used to calculate the rates of pre-anthropogenic, atmospheric Hg accumulation in the Bracebridge Inlet core are given in our paper (9) and yielded an average of 1.3 µg m-2 yr-1. If, however, we simply take the age of the base and the top of the frozen sections of the same peat core, the calculation yields an average rate of 1.6 µg m-2 yr-2. While age-depth modeling is important for quantifying the variation in Hg accumulation rates over time, it does not have a profound effect on the average value calculated.
Note Added after ASAP Publication This paper was released ASAP on December 30, 2004, with an author name misspelled. The corrected version was posted on January 19, 2005.
Supporting Information Available Three photos. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Bindler, R.; Martinez-Cortizas, A.; Blaauw, M. Comment on “Atmospheric mercury accumulation rates between 5900 and 800 calibrated years BP in the High Arctic of Canada recorded by peat hummocks”. Environ. Sci. Technol. 2005, 39, 908-909. (2) Gajewski, K.; Garneau, M.; Bourgeois, J. C. Paleoenvironments of the Canadian High Arctic derived from pollen and plant macrofossils: problems and potentials. Quat. Sci. Rev. 1995, 14, 609-629. (3) Blake, W., Jr. Preliminary account of the glacial history of Bathurst Island, Arctic Archipelago. Geol. Surv. Can. 1964, Paper 64-30, 1-8. (4) Goodsite, M. E.; Shotyk, W. Long-term records of atmospheric deposition of Hg, Cd, Pb and persistent organic pollutant (POPs) in peat cores from Arctic peatlands (Bathurst Island). Institute of Geological Sciences, University of Berne: Switzerland, 2001. VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(5) Blake, W., Jr. Periglacial features and landscape evolution, central Bathurst Island, District of Franklin. Geol. Surv. Can. 1974, Paper 74-1, Part B, 235-244. (6) Ovenden, L. Holocene proxy-climate data from the Canadian Arctic. Geol. Surv. Can. 1988, Paper No. 88-22. (7) Ovenden, L. Peat accumulation in northern wetlands. Quat. Res. 1990, 33, 377-386. (8) Tarnocai, C. Soils of Bathurst, Cornwallis, and adjacent islands, District of Franklin. Geol. Surv. Can. 1976, Paper 76-1B, 137141. (9) Givelet, N.; Roos-Barraclough, F.; Goodsite, M. E.; Cheburkin, A. K.; Shotyk, W. Atmospheric mercury accumulation rates between 5900 and 800 calibrated years BP in the High Arctic of Canada recorded by peat hummocks. Environ. Sci. Technol. 2004, 38 (19), 4964-4972. (10) Blake, W., Jr. Ratios of stable carbon isotopes in some High Arctic plants and lake sediments. J. Paleolimnol. 1991, 6, 157166. (11) Walton, G. F.; Tedrow, J. C. F. A soil pattern of central Bathurst Island, Queen Elizabeth Island, Canada. Biul. Peryglacjalny 1986, 30, 127-139. (12) Shotyk, W. Peat bog archives of atmospheric metal deposition: Geochemical assessment of peat profiles, natural variations in metal concentrations, and metal enrichment factors. Environ. Rev. 1996, 4 (2), 149-183. (13) Shotyk, W. Review of the inorganic geochemistry of peats and peatland waters. Earth-Sci Rev 1988, 25 (2), 95-176. (14) Shotyk, W. Organic soils. In Weathering, Soils, and Paleosols; Martini, I. P., Chesworth, W., Eds.; Elsevier: Amsterdam, 1992; pp 203-224. (15) Givelet, N.; Le Roux, G.; Cheburkin, A. K.; Chen, B.; Frank, J.; Goodsite, M. E.; Kempter, H.; Krachler, M.; Noernberg, T.;
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Rausch, N.; Rheinberger, S.; Roos-Barraclough, F.; Sapkota, A.; Scholz, C.; Shotyk, W. Suggested protocol for collecting, handling and preparing peat cores and peat samples for physical, chemical, mineralogical and isotopic analyses. J. Environ. Monit. 2004, 6, 481-492.
William Shotyk,* Andriy K. Cheburkin
Nicolas
Givelet,
Institute of Environmental Geochemistry University of Heidelberg INF 236 D-69120 Heidelberg, Germany
Michael E. Goodsite Environmental Chemistry Research Group Department of Chemistry University of Southern Denmark Odense University, Campusvej 55 Odense M, Denmark
Fiona Roos-Barraclough Zentralstrasse 68 8212-Neuhausen am Rheinfall, Switzerland ES040107C
and
