1000 years of environmental changes in Falun, Sweden
Lake Sediment as source material
Neele Classen
Student
Degree Thesis in Environmental Sciences i 30 ECTS Masters Level
Report passed: 21.11.2012 Supervisor: Richard Bindler
Abstract
The aim of this study was to get a better knowledge of the metal pollution and the mining history of the Falun area. It adds new information on the geochemistry of the lakes and the beginning of mining in the Falun region, together with the influence of early land use. The main focus is on three lakes Hagtjärnen, Stugutjärnen and Nästjärnen, which were previously dated and analyzed regarding acidification by Anna Ek. Additional supporting information is provided from records from 10 other lakes, which are located at distances between 0-27 km from the Falun Copper mine. Another specific focus is on the lake Tisken, which has been assumed over the past 50 years to represent faithful historical record of mining in the Falun area. In this study this lake record was dated and analyzed, too. The analyses of all the lakes included resulted in four significant phases of environmental change, indicating the start of agriculture and mining, the development of each sector, as well as the sharp increase in pollution in the modern time period. Phase I covers the time period A.D. 700-1000 and represents the time of the early beginning of land use and small scale mining activities. Phase II represents the time between A.D. 1200 to 1450, which is dominated by an ongoing development of mining and a sharp increase in metal concentrations and occurrence of cultivated plants and plants favored by disturbance from A.D. 1450 onwards. The third phase, representing the year A.D. 1540, clearly displays another period of sharp increases among the metal concentrations, which coincides with a peak in Cu production volumes. Phase IV covers the time period A.D. 1750-1900, referred to as Modern time, and features a clear increase in Pb pollution, which is linked to the introduction of tetra ethyl Pb in the 1970s.
Other metals increase also, together with cultivated plants like cereals, indicating an ongoing expansion of mining and agriculture. The results also indicate that Cu was not emitted as far as other elements, like for example Pb, which led to great pollution only in the lakes close to the Falun mine.
Another important finding is that the lake Tisken does not represent a continual historical record, because the sediment is not a chronological sequence and instead likely represents mostly a catastrophic input of debris of mixed age. The C-14 dating shows, that the sediment is mixed and disturbed in Tisken. As a consequence, the long-standing interpretation of Tisken’s sediment record as an archive for the historical start and late development of mining at the Falun copper mine is incorrect
Key-words: Falun, Copper mine, Bergslagen, Tisken, sediment records
Contents
1 Overview………..……….1
1.1 Paleolimnology (background and methods)……….…1
1.2 Mining history in Europe………..……….3
1.3 Mining history in Bergslagen (Sweden)……….………...4
1.4 Mining history in Falun region……….………...………..6
1.5 Introduction to the study………...………7
2 Site description………...……….8
3 Methods………..………11
3.1 Sampling………...……….11
3.2 Sub-Sampling………12
3.3 Analytical Methods………...…………13
3.3.1 Mercury (Hg)……….……….…..…..13
3.3.2 Loss-on-ignition (LOI)………...……….………..…...13
3.3.3 Major and trace metals (XRF)……….………..…..13
3.4 Dating………...…..14
3.5 Statistical Methods………...……14
4 Results………...………...16
4.1 Hagtjärnen……….16
4.2 Stugutjärnen………..18
4.3 Nästjärnen………...………..20
4.4 Djuptjärnen………...22
4.5 Övre Önsbackdammen……….24
4.6 Varbotjärnen………...………..26
4.7 Laktjärnen……….28
4.8 Uvbergstjärnen………..30
4.9 Kvarntjärnen……….32
4.10 Rudtjärnen………...34
4.11 Mörttjärnen……….36
4.12 Abbortjärnen………...38
4.13 Tisken………...40
5 Discussion………41
5.1 Tisken……….41
5.2 Mining history………...42
5.3 Land use……….48
5.4 Significant phases of environmental change………...……50
5.4.1 Phase I………...50
5.4.2 Phase II………...…...50
5.4.3 Phase III………..………..51
5.4.4 Phase IV……….…………...51
5.5 Conclusion……….51
6 References………...…52
7 Appendix……….………...……….55
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1 Overview
1.1 Paleolimology (background and methods)
Paleolimnology, which is a multidisciplinary science, is defined as the study of past conditions and processes in lake and river basins and the interpretation of the histories of these systems. The field of paleolimnology goes back almost two centuries (Last and Smol 2001). The best archives among all terrestrial settings are known to be lacustrine sediments.
They provide records of environmental changes and pollution.
Once a core of lacustrine sediment is extracted the first step in any investigation would be the visual description. Important characteristics are for example the presence/absence of annual laminations, residuals from plants or changes in color (due to oxidations or changes in sediment source such as marine clay). The goal of this visual investigation is to help define the intervals for sub-sampling of the core. After sub-sampling has been performed the samples need to undergo other treatments prior to analyses. Most commonly the wet samples must be dried, before any analyses can be performed. One of the techniques is the freeze- drying with liquid nitrogen. The sample must be in contact with nitrogen at one face only so that it can move through the sediment block as a front. The freeze-drying turns water into ice without producing any ice-crystals. The freeze-dried samples are now ready for analysis (Kemp and Pike 2001). Alternatively the samples can be oven dried, although for some volatile elements such as mercury this can cause some losses in concentration.
One possible procedure is for example the geochemical analysis. The use of geochemical analysis in the field of paleolimnology can be traced back to Mackereth (1966) who attempted to construct an interpretational framework from observations on Holocene sediments in English Lake district. The analysis of the geochemical properties of lake sediments is useful to gain information on the lake, the catchment of the lake and the atmosphere or the atmospheric pollution. Boyle (2001) describes three principles of importance in applying geochemical analyses in paleolimnology. The first principle says that the sediment cores taken from a lake should be viewed as a sequence of soils derived from the catchment to get a better understanding of the stratigraphic changes. The second one implies that the sediment composition is largely governed by the catchment and the third one indicates that peaks in mineral matter concentration correspond with erosion events.
