Environ. Sci. Technol. 2005, 39, 5131-5140
Record of Metal Workshops in Peat Deposits: History and Environmental Impact on the Mont Loze` re Massif, France S . B A R O N , * ,† M . L A V O I E , ‡,§ A . P L O Q U I N , † J. CARIGNAN,† M. PULIDO,‡ AND J.-L. DE BEAULIEU‡ Centre de Recherches Pe´trographiques et Ge´ochimiques, CNRS Nancy, 15 rue Notre Dame des Pauvres, BP 20, 54 501 Vandoeuvre les Nancy, France, and Institut Me´diterrane´en d’EÄ cologie et de Pale´oe´cologie, Case 451, Faculte´ Saint-Je´roˆme, Universite´ d’Aix-Marseille III, Marseille Cedex 20, France
This study aims to document the history of the metallurgical activities on the Mont Loze` re massif in the Ce´ vennes Mountains in Southern France. Many medieval sites of metallurgical wastes (slags) have been reported on the massif. These sites are thought to represent ancient lead workshops. The impact of past metallurgical activity on the environment was studied using geochemical and palynological techniques on a core collected in the Narses Mortes peatland near medieval smelting area. Two main periods of smelting activities during the last 2200 years were revealed by the lead concentration and isotopic composition along the core profile: the first period corresponds to the Gallic period (∼ca. 300 B.C. to ca. 20 A.D.) and the second one to the Medieval period (∼ca. 1000-1300 A.D.). Forest disturbances are associated with lead anomalies for the two metallurgical activities described. The impact of the first metallurgy was moderate during the Gallic period, during which beech and birch were the tree species most affected. The second period corresponds to the observed slag present in the field. Along with agropastoral activities, the medieval smelting activities led to the definitive disappearance of all tree species on the summit zones of Mont Loze` re. The abundance of ore resources and the earlier presence of wood on the massif justify the presence of workshops at this place. The relationship between mines and ores has been documented for the Medieval period. There is no archaeological proof concerning the Gallic activity. Nevertheless, 2500-2100 years ago, the borders of the Gallic Tribe territory, named the Gabales, were the same as the present-day borders of the Loze` re department. Julius Caesar reported the existence of this tribe in 58 B.C. in “De Bello Gallico”, and in Strabon (Book IV, 2.2) the “Gabales silver” and a “treasure of Gabales” are mentioned, but to this day, they have not been found.
Introduction Atmospheric deposition resulting from anthropogenic activity are recorded in natural archives such as ice cores (1, 2), * Corresponding author e-mail:
[email protected]. † Centre de Recherches Pe ´ trographiques et Ge´ochimiques. ‡ Universite ´ d’Aix-Marseille III. § Present address: Centre d’e ´ tudes nordique et De´partement de ge´ographie, Universite´ Laval, Que´bec G1K 7P4, Canada. 10.1021/es048165l CCC: $30.25 Published on Web 06/07/2005
2005 American Chemical Society
peatlands (3-5), marine sediments (6), lake sediments (7, 8), and lichens (9-11). These archives record atmospheric pollution at regional or continental scale. Records at global scale have been reported (12, 13), but very few studies have been conducted at regional scale (14, 15). The knowledge of local history allows more accurate estimation of the sources of the heavy metals accumulated in the environment. In southern France, 60 metallurgical waste (slag) sites have been found on the Mont Loze`re massif suggesting that metallurgical activities occurred in the past (16, 17). Fens are the most common peatlands in southern Europe. This type of peatland is not frequently used to reconstruct changes in past atmospheric deposition because of possible disturbances of the geochemical signal resulting from leaching and groundwaters (18). Although bogs are recognized to be a better geochemical archive, it has recently been shown that fens are also able to record successfully atmospheric deposition without significant distortion (19). This was deduced from the study of ombrotrophic peatlands where the underlying peat was minerotrophic (20, 21). Moreover, West et al. (20) demonstrated that high ash content in peat is not necessarily associated with high lead concentration, suggesting that variations in lead concentrations might not be the result of changes in the mineral fraction sedimentation. As a result, minerotrophic peatlands give the opportunity to establish local and regional environmental reconstructions in areas where bogs are absent. This study presents geochemical and pollen analyses of a core collected in a minerotrophic peatland on the Mont Loze`re massif. Our aim is to document periods of local metallurgical activities during the last millennia and to examine their impact on the long-term forest dynamics. Elemental chemical analyses, lead isotopic compositions, and pollen analyses were used to reconstruct the history of the anthropogenic activities. The lead isotopic signature along the peat core allowed tracing the natural and anthropogenic origins of lead and constraining the role of human activities related to vegetation disturbance observed in the pollen records.
