doi:10.1016/j.gca.2005.06.009 Trace metal profiles in the varved sediment of an Arctic lake P. M. OUTRIDGE, 1,2, * G. A. STERN, 2 P. B. HAMILTON, 3 J. B. PERCIVAL, 1 R. MCNEELY, 1 and W. L. LOCKHART 2 1 Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada 2 Fisheries and Oceans Canada, 501 University Crescent, Winnipeg R3T 2N6, Canada 3 Canadian Museum of Nature, P.O. Box 3443 Station D, Ottawa K1A 6P4, Canada (Received January 18, 2005; accepted in revised form June 8, 2005) Abstract—Varved (annually-laminated) sediments offer a rare and physically undisturbed archive of past trace metal deposition and limnological conditions. Here, a high-resolution 1,300 year record of metal accumulation is presented from a varved lake sediment on Devon Island in the Canadian High Arctic. Down-core concentration profiles of Cd, Cu and Zn were positively correlated (P  0.01) with organic C (Cd, Zn) or with leachable Fe (Cu), while distinct sub-surface peaks of these metals coincided with those of Fe, S and other redox-sensitive elements such as Co, Cr and U. The fluxes of these metals since 1854 were correlated with elements such as Ca, Al and La (P  0.001) which are predominantly of local geological origin. Furthermore, the Cd, Cu and Zn patterns did not match concurrent records in Greenland Summit ice over the last century, nor global industrial emission histories. These facts suggest that inputs from local geological sources, coupled with some degree of post-depositional mobility or association with organic matter inputs, explain the metals’ sedimentary profiles, which were apparently not affected by long-range atmo- spheric metal pollution. Mercury concentrations were strongly correlated with total diatom abundance over the last 400 yrs, especially during the 20th Century when a two-fold increase in Hg concentrations and a four order-of-magnitude increase in diatoms occurred in tandem. Since 1854, 81% of the variation in Hg flux was associated with diatom and Ca fluxes. A similar correspondence between Hg and diatoms was found in a second lake nearby, confirming that the relationship was not unique to the main study lake. Recent Hg increases in Arctic and sub-Arctic lakes have been attributed to global anthropogenic Hg emissions. We propose an alternative hypothesis for High Arctic lakes: the recent Hg increases may be partly or entirely the product of elevated rates of Hg scavenging from the water column caused by markedly greater algal productivity, which in turn was driven by accelerating climate warming during the 20th Century. Given the important environmental assessment and policy implications if the alternative hypothesis is true, the possible effects of climate warming on sedimentary Hg fluxes in this region deserve further study. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION The reliability of lake sediments in recording and preserving the history of atmospheric metal concentrations and deposition is a continuing scientific issue with widespread environmental policy implications. Because lake sediments are widely used to reconstruct temporal changes in metal pollution, the data de- rived from them has had considerable influence in shaping environmental assessments and regulatory actions, especially in Arctic regions where instrumental records of atmospheric metal deposition are sparse (see AMAP 1998, 2002; INAC 2003). However, the interpretation of sedimentary metal profiles in most lakes as an unambiguous record of atmospheric metal deposition has been contested, due to the possibility of post- burial remobilization of metals resulting from diagenesis: phys- ical mixing by wind and wave action, compaction, bioturbation and irrigation and, in extremely low sedimentation environ- ments, diffusive migration of dissolved metals as a result of redox-related concentration gradients (Boudreau 1999; Boyle 2001). Elements which have been demonstrated to undergo redox-mediated migration in lake sediments include As, Co, Cr and V, although it is unclear whether other environmentally- important elements such as Hg, Cd, Cu, Pb and Zn are affected (Boyle 2001). For temperate North America, confidence in the reliability of sediments as metal archives has been enhanced by various forms of corroborative evidence, including findings that sedi- mentary profiles of Hg agree well with known fluvial Hg input histories (Frazier et al. 2000; Lockhart et al. 2000), by model- ling studies which suggest that significant metal remobilization is limited to lakes with very low sediment accumulation rates (Boyle 2001), by Hg trends in a glacier ice core in the western U.S.A. that match continent-wide trends in lake sediment cores (Schuster et al. 2002), and by good agreement between sedi- ment-derived atmospheric Hg flux data and actual measure- ments (Swain et al. 1992). However, at Arctic latitudes, similar corroborative studies are lacking. Furthermore, Arctic lakes often have lower sediment accumulation rates and organic matter contents than many temperate lakes (Gajewski et al. 1995; Engstrom and Swain 1997), conditions which may favour metal remobilization. Extremely low sedimentation rates were the probable underlying cause behind diagenetic redistribution of Hg in Arctic marine sediments (Gobeil et al. 1999). Another potential problem with respect to sediments as chemical archives is the largely-untested assumption that the transport processes of metals from atmosphere to sediments via the catchment and water column have not changed in recent * Author to whom correspondence should be addressed (outridge@ nrcan.gc.ca). Geochimica et Cosmochimica Acta, Vol. 69, No. 20, pp. 4881– 4894, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 4881