The analysis of the geochemical properties alone does not provide adequate information. But together with pollen and diatom data it is possible to get a picture of the long-term environmental changes and the impact of humans, for example the land use, in the surroundings of the lake. Another positive factor is the low cost of the geochemical analysis.
“The principal objective of inorganic geochemistry is the paleoenvironmental reconstruction to establish the link between sediment composition and environment” (Boyle 2001). One way of achieving information on the inorganic geochemistry of a lake is the determination of the total concentration of the chemical elements.
The XRF-technique is one of the procedures that can be used to analyze total element concentrations. Its basis is the photoelectric fluorescence characteristic of secondary x-rays from a sample, which is most common analyzed as a pressed pellet. Other options are the analyses on liquid samples or powders. Samples in this study were analyzed as powders. The samples are stimulated with suitable primary photons. Another approach would be the use of an isotope source system, where the primary photons are provided by radio isotopes that emit
photons in the x-ray energy band. Energies of the secondary x-rays generated within the sample are characteristic for the elements present, and the rate of emission is largely a function of the concentration of the element and absorption of the outgoing x-rays by the sample (Boyle 2000).
The procedure used in this study is a XRF spectrometry and specially wavelength dispersive instruments (WD-XRF), which are more precise, but more time consuming. Energy dispersive instruments (ED-XRF) are somewhat more rapid, but less precise and with poorer detection limits (Boyle 2000). Because of those characteristics the XRF-WD was used for determination of the geochemical properties during this study. Since the XRF technique is not suitable for measurements on mercury another system is needed. In this study, for example, mercury concentrations were measured with a Perkin Elmer mercury analysis system SMS 100, which is a thermal decomposition AAS.
Another analysis of importance is the determination of the organic matter content of the sediment via the loss-on-ignition (LOI) method. The loss-on-ignition method provides information on organic matter (residual remains of plants and animal tissue) and in a second step also inorganic carbon (as calcium carbonate)(Heiri et al. 2001, Santisteban et al. 2004).
The knowledge of the amount of organic matter content in the lake sediments is important for the reconstruction of paleoenvironments, like for the retrieval of information on the regional catchment and environment as well as human-induced changes (Meyers and Teranes 2001).
Another important step, apart from the geochemical analyses, is the dating of a sediment core.
For longer timescales (hundreds to thousands of years) the main approach is radiocarbon dating. This method can be used on for example macrofossils found in a sediment core. Most commonly when applied on terrestrial macrofossils in lake sediments radiocarbon dating gives excellent results. But under special circumstances, for example if no macrofossil is available and a bulk sediment sample must be used instead, radiocarbon dating is not as reliable as wished for, due to e.g., reservoir effects, which would include transport to the lake of older organic carbon from boreal watershed. Because of that, other dating methods are needed to support age-depth modeling. Renberg et al. (2001) suggests that Pb can be used as an atmospheric pollution indicator that contains chronological information in areas with well- established Pb records. This inferred Pb pollution dating could be used for northern Europe with reasonable accuracy at levels AD 0 (Roman peak in atmospheric Pb-pollution fallout 100 BC to AD 200), AD 1000-1200 (the Medieval increase, and occasionally, peak in pollution) and AD 1970 (peak in Pb pollution) in sediment deposits (Renberg et al. 2001).
Pb is, unlike many other heavy metals that are emitted to the atmosphere, essentially non- mobile in lake sediments, which is a basic requirement for a good chronological marker.
Another would be that the marker layer is synchronous over a defined, and preferably large, geographic area (Renberg et al. 2001). Pb consists of four stable isotopes (204Pb, 206Pb, 207Pb and 208Pb), which can be analyzed using inductively coupled plasma-mass spectrometry (ICP- MS). The mixture of these four Pb isotopes varies in the crust of the earth for geological reasons (Zartman and Doe 1981). The ratio of Swedish lake sediments and soils is considered to be relatively high, with a value of typically around 1.5, compared to the global average ratio of the upper continental crust with a value of 1.2 (Zartman and Doe 1981). Another approach on using Pb stable isotopes can be to distinguish between pollution Pb and natural Pb, since both forms of Pb consist of different stable isotopes (Brown 1962).
According to Renberg et al. (2001) plots of isotope ratios and pollution Pb concentrations against sediment depth are the most useful diagrams for inferring ages from atmospheric
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pollution records in lake sediment cores. “The difference in Pb isotope ratios and Pb concentrations between lakes reflects variations in the composition of the local bedrock and soils.” (Brännvall et al. 2001) Erosional events can cause fluctuations in the isotope ratio.
There are two main sources for Pb derived through the atmosphere, soil dust from mainland Europe and pollution from metal productions (Brännvall et al. 2001). Unpolluted soils and pre-industrial sediments from Europe have an average ratio of 1.2, whereas the characteristic
206Pb/207Pb ratio in sulphide ores exploited in ancient times is typically in the range 1.16- 1.187. “Transport of soil dust must have always occurred due to atmospheric circulation patterns and probably accelerated with the introduction and expansion of agriculture in Europe.” (Brännvall et al. 2001) In Sweden, prior to the earliest detectable influence of atmospheric Pb pollution, that is, prior to about 3500 years ago, peat records from southern Sweden show Pb isotopic ratios typical of the average composition of soils and rocks from mainland Europe and the global upper continental crust (Bindler et al. 1999; Klaminder et al.
2003; Klaminder et al. 2010).
Sweden has approximately 90 000 lakes, and a small number of them have varved sediments (annual laminations). The formation of laminations and varves in boreal lakes can only occur under special conditions, such as extensive periods of oxygen-free bottom waters, since laminations are an indicator for altered water quality due to significant emissions (Bindler et al. 2000). Those varved sediments provide perfect opportunities for studies with high-time resolution and accurate chronology.
Varved sediments are discernible and absolute chronologies of past changes, if counted. High resolution subsampling, even to an annual level, is possible from these unmixed sediments (Brännvall et al. 1999). Many remote forested Swedish lakes typically show a low sedimentation rate ̴ 1 mm year-1, these cores can span from about 250 to over 350 years of sediment accumulation (Bindler and Rydberg 2010).
