Subarctic ecosystem responses to climate, catchment and permafrost dynamics in the Holocene

Abstract: This thesis assesses aspects of the wetland development, permafrost dynamics and associated changes in carbon and nutrient cycling of the Stordalen Mire in northern Sweden. Various ecological and biogeochemical analyses of one peat and two lake sediment sequences were conducted, including analyses of organic matter and carbonate content, mosses, diatoms, testate amoebae, pigments, carbon and nitrogen and their stable isotopes, near infrared spectroscopy and biogenic silica. Results revealed that the structural development of the mire occurred during the later part of the Holocene. Peat inception was dated at 4700 cal BP and onset of organic sedimentation in two adjacent lake basins occurred at 3400 and 2650 cal BP. Fen peat accumulated until minimum 2800 cal BP, and after c.2650 cal BP an early permafrost aggradation phase likely caused frost heave and significant changes in the wetland structure and hydrology. Peat is largely missing in the examined core between 2800 and 1350 cal BP, reflecting either environmental stress causing a decrease/cease of peat accumulation and/or erosion of previously formed peat. An increased content of redeposited peat in one of the lakes after c.2100 cal BP, points to mire erosion caused by permafrost decay. A high nutrient/productivity layer in the other lake between 1900 and 1800 cal BP may have been related to the same event in the mire. Sedge peat accumulated from 1350 cal BP. Renewed permafrost aggradation is indicated indirectly around 700 cal BP and directly 120 cal BP from changes in peat building vegetation. Fen peat and transitions between dominating mire vegetation communities were characterized by frequent diatoms and high nutrient concentrations. Permafrost phases were associated with poor fen and bog formation, and thus considerably more acidic conditions in the mire as compared to pH conditions when richer fen communities dominated. This development resulted in more acidic runoff to adjacent lakes and affected carbonate precipitation there. Further, poor catchment retention of nutrients during poor fen/bog stages, probably caused increased fluxes of nutrients out of the system, stimulating primary lake productivity in adjacent lakes. Increased lake productivity in turn caused increased oxygen consumption for decomposition at the lake bottom, and thus anoxic conditions. Thereby an increased flux of phosphorous from the sediment triggered a state of self-sustained eutrophication during two centuries, preceding the onset of 20th century permafrost thaw. Proxy indications of peat surface moisture conditions and lake-water TOC concentration dynamics during the last 100 years were reconstructed by means of testate amoebae assemblages in peat and near infrared spectroscopy and the carbon isotopic composition of lake sediment bulk organic matter. These results revealed a close connection with decadal trends of total annual and summer precipitation as well as single years with anomalously high precipitation, especially in the late summer. The data could thus not be directly linked to monitored trends in active layer thickness.