Physical, chemical and biological (microfossil) analyses of lake sediment cores enable lake, catchment and atmospheric histories to be reconstructed with high resolution over a range of timescales. In the SWAP Palaeolimnology Programme these techniques have been used to trace the recent (post 1800 A.D.) history of a number of specially selected study sites in Norway, Sweden and the U.K. For this purpose a large calibration data-set of diatoms and water chemistry has been assembled and new statistical techniques of pH reconstruction have been developed. In addition, by comparing temporal trends and spatial patterns within and between sites, various explanations for recent lake acidification have been evaluated.
Abstract Abstract Annual laminations — varves — in sediment cores from Järlasjön are described on the basis of microscopic examination and X-radiographic analysis. Dates are inferred from varve-counting and Cs-137 analysis. Four clay layers, which can be related to building-excavations in the surroundings of the lake, are exactly dated. Chemical changes in the recent sediment and in the sediment deposited around the isolation of the lake are discussed.
The response of diatom assemblages to changes in climate and nutrients over a 1200 year timescale was investigated by means of a multiproxy sediment core study involving radiometric dating, diatoms, grain size, loss on ignition and geochemical analysis. Four stages of environmental change were defined, each being consistent with changes in documented human activity and known climate patterns. From ad 750 to 880 relatively high nutrient status and a cold, unstable climate was inferred based on the high abundance of Aulacoseira subarctica and high Na/K and Na/Al values. In the following 1000 years (ad 880–1880) diatom assemblages were dominated by Cyclotella comensis, Cyclotella radiosa and Achnanthidium minutissimum, suggesting a long period of relatively low productivity and, by implication, that climate was the main control on the community during this period. Two climatic phases, namely the ‘Medieval Warm Period’ (MWP) and the ‘Little Ice Age’ (LIA), were apparent. According to shifts in the diatom assemblages and other proxies, it was proposed that the MWP was initiated from ~ad 880 and was terminated by the LIA at ~ad 1350. After ad 1880 the palaeoecological data revealed a period of strong human impact in the catchment and hence higher productivity. This work illustrates the complexity of climate–nutrient interactions and the roles of the two drivers on different timescales and at various stages of the lake’s history: climate impacts were more pronounced when nutrient concentrations were relatively stable prior to ~ad 1880; while nutrients appeared to play a more important role in regulating diatom communities from the mid-eighteenth century.
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