| Summary: | Advisors: Nathan D. Stansell. Committee members: Justin Dodd; Eric Klein. Includes bibliographical references. Includes illustrations and maps. Oxygen isotopes derived from authigenic carbonate from open lake systems record variations in seasonal precipitation source. This study focuses on the sediment record from Lake Nuudsaku in southern Estonia to evaluate how winter versus summer precipitation has changed throughout the Holocene as a result of fluctuating North Atlantic Ocean conditions, primarily the North Atlantic Oscillation (NAO). Estonia receives precipitation with a lower [delta]¹⁸O value from the North Atlantic and Baltic Sea during the winter months. In contrast, during the summer months Estonia receives precipitation with greater [delta]¹⁸O values from warmer North Atlantic waters and from the Mediterranean Sea and Black Sea. Therefore, lower [delta]¹⁸O values in the carbonate record were interpreted as periods of time in which there were increases in the amount of winter precipitation. Oxygen isotope data indicate relatively wet winters during the early Holocene (9960 to 8800 cal yr BP) followed by a shift toward drier winters during the middle of the Holocene (8800 to 4200 cal yr BP). The late Holocene (4200 cal yr BP to the present) was characterized by the wettest winters recorded in the oxygen isotope record. The periods of increased winter precipitation in the Baltic region generally coincided with periods of increased NAO index between 5200 cal yr BP and 1000 cal yr BP. There was an inverse relationship between winter precipitation and NAO index during the Medieval Climate Anomaly and the Little Ice Age (900 to 100 cal yr BP). The positive relationship between NAO index and winter precipitation in Northern Europe is present once again in the modern setting and has persisted since at least AD 1950. M.S. (Master of Science)
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