Holocene climate and atmospheric circulation changes in northern Fennoscandia Interpretations from lacustrine oxygen isotope records

Sammanfattning: This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (?18Olakew) collected between 2001 and 2006 show that ?18O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (?18Op). Changes in local ?18Op depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water ?18O is determined by a combination of the original ?18Olakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various ?18Olakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (?18Odiatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing ?18Op trend was likely forced by a shift from strong zonal westerly airflow (relatively high ?18Op) in the early Holocene to a more meridional flow pattern (relatively low ?18Op). The large ?18Olakew depletion recorded in the ?18O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.