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 spectroscopy 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 varied during the Late Holocene. The comparison between minerogenic inputs and vegetation changes improves the understanding of the inferred climate dynamics. Since at least 5400 cal yr B.P., the paleomonsoon rainfall intensity decreased, as shown by a general decrease in mineral fluxes. This observation is consistent with a gradual weakening of the boreal summer insolation in tropical latitudes. However, the major vegetational change lags behind the 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 replaced by pioneer trees and Poaceae, when the mineral fluxes attained their lowest values. This episode of maximum dryness is attributed to an abrupt climatic event of global significance which is superimposed onto the paleomonsoon variability. It is related to a cold event, which in turn produced a change in the lower atmospheric circulation that was characterized by a strengthening of northern trade winds, probably correlated with sea-surface temperature variations in the eastern tropical Atlantic area. (C) 2001 University of Washington.