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TitleIncreased organic-bound mercury in the sediments of coastal Arctic lakes during Medieval and modern warm periods: Climate as a significant factor in sediment mercury concentrations
AuthorOutridge, P; Stern, G A; Goodsite, M E; Hamilton, P; Sanei, H
SourceArcticNet (ASM2013), programme ; 2013 p. 91
Alt SeriesEarth Sciences Sector, Contribution Series 20130190
MeetingArcticNET Annual Scientific Meeting; Halifax; CA; December 9-13, 2013
Mediapaper; on-line; digital
File formatpdf
ProvinceNunavut; Northwest Territories
AreaDevon Island; Mackenzie Delta Lake
Subjectsenvironmental geology; geochemistry; mercury; mercury geochemistry; algae
Programenvironmental impacts and adaptation in the northern environment, Environmental Geoscience
LinksOnline - En ligne
AbstractThe 'algal scavenging' hypothesis was proposed to explain high correlations found between the concentrations and fluxes of total mercury (Hg) and algal productivity indicators (diatom abundance, 'S2' organic carbon) in the sediments of northern lakes. Historical variations in these parameters over recent centuries matched reconstructed Arctic air temperatures, suggesting a mutual underlying climate effect. If this hypothesis is true, then recent climate warming must be taken into account when attributing the causes of recent sediment Hg increases. Subsequently, many research groups looked for evidence of this effect in other lakes; similarly strong correlations have been reported in >20 pan-Arctic lakes but not in others. However, correlations ultimately have limited power to test hypotheses.

Here we report new evidence from Hg speciation studies (sequential chemical extractions) of sediments in several Canadian Arctic lakes. In one shallow coastal lake on Devon Island, NU, the concentrations of organic matter-bound (i.e. oxidizable) Hg were significantly higher (P<0.001) during the early 15th century (i.e., Medieval Warm Period) than in sediments deposited before and afterwards during cooler periods. Other Hg fractions (mineral, acid-extractable, water-soluble) mostly did not change. In this lake, virtually all of the variation in pre-industrial total Hg concentrations over 500 years was explained by organic-bound Hg (r2=0.99, p<0.001, N=9). An identical result (r2=0.99, p<0.001, N=7) was found in a shallow Mackenzie Delta lake, NWT, over 1,500 years. In both lakes, the variation in organic-Hg was significantly correlated with the algal proxy S2 carbon. During the 20th century in 3 lakes, significant increases occurred in the organic-bound Hg fraction compared to the pre-industrial period, in tandem with increased S2 carbon, diatoms and Arctic air temperatures. Most of the increase in 20th century total Hg concentrations could be explained by the increase in organic-bound Hg and, in turn, by algal biomass. These findings support the scavenging hypothesis, whereby major changes in phytoplankton abundance in a lake's water column may alter sedimentary Hg concentrations. Rapid absorption of available Hg by living algae, or adsorption onto the reactive surfaces of algal detritus and other types of particulate organic matter, followed by their sedimentation is the most likely mechanism.

Human-derived Hg pollution is undoubtedly present in the Arctic. However, these findings indicate that northern lake sediments are not simply recorders of atmospheric Hg deposition, but also record (in Hg) a climate signal. Therefore, interpretation of the 20th century Hg increases as being solely due to long-range pollution, and calculating pollutant Hg fluxes based on these increases, is incorrect. Statistically accounting for the climate effect leads to the conclusion that the atmospheric input of Hg pollution to these lakes is considerably smaller than previously reported.
Summary(Plain Language Summary, not published)
This study indicates that climate change in northern Canada, both during the Medieval Period and recently, has affected the rate of incorporation of mercury into lake sediments. Mercury is a potentially toxic metal of great concern to northern communities. By looking at how mercury is bound up in sediments and how that changes with climate, we were able to show that lake sediments are not simply recorders of airborne mercury deposition, but also record (in mercury) a climate signal. So, interpretation of the recent major increases of mercury in sediments as being only due to local or long-range pollution is incorrect.