Reconstructing the African Humid Period

 
 

The onset and termination of the African Humid Period mark the most dramatic changes in North African climate of the past 20,000 years. During the African Humid Period (~15-5 ka), the modern-day Sahara was the site of multiple large lakes as well as extensive vegetation, animal life and human settlements. These conditions are thought to relate to a precessional increase in local summer insolation, which led to an intensification of the North African summer monsoon.


Several records suggest that the African Humid Period was marked by greatly reduced dust emissions from North Africa. The most highly-resolved dust flux record available, that from ODP site 658, indicates that changes in dust emissions associated with the beginning and end of the period were extremely abrupt, with dust fluxes changing by a factor of two within centuries or less (see figure below; deMenocal et al., QSR 2000; Adkins et al., Paleoceanography 2006). Though the abrupt onset of humid conditions may be tied to high-latitude climate change during the deglaciation, the abrupt increase in dust emissions at the end of the humid period 5,500 years ago remains puzzling: with insolation changing only gradually and no substantial high-latitude changes, what caused this step-like change in North African climate?


 

My current work in collaboration with Peter deMenocal, Gisela Winckler (LDEO) and Jan-Berend Stuut (NIOZ) will determine whether dust fluxes changed with similar timing, abruptness and magnitude throughout North Africa during the transitions into and out of the African Humid Period. By measuring dust fluxes in a north-south transect of 8 cores along the North African margin, we will be able to reconstruct the evolution of the North African dust plume over the last 20,000 years. This work will provide a robust foundation for future model-based explorations of the potential for abrupt changes in monsoonal regions.

(Above) Map of core sites (black dots) and ODP 658 (red) along with satellite-based estimate of relative atmospheric dust content (darker=more dust; adapted from Cole et al., EPSL 2009).


(Below) North African summer insolation over the past 25,000 years and the record of terrigenous (~dust) concentrations and fluxes measured at ODP site 658 (deMenocal et al., 2000). Note the abrupt increase in dust at the end of the AHP at 5.5 ka.