Stormy records from peat bogs in south-west Sweden : implications for regional climatic variability and vegetation changes during the past 6500 years

Abstract: This thesis aims to reconstruct and explain variations in storm activity, humidity, vegetation composition and land cover in south-west Sweden during the past 6500 years. Two ombrotrophic bogs, Store Mosse and Undarsmosse, situated on the coastal plain of Halland, were selected for this purpose. The coastal plain of Halland is positioned in the cyclone path and is therefore particularly sensitive to changing cyclone frequencies and associated storms.
Cyclones have a strong control on the climate in this region, by moderating summer and winter temperatures and being the main source of precipitation. Aeolian sediment influx (ASl) was used as a proxy for storm activity. Bog surface wetness reconstructions were based on organic bulk density values from both study sites, whereas testate amoebae analysis was applied on cores from Undarsmosse bog only. Pollen analysis was carried out to reconstruct vegetation and land use changes. In addition the hypothetical regional scale vegetation composition around the
Store Mosse bog was reconstructed by applying the REVEALS model, which corrects for differences in pollen productivity and dispersal between plant taxa. The land use reconstructions were also used to discriminate between climatic and human induced variations in ASI values, since increased sediment availability resulting from human activity may have affected sand influx into the bogs. The reconstructions of storm activity are similar at both study sites, with peaks of storminess around 4700, 2800-2000, 1500, 1100, 700 and 400-50 cal. yrs BP. These
time periods are in good agreement with, for example, dune re-activation phases in Denmark. A comparison to land use in the region shows that ASI values are highest when the indications for cultivated fields are low, which implies that land use cannot explain the occurrence of ASI peaks. These are therefore most likely causally related to regional scale climatic changes. Humidity shifts were recorded around 4800, 2000 and 400 cal. yrs BP at both sites, which concurs with lake chemistry and lake level reconstructions from southern Sweden. These results were used to propose a regional scale reconstruction of climatic change, and may be used to infer variations in past atmospheric circulation patterns. In addition, a detailed comparison between ASI values and testare amoebae based wetness reconstructions has revealed an enigmatic pattern of increased storm activity during shifts in humidity during the past 1700 years.