Holocene and Younger Dryas speleothem-based paleoclimatic reconstructions from the northern Midwest, United States of America, the Ozark Highlands, United States of America, and the Pokhara Valley region, Nepal.

摘要

Chapter 1 demonstrates the potential impact of pre-infiltration evaporative effects on the oxygen isotopic composition of speleothem calcite while simultaneously demonstrating the consistency of carbon isotopes in preserving vegetation signals. Stalagmite growth rates determined from radiometric dating suggest that the middle Holocene prairie period (∼6000 to 3500 years before present) may have been marked by an increase in cool season infiltration and/or precipitation.; Chapter 2 is a comparison of the timing of the arrival of prairie into different areas in the northern Midwest using the carbon isotopic compositions of speleothems from four caves. A pronounced and sharp boundary between prairie and more mesic elements remained constant for approximately 1000 years during the middle Holocene suggesting that the eastward advance of Pacific air masses responsible for stabilizing prairie was stalled.; Chapter 3 is an attempt to relate the paleoenvironmental information preserved within speleothems from the Ozark Highlands to the limited paleoclimatic datasets currently documented for the region, namely a limited number of pollen spectra and vertebrate sequences.; Chapter 4 includes some of the speleothem data from Chapter 3 but also introduces analyses of speleothems collected from three other Ozark caves. Examination of this larger data set illuminates several small-scale isotopic shifts that may represent regional climatic trends including an early Holocene cool period that overlaps chronologically with a rise in global sea level.; Chapter 5 is an analysis and modeling study of a Younger Dryas-age stalagmite from the Ozark Highlands. Oxygen and carbon isotopic shifts indicate that the North American continental interior underwent significant climatic deterioration during the Younger Dryas with mean annual temperatures approximately 3--5°C cooler than present.; Chapter 6 is an attempt to reconstruct changes in the precipitation associated with Indian summer monsoon in the Pokhara Valley, central Nepal over the past ∼2300 years. In caves hosted by dolomite, aragonite is favored during times of a weakened monsoon while calcite is more likely to crystallize during wetter intervals. Uranium concentrations approaching 25 ppm allow for extremely precise age determinations using U-series mass spectrometry techniques.