Intensification of the NE Trades and Increased Upwelling off the Coast of Mauritania Prior to the Onset of the African Humid Period.
Abstract
Following the last deglaciation and the Younger-Dryas cold event, a warm and moist period of roughly 7,000 years dominated the North African climate. Driven by precessional changes in the Earth's orbit, increased summer insolation and sea-surface temperature during this period resulted in the strengthening of the N African monsoon and an increase in the precipitation over both the Sahel and Sahara regions. Associated with the development of the pronounced continental low pressure center that regulated this change was the migration of the ITCZ to a more northerly position. This relocation of the meteorological equator in turn resulted in a change in the pattern and intensity of the NE trades and, ultimately, upwelling in the coastal cell adjacent to NW Africa. A number of studies, particularly that done by deMenocal et al.(2000) on ODP site 658C (20°45'N, 18°35'W, 2263m), indicate a sustained increase in the relative abundance of CaCO3 in the marine sediment near Mauritania from 14.8 to 5.5 kyr BP, implying a combination of humid conditions and regionally increased upwelling over this entire interval. However, our evaluation of the 230Th-normalized particle flux at a neighboring site, IMAGES core MD03-2705 (18° N, 21° W, 3100m), indicates that the actual preserved rain rate of CaCO3 in this region was much greater (approx. 300% )than LGM levels during the glacial-interglacial transition (17,000-11,000 BP) but decreased rapidly after 11,000 BP to Holocene levels about 15-20% greater than LGM. These results suggest that the NE trades and consequently the upwelling off Mauritania were intensified only during the deglacial. This phase was further punctuated by a drastic drop in CaCO3 flux and a massive increase in dust during the Younger-Dryas. The abrupt decrease in CaCO3 flux that occurred subsequently (approx. 11,000 BP) was approximately coincident with the insolation maximum of N Africa and the onset of the African Humid Period, as determined by terrestrial records and our analysis of the 230Th-normalized clay and 232Th (indicative of the terrigenic fraction in marine sediment) fluxes. As such, the low pressure center that developed over NW Africa at this time may have been substantial enough to have drawn the ITCZ north of MD03-2705 resulting in both a dramatic weakening of the trades-driven upwelling off NW Africa and the humid conditions that prevailed on the continent.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMPP21C1694K
- Keywords:
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- 4901 Abrupt/rapid climate change (1605);
- 4924 Geochemical tracers;
- 4934 Insolation forcing;
- 4946 Milankovitch theory;
- 4964 Upwelling (4279)