LATE QUATERNARY PALEOHYDROLOGY IN THE TROPICS AND SUB-TROPICS

Sparse records and correct interpretation of proxy data are major issues. Paleolake studies often ignore catchment conditions and groundwater flows; pollen spectra may not characterize vegetation patterns, and aridity is masked at wetland sites; ocean sediments are ambiguous about catchment behavior, and submarine fans of major rivers integrate fluvial response across many ecological zones. Use of terrestrial sediments to infer climate change implies that climate forcing determines fluvial behavior, but tectonic forcing, inherited morphology, complex response and human impacts are complications. Large drainage systems (Amazon, Congo, Nile) create major problems for sampling.

Quaternary sediments indicate prolonged deposition during Zone 3 continuing until 27ka. Forest changes and reduced rainfall became severe at the LGM. Fans, braided rivers and ephemeral tributaries that left few deposits, are recorded from Amazonia, Congo, E and W Africa, India and N Australia: 27/23-15/14ka. Recovery of climate was diachronous after 17ka. In Africa, lake overflows and floods occurred at 13.5ka and 11ka. Sedimentation increased x2 in the Ganges delta at 11ka. In W Africa channels were scoured, and gravels deposited after 15ka, in NE Australia fans entrenched after 14ka. From 11-8ka humid climates and rainforest were widely established and meandering channels replaced braided rivers as flow regimes stabilised. TheYD and mid-Holocene aridity affected lakes and rivers, but no change to fluvial style is evident.

Major issues include: 1/ the regionalisation of climate change and fluvial response across the tropics; 2/ the differential and diachronous response between equatorial and arid areas (severity of LGM, progress of wetter post-glacial, Holocene climates, collapse of rainfall in the mid-Holocene); 3/ the nature and timing of the fluvial response to hydrologic change on different timescales. Although lake levels and palaeofloods often indicate immediate responses to water inputs, the evolution of floodplains is more complex, requiring millennia of changing climate and land cover. But the major impact of Quaternary climates on stream and slope hydrology in the tropics is confirmed, and is fundamental to the evaluation of short-term changes in fluvial and hillslope systems.