The “African humid period” and the record of marine upwelling from excess ²³⁰Th in Ocean Drilling Program Hole 658C

Abstract

[1] Using a high-resolution 230 Th normalized record of sediment flux, we document the deglacial and Holocene history of North African aridity and coastal upwelling at Ocean Drilling Program Hole 658C. At both the end of the Younger Dryas and after the 8.2 ka event, there are significant drops in terrigenous accumulation at our site, indicating an increase in the monsoon moisture flux over Africa at this time. At 5.5 ka, there is an abrupt end to the ''African humid period'' and a return to stronger upwelling conditions. For carbonate and opal fluxes the 230 Th normalization completely changes the shape of each record based on percentage variations alone. This site is a clear example of how variations in one sediment component can obscure changes in the others, and it demonstrates the need for radionuclide measurements more generally in paleoceanography. By taking our new records and a large amount of previous data from this site we conclude that increases in African moisture are tightly coupled to decreases in coastal upwelling intensity.Citation: Adkins, J., P. deMenocal, and G. Eshel (2006), The ''African humid period'' and the record of marine upwelling from excess

The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system

“…In good agreement with the Holocene climate evolution modelling experiments have demonstrated an intensification of summer monsoon activity in response to precession-forced changes (Claussen et al, 1999;Schefuß et al, 2003, Marzin andBraconnot, 2009). Proxy data indicate that the Holocene Wet African Period (12 to 5.5 ka) coincided with the regional climate optimum and major insolation change (deMenocal et al, 2000a, b;Adkins et al, 2006;Tjallingii et al, 2008). For MIS 11c, however, we find a somewhat different relation: Wet conditions lasted almost twice as much longer in comparison to the Holocene, from 420 to 409 ka, and passed through an insolation minimum (Fig.…”

“…Because Site 958 is affected by coastal upwelling (Johnson and Stevens, 2000) we can assume that a substantial part of the SST increase during the early interglacial phase of MIS 11c reflects a successive weakening of the coastal upwelling cell caused by weakened trade wind strength. As such conditions are contemporaneous with wetter conditions on land, the period between 420 and 409 ka is therefore directly comparable with the African Wet Period of the early Holocene (deMenocal et al, 2000a, b;Adkins et al, 2006;Helmke et al, 2008;Tjallingii et al, 2008).…”

“…The abrupt change of this arid phase towards more moist conditions during the later terminations is also characteristic for glacial-interglacial transitions in general, and is the result of geographical shifts and intensifications changes within the West African Monsoon system (Balsam et al, 1995;Pflaumann et al, 1998;Tjallingii et al, 2008). Like in the early Holocene monsoon intensification over NW Africa during the early phase of MIS 11c was probably related to a northward-moving ITCZ, which caused a weakening of the trade winds and, accordingly, a decrease in coastal upwelling intensity (Adkins et al, 2006;Weldeab et al, 2007). Due to the combined effects of vegetation cover/enhanced precipitation and diminished trade wind strength, total dust flux off NW Africa is reduced when North Atlantic SSTs rise and vice versa (e.g., Tjallingii et al, 2008).…”

Collapse of an ecological network in Ancient Egypt

“…Termination of the AHP was associated with increasingly weak summer monsoons (6) and the disappearance of many Egyptian species, including spotted hyenas, warthogs, zebra, wildebeest, and water buffalo (7)(8)(9)(10), as well as the onset of dense human settlements in the region (11). A sharp increase in aridification ∼5,000 y B.P.…”

Quaternary Glaciations of the Atlas Mountains, North Africa

“…One hypothesis is that the West African Monsoon was strengthened when sea-surface temperatures in the North Atlantic were much cooler than present, such as during the Late Pleistocene/Early Holocene transition when the vast Laurentide ice sheet was decaying and releasing cold meltwater into the North Atlantic. This resulted in a steep thermal gradient between the North Atlantic Ocean and the Western Sahara, thereby enhancing the West African Monsoon (Renssen et al, 2003)-a situation promoted by cold upwelling in the Atlantic off the coast of North Africa at the end of cold phases such as the Younger Dryas (12.9-11.7 ka) and the 8.2 ka Event (Adkins et al, 2006). This produced a greater latitudinal temperature gradient between the tropics and the mid-latitudes over Africa, which effectively forced the West African Monsoon further north over the current Sahara region.…”