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

Les processus anthropiques et naturels modifient et régulent la concentration et le stockage du carbone (C) et du mercure (Hg) dans les sédiments des lacs boréaux. Cependant, la manière dont ces deux éléments sont couplés dans les sédiments de lacs largement répandus et la manière dont leurs concentrations, leur composition et leur couplage ont pu changer dans les archives sédimentaires restent mal comprises. Ici, nous avons mesuré la concentration de Hg et de C dans les sédiments de surface de 42 lacs (dont 5 lacs inondés par des barrages hydroélectriques) dans trois régions boréales du Québec et nous avons obtenu des carottes de sédiment de ~1m dans deux de ces régions, allant de la région de la Baie James (JB) à l'ouest jusqu’à la Côte-Nord à l'est. Nous avons constaté que la concentration de Hg total (THg) était plus élevée dans les sédiments récents (post-industriels), probablement en raison des perturbations anthropiques ainsi que du contexte environnemental et climatique. La concentration de THg augmente avec un pourcentage plus élevé de C total (%TC) dans les sédiments à l'intérieur des carottes de sédiments et entre elles, avec des pentes plus élevées pour les relations THg-%TC dans les sédiments récents. La fluorescence de la matière organique dissoute (MOD) dans l'eau interstitielle était un bon prédicteur du THg dans les carottes de sédiments JB, probablement en raison d'une affinité préférentielle et le co-transport du THg avec la matière organique de type humique. Il existe une variabilité inter-régionale de la concentration de THg et du pourcentage de matière organique (%OM) dans les sédiments de surface, avec une distinction inter-régionale (HSP, JB, VRD) liée au contexte géographique (par exemple, profondeur du lac, superficie, altitude, chimie de l'eau). Ces résultats suggèrent des niveaux de variation naturels et anthropiques en termes de concentration et de composition de la MO et du Hg s'accumulant dans les sédiments, avec une augmentation préférentielle de l'accumulation du Hg dans les sédiments récents qui est associée à des groupes spécifiques de MO d'origine terrestre. Les réponses des lacs aux changements futurs ne seront peut-être pas homogènes, et une meilleure compréhension des facteurs géographiques du stockage du carbone et du mercure dans les sédiments lacustres permettra d'améliorer la prévision de ces éléments imparfaitement couplés au niveau des bassins versants. / Anthropogenic and natural processes modify and regulate carbon (C) and mercury (Hg) concentration and storage in boreal lakes sediments. However, how these two elements are coupled in sediments of widely distributed lakes and how their concentrations, composition and coupling may have changed over sediment records remain poorly understood. Here, we measured the concentration of Hg and C in surface sediments of 42 lakes (including 5 lakes flooded by hydro-electric damming) in three boreal regions of Quebec, Canada and further obtained ~1m sediment records in two of these regions, ranging from the James Bay (JB) area to the west to the Cote-Nord area to the east. We found that total Hg (THg) concentration was higher in recent sediments (post-industrial), likely because of anthropogenic disturbance as well as environmental and climatic context. THg concentration increased with a higher percent total C (%TC) in sediments within and among sediment cores, with higher slopes for the THg-%TC relationships for recent sediments. Porewater dissolved organic matter (DOM) fluorescence was a good predictor of THg in JB sediment core, likely because of a preferential affinity and co-transport of THg with humic-like material. There was inter-regional variability in THg concentration and percent organic matter (%OM) in surface sediment with inter-regional distinction (HSP, JB, VRD) related to geographic context (e.g. lake depth, area, elevation, water chemistry). These results suggest natural and anthropogenic levels of variation both in terms of the concentration and composition of OM and Hg accumulating in sediments, with a preferential increase in Hg accumulation in recent sediments that is associated with specific groups of terrestrial-derived OM. Lake responses to future changes may thus not be homogeneous, and a better understanding of the geographic drivers of C and Hg storage in lake sediments will help improve prediction of these imperfectly coupled elements at the watershed level.