1.2 Mining history in Europe
It is believed that widespread heavy metal pollution started with the Industrial Revolution during the 19th century. But over the last two decades studies have shown, that there has been large-scale pollution before that.
The discovery of metallurgy has played a major role in the development of ancient civilizations. Pattersson (1971) and Darling (1990) point out that man began to mine and extract metals on a significant scale as early as ̴ 7000 years ago, which represents the time were the use of smelting processes was discovered. The time after the discovery was effected by an increasing demand and supply for metals. Especially gold (Au), silver (Ag), Cu (Copper) and lead (Pb) mining increased together with the fast development of ancient civilizations (Hong et al. 1996).
Pb has played a key role in the history of humans as early as six millennia ago. Therefore it is believed that Pb was the first metal to be extracted from ores (Nriagu 1983). Those early mining activities were small scale, but only one millennium later significant Pb production started due to the discovery of new techniques of smelting Pb-Ag alloys from Pb sulfide ores and cupeling silver from the alloys. Pb production then rose continuously during the Cu, Bronze and Iron ages (Hong et al. 1994). For example a pronounced maximum of about 800.000 metric tons per year (approximately the rate at the time of the industrial revolution) was reached during the flourishing of Roman power and influence around two millennia ago
(Hong et al. 1994). The production volume of Pb of this early time period (earliest time to ca.
200 B.C.) is estimated to be about 39 million tons. But only the volume produced under the Roman Empire exceeds almost 50 % (18 million tons) of all Pb produced in ancient times (Waldron 1974; Nriagu 1983).
Pb may have been the first metal extracted from ores, but a demand for Cu can be traced back to a time between 5400 and 4000 B.C. This period is referred to as the Middle and Late Neolithic cultures, and Cu in form of artifacts, were found in settlements and cemeteries from that time period throughout southeastern Europe. The experimentation with the metallic minerals in southeastern Europe during the Middle and Late Neolithic periods appears to have resulted in the gradual assimilation of knowledge, which would have lead to the mass production of Cu and its alloys in the European “Bronze Age”, beginning approximately 2000 B.C. (Glumac and Tod 1989). Due to those findings it can be said, that Cu production became substantial ̴ 5000 years ago and increased, like Pb extraction, through the development of new smelting techniques for example for smelting carbonate and oxide ores. Production volumes of Cu were as high as ̴ 500,000 metric tons between ̴ 4000 to 7000 years ago (Hong et al.
1996). The first peak in Cu production, again similar to Pb, was reached during the Roman period, due to the introduction of coinage and the demand for Cu alloys for military purposes.
The fall of the Roman Empire lead to a decline in Cu production. But already during the medieval times production rates increased again and mining started in several parts of Europe, for example in Falun in Sweden in the 13th century.
The great expansion of production of metal ores in Europe, in the time period between 1450 and 1520, was triggered by the intense demand for metal, especially iron (Fe), Cu and Ag, but also tin and Pb, for coinage and military purposes (Braunstein 1998). The average production of Cu was ̴ 400 tons/year during the late Bronze Age (2000-700 B.C) (Hong et al. 1996).
During the Iron Age the introduction of coinage ̴ 600 B.C. stimulated the exploitation of precious metals like Au and Ag, and Cu for base- metal coins. The total average production rate during that time was ̴ 2000 tons/ year. Because of extensive smelting of sulfide Cu ores Cu production increased during the Roman times (̴ 250 B.C.- A.D. 350), with an average production of ̴7000 tons/year, which represents ̴ 10% of the total Cu produced before A.D.
1900 (Patterson 1971).
Pollutant Cu was probably emitted to the atmosphere as small-sized aerosols during the high- temperature steps of Cu production processes. The cumulative large-scale Cu pollution of the atmosphere of the Northern Hemisphere was much larger before the Industrial Revolution than after it (Hong et al. 1996).
1.3 Mining history in Bergslagen (Sweden)
Geijerstam and Nisser (2011) summarized the current understanding of the early history of mining in Bergslagen and Falun. Their work is based largely on Swedish sources that have documented much of the known historical documents (REFs such as Lindroth) and the archaeological evidence that has come to light since the 1980s (Magnusson, Petersson- Jensen). Below follows a brief summary of the important details, especially as they relate to Bergslagen and Falun.
Sweden is known for its long history as a producer of metals. Mining in Sweden was possible early in time because of its geographical advantages like rich deposits of high-grade ore, extensive forests yielding charcoal for metallurgical processes and abundant hydropower.
Ore, forest and running water are the three natural resources that were essential prerequisites
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for Swedish mining. The land area of Sweden consists of up to ̴ 55 % of forest and woodland.
These forests have been of great economic importance to mining in Sweden from time immemorial. Charcoal was used in both the production and the working of metals. Timber has been employed in building structures in mines and ironworks. In the mining areas the wood used for charcoal burning made up around 80- 90% of total volume felled. The mining activities were certainly responsible for a considerable amount of depletion of the forests in that area, but there was never any real overexploitation at any point of time, since the use of wood was regulated in the area around Falun. During the late 19th century the forests were even actively replanted.
The earliest traces of mining in Sweden are found in Bergslagen, to be accurate in Falun. The region of Bergslagen was the first one where mining started, since all the three requirements for mining were fulfilled. Bergslagen was sparsely populated before the introduction of mining. The area was seasonally used for farming, haymaking, fishing and hunting. Only in the 13th century, during the beginning of mining, people settled in Bergslagen because of the prospective of work. The region of Bergslagen had in total about fifty large to medium-sized mines and in the majority of those (thirty) Fe was mined, whereas the others were used for mining of sulphide and ore. The number of medieval Fe, Cu and Ag furnaces in this region is however known to be at least 700 and they were all in operation for different periods of time.
The Fe furnaces predominated in Bergslagen, but Cu ore was processed large scale in Falun.
The oldest blast furnaces found are from the 12th century.
