An Oceanic Perspective on the African Humid Period

Data gathered from cores drilled off NW Africa have been instrumental in understanding recent climate change in northern Africa, including the most recent African Humid Period (AHP). Early work by deMenocal et al. (2000, Quat. Sci. Rev. 19: 347-361) showed that dust (as terrigenous percent) increased and then decreased abruptly at the onset and termination of the AHP, and linked that on/off switch to a threshold crossing in precessional index. However, this perspective differs from recent lacustrine and palynological evidence from Lake Yoa in the eastern Sahara suggesting a more gradual transition out of the AHP (Kroepelin et al., 2008, Science 320: 765-768). We will present major and trace element and radiogenic isotope data from the last ~25 kyr at ODP 658C off Mauritania. Large and abrupt shifts, notably in Sr isotope ratios, are contemporaneous with changes observed in dust flux and cannot be explained simply by changes in geologic source terrane or grain size sorting that might result from a shift in wind direction or wind speed. We envisage the addition of highly weathered, authigenic mineral phase(s) formed in lakes during the AHP. Additionally, we will present new data from the recent CHEETA (Changes in the Holocene Environment of the Eastern Tropical Atlantic) sediment coring cruise, a transect of 28 coring stations from Gibraltar to Senegal. Preliminary results suggest that the abrupt termination of the AHP near 5.5 ka BP can be traced from Senegal to the Canary Islands (roughly 18-28°N), suggesting that the end of the AHP was indeed abrupt along the western African margin whereas it was more gradual in the eastern Sahara.