Limnological responses to late Holocene permafrost dynamics at the Stordalen mire, Abisko, northern Sweden

Sammanfattning: A composite sediment sequence, consisting of a Russian core and an overlapping freeze core, comprising the uppermost meter of the sediment record, was retrieved from Lake Inre Abborrtjärn in the Torneträsk valley in Lappland, northern Sweden. The core was subsampled for analysis of organic matter content, inorganic carbon content and analysis of the sediment composition using a technique known as structural analysis, resulting in a high-resolution data set spanning the last 2000 years. The chronology of the sediment sequence is based on radiocarbon dating and 210Pb dating of the uppermost sediments. The aims of the study were to evaluate possible connections between sediment stratigraphic changes and variations in permafrost in the adjacent mire complex at Stordalen, with a main focus on variations in the presence of calcareous laminations in the sediments and their coupling to water chemistry (pH and alkalinity) of the lake. Since both Lake Inre Abborrtjärn and the Stordalen mire are situated in an area with abundant carbonate-rich bedrock exposures, the impacts of modern catchment hydrology as well as the influence on pH exerted by ombrotrophic mires and fens were assessed based on a water chemical survey. The stratigraphic records were also compared to meteorological data from the Abisko Scientific Research Station and temperature reconstructions from the Torneträsk area and the Northern Hemisphere beyond the short instrumental records. The study shows that the calcareous laminations originate from calcium carbonate precipitation by Characean algae (Chara sp.) as a result of photosynthesis during the summer months. The precipitation of calcium carbonate is dependent on lake-water pH and thus related to variations in the chemical composition of catchment runoff. From c. 1240 AD an expansion of ombrotrophic peat and permafrost aggradation at the Stordalen mire lowered the pH of the lake-water as a result of increased inflow of humic waters and DOC. Two periods of permafrost expansion were recognized in the sediment record, the first period at c. 1240-1425 AD and the second at c. 1550-1850 AD, with a period of climatic amelioration in between when the influence of ombrotrophic peat on lake-water pH decreased. From c. 1850 AD and onwards the permafrost gradually started to disintegrate with an acceleration during the last decades. Thus, changes in lake-water chemistry as recorded in sediment sequences may provide detailed information on past variations in permafrost occurrences in subarctic settings.