333 years of copper mining in the Røros region of the Mid-Scandic highlands Written sources versus natural archives - PDF

Journal of Nordic Archaeological Science 17, pp (2010) 333 years of copper mining in the Røros region of the Mid-Scandic highlands Written sources versus natural archives Lisbeth Prøsch-Danielsen*

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Journal of Nordic Archaeological Science 17, pp (2010) 333 years of copper mining in the Røros region of the Mid-Scandic highlands Written sources versus natural archives Lisbeth Prøsch-Danielsen* 1 & Rolf Sørensen 2 * Corresponding author 1 University of Stavanger, Museum of Archaeology (UiS-AM), N-4036 Stavanger, Norway 2 Department of Plant and Environmental Sciences, University of Life Sciences (UMB), P.O. Box 5003, N-1432 Aas, Norway The progression of industrial, rural and agrarian development is studied from sediment cores collected from two kettlehole lakes, Doktortjønna and Sandtjønna, in the vicinity of the mining town of Røros, focusing on the period The results of palynological and geochemical analyses are compared with well-known historical documents to test whether the records from these natural archives match the documentary evidences. Vigorous forest clearance took place during the first 25 years of copper mining, but the evidence of escalating deforestation is less pronounced in the pollen diagram, probably due to the presence of long-distance tree (AP) pollen. On the other hand, the non-arboreal tree pollen (NAP) curves do reflect the dispersal of the agrarian plots fairly well. Forest regeneration is observed in the pollen record after the introduction of coal for use in the roasting ovens in Abrupt increases in Cu, Zn and Pb reflect air pollution and the commencement of ore roasting. The presence of cations and variations in their content mirrors the production of the copperworks and shifts in methods, and can be used here for dating purposes. A decrease in loss-on-ignition (LOI), with a delay of c. 50 years, serves as an indication of soil erosion caused by intensive land use. Sedimentation rates increased from 0.2 to 1.2 mm/yr. The source area for the sand was the field of sand-dunes (the Kvitsanden desert ) formed during deforestation, possibly aided by vegetation damage caused by sulphuric fumes. The SO 2 emissions during periods of high production may have been 5 000 tonnes/yr. Keywords: copper mining, palaeo-environment, human impact, deforestation, air pollution, geochemical parameters Introduction and historical review The mining town of Røros, established in 1644, is situated in the county of Sør-Trøndelag in central Norway, 35 km west of the Swedish border (Fig. 1). It lies at the margin of a large undulating plateau (Rørosvidda) in the southern central part of the Scandic Mountains and is situated on a southwest-facing slope rising from a rather wide and flat valley bottom marked by the confluence of three main watercourses: Glåma, Håelva and Hitterelva. The last-mentioned river was also decisive in determining the location of the Bergstaden (mining-town of Røros), as its discharge was sufficiently large to operate the bellows for the smelting works. The aim of this paper is to describe and verify how the effects of 333 years of well-documented mining activities (involving land use, deforestation and pollution) can be traced from natural archives in the farming landscape associated with the mining community 37 lisbeth prøsch-danielsen & rolf sørensen Sweden Trondheim Røros 500m C Norway Oslo Falun Stockholm A 662,2 665, ,5 Sandtjønna 650 Glåma B Åstj. Kvitsanden C Hittersjøen Åsen N 624,5 Doktortjønna Airfield Småsetran Railway Røros st. Håelva Hitterelva Gjettjønna Røros bergstad The smelter Figure 1. The key map, A, shows the location of the town of Røros in central Scandinavia. Map B shows the extent of the town, with major communication lines, the location of the smelter ( ), and the environmentally protected area of Kvitsanden. The black area adjacent to the smelter represents the present extent of mine waste. The position of map C is framed. The catchment of Lake Doktortjønna is delimited with finely stippled lines on map C, and the sampling points are marked with black dots. The upper limit of glaciolacustrine sediments is marked with a heavy stippled line. around the Røros township. To test this, sediments from two small lakes, Doktortjønna and Sandtjønna (Fig. 1c), lying within the agrarian area were analysed in terms of pollen, loss on ignition (LOI), grain size and geochemical parameters indicative of air pollution. Similar investigations in Central Sweden have confirmed that lake sediments are reliable archives of pollution from metal smelters (Åström & Nylund 2000; Becker et al. 2001; Ek et al. 2001; Ek & Renberg 2001; Hammarlund et al. 2008). Prehistoric copper mining activities have also been reported in studies of blanket peats in the British Isles (Mighall et al. 38 copper mining in røros Figure 2. A view of Røros in 1915, with the church to the left and the smokestack of the smelter (acting as a point source for pollution) close to it. Lake Doktortjønna is seen in the foreground. The farm of Åsen and its hayfields with adjacent barns (outhouses) are located on the slopes towards the town. Photo: Harald Pedersen (Røros Museum, archives). 