Late Quaternary environmental changes and the alluvial record in humid tropical environments

Ice core and ocean-drilling records now confirm the evidence from lakes, rivers and mires, concerning temperature depression in the humid tropics during the LGM by at least 5 degrees C on land and this accords with the recent CCM1 model output. There is less agreement about fluctuation in precipitation amount, intensity and seasonality during the late Quaternary. Pollen records remain indeterminate because of the absence of sites in lowland rainforest locations and a widespread hiatus in sedimentation spanning the LGM. Alluvial sediments have often been ignored by specialists from other disciplines, bur they also provide a Valuable source of palaeoenvironmental information that reflects catchment-wide conditions. Current evidence shows that prolonged sedimentation took place in Stage 3 at sites in S America and SE Asia, and this was followed by incision. For up to 9 ka, spanning the LGM streams either produced little sediment or were characterised by braided channels and sometimes fan deposits, until the transition towards an interglacial mode took place after 12,700 BP (C-14). In many places, this transition was marked by large floods, incision and deposition of coarse sediment. But as the Holocene climates stabilised and fluctuations of runoff reduced in amplitude, so meandering rivers developed floodplains with thick overbank silts and clays. These changes have regional and intercontinental patterns, not everywhere identical in bracketing dates, but demonstrating major changes to the fluvial systems as a result of profound changes to the hydrology of forested tropics during the last 60 ka. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.