During the Early Middle Ages about 20 Cu furnaces belonged to the Great Copper Mine (Kopparberget) in Falun. The oldest known, mentioned as early as 1357, was on the northeast shore of Hosjön. The first furnaces within the town of Falun are mentioned in 1414 (Främby).
In the 17th century there were no fewer than 132 Cu furnaces. Most lay close to the mine and the town, but some were up to 20 km away. Trade in Cu reached its climax in the mid -17th century. From A.D. 1700 both production and export of defined Cu declined as a result of difficulty in extracting the ore at Falun and of keener foreign competition like England, which became the big producer of Cu. Another reason for the decline in Cu production was the big collapse of the mine in 1687, when the Stora Stöten or Great Pit was formed (Ek et al. 2001 a). “When the “Great Pit” was formed in Falun the amount of leaching must have increased enormously. As the whole cave-in is composed of large blocks of rock, water and oxygen can readily penetrate it.” (Qvarfort 1984)
Fe was also important for Sweden, who was a sizeable exporter already in the middle Ages.
The Fe production in Sweden started ̴ 2,500 years ago and was depended on the location, which needed to be close to technical development, which in return has been related to natural conditions, social change and market needs. During the 17th century and 18th century Sweden became one of the leading producers of Fe. Lübeck, the leading port of the Hanseatic League of German merchants and cities in the North Sea and Baltic Areas, was the main port for the import of Swedish Fe until the middle of the 16th century. This association, the Hansa, played an important role in trade and politics in northern Europe, until the 16th century. In the mid-1600s Fe and Cu accounted for 80 % of the total value of Sweden’s exports. The share of Fe was 50 % and that of Cu 30 %.
During the 19th century Bergslagen retained its position as the main mining region, but there were great changes during the second half of the century, when many blast furnaces and forges were closed. Around that time large-scale exploitation of the ore fields in the north began. By the start of the 19th century Sweden’s gold age as an exporter of Cu was over.
During the 19th century Falun mine became the biggest Au mine in the country and the
second biggest Ag mine and during the 20th century large volumes of sulphur pyrites, zinc (Zn), Fe, Pb and Cu were produced. Altogether more than 30 million tons of ore were mined in Falun, yielding about 350 000-400 000 tons of Cu (Ek et al. 2001 b).
Today Sweden is still one of the leading European producers of the base metals Cu, Zn and Pb and the precious metals Au and Ag. Even though the number of Fe works has fallen from 500 to around 30 today over the last 150 years, the production has not fallen; on the contrary it has become greater and more efficient. The ores of Bergslagen occur in metamorphosed felsic volcanic and sedimentary surface rocks, which are about 1.9 billion years old. The stratiform calc-silicate deposits (banded skarns) are thin to medium-bedded and are a peculiar, but important component of the supra crustal successions in the Paleoproterozoic Bergslagen area. The word “skarn” describes the strong affinity between calc-silicate rocks and the ore in Bergslagen and it was introduced in the 19th century, originally referring to petrologically exotic garnet and pyroxene rocks proximal to ore deposits (Törnebohm 1875). The skarn beds in Bergslagen are mineralogically variable and dominantly composed of grandite, spessartine, epidote, actinolite, quartz, clinopyroxene, and locally magnetite (Jansson and Allen 2011).
1.4 Mining history in Falun region (Great Copper Mountain)
At the Falun mine complex sulphide ores containing Zn, Cu, Pb, sulphur, pyrites, Ag and Au were extracted. The ore in Falun occurs in granites and leptites of Precambrian age.
The boreal forest in Falun is typically a mixture of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies), along with birch (Betula pubescens). In more open and disturbed areas juniper (Juniperus communis) may also be found. Under the forest canopy a field layer consisting of heather (Calluna vulgaris), bilberry (Vaccinium uliginosum), blueberry (V.
myrtillus), lingonberry (V. vitis-idaea L.) and crowberry (Empetrum nigrum) can be found (Bindler et al. 2008). These conditions together with the availability of water sources such as numerous lakes and rivers enabled the formation of the Falun mining district.
It is believed that rock ore has been extracted at the Cu Mountain in Falun since the 8th century, if not earlier. Mining and use of ore as the raw material for metal production, was established in large areas of Bergslagen during the 12th century and was of substantial financial and social importance in the 13th century. The establishment of mining coincided with important changes in Sweden, for example in the society with the introduction of Christianity and the unification of the nation and growth of the towns of the Mälaren Valley.
The production volume of the Falun Cu mine is well recorded. Therefore it is known that the Falun mine produced 90 % (3,000 tons) of the Cu ore mined in Sweden in the mid-17th century. More than half of production volume, to be exact 2,600 tons, went for export. The Cu ore from Falun was roasted in furnaces adjacent to the mine. From there it was hauled to one of the 130 peasant miners’ furnaces in and around the town at that time. From 1637 the raw Cu was taken the 60 km to the new Cu works at Avesta.
The old town of Falun is mainly build on dump-heaps and leached material from these, along with the pump water from the mine, have long contributed to the pollution of the water and the sediments of the surrounding lakes. Additional pollution came from the people of the town in form of wastewater and through airborne metals from the melting works in this area (Qvarfort 1984). The surroundings of the mine include the catchment of the river Dalälven, which drains an area north of Malären, Sweden’s third largest lake, into the Gulf of Bothnia (Bindler et al. 2009).
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There has been both surface and underground mining in an area about 500 m2 and down to a depth of 400 m. Before the closure in 1992 about 28 of millions of tons of ore had been extracted. Today the mine is 360 m long, 220 m wide and 95 m deep (Forshell 1992).
