Miocene–Pliocene surface-water hydrography of the eastern equatorial Atlantic

Abstract

Stable 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 δ 18 O 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.

Changes in Caribbean surface hydrography during the Pliocene shoaling of the Central American Seaway

“…These changes document the reorganization of tropical Pacific circulation most probably in response to the restriction of the CAS, as these shifts occurred along with the development of the Atlantic‐Pacific salinity contrast. In the tropical Atlantic, major changes in surface hydrography at 4.4–4.0 Ma [ Chaisson and Ravelo , 1997; Norris , 1998] were also linked to a southward movement of the ITCZ [ Billups et al , 1999].…”

Link between oceanic heat transport, thermohaline circulation, and the Intertropical Convergence Zone in the early Pliocene Atlantic

“…Age assignments for Sites 851 and 925 reflect tuning to Northern Hemisphere summer insolation (Shackleton et al, 1995a;Tiedemann and Franz, 1997, respectively). We use biostratigraphic data for Sites 806 (Mayer et al, 1993) and 959 (Norris, 1998) on the Berggren et al (1995) time scale. The time scales of Shackleton et al (1995a) and Berggren et al (1995)…”

“…Previous studies have demonstrated how the δ 18 O values of planktonic foraminifera can be used to reconstruct past changes in the relative depths of the tropical mixed layer and thermocline (Ravelo and Fairbanks, 1992;Ravelo and Shackleton, 1995;Cannariato and Ravelo, 1997;Norris, 1998). In environments with deep mixed layers throughout the year, such as the western equatorial Atlantic and Pacific, the photic-zone temperature gradient (and hence the vertical foraminiferal δ 18 O gradient) is small, because the thermocline is below the base of the photic zone (Fig.…”

“…The western Atlantic (Ocean Drilling Program [ODP] Site 925, Billups et al, 1998a) and Pacific (Site 806, this study) 1 sites are located in regions that supply warm and saline waters to the western boundary currents of these oceans. The eastern Pacific (Site 851, Cannariato and Ravelo, 1997) and Atlantic (Site 959, Norris, 1998) sites sample the eastern equatorial surface hydrography, which is highly sensitive to changes in the intensity and direction of the trade winds. By characterizing changes in the surface hydrography at each site (e.g., mixed-layer depth), we draw inferences about the character of the wind field and the position of the Intertropical Convergence Zone.…”

Pliocene Changes in Tropical East Pacific Upper Ocean Stratification: Response to Tropical Gateways?

“…Moreover, support for an early Pliocene southward shift of the ITCZ comes from studies that investigated Atlantic and Pacific changes in tropical wind field and surface hydrography (Hovan, 1995;Farrell et al, 1995;Cannariato and Ravelo, 1997;Chaisson and Ravelo, 1997;Norris, 1998;Billups et al, 1998Billups et al, , 1999. Similar to the modern seasonal cycle, a shift to a more southern position of the ITCZ in the TEP would have weakened the southeast trades and thus significantly reduced the strength of the NECC and the SEC.…”