Correspondence Comment on “Atmospheric Mercury Accumulation Rates between 5900 and 800 Calibrated Years BP in the High Arctic of Canada Recorded by Peat Hummocks” A continuing challenge in studies of mercury biogeochemistry is the need to assess natural sources and fluxes and to assess long-term biogeochemical processes. Given the consensus of a global impact of anthropogenic activities on the presentday global cycle (1), it is however difficult to directly quantify the natural component. One approach is to study natural environmental archives (such as peat, lake sediments, and glacial ice), which can provide valuable insights into longterm processes and natural, pre-anthropogenic mercury fluxes. At present there is a particular focus on the peat archive preserved in ombrotrophic (rain-fed) bogs because of their potential for reconstructing “absolute” atmospheric deposition rates. Although there are concerns that the quantitative record of mercury in peat may be affected by peat decomposition (2), there is good coherence among the increasing number of peat studies that suggest a pre-anthropogenic accumulation rate of about 1 µg of Hg m-2 yr-1, which is 10-20 times lower than recent mercury deposition rates. This rate is based on bog and mire sites (2-10) and also on the glacial record (11). The minerogenic peat hummocks investigated by Givelet et al. (12) present an interesting potential for reconstructing mercury accumulation at a high-latitude site on Bathurst Island in the Canadian High Arctic. But although their estimated accumulation rate of 0.5-1.5 µg of Hg m-2 yr-1 agrees with other estimates, there is significant reason to question the inherent viability of these arctic peat cores as reliable geochemical archives of past deposition. As the authors point out, desiccation cracks are a typical feature of arctic peatlands; consequently, the uppermost peat in each core is rejected due to concerns for infilling of old carbon and downward penetration of “modern” mercury. However, the now-frozen peat in deeper layers was itself once located in the active layer and likely subject to the same processes, yet the authors assume that these older layers have been unaffected. More critically, the age-depth modeling of the cores from Bracebridge Inlet (BI) and Museum Station (MS) exemplifies the uncertainty of these cores as reliable archives. Reliable chronologies are important in their own respect, but they are vitally important in the discussed paper as its primary goal is to reconstruct deposition rates; consequently, good care must be taken with the age models. For the BI core, nine levels from the 12 radiocarbon dates in the 90-cm core are used to develop an age-depth model for the 21-cm-thick section (from 14 to 35 cm depth) that the authors consider as actually reliable. This section itself only contains three dated levels, and two of these levels (20.5 and 31.5 cm depth) as well as the next dated level below at 43.5 cm show generally constant ages (BC 3263-2963, BC 3386-3093, and BC 31092878, respectively). This problem of constant radiocarbon ages was the motivation for not considering the peat below 35 cm, yet approximately 70% of the 21-cm section used for the reconstruction also displays a constant age (Figure 1). In contrast to the constant-age problems in the BI core, the MS core exhibits an overlapping radiocarbon stratigraphy; 908
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FIGURE 1. Calibrated ages and δ13C vs depth in the Bathurst Island peat cores (redrawn from ref 12). Sections used in the mercury accumulation reconstructions are marked. In the BI core (top panel), two sections of generally constant radiocarbon ages are circled. The overlapping ages in the MS core (bottom panel) would suggest a plug-like flow of peat (14). here, only 4 of 14 radiocarbon dates are used. Although the authors suggest infilling of older organic material as the likely explanation for the inconsistent ages, a more plausible explanation is the briefly mentioned alternative of perturbation due to permafrost dynamics, namely, slumping, solifluction, or gelifluction of peat material. Summer thawing of the active layer in permafrost areas can produce saturated soils that flow, even on very gentle slopes (as little as 2°; 13). Such a massive sliding of material (i.e., a plug-like flow; 14) can contribute to hummock formation in cold permafrost areas, such as Bathurst Island, and could reasonably explain the discontinuity in the MS core and the overlapping agedepth profile (Figure 1). The presence of peat polygons, fractured blocks, and gullies is evidence of active mass movement in the study area. The δ13C values in the cores also show strong excursions, which seem to occur at the age discontinuities. These changes in isotopic values (up to 6‰) suggest important changes in the peat organic matter, such as vegetation changes or changes in the degree of peat decomposition. For example, during peat decomposition carbohydrates (∼-25 to -24‰) are rapidly degraded and lipids (∼-30‰ or lower) are preferentially preserved so the peat tends to develop more negative δ13C in more decomposed layers (e.g., ref 15). The more degraded peat may have a different behavior when frozen due to its more compact structure and higher water 10.1021/es040100v CCC: $30.25
2005 American Chemical Society Published on Web 12/30/2004
retention, so perhaps these sections move as blocks when solifluction occurs. Given the strong association of mercury with organic matter, the complexity of the 14C and δ13C data suggests that the vertical stratigraphy of mercury in the cores may not only be a function of variations in deposition over time. An intriguing speculation is that the ages of the surfaces of the two mires seem to belong to two distinctive climatic periods: the Medieval Warm Period (MWP) in BI and the Little Ice Age in MS. That the MS peat core suggests an overlapping stratigraphy also presents an interesting question: if this is the result of a plug-flow, why and when did it occur? If the discontinuity in stratigraphy at about 25 cm represents the former surface at the time of the possible event, then this suggests a date in the 13th century (possibly MWP?). The concept of studying arctic peatlands as archives is exciting, but before a proper discussion of mercury accumulation rates in arctic peat cores can be made, it is important that a more thorough evaluation of the validity of these cores as archives be done. While problems with chronology, such as those detailed above, do not inherently exclude the possibility of estimating metal accumulation rates and by inference atmospheric deposition rates from these peat records, the current approach in the Bathurst Island cores does not produce a sufficiently robust age-depth model upon which to base a reconstruction.
Literature Cited (1) Fitzgerald, W. F.; Engstrom, D. R.; Mason, R. P.; Nater, E. A. The case for atmospheric mercury contamination in remote areas. Environ. Sci. Technol. 1998, 32, 1-7. (2) Biester, H.; Martı´nez Cortizas, A.; Birkenstock, S.; Kilian, R. Effect of peat decomposition and mass loss on historic mercury records in peat bogs from Patagonia. Environ. Sci. Technol. 2003, 37, 32-39. (3) Martı´nez-Cortizas, A.; Pontevedra-Pombal, X.; Garcı´a-Rodeja, E.; No´voa Mun ˜ os, J. C.; Shotyk, W. Mercury in a Spanish peat bog: archive of climate change and atmospheric metal pollution. Science 1999, 284, 939-942. (4) Biester, H.; Kilian, R.; Franzen, C.; Woda, C.; Mangini, A.; Scholer, H. F. Elevated mercury accumulation in a peat bog of the Magellanic Moorlands, Chile (53[deg]S)san anthropogenic signal from the Southern Hemisphere. Earth Planet. Sci. Lett. 2002, 201, 609-620. (5) Roos-Barraclough, F.; Martinez Cortizas, A.; Garcia-Rodeja, E.; Shotyk, W. A 14500 year record of the accumulation of atmospheric mercury in peat: volcanic signals, anthropogenic influences and a correlation to bromine accumulation. Earth Planet. Sci. Lett. 2002, 6334, 1-18. (6) Roos-Barraclough, F.; Shotyk, W. Millennial-scale records of atmospheric mercury deposition obtained from ombrotrophic and minerotrophic peatlands in the Swiss Jura mountains. Environ. Sci. Technol. 2003, 37, 235-244.
(7) Bindler, R. Estimating the natural background atmospheric deposition rate of mercury utilizing ombrotrophic bogs in south Sweden. Environ. Sci. Technol. 2003, 37, 40-46. (8) Bindler, R.; Klarqvist, M.; Klaminder, J.; Fo¨rster, J. Does withinbog spatial variability of mercury and lead constrain reconstructions of absolute atmospheric deposition rates from single peat records? The example of Store Mosse. Global Biogeochem. Cycle 2004, 18, GB3020, doi 10.1029/2004GB002270. (9) Givelet, N.; Shotyk, W.; Roos-Barraclough, F. Rates and predominant anthropogenic sources of atmospheric mercury accumulation in southern Ontario recorded by peat cores from three bogs: comparison with natural “background” values (past 8,000 years). J. Environ. Monit. 2003, 5, 935-949. (10) Shotyk, W.; Goodsite, M. E.; Roos-Barraclough, F.; Frei, R.; Heinemeier, J.; Asmund, G.; Lohse, C.; Hansen, T. S. Anthropogenic contributions to atmospheric Hg, Pb and As accumulation recorded by peat cores from southern Greenland and Denmark dated using the 14C “bomb pulse curve”. Geochim. Cosmochim. Acta 2003, 67, 3991-4011. (11) Schuster, P. F.; Krabbenhoft, D. P.; Naftz, D. L.; Cecil, L. D.; Olson, M. L.; Dewild, J. F.; Susong, D. D.; Green, J. R.; Abbott, M. L. Atmospheric mercury deposition during the last 270 years: a glacial ice core record of natural and anthropogenic sources. Environ. Sci. Technol. 2002, 36, 2303-2310. (12) Givelet, N.; Roos-Barraclough, F.; Goodsite, M. E.; Cheburkin, A.; 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, 4964-4972. (13) Matsuoka, N. Solifluction rates, processes and landforms: a global review. Earth-Sci. Rev. 2001, 55, 107-134. (14) Gorbunov, A. P.; Seversky, E. V. Solifluction in the mountains of Central Asia: distribution, morphology, processes. Permafrost Periglaciol. Process. 1999, 10, 81-89. (15) Ficken, K. J.; Barber, K. E.; Eglinton, G. Lipid biomarker, [delta]13C and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia. Org. Geochem. 1998, 28, 217-237.
Richard Bindler* Department of Ecology and Environmental Science Umeå University SE-901 87 Umeå, Sweden
Antonio Martı´nez Cortizas Departamento de Edafologia y Quimica Agricola Universidad Santiago de Compostela (USC) Facultad de Biologia, Campus Sur s/n 15782 Santiago de Compostela, Spain
Maarten Blaauw Centro de Investigacion en Matematicas (CIMAT) A. P. 402 Guanajuato, Gto. C.P. 36000, Mexico ES040100V
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