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Trace metal profiles in a varved Arctic lake sediment

Geochimica et Cosmochimica Acta
DOI: 10.1016/j.gca.2004.04.010
  • Biology
  • Earth Science
  • Ecology
  • Geography


Abstract Varved (annually-laminated) sediments offer a rare and physically undisturbed archive of past trace metal deposition and limnological conditions. Here, a high-resolution 1,000+ year record of metal accumulation is presented from a varved lake sediment in the Canadian High Arctic. These data are especially relevant to the debate about whether lake sediments in the remote Arctic preserve atmospheric metal deposition histories. Down-core metal profiles were significantly correlated (P < 0.01) with organic C (Cd, Zn) or with leachable Fe (Cu), while distinct sub-surface peaks of Cd and Cu and, to a lesser extent, Zn coincided with those of Fe, S and other redox-sensitive elements such as Co, Cr and U. Furthermore, the profiles of Cd, Cu and Zn did not match their record in the Greenland ice sheet over the last century nor short- and long-term industrial emission trends. These facts suggest that their sedimentary profiles are more strongly influenced either by an association with organic matter inputs, or by post-depositional migration (diagenesis), than by atmospheric deposition. In contrast, the Hg concentration and flux profiles were strongly correlated with total diatom abundance during recent centuries, particularly during the 20 th Century when a two-fold Hg increase and a four orders-of-magnitude diatom increase occurred in tandem. A similar correspondence between Hg and diatom trends was found in a second lake nearby, confirming that the relationship was not unique to the main study lake. Recent Hg increases in other Arctic and sub-Arctic lakes have been attributed to global anthropogenic Hg emissions. We propose an alternative hypothesis for High Arctic lakes: the elevated sediment Hg may be due to increased scavenging of dissolved Hg from the water column caused by markedly greater diatom productivity, which in turn is being driven by climate warming that began in the late-18 th Century and accelerated in the 20 th 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.

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