Geomorphic response to rapid climatic and hydrologic change during the late pleistocene and early holocene in the humid and sub-humid tropics

Abstract

The data base concerning late Quaternary environmental change in the humid tropics is dependent on records from scattered sites; the intensity and duration of wet-dry oscillations remain speculative, and the responses of hillslopes and river systems are based on over-simplified models. However, available evidence indicates that prolonged aridity affected all but a few favoured core areas of equatorial climate after 20,000 BP, lasting 5–7 ka. Dry conditions at the LGM were marked by semi-arid landforms and reduced steam activity. Large palaeofloods occurred after 13,000 BE but dry conditions returned after 11,000 BP, before the early Holocene pluvial led to abundant sedimentation lasting nearly 2 ka after 9500 BP, in Africa, Amazonia and Australasia. Erosion (channel cutting) and flood deposition occurred at the Pleistocene-Holocene transition, followed by multiple shifts from lateral to vertical accretion (fill-cut episodes) during the early Holocene, while thick overbank deposits formed during the later Holocene. Re-establishment of the lowland rainforests was delayed until after 9000 BP in Africa, Australia and Brazil and several wet-dry oscillations followed in the mid Holocene period. Hillslope activity at the LGM was marked by local sediment transfers and fan formation; at the termination sediment fluxes increased rapidly, but mass movement probably peaked after 10,000 BP Sediment delivery to stream channels was not immediate and large sediment stores remain on the landscape today, many in areas of potential sensitivity of erosion.