Late Quaternary upwelling off tropical NW Africa: new micropalaeontological evidence from ODP Hole 658C

Abstract

Planktonic foraminifera and radiolaria have been analysed in a Late Quaternary (40-0 ka) sediment sequence from Ocean Drilling Program (ODP) Hole 658C located under a coastal upwelling system near Cap Blanc, offshore northwest Africa, in order to document the palaeoceanographic history of the area. Temporal variations in species abundance and faunal assemblage analysis reveal a tripartite phased sequence of palaeoceanographic change through the Late Quaternary. Phase 1 spans 40-14.5 ka and is characterised by moderate upwelling, but Heinrich event 2 is distinguished as a brief episode of strengthened upwelling. Phase 2 begins with a change in a number of variables at ca. 14.5 ka and extends to ca. 5.5 ka. This phase is characterised by a general strengthening of upwelling, but may be subdivided into three minor phases including (a) the recognition of the Younger Dryas, marked by a temporary reduction in upwelling strength, followed by (b) an intensification of upwelling, and (c) upwelling and high productivity between 8 and 5.5 ka. This phase of upwelling corresponds with maximum Holocene cooling, possibly triggered by the collapse of the Laurentide ice sheet. Phase 3 extends from 5.5 to 0 ka and is characterised by weak upwelling and significant calcite dissolution. These phases are related to climatic events, particularly the African Humid Period (AHP), which is coincident with Phase 2. The AHP is characterised by increased precipitation, linked to an intensification of the African monsoon that enhances North East Trade Wind-driven coastal upwelling and is associated with the expansion of continental vegetation across North Africa.

Upwelling

“…Arid conditions with more upwelling also characterized the cold Younger Dryas period. These data contradict interpretations based on foraminifera and radiolaria (Haslett and Smart, 2006) but make more sense in terms of the observed fluxes of terrigenous material (Adkins et al, 2006;Adkins et al, 2007).…”

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

“…The overall correspondence between the alkenone SST and Th‐normalized flux data shown in Figure 8 provides compelling evidence linking the deglacial and Holocene evolution of NW African climate and coastal upwelling. Our flux based inferences of weaker upwelling during the AHP are in direct contrast to foram‐ and radiolarian‐based upwelling indices [ Haslett and Smart , 2006]. Our resolution to this difference is presented below.…”

Warming and Cooling: The Medieval Climate Anomaly in Africa and Arabia

“…Similar as in Morocco, MCA palaeoproductivity was high, as evidenced by diatom assemblages (Knaack, ) and an increase in opal (deMenocal et al, ) (Figure ). Previous temperature reconstructions for this core based on foraminiferal assemblages may be unreliable; see details in the supporting information (deMenocal et al, ; Haslett & Smart, ).…”