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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Speleothem, Holocene, Oxygen Isotope, South America, Paleoclimatology
Isotopic investigations of speleothem CR1 from Cristal Cave in southeastern Brazil have established a high-resolution record of climate change extending 4,100 years prior to sample collection in 2007. A total of 14 subsamples were collected from CR1 for U/Th age determination. ICP-MS analysis yielded very precise ages with analytical errors (2σ) averaging ± 13 years. An initial growth rate of .062 mm y-1 for the first 2860 years is followed by a rate of .08 mm y-1 for the remaining growth period allowing for sampling of δ18O at sub-decadal resolution. Stable isotope analyses show a large range of δ18O values between -7.5 ‰ to -4 ‰. The data show a trend of steadily decreasing values over the past 4,000 y BP. The exception to this trend is the last 150 years BP when some of the least negative and most variable values for the record are observed.
Variations in speleothem δ18O in southern Brazil have been shown to reflect changes in rainfall δ18O, which in turn indicate changes in rainfall source or rainfall amount (Cruz et al., 2006). In Southeastern Brazil, δ18O is controlled mainly by moisture source location, in particular South American monsoonal versus extratropical sources (Cruz et al., 2005). The relative contribution of monsoonal and extratropical moisture help to define the δ18O of regional precipitation via the mean location and southward extent of Hadley cell convective activity associated with the South American summer monsoon (Cruz et al., 2005). For example, decreases in precipitation δ18O are often interpreted as a decreased contribution of winter versus summer precipitation (Cruz et al., 2005). Assuming that the modern (observed) relationship between the seasonality and moisture source location effect occurring in southeastern Brazil have functioned for the past ~ 4,100 years, trends of more negative values towards modern day are likely due to increased summer precipitation and/or an overall increase in total yearly precipitation. The more enriched values of the past 100 years suggest a recent decrease in summer and/or total rainfall.
Stephen J. Burns