Fate of natural and anthropogenic particles in peat bogs
Investigating atmospheric deposition over a scale of millennial period is crucial because humans are emitting more and more synthetic and natural compounds (i.e.: pollutants and/or dust) to the environment through the atmosphere. It is therefore necessary to determine the background deposition rate of these compounds, to assess their natural variations (ie.: temporal and/or spatial) and to understand the effects of the increased atmospheric depositions induced by humans on the environment. The primary aim of this work was to improve the understanding of the processes affecting the fate of anthropogenic and natural particles in peat bogs, to see which geochemical processes can affect the suitability and accuracy of peat bogs as archives of atmospheric deposition, and also the effects of these inputs on the bog ecosystem. To test whether peat bogs are accurate archives of Pb atmospheric deposition, Pb distribution was investigated in Kohlhütte Moor (KM), a bog in Southern Germany. Pb is a toxic element extensively dispersed by human activities (Chap.2.1). The atmospheric Pb record assessed using a peat core was compared with other archives, including a snow pack and previously studied Swiss peat cores (Chap.2.2). To this end, new improved methods both methodically (Chap.1.1) and analytically (Chap.1.2 and Chap.1.3) were developed. Also particular attention was given to age dating using 210Pb and 14C and the estimation of the Pb accumulation rate (AR) based on these ages. The most recent Pb AR in KM (2.5 mg m-2 y-1) is similar to that obtained from the snow pack on the bog surface (1 to 4 mg m-2 y-1). The isotopic composition of Pb was measured in both the modern and ancient peat samples as well as in the snow samples, and clearly shows that recent inputs are dominated by anthropogenic Pb. The chronology and isotopic composition of atmospheric Pb accumulation recorded by the peat from the Black Forest is similar to the chronologies reported earlier using peat cores from various Swiss peat bogs and point to a common Pb source to the region for the past 200 years. Taken together, the results show that peat cores from ombrotrophic bogs can yield accurate records of atmospheric Pb deposition, provided that the cores are carefully collected, handled, prepared, and analysed using appropriate methods. In addition, the rates of atmospheric Pb accumulation at least for the last 6000 years were quantified using peat cores from KM and Lindow Bog (LDW), England. In KM, the most recent Pb accumulation rate (2.5 mg m-2 y-1) is 50 to 200 times greater than the “natural” average background rate of atmospheric Pb accumulation (~20 µg m-2 y-1) calculated using “pre-anthropogenic” samples from the same site. The core from LDW (Chap.2.3) shows the potential of carefully studying Pb distribution in a peat profile for regional paleo-ecology and archaeology. Using the Pb/Ti ratio to calculate the rates of anthropogenic, atmospheric Pb deposition, the core reveals Pb contamination first appearing in peat samples dating from ca. 900 B.C. which clearly pre-date Roman mining activities. The timing of the ancient and medieval Pb pollution is also directly related to socio-economical events. 208Pb/206Pb and 206Pb/207Pb data indicate that English ores were the predominant sources for atmospheric deposition in England during the pre-Roman, Roman, and medieval periods. To test whether peat bogs are suitable archives of atmospheric deposition of minerals and after a review of the different possible dissolution mechanisms influencing inorganic particles (Chap.3.1), mineral distributions in the upper part of KM were investigated (Chap.3.2). Similar minerals to the local granite were identified using X-Ray-Diffraction: most of the minerals are therefore of local origin. The distribution of quartz and feldspars is unaffected by the low pH and the abundance of organic acids, possibly due to the early formation of a siliceous layer and/or coating by humic acids. Therefore the preservation of quartz and feldspars in ombrotrophic peat might make bogs useful archives of the changing rates of atmospheric dust since the Last Glacial. In addition, other minerals, calcite and apatite, were identified but only in the topmost samples of the bog. They probably dissolve comparatively quickly and thereby influence the chemistry of the surface of the bog releasing nutrients and therefore influencing the botanical composition of the bog and the rates of plant growth.
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