Late quaternary paleoclimate in semiarid northeastern Brazil from U-Series dating of travertine and water-table speleothems

Fossil travertine and subaqueous speleothems in presently dry caves suggest periods of enhanced ground-water recharge during the Quaternary Period in semiarid northeastern Brazil. Travertine deposits and water-table speleothems were dated by the Th-230/U-234 technique, yielding new evidence for the timing of past periods of increased rainfall. The travertine was deposited in two distinct phases, extensive in situ deposits during marine isotope stage 2 and fragmentary, indurated deposits dating to ca. 400,000 yr B.P (possibly marine isotope stage 10 or 12), Dating of water-table speleothems showed that the regional water table was 13 +/- 1 m above the present level at the last glacial maximum. A second site yielded ages suggesting a still higher water table during marine isotope stage 6. These new data demonstrate that northeastern Brazil did not follow the general pattern of a dry last glacial maximum, as suggested by other continental records in Amazonia and southeastern Brazil, and as depicted in recent General Circulation Model(GCM) simulations. Our results thus indicate that significant variability in regional rainfall patterns occurred in the area during glaciations and caution against overgeneralization of paleoclimate patterns. (C) 2001 University of Washington.