CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on Climate Change and Mercury Accumulation in Canadian High and Subarctic Lakes
T
he hypothesis that recent climate warming has significantly increased mercury (Hg) fluxes to Arctic lake sediments through increased algal scavenging of Hg from the water column 1 3 was recently evaluated by Kirk et al.,4 who examined relationships between sedimentary Hg and “S2” carbon profiles in 14 Arctic and sub-Arctic lakes. S2 carbon has been used as a proxy of algalderived organic matter (OM) concentrations in northern Hg study lakes,2,3,5 7 and is analyzed with the Rock-Eval 6 technique which provides a thermolytic measure of plant kerogens. Kirk et al. found no association between Hg and S2 in half of the lakes, and attributed significant correlations in the other half to spurious artifacts. The scavenging hypothesis requires rigorous testing because of its potential importance for Hg science and policy issues in northern regions. However, the evaluation by Kirk et al. falls short of the required rigor due to errors in calculation and incorrect interpretation of Rock-Eval data. Furthermore, their conclusion that no correlation implies no association is based on the unsupportable premise that the presence (or absence) of a correlation between Hg and S2, without corroborating evidence, is definitive evidence for the presence (or absence) of increased algal Hg scavenging. This premise misrepresents previous work. First, all plants contain organic moieties that volatilize as S2 carbon.8,9 Thus, S2 compounds are not uniquely algal, as stated by Kirk et al., although they are present in higher concentrations in algae.10 The actual algal contribution to S2 requires corroboration by organic petrology, fossil algae enumeration (diatoms are one proxy) or other indicators. In some High Arctic lakes, such as Amituk and DV-09 in Nunavut,2 or “H2” on Svalbard,7 S2 is effectively equivalent to algal OM because other plant sources are negligible. Jiang et al.,7 who compared sediment S2 profiles with other algal OM proxies including diatom abundance, C/N ratios, and specific pigments, concluded that all gave reliable, corroborative measures of increasing algal productivity. But depending on the complexity of a lake’s organic depositional environment (OM sources, types and fluxes), S2 in other Arctic and particularly in sub-Arctic lakes may be significantly influenced by inputs from other plant types. In settings like these, sedimentary S2 alone is not a definitive indicator of algal productivity, and must be corroborated with other proxies (e.g., refs 3,5,6). Second, the formula presented in Kirk et al. to calculate residual carbon (RC) is incorrect (see 8 for correct formula). This error affects interpretation of S2/RC as an indicator of OM decomposition, which featured in Kirk et al.’s discussion. Third, residual carbon does not exclusively represent “allochthonous inputs of organic matter”. In fact a significant fraction of fresh algal OM is refractory, and will contribute to RC yield during Rock-Eval analysis. Petrology is a powerful tool to distinguish plant sources (e.g., refs 3,5,6), but was not employed by Kirk et al. Fourth, the type of kerogen in recent sediments as represented by Hydrogen Index and Oxygen Index plots (a Van Krevelen diagram) is not uniquely controlled by the input of terrestrial r 2011 American Chemical Society
OM. Total OM in some oligotrophic High Arctic lakes may be erroneously classified as Type III (terrestrial) kerogen due to low algal productivity and oxidation of this OM within the water column prior to its sedimentation. Again, petrology and other proxies will help to improve interpretation. Lastly, S2/RC profiles were used simplistically as an OM decomposition indicator, leading to the conclusion that significant Hg S2 correlations were an artifact caused by the coincidence of down-core OM decomposition and upward Hg increase due to modern anthropogenic inputs. But the reported scale of TOC burn-down in northern lake sediments appears to be an order of magnitude smaller than the 270 310% increase in S2 carbon levels since ∼1850 in Canadian Arctic lakes;2 therefore, most of the S2 increase probably resulted from elevated algal productivity.2,3,5 7 G€alman et al.11 reported a TOC loss of about 23% over 27 years in a Swedish lake sediment, but most of this (20%) occurred within the first five years after deposition, presumably in the oxic upper sediments, with negligible degradation thereafter. Crucially, TOC or S2 loss alone cannot disprove the scavenging hypothesis. The key parameter is the amount of algalderived OM. Even if one discounts S2 data, there is abundant evidence that algal productivity has increased in northern lakes, and increasing sediment Hg profiles in the Arctic are significantly correlated with other algal OM proxies including diatom abundance,1 3,6,7 organic petrography indicators,3,5,6 and algal pigments.7 Indeed, Kirk et al.4 (p 964) concurred that “changes in algal community assemblage ... likely reflect increased lake primary productivity”. OM decomposition near the present top of sediment columns also does not account for significant Hg:S2 correlations deeper in certain sediment cores, corresponding to naturally enhanced algal productivity during different preindustrial periods. For example, fluctuations in Hg, S2 and diatom concentrations occurred in tandem during long-term geomorphological evolution of Nesbitt Lake, Northwest Territories,6 and during the Medieval Warm Period in Lake DV-09, Nunavut, the smallest and most climate-sensitive of the Hg study lakes.2 A further criticism by Kirk et al. related to the use of total diatom abundance versus relative algal abundance. Diatom enumeration is a supplementary proxy of total algal density and productivity.1 3,6,7 Diatoms and Cyanoprokaryota are the most common algal groups in Arctic freshwaters.12 14 Absolute diatom abundance data can be readily tested statistically against S2 and Hg, whereas relative abundances are more problematic because the proportion of any one taxon is influenced by the varying abundances of other taxa (e.g., see cautions by Filzmoser et al.15 regarding proportional data). Diatoms also may preserve better in many freshwater sediments than other algal fossils and so give a more accurate picture of historical algal densities. Thus,
Published: July 08, 2011 6703
dx.doi.org/10.1021/es2014709 | Environ. Sci. Technol. 2011, 45, 6703–6704
Environmental Science & Technology to date no evidence has been presented to show that total diatom abundance significantly misrepresents changes in total algal productivity. P. M. Outridge,†,‡,* H. Sanei,‡,§ G. A. Stern,‡,|| M. Goodsite,^ P. B. Hamilton,# J. Carrie,‡ F. Goodarzi,§ and R. W. Macdonald‡,3 †
Geological Survey of Canada, Ottawa, Canada;
‡
Department of Environment and Geography, University of Manitoba, Winnipeg, Canada; Geological Survey of Canada, Calgary, Canada;
)
§
Department of Fisheries and Oceans, Winnipeg, Canada;
^
Department of Atmospheric Environment, NERI Aarhus University, Roskilde, Denmark;
#
Canadian Museum of Nature, Ottawa, Canada; Department of Fisheries and Oceans, Sidney, Canada
3
CORRESPONDENCE/REBUTTAL
Brooks, S. J.; Fallu, M.-A.; Hughes, M.; Keatley, B. E.; Laing, T. E.; Michelutti, N.; Nazarova, L.; Nyman, M.; Paterson, A. M.; Perren, B.; Quinlan, R.; Rautio, M.; Saulnier-Talbot, E.; Siitonen, S.; Solovieva, N.; Weckstr€om, J. Climate-driven regime shifts in the biological communities of Arctic lakes. Proc. Natl. Acad. Sci. 2005, 102, 4397–4402. (13) Gajewski, K.; Hamilton, P. B.; McNeely, R. A high resolution proxy-climate record from an arctic lake with annually-laminated sediments on Devon Island, Nunavut, Canada. J. Paleolimnol. 1997, 17, 215–225. (14) Douglas, M. S. V.; Hamilton, P. B.; Pienitz, R.; Smol, J. P. Algal indicators of environmental change in Arctic and Antarctic lakes and ponds. In Long-Term Environmental Change in Arctic and Antarctic Lakes; Pienitz, R; Douglas, M; Smol, J,Eds.; Springer: The Netherlands, 2004; Vol. 8, pp 117 157. (15) Filzmoser, P.; Hron, K.; Reimann, C. Univariate statistical analysis of environmental (compositional) data: Problems and possibilities. Sci. Total Environ. 2009, 407, 6100–6108.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +1 (613) 996-3958; e-mail:
[email protected].
’ REFERENCES (1) Outridge, P. M.; Stern, G. A.; Hamilton, P. B.; Percival, J. B.; McNeely, R.; Lockhart, W. L. Trace metal profiles in the varved sediment of an Arctic lake. Geochim. Cosmochim. Acta 2005, 69, 4881–4894. (2) Outridge, P. M.; Sanei, H.; Stern, G. A.; Hamilton, P. B.; Goodarzi, F. Evidence for control of mercury accumulation rates in Canadian High Arctic lake sediments by variations of aquatic primary productivity. Environ. Sci. Technol. 2007, 41, 5259–5265. (3) Stern, G. A.; Sanei, H.; Roach, P.; Delaronde, J.; Outridge, P. M. Historical interrelated variations of mercury and aquatic organic matter in lake sediment cores from a subarctic lake in Yukon, Canada: Further evidence toward the algal-mercury scavenging hypothesis. Environ. Sci. Technol. 2009 43, 7684 7690. (4) Kirk, J. L.; Muir, D. C. G.; Antoniades, D.; Douglas, M. S. V.; Evans, M. S.; Jackson, T. A.; Kling, H.; Lamoureux, S.; Lim, D. S. S.; Pienitz, R.; Smol, J. P.; Stewart, K.; Wang, X.; Yang, F. Climate change and mercury accumulation in Canadian High and Subarctic lakes. Environ. Sci. Technol. 2011, 45, 964–970. (5) Carrie, J.; Wang, F.; Sanei, H.; Macdonald, R. W.; Outridge, P. M.; Stern, G. A. Increasing contaminant burdens in an Arctic fish, burbot (Lota lota), in a warming climate. Environ. Sci. Technol. 2010, 44, 316–322. (6) Sanei, H.; Outridge, P. M.; Dallimore, A.; Hamilton, P. B. Mercury organic matter relationships in pre-pollution sediments of thermokarst lakes from the Mackenzie River Delta, Canada: the role of depositional environment. Biogeochem. 2010, DOI: 10.1007/s10533010-9543-1. (7) Jiang, S.; Liu, X.; Chen, Q. Distribution of total mercury and methylmercury in lake sediments in Arctic Ny-Ålesund. Chemosphere 2011, 83, 1108 1116. (8) Lafargue, E.; Marquis, F.; Pillot., D. Rock-Eval 6 applications in hydrocarbon exploration, production and soil contamination studies. Rev. Inst. Fr. Pet. 1998, 53, 421–437. (9) Tyson, R. V. Sedimentary Organic Matter: Organic Facies and Palynofacies; Chapman and Hall: London, 1995. (10) Taylor, G. H.; Teichm€uller, M.; Davis, A.; Diessel, C. F. K.; Littke, R.; Robert, P. Organic Petrology; Gebr€uder Borntraeger: Berlin, 1998; p 704. (11) G€alman, V.; Rydberg, J.; de-Luna, S. S.; Bindler, R.; Renberg, I. Carbon and nitrogen loss rates during aging of lake sediment: Changes over 27 years studied in varved lake sediment. Limnol. Oceanogr. 2008, 53, 1076–1082. (12) Smol, J. P.; Wolfe, A. P.; Birks, H. J. B.; Douglas, M. S. V.; Jones, V. J.; Korhola, A.; Pienitz, R.; R€uhland, K.; Sorvari, S.; Antoniades, D.; 6704
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