Peat bog archives of atmospheric metal deposition: geochemical evaluation of peat profiles, natural variations in metal concentrations, and metal enrichment factors

Abstract

The surface layers in ombrotrophic peat bogs are fed solely by atmospheric deposition; there is no chemical influence by groundwaters and other mineral soil waters. Bogs that have remained in this condition for the past hundreds or thousands of years may act as archival repositories of a wide variety of atmospheric constituents. Thus, peat cores taken from such bogs have the potential to provide detailed records of the changing rates of atmospheric metal deposition. However, not all bogs are appropriate for this type of investigation and the emphasis of this paper is on the geochemical factors that must be considered to select proper sites. Specifically, the objectives of this review are to (i) outline some of the geochemical characteristics that can be used to establish the existence of an ombrotrophic zone and illustrate the importance of this condition, (ii) provide examples of variations in bulk density and mineral matter contents and show how this can affect metal concentration profiles, and (iii) explain how natural variations in metal concentrations can be separated from anthropogenic inputs. Examples are taken mainly from two Sphagnum bogs in the Jura Mountains of Switzerland. At La Tourbière des Genevez (TGe), the geochemical properties of both the solid and aqueous phases indicate that there exists only a thin veneer of ombrotrophic peat, extending no deeper than 20 cm and representing less than 50 years of peat accumulation; the remainder of the profile is essentially minerotrophic. Here there is a striking natural As enrichment with increasing depth, which clearly shows the great danger in using inappropriate peat samples (such as those from peatlands that are mainly minerotrophic and whose metal budgets may be dominated by mineral-water interactions and groundwater flow patterns) for studying atmospheric processes. At Étang de la Gruyère (EGr), the geochemical data reveal an ombrotrophic layer that extends down to 250 cm and is estimated to represent 5000 years of peat accumulation. The uppermost 102 cm of this profile (core 2f) was studied in some detail and found to represent 2110 ± 30 years of peat formation. This core preserves the changing record of atmospheric Pb deposition since the Roman period. Normalizing metal concentrations to Sc provides an effective means to correct for variations in bulk density and ash content and to separate natural from anthropogenic metal inputs. In core 2f the minimum Pb/Sc ratios are at least seven times higher than the crustal ratio, showing that human activity has had a significant effect on the atmospheric Pb fluxes throughout the past 2110 years. The background Pb concentrations at EGr are found only at depths below 150 cm. The results from this profile emphasize the need to examine complete, ombrotrophic peat cores, not only to identify possible natural sources of the metals to the peat but also to quantify the true impact of human activity on atmospheric metal deposition.Key words: heavy metals, atmospheric deposition, peat cores, ombrotrophic bogs, historical records, Pb.