Date of Award
Master of Science
Geography and Environmental Resources
My study was designed to identify the relationships between climate variables, such as precipitation and temperature, and δ 18 O values of tree ring &alpha-cellulose extracted from exactly dated tree rings of Pterocarpus angolensis growing in the arid to semiarid Mzola region of western Zimbabwe. This species is known to be sensitive to seasonal variation in rainfall. In this region, the wet season occurs during the austral summer from mid November to early April followed by a dry winter season from around June through October. Overall, the total annual rainfall exhibits a high degree of spatial and temporal variation with a mean of less than 600 mm per year. I applied the Modified Brendel technique to isolate &alpha-cellulose from raw wood samples extracted from two P. angolensis trees and measured the α-cellulose δ 18 O values using continuous flow isotope ratio mass spectrometry. I developed a 30-year (1955-1984) &alpha-cellulose δ 18 O chronology and correlated it with tree-ring width, meteoric water δ 18 O values, monthly and seasonal precipitation totals, and mean seasonal temperature. The δ 18 O values of meteoric water for this region were obtained from the Global Network of Isotopes in Precipitation (GNIP) and correlated with the δ 18 O values of tree ring &alpha-cellulose. The strongest correlations were observed between &alpha-cellulose δ 18 O values and February total precipitation (r = -0.49, p = 0.006) and to a lesser degree total wet season (NDJFMA) precipitation, In particular, unusually rainy wet seasons were significantly correlated (r = -0.79, P = 0.007) with &alpha-cellulose δ 18 O. This relationship is consistent with 18 O-depleted values measured in summer precipitation during periods of high rainfall, which is most likely the result of the isotopic amount effect reported in tropical regions. I also identified a positive correlation (r = 0.49, p = 0.03) between &alpha-cellulose δ 18 O and the δ 18 O values of meteoric water, and investigated the possibility of an isotopic temperature effect for δ 18 O in meteoric water, which also may be reflected in the δ 18 O values in tree ring &alpha-cellulose. The strongest correlation with mean temperature was observed during the wet summer season (r = 0.56, p = 0.01). My results suggest that the δ 18 O values of P. angolensis tree rings can be used as natural indicators of paleoclimate in southern Africa.
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