Paleohydrological changes during the last deglaciation in Northern Brazil

We here report a reconstruction of hydrological balance variations in Northern Brazil for the last 20 ka deduced from the delta D values of aquatic and land plant molecules extracted from the sediment infill of Lake Caco. Our reconstructed precipitation, lake water isotope ratio and evaporation-evapotranspiration isotope effect allows us to obtain an estimate of moisture balance, and, to a lesser extent, precipitation amount and seasonality changes. During the end of the Last Glacial Maximum (LGM, between ca 20 and 17.3 ka), high delta D values and smaller fractionation of leaf waxes indicate an and to semi-arid climate with a long lasting dry season. An abrupt change towards much wetter conditions occurred within ca 500 years from 17.3 to 16.8 ka, as shown by a 50 parts per thousand decrease in D/H ratios and a marked increase in H isotopic fractionation of leaf waxes. This abrupt isotopic change coincides with a major transformation from savanna-dominated vegetation to humid rain forest around the lake, based on pollen data. Comparisons with other paleo-precipitation records from South American sites indicate that Lateglacial humid conditions were controlled by intensification of the ITCZ and/or a southward shift of its mean position across our study site. Our isotope data show only a small rise in aridity during Younger Dryas event (13-11.5 ka). Although the Holocene was not screened in details, D/H ratios of terrestrial and aquatic compounds show near constant offsets, suggesting stable and relatively humid climate conditions during this period. (c) 2007 Elsevier Ltd. All rights reserved.