Late Quaternary climate and environmental change in the summer rainfall region of South Africa : A study using trees and wetland peat cores as natural archives

Sammanfattning: This thesis contributes with information on past climate and environmental changes in South Africa’s summer rainfall region. The study is based on multi-proxy analyses on wetland peat cores and analyses of stable isotope composition (?13C, ?18O) and wood anatomy in cross sections from subtropical trees (Breonadia salicina). The peat archive covers the last 16 ka (ka; 1000 cal yrs BP) and was analysed in terms of fossil pollen, charcoal, diatoms, phytoliths and stable isotope composition. The peat record infers relatively wet climate conditions at c. 13.7-12.8 ka, 10.5-9.5 ka and 2.5-0.5 ka, and drier conditions at c. 16-13.7 ka, 12.8-10.5 ka and during mid-Holocene. Tentatively, cooler late Pleistocene temperatures shifted towards warmer after c. 9.5 ka. The study of B. salicina demonstrates the paleo-climatic value of subtropical trees despite absence of annual tree rings. An age model was constructed from radiocarbon dating and calibration by wiggle matching. ?13C in B. salicina shows a co-variation with annual rainfall amounts, suggesting that it may be considered a regional climate-proxy. ?18O is mainly influenced by local factors, but acts as a useful complement when interpreting ?13C. Together with other regional, high-resolution records, the 600 year long ?13C-record suggests dry climate conditions in northern South Africa during the 1700s and mid-1500s AD. Inferred climate and environmental changes are suggested being a response to expansion, contraction and latitudinal shifts of the tropical, subtropical and mid-latitude atmospheric circulation cells. An observed inverse humidity pattern between southern and equatorial Africa suggests that ENSO-like teleconnections may be a possible forcing mechanism in a decadal to centennial time perspective.