Effect of historical land-use on lake-water carbon and geochemistry: : A multi-proxy study of two lake sediment profiles in Dalarna throughout the Holocene

Sammanfattning: This study examines changes in lake-water total organic carbon (LW-TOC) and lake sediment geochemistry in two lakes, Stångtjärnen and Holtjärnen in (Dalarna, Sweden), during the Holocene and the role of the historic forest grazing and farming (fäbod-system). The aims of the study were to: 1. Discern the effects of natural processes on the lake’s biogeochemistry in different position in the landscape. 2. Identify the effects and differences in intensity of historic land-use on the lakes. A multi-proxy study was conducted encompassing multi-element (15) geochemistry, biogenic silica, LW-TOC, chlorophyll a and published pollen records. The first lake, Stångtjärnen, is shaped and influenced by surrounding mires, which developed shortly after deglaciation and stabilized the LW-TOC at 19 mg L-1 throughout most of the Holocene, while Holtjärnen, a small upland lake, changed from a productive lake (BSi: 35 %), low humic (LW-TOC: 8 mg L-1) to a less productive (BSi: 4 %) more humic lake (LW-TOC: 12 mg L-1) in 7300 BP. The intensification of agricultural land-use (e.g. hay-making) in Stångtjärnen reduced the concentrations of organic associated elements (Br, Cl) and LW-TOC and increased lithogenic elements (K, Ti), while Holtjärnen showed less anthropogenic influence. The comparison between the two lakes displayed the intensive influence of land-use on the Stångtjärnen catchment, showcased by changes in the sediment geochemistry, vegetation composition and the extent of the forest-grazing system in a landscape perspective. In response to the changes of the Holocene, Stångtjärnen’s mires became the main influence, while Holtjärnen was more sensitive to changes.