FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

decomposition of organic matter
The bulk density of the samples at 47 and 21 cm in the GL profile differ only by a
factor of 1.7x (Fig. 2a), in contrast with the differences in Hg concentrations (15x). We assume
that the bulk density of a given type of peat is a reflection of the extent of its decomposition
(degree of humification). Support for this assumption comes from a detailed spectroscopic
characterisation of organic matter in the Swiss peat bog profile at Etang de la Gruère (Cocozza
et al., 2003). Bulk density, therefore, helps to take into account any changes in metal
concentrations which take place during the decomposition of organic matter. In a recent paper
about Hg accumulation rates in peat cores from Patagonia, Biester et al. (2003) suggested that
bulk density is not an adequate parameter to express changes in peat humification, and that Hg
accumulation rates (calculated as we have described here) should be corrected for humification
to take into account mass loss during decay. However, in the GL peat profile, the difference in
bulk density at 47 versus 21 cm (1.6x) is similar to the difference in Zr concentrations (1.7x).
Zirconium is a conservative element which resides almost exclusively in zircon, and this element
(along with other conservative trace metals) should increase in concentration with increasing
extent of peat decay. In fact, the differences in Zr concentrations are comparable to the
differences in bulk density; taken together these data suggest that the differences in humification
alone cannot explain differences in Hg concentrations by more than a factor of two. As a
consequence of these data and arguments, it is unlikely that physical processes such as organic
matter decomposition can explain the magnitude of the variation in Hg concentrations with depth
in the GL profile.
redox-related processes in the oxic zone
It has been suggested that redox-related transformations of Fe and Mn in marine and
lacustrine sediments may contribute to Hg enrichments in the surface layers of these sediments.
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