Orbital and Millennial-Scale Precipitation Changes in Brazil from Speleothem Records

Abstract

Paleorainfall variability on orbital and millennia] time scales is discussed for the last glacial period and the Holocene, based on a multi-proxy study of speleothem records from Brazil. Oxygen isotope (delta(18)O) records from Botuvera and Santana eaves, precisely dated by U-series methods, indicate stronger summer monsoon circulation in subtropical Brazil during periods of high summer insolation in the southern hemisphere. In addition, variations in Mg/Ca and Sr/Ca ratios from speleothems confirm that this monsoon intensification led to an increase in the long-term mean rainfall during insolation maxima. However, they also suggest that glacial boundary conditions, especially ice volume buildup in the northern hemisphere, promoted an additional displacement of the monsoon system to the south, which produced rather wet conditions during the period from approximately 70 to 17 ka B.P., in particular at the height of the Last Glacial Maximum (LGM). These delta(18)O records, together with speleothem growth intervals from northeastern Brazil, have also revealed new insights into the influence of the northern hemisphere millennial-scale events on the tropical hydrological cycle in South America. This teleconnection pattern is expressed by an out-of-phase relationship between precipitation changes inferred from speleothem records in Brazil and China, particularly during Heinrich events and the Younger Dryas. We argue that the pronounced hemispheric asymmetry of moisture is a reflection of the impact of meridional overturning circulation conditions on the position and intensity of the intertropical convergence zone (ITCZ).