There are several studies, which are tracing the beginning of the mining in Falun. Evidence is represented through a first written record about the Falun Cu mine, which dates back to 1288 and was written by Bishop Peter Eriksson of Västerås (Thunberg 1922). Studies in the field of paleolimology and paleoecology go back further in time. For example, Lundqvist (1963) estimated the start of mining and processing of Cu ore in Falun to have begun around the year 1080, because of the results of the dating of undisturbed peat, for example. The peat he used for the determination of the age of mining in Falun was extracted from a mire with the name
“Tisksjöbergets myr”. This mire stratigraphy was persevered, with approximately 4 m of waste material on top of it, in the slope down to Stora Stöten (the Great Pit). Once this waste material was dumped on the mire the development of it ended. Lundqvist performed pollen analysis and C-14 dating on sequences from this stratigraphy. His hypothesis was, that the surface of the mire represents the time of the start of mining in Falun. The pollen analysis resulted in the year A.D. 1000, whereas additional C14-datings revealed an age of A.D.1080±60.
Lundqvist performed his analysis really early in time (1963) and methods in both pollen analysis and C14-dating have improved since then. More pollen types are identified now and C14-dating techniques can work on a higher resolution than before. Those reasons motivated Eriksson and Qvarfort in the year 1996 to redo the analysis on the mire stratigraphy, with focus on charcoal particles, derived from fires used in the fire setting, and a change in vegetation due to land-use. Another attempt was to identify the disturbed levels of the mire stratigraphy as well as the ones that record the earliest mining activities, by using C14-dating, loss-on-ignition, and again pollen analysis. They found out that the charcoal particles, which are associated with the first quarrying technique, increased sharply at a depth dated as 1460±100 C14-years BP (i.e. A.D. 589±97), which indicates a time of active mining. Another sample taken from 3 cm depth was dated 1665±55 C14-years BP (A.D. 394±73) and may be interpreted as preceding mining activity. According to those findings Eriksson and Qvarfort (1996) were able to prove that mining took place even earlier than Lundqvist assumed, although the first mining activities must have been very small scale, since no change in the vegetation assemble could be identified through the pollen analysis.
Another study based on the Lundqvist paper (1963), was conducted by Qvarfort in 1984. He also found out that the mining in the Falun area started earlier than determined by Lundqvist.
He analyzed sediment cores taken from Lake Tisken and Lake Runn and reported a first increase in the metal content at about A.D. 700. The lake Tisken was indirectly dated earlier by Lundqvist (1963) by interfering the age with the dated peat record. Taken together both studies, Eriksson et al. (1996) and Qvarfort (1984), provide considerable evidence that mining in Falun started at least 200 years earlier (i.e. 700-800 A.D.) than estimated by Lundqvist (1963).
Similar to findings in other parts in Europe it is believed that Cu bearing ores in Falun were already worked on as early as the Bronze Age and in the late Medieval time (Forshell 1992).
Taken all those results together it is safe to say that the Falun Cu Mine has been in continuous use for at least thousand years.
1.5 Introduction to the study
Lake sediments provide most of the time a continuous record of ecological and geochemical environmental changes of an area, unlike historical and archaeological sources. Since the Falun Cu mine was in continuously usage for about 1000 years the surrounding area can provide a unique opportunity for studies on long-term effects of pollution.This study focuses on the changes in land use and the history of mining in Falun, by upgrading and expanding the analytical results of Ek et al. (2001 a, b) in combination with newly analyzed samples of the same cores they used. Ek et al. (2001 a, b) main focus was on the acidification of the lakes in the Falun area.
As mentioned above Ek’s studies focused mainly on the early anthropogenic acidification of lakes and to some extent on the effects of agriculture, forestry and mining. Sediment records were only used for determination of the historical and geographical distribution of airborne pollutants. This study adds some more information on the geochemistry of the lakes and the beginning of mining in the Falun region, together with the early land use. Most attention is paid to the three lakes Hagtjärnen, Stugutjärnen and Nästjärnen, which were previously dated through a mixture of pollen analysis, ISCP, Pb pollution trends and production history in Falun. Additional supporting information is provided through 10 other lakes including Tisken, which are located at distances between 0-27 km from the Falun Copper mine. Ek et al. (2001 a, b) introduced some of those lakes in her studies. Another focus is on the lake Tisken, which has been used as historic mining record of Falun since the early findings of Lundqvist (1963).
The lake was dated again and it is critical scrutinized if it is entitled to be used as a record of this region.
8 Site description
The Falun Copper mine (60°36’ N, 15°37’ E) lies in a mineral-rich area in central Sweden (Figure 1).
Figure 1: The red square marks the location of Falun within Sweden (ENIRO, 2012; modified by the author)
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The bedrock of the Falun area consists of a leptite formation of Precambrian age, with large intrusions of granitoids (SGU, 1987 a, b). The main ore mineral is pyrite, together with some chalcopyrite, sphalerite, and galena (Qvarfort 1984). According to Qvarfort (1984) the pyrite ore forms compact, lens-shaped masses and has to a large extent replaced limestone or dolomites which are now found as scattered remnants in the ore body. Additional in some sulphide minerals can occur as impregnations in quartzite. Qvarfort (1984) also points out that most of the ore bodies in Falun are bordered by sköld-zone, which is a mixture ore biotite, chlorite, and talc with large crystals of chalcopyrite and other sulphide minerals.
During the last decades of mining the average content of the ore was 28- 30% sulfur, 6% Zn, 2% Pb and 0.5% Cu. But the Cu content used to be up to 7% during the earlier centuries (Ek et al. 2001 b). The water used for the power of the furnaces was provided from the lakes closest to the mine.
An increasing need of water let to construction of dames. The lakes closest to the mine were dammed to a height of 1- 2 m during the early 17th century (Lindroth 1955). The catchments in this area were not only affected by the mine, but also by grazing animals, cultivation and tree felling (Ek et al. 2001 a).
Today the Bergslagen region, which is part of the southern boreal zone, is dominated by managed coniferous forest with equal proportions of Picea abies (Norway spruce) and Pinus sylvestris (Scots Pine). Furthermore deciduous trees like Betula pendula (silver birch), Alnus incana (grey alder), Sorbus aucuparia (rowan) and Populus tremula (aspen) can be found at low frequencies. The trees Corylus avellana (hazel), Quercus robur (oak), Ulmus glabra (elm), Fraxinus excelsior (ash) and Tilia cordata (lime) may appear occasional (Hammarlund et al. 2007). In former days this area showed a deforested landscape with cultivated land and cattle grazing (Ek et al. 2001 b). Regarding the climate, the mean annual temperature in this region is ̴ 4°C and the mean annual precipitation is approximately 700 mm (Raab 1995). The lakes in the Falun area are ice covered between 75- 100 days (end of November until late April) of the year (Geijerstam and Nisser 2011).
Figure 1 displays all of the thirteen lakes, which are included in this study. The locations of the Falun Cu Mine, as well as some of the major smelting sites are also shown in that map.
Figure 2: The studied lakes (•), the Falun Cu mine (★), and the major smelting sites (⏏) around Falun according to Ek et al. (2001 b) (Google maps, 2012; modified by the author) The Swedish National Heritage Board (2012) has completed a registry of archaeological and historical sites of interest. Along with a description of each lake (Figure 2), details from their database of relevance to each lake and its catchment are included.
The first lake of interest is Lake Hagtjärnen, 10 km southeast from the mine, with a lake area of 0.1 km2 and a maximum water depth of 7.0 m. The lake is located at a 500 m distance to Staberg, where smelters were operating. Another mining area was just 780 m to the south of the lake. The lake Stugutjärnen, located 5.5 km southwest from the mine, has an area of 0.07 km2 and a maximum water depth of 10.9 m. Aspeboda, a region where many smelters were operating, is just 2 km away from the lake. The next lake, Nästjärnen, located 4 km northwest of the Falun mine, is one of the lakes dammed in the 17th century. The maximum water depth is 6.4 m and the lake area is 0.1 km2. For a couple of hundred years around the 17th century a small smelter was situated 150 m downstream of the outflow from the lake, which used the water for power. Just 194 m to the north a mining area was in operation.
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The next lake, Djuptjärnen, has an area of 0.09 km2 and a maximum water depth of 13.9 m.
This lake lies at a distance of 6 km in northwest direction from the Falun mine. Djuptjärnen was dammed later in the 17th century. Mining areas were located at distances of 199 m to the north and 415 m to the east. The lake Övre Önsbackdammen is, together with Tisken, closest to the mine with a distance of just 3 km to the west. The lake area is 0.19 km2 and the maximum water depth 2.6 m. This lake was dammed really early and was originally just a small lake or even only a brook. Several smelters were operating around the shores of this lake. The lake Varbotjärnen, 6.5 km north from the mine, has a lake area of 0.05 km2 and a maximum water depth of 15.6 m. Bergsgården, a large smelting area, is just 500 m away from the lake. Varbotjärnen lies in close proximity to the big lake Varpan, on which shoreline several mining areas can be found, like Bergsgården and in Osterå. Osterå is just 2.5 km east of Varbotjärnen and could easily have contributed to the pollution in the lake. The lake Laktjärnen, 10 km northeast of the mine, has an lake area of 0.1 km2 and a maximum water depth of 11.6 m. Pastures surround the lake these days. Another lake, Uvbergstjärnen, is located at a distance of 13 km north from the Falun mine. This lake has an area of 0.05 km2 and a maximum water depth of 7.9 m. A big lake, called Rögsjön, is located north and west to Uvbergstjärnen. Just 1 km southwest remains of a once industrial used forest can be found.
Those forest residuals are located close to a small river with the name Rogsån. This river was dammed at the border to the lake Rogsjön. The area around Uvnäs, a village close to Uvbergstjärnen with 2 km distance to the southeast, consists of farming and agriculture grounds these days. At a distance of 84 m (NE) a mining area was located. The lake Kvarntjärnen has a lake area of 0.14 km2 and a maximum water depth of 7.1 m. It is situated 17 km northeast from the Falun Cu mine. A dam dominates the southern shore of the lake.
The lake Rudtjärnen has a lake area of 0.05 km2 and a maximum water depth of 4.5 m. This lake is situated at a distance of 17 km north from the Falun mine. Deforestation and clearances can be found around the lake nowadays. Southeast, at a distance of 987 m, a mining area was located. The big lake Rögsjön, which was mentioned before, lies 1.5 km south of Rudtjärnen. A mill was operating just 2.5 km west of Rudtjärnen, next to a stream called Lurån. It originates from another big lake, named Utgrycken, northwest of Rudtjärnen.
Some pits can also be found around the lake Utgrycken. The lake Mörttjärnen is furthest away from the Falun Cu mine, with a distance of 27 km to the north. The lake area is 0.15 km2 and the maximum water depth 6.9 km. The catchment of the lake is characterized by small hills, pastures and mires. The river Lungsjöån lies only 2 km to the east and contains a sawmill.
Mörttjärnen has been so little influenced by mining that it is safe to say, that it is a remote lake. The lake Abbortjärnen, 26 km north of the Falun mine, has a lake area of 0.01 km2 and a maximum water depth of 7.2 m. No mining influences can be found around this lake, similar to the lake Mörttjärnen. The area around Abbortjärnen is dominated by pastures and mires.
The lake Utgrycken is 1.5 km southwest of Abbortjärnen located. Those tributes make Abbortjärnen a remote lake.
The last lake of interest is lake Tisken, which is situated closest to the Falun Cu mine, with a distance of less than 1 km. Tisken has a lake area of 0.5 km2 and a maximum depth of 2 m and was originally a stream connecting Varpan to Runn. During the 16th century the lake was dammed about one meter above Lake Runn. The inflow comes from the Faluån River in the north, whereas the outflow leaves in south direction towards Runn (Qvarfort 1984).
3 Methods 3.1 Sampling
Ek et al. (2001 b) cored thirteen lakes at distances ranging from 1 to 27 km from the mine in the winters of 1994- 1996.
The multiple, over-lapping cores were retrieved from the deepest parts of the lakes. The upper 0-28 cm of the sediment were taken either with a HTH Kajak gravity corer or freeze corer (Renberg and Hansson, 2008). Those cores were sub-sampled in 0.5 cm slices down to 5 cm depth and thereafter in 1 cm slices. Sequences of different length (depending on the depth of the lake) of the underlying consolidated sediments were taken with a Russian peat corer (length 1 m, diameter 8 cm.) (Table 1). The sediment cores from the lake Tisken were taken with a Russian corer during a different occasion in the year 1998. This lake was not included in previous studies of Ek et al. 2001.