2002) and France (Jouffroy-Bapicot et al. 2007). The results will be compared and matched with well-documented historical events in the town of Røros as recorded in local histories (Kvikne 1942, Øisang 1946), which are in turn based on works such as Schøning (1910), Brinchmann & Agerholt (1926), J. R. Prytz (1953) and archives and weekly internal accounts of the copperworks as compiled by Hiort (1886), Hiort, Krags & Aas (1903) and Voss (1911). The question to be answered is whether the impacts recorded in the natural archives are comparable in substance and time with the written sources. Røros was designated a UNESCO World Heritage Site in 1980 on account of the considerable cultural treasures contained in its unique mining environment and its fine wooden architecture. In 1994 a group recommended that the site should be extended to include the historical industrial and farming landscape associated with the mining community in the surroundings of the town, the Circumference, an area of radius 44 km and covering km 2 (Larsen et al. 1994; Grytli 1996; Jones 1999). This extension plan has in effect been a subject of continuous debate since 1993 (Larsen et al. 1994; Sletten 2007a). As part of this process, the Outhouse Project was started in 1996, but its goal is only to renovate the buildings connected with the hayfields (Sletten 2007b). As pointed out by Daugstad (2001), the authorities are faced with a multitude of challenges for integrated environmental management. We also hope that our paper can be a tool in this process. Land-use history The first human exploitation of this sub-alpine plateau was by Stone Age hunters, as revealed by several habitation sites, stray finds and pitfall traps, especially along the watercourses (Daugstad et al. 1999). Scattered permanent farms or summer farms existed in Røros and its immediate surroundings during the 39 lisbeth prøsch-danielsen & rolf sørensen Migration Period, the Viking Age and the Early Medieval Period, but these settlements were abandoned after the plague in (Sandnes 1971). Today only burial mounds, pitfall traps and traces of ancient summer farms and bloomery iron production are left (Daugstad et al. 1999). After the Reformation, but prior to the mining settlements created in 1644, there were only a few farms in the area (Kvikne 1942; Øisang 1946). One of these was Åsen (Figs. 1c, 2), now located on the edge of the town. The discovery of copper ore in the region led in 1645 to the start of mining and metal production, which lasted until 1977 (Nissen 1976; Jones 1999). The number of inhabitants increased quickly, and so did the need for agricultural plots on the edge of the town, for the cultivation of both crops and hay, not least in order to provide winter fodder for the many draught animals needed in the mining community. Regular summer dairy farming also developed from the 18th century onwards. The community was thus based on a mixed economy, with a close link between mining and farming. The roasting of the ore and further smelting produced large quantities of fumes that spread over the surrounding, and serious air pollution was reported in and around Røros, where the first smelter was built (Øisang 1946; Ødegaard 1984; Borgos 1994). Large quantities of timber and charcoal were consumed in the production of copper (Ødegaard 1984; Espelund 1998), and rapid deforestation took place in and around the mining town. After only 10 years of operation, m 3 of timber had been cut down in the vicinity (c km 2 ) for fuel alone. The resulting lack of fuel forced the industry to move further out, and 15 new smelters were established, 10 of them within the projected extension to the World Heritage Site (Gjestland 1995). When the railway was completed in 1877, charcoal was replaced by coal and natural forest regeneration started. Copper production and air pollution Copper production rose slowly over the first 80 years to about 200 metric tonnes per year, but increased after 1730 to reach a peak of 600 t/yr in 1775, declining thereafter to about 350 t/yr. After 1888, when the smelting process was centralised to Røros and the Bessemer process was introduced, production increased once more, to about 700 t/yr. There was no production between 1920 and 1925, but on resumption it increased rapidly, with an exceptional record of tons/yr t/yr in the late 1940s (Fig. 3). The works was finally closed in Due to the high sulphur content of the ore, the first step in extracting the copper was an open-air roasting process where the sulphides were oxidised, releasing considerable amounts of SO 2. Until 1850 this process was done outside the smelting works, close to the river Hitterelva. To remove all impurities (resulting in 98 99% pure copper), three oxidations and two reductions were needed. The reductions, yielding products such as Cu 2 S, FeS and slag, were performed inside the smelting works (Ødegaard 1984; Espelund 1998). In contrast to the smelting works at Falun and in Germany (Ødegaard 1984; Ek et al. 2001), the one at Røros already had a chimney before 1700, and therefore acted as a point source of air pollution. The damage to the vegetation was especially pronounced Centralized smelting and the Bessemer method Figure 3. Annual copper production in tonnes per year (t/yr) during the 333 years of mining and smelting activities at Røros. Modified from Ødegaard (Røros Museum, archives). 