Late Quaternary climate changes in Brazil recorded by speleothems

Cave calcite and surface spring deposits from various locations in Brazil were studied to reconstruct changes in climate and vegetation during the last interglacial-glacial cycle. A record of speleothem growth periods from northeastern Brazil and a record of speleothem oxygen isotopic ratio (18O/16O) from southern Brazil were established with uranium-thorium disequilibrium dating and stable isotopic analysis methods. Both records show significant millennial-scale variations and indicate that climate in these two areas, expressed as changes in meteoric precipitation, has undergone abrupt oscillations in the past. The cave records were compared to other global paleoclimatic records, based on their independent chronologies, to understand mechanisms of abrupt climate events. While the two Brazilian records are positively correlated (in-phase precipitation changes), both are anti-correlated with contemporaneous records in the northern low latitudes (anti-phase precipitation changes) on millennial time scales.; These observations suggest that a rainfall seesaw may have existed between the southern and northern low latitudes on millennial time scales in the past. This interhemipheric anti-phasing of rainfall is most likely tied to North Atlantic thermohaline circulation changes, which affect not only North Atlantic climate but also the displacement of the mean position of the intertropical convergence zone (ITCZ) and associated asymmetry of the Hadley cell. The Brazilian speleothem records further indicate that millennial-scale abrupt climate change is largely involved with meridional rather than zonal atmosphere-ocean dynamics, therefore, unlikely to have been triggered by a tropical source. The modern appearance of a strong El Nino-Southern Oscillation (ENSO) signal in tropical Atlantic climate was perhaps induced by an intermediate state of the mean ITCZ position and relatively weak asymmetry of the Hadley circulation, and can be monitored by the differences in speleothem calcite oxygen isotopic compositions. In addition, U-Th dates of speleothems and travertines from currently semiarid northeastern Brazil, combined with geomorphic and paleobotanic features, can provide a regional history of major environmental changes, such as rainforest expansion and shrinkage.*; *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat.