Varve chronology, proxy calibration, and Holocene climate of Minnesota

Paleoclimatic studies are essential to our understanding of climatic changes because current and future climate can only be rigorously evaluated in the context of natural climatic variation. In this dissertation two general topics of paleoclimatic studies were investigated: (1) the accuracy of a varve chronology, and (2) the reliability of commonly-used proxy indicators for sediment-based paleoclimatic reconstruction. In addition, Holocene climatic patterns and potential mechanisms were discussed based on the analyses of sediment cores from Steel Lake (SL) and West Olaf Lake (WOL), Minnesota.; Varved sediments are valuable for paleoclimatic studies because of their potential to provide calendar-year-resolution chronologies. However, varve chronologies are susceptible to errors. Thus, an independent age control is needed to verify varve counts. Nine high-quality 14C dates were used to compare with the 3000-year SL varve chronology. The comparison revealed ∼240 years of varve error concentrated in two stratigraphic intervals with ambiguous laminations. Corrections were applied to the raw varve counts, which resulted in a varve chronology in good agreement with the 14 C chronology based on the OxCal V_Sequency analysis.; The accuracy of paleoclimatic inferences relies on an understanding of which climatic variables control proxy indicators. The SL varved sediment provides an excellent opportunity for proxy indicator calibration. Comparison of proxy indicators with weather variables revealed that the calcite oxygen- and carbon-isotopic compositions, varve thickness, sediment accumulation rate, and biogenic silica are sensitive to temperature and moisture variations. In conjunction with calibration studies from other regions, our result demonstrated that proxy calibration is region specific.; Despite the numerous Holocene paleoclimatic records from the North American continent, few existing records from the American Midwest have adequate chronological controls, marking it problematic to link regional climatic changes to possible larger-scale forcing. With solid 14C chronologies, the multi-proxy records from SL and WOL revealed (1) that the abrupt transition from the early- to mid-Holocene postdated the 8.2 ka event at the 95% confidence level of 14C dates, (2) that the two mid-Holocene climatic stages likely resulted from a regional reconfiguration of air masses, and (3) that millennial-scale climatic cycles did not occur in the midwestem United States.