THE CHRONOLOGY OF ANTHROPOGENIC, ATMOSPHERIC Pb DEPOSITION RECORDED BY 3 MINEROGENIC PEATLANDS FROM SWITZERLAND

William Shotyk (shotyk@geo.unibe.ch) Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland tel +41 (31) 631 8761 FAX 631 4843

ABSTRACT

Peat cores were collected from 3 minerogenic peatlands in Switzerland: La Tourbière des Genevez (TGE), in Canton Jura; Gola di Lago (GDL), in Canton Ticino, and Mauntschas (MAU), in Canton Grisons. Chemical analyses of the pore waters and the peats document the increasingly minerogenic character with depth. Despite this circumstance, Pb concentrations are greatest in the surface layers, and decrease with depth. Thus, dissolution of the basal sediments at these sites appears to be an unimportant source of Pb, compared with atmospheric deposition. Twenty-one new radiocarbon age dates for the 3 cores provide a chronology of Pb enrichment, and these are consistent with the record of atmospheric Pb deposition recorded by the ombrotrophic peat bog "Etang de la Gruère" (EGR). Minerogenic peat deposits such as those described here, therefore, can serve as reliable archives of atmospheric Pb deposition, provided that mineral dissolution in the underlying sediments does not contribute measurably to the Pb inventory of the peat profile.

INTRODUCTION

There are two principal kinds of peatlands: minerogenic peat deposits such as fens, swamps, and marshes which are fed mainly by groundwater, and ombrogenic bogs which are nourished exclusively by rainwater and dust (Clymo, 1987). The inorganic solids in minerogenic peatlands are ultimately derived from rock-water interactions (mainly mineral dissolution reactions) in surrounding soils and underlying sediments (Shotyk, 1988). In a raised bog, the surface peat layers of the dome are well above the elevation of surrounding mineral soils, and elements are supplied only by the atmosphere: lithogenic elements such as Al, Ti, and Fe primarily by atmospheric soil dust, nutrients such as N, P, S, Cl, and Se by rainwater, and “heavy metals” such as Pb, Cd, Hg, and As by aerosols released by various anthropogenic activities. Recent studies have shown that Pb deposited on the surface of ombrogenic bogs is effectively immobilized (Shotyk et al., 1997; Weiss et al., 1999a,b), allowing peat bogs to be used as archives to reconstruct historical records of atmospheric Pb deposition (Shotyk et al., 1998).

Unfortunately, raised bogs are thought to be restricted by climate to areas of abundant atmospheric precipitation, low evaporation, and (mainly) cool temperatures (Moore and Bellamy, 1974). One consequence of geography and climate is that ombrogenic bogs are not always present where they are sought to serve as archives of atmospheric metal pollution. Also, in central and eastern Europe, many countries have lost 90-95% of their raised bogs due to drainage and development. If minerogenic peat deposits could be used as archives of atmospheric metal deposition, it would significantly broaden our understanding of the geographic distribution of atmospheric metal pollution.

During the course of our studies of Swiss peat bogs, we have measured Pb concentrations in many peat deposits, some of which are minerogenic. The shapes of these Pb concentration profiles suggest that atmospheric sources of Pb to the peat deposits are quantitatively more important than Pb supplied by fluid-mineral interactions. The main goal of the present study is to examine the Pb inventories in three of these peat profiles in more detail, and to compare their chronologies of Pb accumulation with the record of atmospheric Pb deposition recorded by the ombrotrophic peat bog at Etang de la Gruère (Shotyk et al., 1998).

MATERIALS AND METHODS

Description of the peat deposits, sample collection, and preparation

The minerogenic peat deposits selected for study are “La Tourbière des Genevez” in Canton Jura (Shotyk, 1996), “Gola di Lago” in Canton Ticino (Shotyk 1996) and “Mauntschas” in Canton Grisons (Shotyk et al., 2000). The top ca. 100 cm of each bog was collected and the samples prepared as described previously (Shotyk, 1996).

Preparation and analyses of pore waters

Selected samples of fresh peat were placed in plastic bags which were squeezed by hand to express the porewater. All water samples were filtered and analyzed using ion chromatography for major element anions and cations as described previously (Shotyk, 1996).

Chemical analyses of solid peat samples

Selected trace elements (K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Br, Rb, Sr, Y, Zr and Pb) were measured using the Energy-dispersive Miniprobe Multielement Analyzer (EMMA) with Mo Kâ (energy of 19.6 KeV) as the exciting wavelength (EMMA Analytical Inc., Elmvale, Ontario, Canada). The instrument was calibrated for Pb using the following certified standard reference plant materials (with Pb concentrations in ìg/g): NIST SRM 1547 Peach Leaves (0.87); NIST SRM 1575 Pine Needles (10.8); BCR 62 Olive Leaves (25.0); BCR 60 Aquatic Plant (63.8). The lower limits of detection (LLD) for Sr, Zr, and Pb were 0.6, 2.5, and 0.6 µg/g, respectively. Due to the lack of suitable SRMs for Y and Zr in plant material, the calibration for these elements was performed using liquid AAS standards as described in detail elsewhere (Shotyk et al., 2000). The accuracy of the EMMA XRF analyses of Sr and Zr was determined by analyzing South African Coal SARM 19 and 20 and was found to be +4.7% Sr and +10.4% Zr.

Age dating of peats using ¹⁴C

A number of peat samples were dated using ¹⁴C decay counting (Physics Institute, University of Berne) as described elsewhere (Shotyk et al., 1998).

