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The abrupt lithologic change from Neocomian pelagic carbonate rocks to widespread organic-rich shales of middle Cretaceous age and the return to mostly oxidized sediments in the Cenomanian suggest significant changes in paleocirculation and productivity patterns. A quantitative study of calcareous nannofossils from DSDP and IPOD cores and selected land samples allowed delineation of paleo-oceanographic and paleoecologic patterns. Major factors controlling the distribution of calcareous nannoplankton are: (1) surface-water temperature and polar or equatorial temperature gradients, (2) aggressiveness of bottom water and associated position of the calcite compensation depth (CCD), and (3) diagenesis. Paleobiogeographic patterns of calcareous nannofossils change significantly during the middle Cretaceous.
In the Barremian to early Aptian, both Atlantic basins showed sluggish surface circulation resulting in a broad tropical assemblage and a weakly developed austral assemblage. During the late Aptian through middle Albian carbonate dissolution was most pronounced. Neither austral nor boreal nannoplankton assemblages could be distinguished. During late Albian to Cenomanian time boreal and austral assemblages are well developed and tropical assemblages are compressed with respect to latitude. Gyre-margin assemblages become more dominant and there are indications of more widespread upwelling in the eastern North Atlantic and over the Walvis-Rio Grande Ridge system.
This would indicate that stronger polar-equatorial temperature gradients developed in the latest Albian to early Cenomanian. More vigorous deep- and surface-water circulation would result in better ventilation of the deep ocean and the deposition of more oxidized sediments. Oxygen isotope determinations support the underlying assumption for this hypothesis. Nannofossil paleobiogeographic studies thus allowed us to document a comprehensive picture of middle Cretaceous paleo-oceanography in the Atlantic.
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