40 copper mining in røros after 1888, when the efficient Bessemer smelting process was introduced (Fig. 3). The SO 2 gas formed acid rain at times of high precipitation and low pressure, and damage to the vegetation and the hayfields that had developed parallel to the smelting works was soon reported (Borgos 1994). The problem had become so serious after 1888 that the roasting process had to be closed down for the grass-growing season from 3rd May to 15th September each year. In 1905 the copperworks took over the most seriously damaged hayfields and paid the owners a small compensation. In addition, the company agreed to close down for a month in the summer every year to allow the inhabitants to move to their summer farms and hayfields. This scheme had the dual purpose of preventing emissions of SO 2 during the most favourable grass-growing season and enabling the workers to provide winter fodder (harvested by scything and mowing) for the draught animals and cows which were so essential to the community. Environmental setting and site description Geology The basis for the mining industry in the Røros region was a system of strata-bound sulphide deposits in the complex of volcanic and sedimentary metamorphic rocks (Nilsen 1988; Nilsen & Wolff 1989). These abundant ore deposits, mainly associated with gabbros, contained zinc (ZnS), copper (CuFeS 2 ) and lead (PbS) in varying proportions, with pyrite (FeS 2 ) and pyrrhotite (FeS) also common. Copper (Cu) was the only metal extracted commercially during the entire production period, however, although zinc (Zn) is normally the dominant element in sulphide ores. The average Cu content of the ore was 2.7% and Zn 4.2 5%, with only small amounts of Pb (Bjerkgård et al. 1999). Thick basal tills cover much of the region, and a major esker system runs through the town of Røros. The Kvitsanden dunes, regarded as the northernmost desert in Europe, are located on this esker. Glaciolacustrine sediments (fine sand and silt) occur sporadically below 665 m in the Glåma valley (Holmsen 1956; Reite 1997), see Figure 1c. Climate The climate at Røros is transitional between sub-oceanic and sub-continental, characterised by relatively low winter temperatures (a January mean of C), high summer temperatures (a July mean of C ) and little precipitation (a mean annual precipitation of 646 mm) (figures for the normal period , Aune 1993). Radiation fog is created in autumn and winter by cold air being forced down into the Hitterelva valley bottom through temperature inversion, and this also affected the distribution of air pollution. Vegetation Botanically, Røros lies in the boreal upland area at the transition between the middle and northern boreal vegetation zones, the latter being characterised by mires, birch woodland (Betula pubescens) and stunted coniferous woodland, here Scots pine (Pinus sylvestris) (Moen 1999). Agriculture is quite marginal owing to the short growing season (Borgos 1994), and this marginal position was further exacerbated by the escalating deforestation from 1645 onwards. Catchment and lake description Lake Doktortjønna is situated m a.s.l. (Fig. 1c) and occupies a kettlehole on the margin of the modern urban area. It is located 400 m west of the farm of Åsen, among strips of former pastures, agricultural land and hayfields, and 800 m west of the smelting works. The lake measures 150x80 m and has a maximum water depth of 6.25 m. Its sediments were sampled down to 8.55 m. Lake Doktortjønna represents a small pollen source area, definable as a local and/or extra-local site (according to Jacobson & Bradshaw 1981). The catchment covers c. 51 ha, an inflow takes place by surface run-off from the till area, presumably augmented by a groundwater input from the esker bounding the basin to the west and north. The lake lacks an outlet and has subsurface drainage through the permeable river terrace sediments. Lake Sandtjønna also occupies a small kettlehole, situated m a.s.l., and is located some 600 m to the northwest on the same river terrace as Doktortjønna. The lake is less than half the size of Doktortjønna and the catchment that lies mainly within the glaciofluvial esker system, is only 8.7 ha. There is no surface inflow, but considerable groundwater seepage takes place from the esker system, so that the effective catchment is larger than the topographic basin. The sampling point is shown in Figure 1c. Supplementary samples were obtained to date the commencement of sand movements, as the lake lies on the southern fringe of the desert of Kvitsanden. 