Sediment leaching experiments

Four g of each sediment sample was reacted with 40 mls of 2M NH₄Ac (pH 4.96) made using trace metal grade CH₃COOH and NH₄OH (Merck Suprapur) and diluted with 18 MÙ deionized water. Samples were shaken intermittently until there was no further, visible reaction, then allowed to sit overnight. Samples were vacuum filtered through 0.2 ìm polysulfone membrane filters (Gelman) and the filtrate analyzed for Pb and Sr using ICP-OES. The residue was collected, dried, and selected trace elements using the Energy-dispersive Miniprobe Multielement Analyzer (EMMA) as described earlier.

RESULTS AND DISCUSSION

Distinguishing ombrogenic from minerogenic peats

Unlike EGR which is ombrogenic, the porewaters at TGE, GDL, and MAU all contain significantly higher concentrations of Ca compared with rainwater values. Average Sr concentrations in the peats are 19.4 ± 12.7 µg/g at TGE, 41.3 ± 14.8 µg/g at GDL, and 197.9 ± 72.0 µg/g at MAU (Figure 1). For comparison, in the ombrogenic bog at EGR, the uppermost 100 cm averages 6.8 ± 2.0 µg/g Sr. Taken together, these results demonstrate quite clearly that TGE, GDL, and MAU are minerogenic.

Pb concentration profiles

The Pb concentrations in the minerogenic peat profiles (TGE, GDL, and MAU) are greatest in the surface and subsurface, and decrease with depth. By far the highest concentrations of Pb are found at GDL which is south of the Alps and strongly influenced by industrial activity in northern Italy (Weiss et al., 1999a).

Figure 1 Sr concentration profiles at EGR which is ombrogenic, compared with the three minerogenic sites: TGE, GDL, and MAU.

Pb enrichments

To take into account the variations in amount of mineral material on the Pb concentration profiles, Pb concentrations were normalized to Ti. This procedure yields the metal Enrichment Factor which was calculated as

EF = ([Pb]/[Ti])_peat / ([Pb]/[Ti])_{Earth`s crust}

where [Pb] refers to the total concentration of Pb measured in the peat sample (µg/g) and [Ti] to the total concentration of Ti. The calculated EFs indicate the extent of Pb enrichment in the peats, relative to the abundance of the Pb in the Earth`s crust where Pb = 14.8 µg/g and Ti = 4010 µg/g (Wedepohl, 1995). These calculations fail to reveal significant Pb enrichment in peats older than ca. 3000 ¹⁴C yr BP in any of the peat profiles. In fact, significant Pb enrichments are found only in peat samples younger than this time.

Chronology of Pb enrichments

The radiocarbon age dates show that the Pb enrichments in the peat profiles, although found at varying depths, have similar chronologies (Figure 2). These profiles are in good agreement with the historical record of atmospheric Pb deposition in the ombrogenic bog at EGR (Shotyk et al., 1998), and reveal the distinct Pb enrichment corresponding to the Roman Period (ancient lead mining) as well as that of the Medieval Period (from German silver mining). The present study is devoted to Pb enrichments in the deepest, oldest peat layers, thus the chronologies of Pb enrichments at the tops of the cores are not shown.

Figure 2 Pb EF (Ti, crust) at the ombrogenic peat bog EGR, and at the three minerogenic sites TGE, GDL, and MAU. All age dates shown are given as conventional radiocarbon years BP.

Sediment dissolution experiments

Sediment samples taken from 3 depths below the peat deposit at TGE appear to show differences in abundance of carbonate minerals. In samples from 169-171 cm and 185-187 cm, there is no reaction with 10% HCl; however, the deepest layer sampled (197-199 cm) effervesced strongly. Chemical analyses of the untreated sediments show that the deeper, unweathered layer contains more than 10% by weight Ca compared with less than 0.4 % Ca in the overlying, leached layers. Assuming that these sediments have a common origin, the differences suggest that in situ chemical weathering of the sediments has led to a preferential dissolution of the carbonate minerals in the uppermost sediment layers. The strong enrichment of Zr in the naturally weathered layers (ie. the samples from 169-171 cm and 185-187 cm which do not react with HCl) compared with fresh sediment, and the loss of 95% of the Mn, supports this interpretation. After reacting with NH₄Ac, there is a considerable loss in Ca from all samples.

In contrast, there is no significant difference in the Pb concentrations of unweathered sediment at TGE (197-199 cm) before (24.6 Pb µg/g) or after (25.9 µg/g) reaction with NH₄Ac; this suggests that the reactive, carbonate minerals are not an important host phase for Pb. In the sediments from EGR, reaction with NH₄Ac certainly removes a large part of the Ca, but Pb is not significantly affected.

SUMMARY

Some simple lab experiments have shown that carbonate dissolution dominates the weathering of the sediments underlying three minerogenic peat deposits (TGE, GDL, and MAU), but this fraction does not contain significant concentrations of Pb. In effect, the entire inventory of Pb in these peat profiles can be explained by atmospheric deposition alone. The chronology of Pb enrichment in each of these cores is very similar to that published previously for the ombrogenic bog at EGR (Shotyk et al., 1998). Minerogenic peat deposits such as those described here, therefore, can serve as reliable archives of atmospheric Pb deposition, provided that mineral dissolution in the underlying sediments does not contribute measurably to the Pb inventory of the peat profile.

ACKNOWLEDGEMENTS

Financial support from the Swiss National Science Foundation (Grant 21-55669-98) is sincerely appreciated. Dr. Andriy Cheburkin of EMMA Analytical Inc., Elmvale, Ontario, designed and built the miniprobe XRF analyzer, and performed all of the measurements of trace metals in the peat samples. Philipp Steinmann, Martin Otz, and Dominik Weiss helped with peat sample preparation and pore water analyses. Steve Reese of the Radiocarbon Lab, University of Berne, measured all of the ¹⁴C age dates.

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