Materials and Methods Studied Area and Site. Mont Loze`re is located in the Ce´vennes National Park (French Massif Central) and is a part of an important mining district. It is a 300 million year old granitic massif, surrounded by various Pb mineralizations. Slag sites are found exclusively on the western part of the massif in an area of 8 km2, in the restricted altitude range of 1340-1430 m (Figure 1) where the plant cover is very poor, likely because of metal pollution. The silicate matrix of slag contains 25% Pb in average (17). All sites show the same slag typology: black slags are often vitreous and brittle, whereas white slags are crystallized and crumbly. These represents the waste of smelting workshop sites for lead and silver making (17). However, silver extraction (used to coin) was not done on the massif as shown by the lack of cupellation wastes on the workshop sites. Nine radiocarbon ages obtained from charcoal (beech) found in archaeological excavations associated with slags gave ages suggesting that metallurgical activities occurred during the Medieval period (17). A recent lead isotope study clarified the relationship between the surrounding old mines and slags on the massif (22): the mines which provided the ores are not the ones close to slags but ones located on the south-southeastern part of the Mont Loze`re massif. The “Narses Mortes Peatland” (44°26′ N, 3°36′ W; 1400 m asl) is located near two smelting slag sites (Figure 1) where VOL. 39, NO. 14, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Location of the Mont Loze` re massif (Ce´ vennes mountains) in southern France. The locations of the Narses Mortes Peatland and the granites samples are also indicated. an archaeological excavation was engaged (17, 23). It is a 21 ha treeless fen dominated by Sphagnum mosses and Molinia caerula (L.) Moench. A microtopographic pattern of hollows and hummocks characterizes the site. The organic sediment thickness reaches a maximum of 140 cm, and a radiocarbon date of 8150 14C yr B.P. was obtained for the onset of peat inception at the center of the peatland (24). The surrounding vegetation consists of a heatland dominated by Calluna vulgaris (L.) Hull, in association with Vaccinium myrtillus L., Cytisus scorparius (L.) Link, and Nardus stricta L. Scots pine (Pinus sylvestris L.) is the only tree species and was planted during the second half of the XIXth century. Sampling. According to palynological studies conducted at the studied site (24, 25), the marginal peat is more appropriate than the central part of the peatland to study in detail the anthropogenic period. In May 2002, a 140 cm-long peat profile was extracted at the western margin of the peatland i.e. out of the way of water draining the workshop 3-3′. The upper part of the profile consists of a 15 × 15 × 75 cm monolith, whereas the deeper sediments (76-140 cm) consist of a core collected using a modified Russian peat sampler. Sediments were wrapped in a plastic film and transported to the laboratory, where they were stored at 5 °C. Granite samples were also collected for an estimation of the local crustal composition. Peat Sample Treatment. The superficial part of the sediments as well as roots and living plant materials were discarded in order to remove potential contamination. The profile was divided into two longitudinal parts: one for the geochemical analysis and the other one for the palynological analysis and radiocarbon dating. The profile was sectioned into 2-cm thick slices. The upper part of the core (21-0 cm depth) has not been sampled for geochemistry because of the abundant roots and living plants. These last samples would have been inconsistent with routine analytical protocol 5132
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applied here. Samples for geochemistry were dried at 30 °C during 5 days in a polypropylene box for conservation until the chemical analysis. Each sample was powdered in an agate mortar. After powdering, samples were sifted in an inorganic sieve (500 µm mesh) in order to obtain a homogeneous powder without roots or large components of flora. Ash Content. Ash content was determined by gravity using 1 g of sediment previously dried at 105 °C. Ashing was done by heating at 550 °C overnight in order to obtain a white color as indication that all the organic matter was burned. Elemental Chemical Analysis of Peat Samples. Elemental chemical composition was measured using a Thermo Elemental ICP-MS X7. Suprapure reagents were used for the preparation of samples, such as distilled-deionized water, distilled acids, and a homemade synthesized Li metaborate flux. All samples were digested using alkali fusion. After cooling, the fusion glass was dissolved with nitric acid and introduced in the mass spectrometer (26). Procedural blanks were carried out and were negligible (26). For the samples having predominant organic matter content, the accuracy and the reproducibility were verified by using the BCR-CRM 482 lichens reference material and are as reported by Doucet and Carignan (10). The accuracy and the reproducibility of high ash content samples were verified by using geological reference materials and are as reported by Carignan et al. (26). Lead Isotopic Measurements. Dried peat samples (30300 mg, according to lead concentration in each sample) were dissolved in a Teflon vessel using 2 mL of concentrated HNO3 and 0.5 mL of 30% H2O2. The Teflon vessels were sealed and left at room-temperature overnight. After evaporation at 110 °C, the residue was taken back with 1 mL of concentrated HNO3, 0.5 mL of H2O2, and 1 mL of concentrated HF (all Merck Suprapur quality) and set at 80 °C overnight.
TABLE 1: Radiocarbon Ages of the Narses Mortes Peat Core depth, cm
radiocarbon age 14C yr BP
calibrated age (2σ) (calendar year)
lab name
40-42 48-50 52-54 60-62 70-72 90-92 118-120
850+/-30 1330+/-35 1265+/-35 1460+/-35 1635+/-35 1950+/-40 2200+/-40
1156 A.D.-1264 A.D. 640 A.D.-780 A.D. 660 A.D.-870 A.D. 540 A.D.-660 A.D. 340 A.D.-540 A.D. 50 B.C.-140 A.D. 390 B.C.-160 B.C.
POZ-7045 POZ-2012 POZ-2014 POZ-2015 POZ-2016 POZ-1957 POZ-1958
H2O2 was added to the Teflon vessel step by step in order to ovoid effervescence of the organic matter. Samples were digested in a clean room laboratory under a laminar flow hood to avoid any contamination. After the last evaporation, the residue was taken up in 1 mL of 0.9 M HBr and stayed at the room-temperature overnight in order to homogenize the solution with the residue. The samples were then ultrasonized for 1 h. After centrifugation, Pb was separated from the other elements by ion exchange using the AG1X8 resin (27). After separation, the solution was evaporated at 80 °C, and the residue was taken back in 3 mL of 0.3 M HNO3. The lead isotopic composition was measured with a MC-ICP-MS (Isoprobe, Micromass, now GV Instruments) equipped with 9 Faraday cups allowing the measurement of all the Pb isotopes, Tl isotopes, and 200Hg simultaneously. The reference materials, NIST 981 Pb and NIST 997 Tl, were used to correct for instrumental mass bias, according to the empirical technique used by Mare´chal et al. (28) and reported by White et al. (29) for lead applications. This technique is based on the relationship measured between Pb and Tl mass bias. Reference values used for both reference materials were taken from Thirlwall (30). A Pb/Tl ratio of 10 was used for both the reference solution and samples. Repeated measurements of the NIST NBS 981 Pb reference material yielded accurate recalculated values (using the Pb-Tl relationship) with a reproducibility (2*standard deviations) better than 150 ppm for all the reported Pb isotope ratios. The uncertainties are better than 180 ppm (2*standard deviations) for all the reported Pb isotope ratios. Pollen Analysis. Subsamples were collected at 2 cm (074 cm) and 4 cm (74-138 cm) intervals for pollen analysis. They were processed following standard methods. Pollen counting was done at 500× magnification and at 1000× for critical determinations. At least 300 grains of terrestrial vascular plants (pollen sum) were counted for every pollen spectrum. Results are expressed in pollen percentages. Only some selected species are presented in the pollen diagram. Whole data will be published elsewhere by M. Pulido (25). Radiocarbon Ages. Eight 2-cm thick samples were dated by acceleration mass spectrometry (AMS) at the Poznan Radiocarbon Laboratory, Poland (Table 1). For the choice of dated levels, special attention was paid to lead anomalies and signs of forest disturbance in the pollen diagram. Radiocarbon ages (14C yr B.P.) were calibrated (yr B.C.-A.D.) using the OxCal3 program (31, 32). A contemporary age of 2002 A.D. was attributed to the top of the profile (0 cm). Net peat accumulation rates (cm‚yr-1) were calculated by linear interpolation.
Results and Interpretations Net Peat Accumulation Rates. The matrix of the peat mainly consists of herbaceous plant remains. The onset of peat accumulation (139 cm) was not dated, and the oldest age obtained for the profile (118-120 cm) was 390-160 B.C. (Table 1). Net “sediment” accumulation rates varied throughout the profile, ranging from 0.024 to 0.096 cm yr-1 (Figure 2).
FIGURE 2. Age-depth model for the lateral core of Narses Mortes Peatlands. The error bars are given at 95% confidence level. Ash Content. The ash content varied from 8 to 85% (Figure 3a). The deepest part of the profile (133-139 cm) presents the highest ash content (saprolite), whereas samples from 23 to 27 cm have the lowest values. There is a general decrease with depth with some fluctuations. The total measured mineral elements have the same pattern with depth suggesting that the peat bog is minerotrophic. The Al concentration will be used for crustal normalization of metals. Heavy Metals. The elemental concentrations of As, Cd, Pb, Sb, Zn, and Al measured in peat samples and surrounding granites are summarized in Table 2. Systematic variations along the profile are mostly found for Pb concentrations which present two “rich-zones”, a large one from ∼120 to 80 cm depth, the peak concentration being at 113 cm, and another one from about 45 cm depth to the subsurface (23 cm), the peak concentration being at 35 cm. Arsenic concentrations present a pattern similar to Pb but with the first peak being slightly displaced toward a deeper position, centered around 120 cm. The other metals (Cd, Sb, and Zn) present a systematic increase in concentration only in the upper part of the profile (35-23 cm), the remaining samples having similar or lower concentrations erratically distributed from 35 cm to the bottom of the profile. The mean concentrations in As, Cd, Pb, Sb, Zn, and Al of the Mont Loze`re granite were calculated from the three samples C, F, and G (Table 2). They are 7.1 ppm,
