MILLENNIAL-SCALE CHANGES IN PALEOPLUVIAL PHASES INFERRED FROM SPELEOTHEM DEPOSITS IN NORTHEASTERN BRAZIL

Records of millennial-scale climate oscillations from the southern tropics exist but are still sparse. More data are needed to establish a satisfactory mechanism for these variations. Speleothems of arid and semiarid regions may cease to grow during dry intervals and therefore record only periods of relatively wet climate. Constraints on these past pluvial periods are possible provided the ages of correlated speleothem growth can be accurately determined using U-series dating techniques. We have analyzed stalagmite subsamples from caves in semiarid northern Bahia state, northeastern Brazil. In this location, speleothem deposition does not presently occur, which delineates any dated subsample as a time when climate was wetter than today. Determined with mass spectrometric techniques, our preliminary analysis of sample ages shows that all speleothems grew during glacial periods. The pattern of speleothem growth indicates that pluvial phases in semiarid northeastern Brazil are coincident with higher insolation at 10oS during the austral summer. In addition, short speleothem growth intervals show millennial-scale changes, which can be correlated to periods of cold and dry stadials in Greenland ice core records, such as Heinrich events 1, 4 and 6. This correlation may indicate a climatic teleconnection between the tropics and northern high latitudes. Abruptly enhanced precipitation in this region is generally in accordance with dramatically weakened North Brazil Current observed in the continental margin sediment cores. This may be due to the southward shift of tropical Atlantic warm water associated with thermohaline circulation shutdown. In combination with increased insolation, this shift may cause positive anomalies on sea surface temperatures in the southwestern tropical Atlantic, change easterly trade wind strength and enhance zonal moisture transport to the continental areas. Synchronicity between these Brazilian pluvial phases and times of relatively weak East Asian summer monsoon (recorded in Chinese speleothems) further illustrates the significant contribution the tropics give to these abrupt climate variations, acting either as one of control sources or as an amplifier.

P.S. Financial support from the INQUA Executive Committee