Late Holocene paleoclimatic changes in Western Central Africa inferred from mineral abundance in dated sediments from Lake Ossa (southwest Cameroon)

Minerals derived from catchment soils were determined using FTIR spectrosco py in the well-dated core OW4 from Lake Ossa, a lowland rainforest area in Cameroon. This quantification provides a hydrologic record indicating that the magnitude of runoff events, and by inference, rainfall pattern, has var ied during the Late Holocene. The comparison between minerogenic inputs and vegetation changes improves the understanding of the inferred climate dyna mics. Since at least 5400 cal yr B.P., the paleomonsoon rainfall intensity decreased, as shown by a general decrease in mineral fluxes. This observati on is consistent with a gradual weakening of the boreal summer insolation i n tropical latitudes. However, the major vegetational change lags behind th e onset of the decrease in mineral fluxes. From 2800 to ca 1000 cal yr B.P. , the forest receded: the amount of rainforest taxa decreased and is replac ed by pioneer trees and Poaceae, when the mineral fluxes attained their low est values. This episode of maximum dryness is attributed to an abrupt clim atic event of global significance which is superimposed onto the paleomonso on variability. It is related to a cold event, which in turn produced a cha nge in the lower atmospheric circulation that was characterized by a streng thening of northern trade winds, probably correlated with sea-surface tempe rature variations in the eastern tropical Atlantic area. (C) 2001 Universit y of Washington.