40. MIOCENE–PLIOCENE SURFACE-WATER HYDROGRAPHY OF THE EASTERN EQUATORIAL ATLANTIC1Richard D. Norris2 |
ABSTRACTStable isotope analyses of four species of planktonic foraminifers and the benthic foraminifer, Cibicides wuellerstorfi, from Ocean Drilling Program Site 959 show that the Gulf of Guinea had a strong, shallow thermocline during the latest Miocene and early Pliocene prior to the first appearance of the Guinea Current at about 4.9 Ma. Gradients of d18O between the surface-water species, Globigerinoides sacculifer, and the thermocline species, Neogloboquadrina dutertrei and Globorotalia margaritae, were about 2‰ in the late Miocene and early Pliocene, but decreased to less than 0.5‰ in several large swings beginning about 4.9 Ma. A decreased oxygen isotope gradient in the upper ocean after 4.9 Ma is consistent with the initial establishment of the Guinea Current although several large swings in the oxygen isotope gradient shortly after this suggest that the Guinea Current did not become a permanent feature until about 4.3 Ma. The initiation of the Guinea Current in the Gulf of Guinea suggests that the Intertropical Convergence Zone (ITCZ) migrated south to near its present location at ~4.9 Ma and is in agreement with evidence for a southward shift in the ITCZ over the Pacific between 4-5 Ma. Modern hydrographic studies suggest that the position of the ITCZ modulates the strength of the North Equatorial Counter Current whose eastern extension forms the Guinea Current in the Gulf of Guinea. Notably, the interval ~4.7 to 4.3 Ma is associated with the return of a strong, shallow thermocline in the eastern Atlantic, which suggests a brief, northward drift of the ITCZ at this time and the disappearance of the Guinea Current. The Guinea Current returned to the Gulf after 4.3 Ma. The highest sea-surface temperature (SST) occurred between ~4.4 and 5.5 Ma in the Gulf of Guinea. Warm surface waters coupled with a shallow thermocline during much of this period may have contributed to a strong West African monsoon and relatively moist conditions over sub-Saharan Africa. In contrast, the large oscillations in SST and upwelling after ~4.5 Ma may have contributed to greater variability in the monsoon and the widespread increase in terrigenous fluxes observed in deep-sea cores within the eastern Atlantic about this time. |
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