Late Pliocene stratigraphic distribution of Globoconella taxa in the North Atlantic

Quantitative records of Globorotalia puncticulata and Globorotalia inflata, the last two members of the Globorotalia (Globoconella) lineage, obtained from North Atlantic sediments collected at DSDP Site 552, ODP Site 659 and ODP Site 665, are used to examine fluctuations in the biogeographic distribution of these species in the Late Pliocene between 3 and 2 Ma. Abundance data indicate that prior to the expansion of Northern Hemisphere glaciation at about 2.5 Ma, Gr. puncticulata was an important component of the planktonic foraminiferal fauna and had a geographic distribution ranging from 2°N to at least 56°N in the North Atlantic. A previously undescribed 6 chambered variant of Gr. puncticulata is found at both Sites 659 and 665. The stratigraphic distribution of this morphotype is restricted, first occurring at 2.9 Ma and then disappearing when glacial intensity increased at 2.75 Ma (isotope stage 110). Similar declines in Gr. puncticulata abundances occurred during glacial isotope stages 102, 100, and 98 immediately prior to the extinction of Gr. puncticulata during glacial isotope stage 96. It appears that this extinction event was latitudinally diachronous within the North Atlantic, occurring earliest in the north at Site 552 (2.453 Ma), then at Site 659 (2.443 Ma) and later still in the Site 665 equatorial record (2.438 Ma). At Site 665 the first record of Gr. inflata occurs during glacial isotope stage 94 (2.416 Ma), shortly after the extinction of Gr. puncticulata. In the mid latitude North Atlantic there was a 340,000 year period following the disappearance of Gr. puncticulata when the Globoconella lineage was absent (the Gr. inflata gap). The Gr. inflata population found in the equatorial Atlantic must therefore have been introduced from the South Atlantic, probably by the South Equatorial Current. Faunal records from Sites 552 and 659 show that it was not until glacial isotope stage 78 (2.10 Ma) that Gr. inflata became widely established in the North Atlantic. Prior to this large-scale migration event, there were two limited colonisation events during glacial isotope stages 86 and 82 when Gr. inflata populations reached as far as Site 659 in the eastern North Atlantic. These incursions are believed to be reflect the entrainment of Gr. inflata within South Atlantic Central Water and the northward subsurface transport of individuals to the coastal upwelling zone off northwest Africa. It seems likely that the same mechanism was responsible for the re-establishment of the Globoconella lineage in the North Atlantic at 2.10 Ma, but in this instance additional factors, such as enhanced glacial circulation patterns and ecological changes within planktonic foraminiferal faunas, resulted in the successful expansion of Gr. inflata across the North Atlantic and the Mediterranean.

Chert intervals in DSDP and ODP sites (Table 1)

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

Age estimates of Discoaster datums and corresponding isotope stages (Table 2)

High-resolution records (2 7 kyr) of Upper Pliocene Discoaster abundances obtained from six ODP/DSDP sites are assessed independently using oxygen isotope stratigraphy. Four Atlantic Ocean sites (DSDP Sites 552 and 607, and ODP Sites 659 and 662) comprise a transect from 56°N to 1°S and provide a record of latitudinal variations in Diseoaster biogeography. Low-latitude sites in the Atlantic (ODP Site 662), Pacific (ODP Site 677), and Indian (ODP Site 709) oceans provide additional information about variability in Discoaster abundance patterns within the equatorial region. A common chronology, based on the astronomical time scale developed for ODP Site 677, has been established for all the sites. By integrating oxygen isotope data and Discoaster abundance records at each site we are able to independently evaluate the temporal and spatial distribution of D. brouweri and D. triradiatus in the 500 kyr prior to the extinction of the discoasters near the base of the Olduvai subchron. Major decreases in abundance are evident during some of the more intense late Pliocene glacial events. In particular, glacial isotope stages 82, 96, 98 and 100 are associated with distinct abundance minima. At these times, large-scale changes in surface hydrographic conditions appear to have suppressed Discoaster numbers on a global scale. The increase in abundance of D. triradiatus, which precedes the extinction of the discoasters by around 200 kyr, may also be related to the intensification of environmental pressures that accompanied the build-up of Northern Hemisphere ice sheets during the late Pliocene. In spite of contrasting geographic and oceanographic settings, the various D. brouweri and D. triradiatus records are remarkably similar. This demonstrates that the acme and extinction events are excellent biostratigraphic datums. The simultaneous extinction of D. brouweri and D. triradiatus at 1.95 Ma were synchronous events at both a regional scale within the Atlantic, and on a global scale between the three major oceans. However, the start of the D. triradiatus acme appears to have been diachronous, occurring some 40 kyr earlier in the Atlantic than in the Indo-Pacific, and hence the stratigraphic usefulness of this datum is regional rather than global.