3.2 Sub-Sampling
The uppermost ̴ 1 m of sediment was sub-sampled in 2-cm intervals, whereas the deeper cores were sub-sampled in 10-cm intervals. Additional to those intervals sub-samples were extracted from spots where a change of color or material was visible. Sub-sampling on Tisken was done continuously throughout the whole core with bulk samples of 5 to 10 cm. The cores of the lakes Djuptjärnen, Varbotjärnen, Uvbergstjärnen, Laktjärnen and Hagtjärnen showed laminations. Marine clay was visible at the deepest cores of Övre Önsbackdammen, Uvbergstjärnen, Kvarntjärnen, Mörttjärnen and Tisken. Rust was visible in the cores of Djuptjärnen, Varbotjärnen, and Laktjärnen and especially in Tisken.
Not decomposed parts of vegetation, like roots and bark, could be found in the cores of Övre Önsbackdammen, Rudtjärnen and Tisken. Besides that the core from Lake Djuptjärnen contained remains of fur.
Furthermore two macrofossils and one bulk sediment sample were taken from core 3 (lake Tisken) at a depth of 187.5 cm, 192.5 cm and 227.5 cm for C14-dating. Dating on those samples was performed at the Tandem Laboratory, Uppsala University, Sweden. Before further treatment all sub-samples were freeze-dried and then homogenized.
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Table 1: Number of cores and the depth of extraction for each lake
Lake Cores Depth (m)
Tisken 3 0.7- 3.2
Hagtjärnen 4 7- 10.4
Nästjärnen 4 6.4- 9.5
Stugutjärnen 5 10.9- 14.8
Djuptjärnen 3 13.9- 16.5
Varbotjärnen 3 15.6- 18.2
Uvbergstjärnen 5 7.9- 12.1
Övre Önsbackdammen 5 2.6- 6.5
Rudtjärnen 5 4.5- 8.7
Kvarntjärnen 4 7.1- 10.5
Mörttjärnen 4 0- 9.6
Abbortjärnen 5 7.2- 11
Laktjärnen 5 11.6- 15.9
3.3 Analytical methods 3.3.1 Mercury (Hg)
Mercury concentrations were determined with the Perkin Elmer mercury analysis system SMS100. This system is a thermal decomposition AAS, which requires no sample pre- treatment. The sample weight for the mercury analysis was ∼ 0.5 g, with a replicate for approximately 10 % of the samples. The accuracy and precision of the measurements were assured using the standard reference material NCS ZC 73002 and NCS DC 73309 where measured values were within the certified ranges at all times. QC- Mess 3 was used as a calibration standard.
3.3.2 Loss-on-ignition (LOI)
Loss-on-ignition (Heiri et al. 2001) was measured on all samples, except for samples of Hagtjärnen, Stugutjärnen, Nästjärnen, because for those lakes only the existing results from Ek et al. (2001) were used. The measurements were performed with the Perkin Elmer mercury analysis system SMS100, parallel to the measurements on mercury. The temperature needed for combustion should be at least 550°C, and the Perkin Elmer provides this requirement.
3.3.3 Major and trace metal geochemistry (XRF)
Determinations of major and trace metal geochemistry were performed with a wave-length dispersive x-ray fluorescence spectrometry, which is a rapid non-destructive method providing data on > 20 elements including lithogenic and trace elements, such as Na, Mg, K, Ca, Rb, Zr, Sr, Ti, Al, Ba, Si, S, P, Mn, Fe, As, Ni, Cu, V, Zn and Pb. The detection limits for the trace metals ranges typically between 5-10 ppm. The results are reported in total element concentration. Certified reference materials (LU) were measured along with the samples, which had powder consistence and a weight of ∼0.7 g. “Major elements are measured with the
greatest precision, which typically improves with atomic number and concentration (Boyle 2001).
3.4 Dating
According to Lundqvist (1998) the deglaciation in central Sweden took place at c. 11,000 cal.
years B.P. and the highest postglacial shore-level developed at around 170 m.a.s.l. during the Yoldia Sea stage of the Baltic basin. The lakes of this study were still covered by ice 11,000 years ago, followed by a phase of formation between 9,000 and 10,000 years ago. The current lake boundaries were established sometime around 8,000 years ago (Figure 3).
For three lakes (Hagtjärnen, Stugutjärnen and Nästjärnen) Ek et al. (2001) had dated 2- 3 levels based on C-14, but they did not develop any age-depth-models. Here, the radiocarbon dates in combination with the modeling program CLAM (Blaauw 2010) age–depth models were developed by me. For the new C-14 dates from Tisken, the conflicting ages dates indicate sediment mixing and no modeling could be done (see Results and Discussion for further description).
Figure 3: Shore line and ice cover (A) 11,000 years ago; (B) 10,000 years ago. The red square represents the location of Falun (SGU 2012)
3.3 Statistical methods
A B
é N
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Once the analyses are done the next step is the statistical analysis of the variables. The most basic steps in the statistical analysis are the graphical representation of the inter-element relationship, together with the correlation matrix. For each lake a correlation matrix was used to determine simple associations among elements. It is useful tool to describe correlation between geochemical values. Correlations can range between +1/ -1 (strongly correlated) and 0 (not correlated). Where R2 values are very high, regression is a useful tool for determining element ratios in components of the sediment (Boyle 2001).
The linear ordination technique of principal components analysis (PCA) was used to explore major patterns in the XRF-derived dataset of inorganic elements, Hg and LOI and to reduce the number of variables, using the software SPSS (IBM SPSS Statistics, Version 20, IBM Cooperation’s and its licensors 1989, 2011). Variables were centered and standardized for the principal component analysis. PCA, a true eigenvector-based multivariate analysis, transforms a number of possibly correlated variables into a smaller number of uncorrelated variables called principal components. The first principal component accounts for as much of the variability in the data as possible, and each succeeding component accounts for as much of the remaining variability as possible.