41 lisbeth prøsch-danielsen & rolf sørensen Figure 4. Percentage pollen diagram for Lake Doktortjønna. The black curves show pollen percentages and the grey curves have 10x magnification. 42 copper mining in røros Methods Fieldwork was carried out during the wintertime in 1997, 2004 and Pollen analyses Material for pollen analysis and radiocarbon dating was obtained using a 75 mm diameter Russian corer. The pollen samples were prepared by the standard acetolysis method with HF treatment (Fægri & Iversen 1989) and the grains were identified with the aid of keys (Fægri & Iversen 1989) and reference material available in the Museum of Archaeology, Stavanger. The nomenclature follows Lid & Lid (1994). Microscopic charcoal particles larger than 25 µm and algae were also routinely counted. The pollen data were processed and the percentage diagram (Fig. 4) drawn using the computer program CORE 2.0 (Natvik & Kaland 1994). The pollen sum, ΣP, used as a basis for the percentage calculations comprised only terrestrial pollen. Elsewhere the basis for the calculation was ΣP+X, where X is the constituent in question. The diagram defines the local pollen assemblage zones (LPAZs), numbered from the base upwards. Sampling and sediment analyses Doktortjønna: Samples for loss on ignition (LOI), grain size and geochemical analyses were collected from the upper 30 cm of the sediments with a gravity corer fitted with 44 mm internal diameter PVC tubes (Skogheim 1979). The 25 to 100 cm sediment depth interval was sampled with a modified Livingstone piston corer using PVC tubes with an internal diameter of 63 mm. The two cores were correlated visually (sand layers) and by reference to LOI and grain-size. The cores were cut into 3 cm slices, except for the two uppermost samples, where the water content was high and 5 cm thick samples were extruded from the cores. The wet weight was determined and the samples dried at c. 40 C for at least 48 hrs. The dry samples were gently disaggregated in an agate mortar and stored in sealed plastic bags for further chemical analyses. Sandtjønna: Samples were collected with a Russian corer of diameter 7.5 cm and length 75 cm. The core was cut into 4 cm slices, except for some thinner sand layers between the depths of 15 and 45 cm. The samples were treated as above. Water content and loss on ignition (LOI) Another set of samples were dried at 105 C for c.12 hrs and the water content determined. LOI was determined after ignition at 550 C for 2 hrs. LOI was calculated from the sum of mineral matter and ash. Grain size The ignited samples were wet-sieved through stainless steel sieves with mesh sizes of: 2, 1, 0.5, 0.25, and mm. The approximate distribution of silt fractions ( 63 µm) was determined with a hydrometer. Geochemistry Samples for the geochemical analyses were sieved through a plastic screen with c mm openings. Metal cations except for lead (Pb) were determined with a slightly modified version of the method used by the Norwegian Geological Survey (Norsk Standard - NS 4770) in order to ensure comparability with similar earlier analyses from the region (Ottesen et al. 2000; Reimann et al. 2003). The dried samples were boiled in 7n HNO 3 for 3 hrs and analysed on an ICP OES instrument (Thermo Jarrell Ash Corporation). Lead (Pb) was analysed with a Perkin-Elmer SIMAA6000 Table 1. Radiocarbon dates from Lake Doktortjønna, Røros, relevant to the present study. Calibrated ages AD/BC after Stuiver et al. (1998), OxCal v3.9, Bronk Ramsey (2003). Lab. ref. Material Depth in cm Uncal yr bp Cal yr ad/bc Average age, corrected for reservoir age: TUa-1738A Sandy silty gyttja ±85 ad Not considered TUa-1739A Fine detrital gyttja ±65 ad ad 1425±95 TUa-1776A Fine detrital gyttja ± bc 360±140 bc TUa-2436A Macroscopic plant ± bc Not corrected fragments TUa-1775A Fine detrital gyttja Another core 6765± bc Not corrected 43 lisbeth prøsch-danielsen & rolf sørensen AAS instrument using a graphite furnace. The methods extract c. 90% of total Cu, c. 60% of total Zn and c. 25% of total Pb. The analyses were carried out at the Department of Plant and Environmental Sciences, Norwegian University of Life Sciences. Radiocarbon dates Three bulk samples of gyttja and one sample containing macroscopic plant fragments were picked out of the Doktortjønna cores for dating. The NaOH-soluble (A) fractions were dated by accelerator methods (AMS). The sample from a depth of cm in the sediment was washed through a 1 mm sieve with distilled water in order to collect plant macrofossils. The dates are expressed in conventional 14 C years BP and calibrated according to Stuiver et al. (1998), Ox- Cal v3.9 (Bronk Ramsey 2003), expressed in terms of a 68.2% confidence interval (Table 1). A reservoir age of approximately 300 years was estimated by comparing the 14 C date for TUa-1739A with the geochemical data. This will be discussed below. Results and interpretations Only t
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