Amazonian paleoecological histories: one hill, three watersheds

Data from the Hill of Six Lakes, in the northwestern Brazilian Amazon region, provide three records of paleoclimatic and vegetation change in lowland Amazonia that span the last 170,000 years. Three lakes, Verde, Pata and Dragão, which occupy separate watersheds on the hill, provide the most detailed image yet obtained of ice-age conditions in lowland Amazonia. Well-dated sedimentary records for fossil palynological, charcoal, cation, and pigment, data are presented.The data indicate the continuous presence of mesic forest throughout the last ice age. Oscillations of lake level are recorded and the lowstands are attributed to reduced precipitation inputs to systems delicately balanced between water loss (leakage through the floor of the basin) and gain (precipitation). Gross stratigraphy, algal remains, and paleochemistry suggest that the stands of high and low lake level were cyclic, apparently correlating precessional orbital variation. Times of lake lowstand coincide with wet season (December-January-February, DJF) insolation minima. The strongest of eight lowstand cycles occurred ca. 35,000 to 27,000 cal BP.Even during lowstand episodes, pollen is well preserved and provides a clear signal of uninterrupted forest cover. The principal lowland elements are continuously present in the record, suggesting the long-term (Quaternary) availability of the lowland forest biome in this region. However, during the late Pleistocene the forest changed in composition with the expansion or invasion of montane floral elements creating communities of the mesic forest biome without modern analogs. While precipitation cycles were causing lake levels to rise and fall, the principal influence on vegetation appears to have been cooling. In the late Pleistocene, the population expansion of montane elements at lower elevations is consistent with a cooling of 4-5 °C.