It should be considered which rotation method to use. The unrotated output of the principal component analysis leads to data compression, meaning that most of the variance is accounted on the first factor. One way to avoid that pattern is the varimax rotation, which leads to an output of either small or large loadings of any variable. This method makes an easy identification of each variable with a single factor possible, whereas the unrotated method is less understandable. The varimax rotation is the most common method and was applied in this study.
The communality is another tool to achieve an even better understanding of the factor loadings. The communality is the sum of the squared factor loadings for all factors for a given variable, which would be an element in this case, which is the variance in that variable accounted for by all the factors. The variance in a given variable explained by all the factors jointly is given in percentage (Wikipedia 2012). Percentages under 80 % should be used careful.
4 Results 4. 1 Hagtjärnen
Figure 4: Results for selected analyses (Cl, Na, Ti, S, Fe, Mn, Cu, Zn, Hg) and for the first three significant components (PC1- PC3). The lines represent a phase of change within the concentrations.
The results of the geochemical analysis of Hagtjärnen show two important phases of change throughout the cores (Figure 4). The first change occurs at a depth of 100 cm, which corresponds with the year A.D. 709. At that point a rapid decline in concentrations is visible for several elements such as Na, Ti, and Cu and to a small extent Zn, whereas elements like S, Fe and Mn show an increase in concentration. Pollen from apophytes (plants favored by disturbance, like heather) and anthropochors (cultivated plants, like cereals) also show a sharp increasing change at 100 cm, possibly marking the start of agricultural land use. Figures regarding the pollen analysis are located in the discussion part.
The next change occurs at a depth of 40 cm, which corresponds with the year A.D. 1450.
Mineral matter (Na, Ti) but also S, Zn and Hg increase at that depth. Other elements like Fe and Cu, as well as LOI decrease at that point of depth.
The first principal component (PC1) shows a similar graph as the mineral matter elements, whereas PC2 displays some of the patterns of the ore related elements, with the sharp increases at 100 and 40 cm. PC3 displays patterns of both, elements bound to organic matter and elements, which are ore related. The phases of change are also visible in that third component.
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According to the correlation matrix Hagtjärnen’s geochemical values show a strong (r ≥ 0.8), positive correlation among elements, such as Mg, K, Al, Ti, Ba, Sr, Y, Rb, Zr, As and Pb. The correlation matrix showed a few errors and should be evaluated with consideration. Four significant principal components were determined, without the elements Na and Hg. PC1 explains as much as 35.4 % of the variance, PC2 27.9 %, PC3 20.1 % and PC4 7.6 %. The element Y shows a communality below 80 %.
Table 2: The four significant principal components of Lake Hagtjärnen together with the communality of each element. (Bold: variance is mostly explained by that component)
PC1 PC2 PC3 PC4 Communality
Mg 0,947 0,247 -0,137 -0,011 98%
K 0,969 -0,157 -0,011 -0,027 96%
Ca -0,051 0,930 0,176 0,063 90%
Al 0,962 -0,052 -0,022 -0,133 95%
Si -0,042 0,729 0,551 -0,315 94%
Ti 0,960 -0,226 -0,069 0,020 98%
Fe -0,084 0,179 0,880 0,331 92%
Mn 0,095 0,861 0,384 0,156 92%
Ba 0,908 -0,153 -0,225 0,144 92%
Sr 0,887 0,031 0,269 0,191 90%
Y 0,747 -0,164 -0,009 0,348 71%
S -0,689 0,155 0,682 -0,049 97%
P -0,179 0,793 0,023 0,382 81%
Cl -0,320 0,834 0,054 0,102 81%
Br 0,215 0,268 0,050 0,893 92%
Cu -0,037 0,279 0,930 -0,135 96%
Zn -0,243 0,816 0,434 -0,124 93%
Ni -0,002 0,573 0,783 -0,036 94%
V 0,373 0,667 0,521 0,138 87%
For Hagtjärnen PC1 shows positive scores for lithogenic elements (mineral matter), such as Mg, K, Al, Ti, Ba, Sr and Y and one negative score for S, an element associated with organic matter. PC2 shows positive scores for elements like Ca, Si, Mn, P, Cl, Zn and V. PC3 shows positive scores for ore related elements like Fe, Cu and Ni. PC4 displays a positive score for the element Br. The principal components 1-4 are displayed in Table 2.
4.2 Stugutjärnen
Figure 5: Results for selected analyses (Cl, Na, Ti, S, Fe, Mn, Cu, Pb, Zn) and for the first three significant components (PC1- PC3). The lines represent a phase of change within the concentrations.
Geochemical analyses on Stugutjärnen resulted in two important phases as well.
Measurements on LOI and Hg were not performed for Stugutjärnen (Figure 5).
The first significant change occurs at a depth of 55 cm, which corresponds with the year A.D.
1232. A decrease in mineral matter (Na, Ti) and an increase in organic matter (S), and in ore related metals (Fe, Cu, Zn) appears at the same time. The second important change is visible at 36.5 cm, which corresponds with the year A.D. 1450. This change is also visible in the lake Hagtjärnen, as mentioned above. At 36.5 cm another decline in mineral matter is apparent, and the elements S, Fe and Zn increase. Other elements like Mn, Cu and Pb show no significant changes at that point. PC1 shows a similar pattern like the mineral matter, with a decrease at 55 cm. The second principal component (PC2) shows an opposed pattern to some ore related metals, whereas PC3 is mostly driven by the element Mn.
Once again is the strongest correlation among the lithogenic elements (Na, Mg, K, Ti, Rb and Zr) with r ≥ 0.7 according to the correlation matrix. The principal component analysis resulted in four significant components. The first component (PC1) explains 30.7 % of the variance, the second component (PC2) 29.4 %, the third component 15.9 % and the fourth component (PC4) 9.8 %. The communality of the elements Si, Ba, As and V is below 80 %.
