Early Cenozoic calcareous nannoplankton biogeography of the Atlantic Ocean

Biogeographic patterns of Early Cenozoic calcareous nannoplankton assemblages are delineated for the North and South Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Nannoplankton assemblages are defined byQ-mode Varimax Factor and Oblique Factor Analyses of census data on 44 taxa from 113 deep-sea and land-based samples. Examination of their latitudinal distribution through time allows recognition of those assemblages which can be used as environmental indicators. Comparison of the distributions of contemporaneous nannoplankton assemblages with the distribution of the appropriate environmental indicator assemblage permits their classification as either low-, mid-, or high-latitude nannoflora. Early Paleocene is characterized by a high-latitude Thoracosphaerid-Markalius astroporus Assemblage and a mid- to low-latitude Braarudosphaerid Assemblage. Eight Middle Paleocene-Early Eocene nannoplankton assemblages are identified and grouped according to their relative environmental distribution: (1) Low-latitude nannoflora: theToweius craticulus-Coccolithus pelagicus Assemblage, the Discoaster-Cyclococcolithus formosus Assemblage, theToweius craticulus-Ericsonia subpertusa-Discoaster Assemblage, and the Fasciculith-Discoaster Assemblage. (2) Mid-latitude nannoflora: theEricsonia subpertusa Assemblage and theCoccolithus pelagicus Assemblage. (3) High-latitude nannoflora: thePrinsius martinii Assemblage and theP. bisulcus Assemblage. These groupings are indicated by comparison of the distribution of our Paleocene-Early Eocene environmental indicator, the high-latitudePrinsius martinii Assemblage, with the distributions of contemporaneous assemblages. Seven Eocene nannoplankton assemblages are identified: (1) Low-latitude nannoflora: the Reticulofenestrid Assemblage and theCyclococcolithus formosus-Sphenolith Assemblage. (2) Mid-latitude nannoflora: the Discoaster Assemblage, theReticulofenestra umbilica-R. bisecta-Coccolithus pelagicus Assemblage, and theCribrocentrum reticulatum Assemblage. (3) High-latitude nannoflora: theToweius craticulus-Coccolithus pelagicus Assemblage, and theC. pelagicus-Cyclococcolithus formosus-C. aff.gammation Assemblage. Our Eocene environmental indicator is the low-latitude Reticulofenestrid Assemblage. Five Oligocene nannoplankton assemblages are identified: (1) Low-latitude nannoflora: the Sphenolith-Discoaster Assemblage. (2) Mid-latitude nannoflora: theCyclococcolithus neogammation Assemblage and theDictyococcites hesslandii Assemblage. (3) High-latitude nannoflora: the Reticulofenestrid-R. bisecta Assemblage and theCoccolithus pelagicus Assemblages. Our Oligocene environmental indicator is the low-latitude Sphenolith-Discoaster Assemblage. If it is assumed (1) that the latitudinal differentiation of calcareous nannoplankton assemblages we observe in the Early Cenozoic is related to a latitudinal temperature gradient, and (2) that the ecological preferences of these assemblages remain stable through time, then the latitudinal nannofloral migrations we recognize delineate paleotemperature changes: The maximum equatorward migration (cooling) of high- and mid-latitude nannofloras in the Paleocene occurs at about 58 m.y. B.P. This is followed by their poleward migration, the disappearance of the high-latitude nannoflora, and the appearance of a new low-latitude nannoflora. A major poleward migration (warming) occurs at about 49 m.y. B.P. The Middle Eocene is characterized by the return of high-latitude nannofloras into mid-latitudes, with the maximum equatorward migration (cooling) occurring at 48-43 m.y. B.P. Low-latitude nannoflora again gradually invade high latitudes through the Late Eocene, indicating a second major Eocene warming by at least 38 m.y. B.P. There is a well-defined migration of high-latitude nannoflora into the mid-latitudes during the Middle Oligocene, with the maximum indicated cooling between 32 and 27 m.y. B.P.; an earlier, though minor, Oligocene cooling may have occurred at about 36 m.y. B.P. During the Late Oligocene, low-latitude nannoflora migrate into high latitudes, indicating a warming by 26 m.y. B.P. These inferred paleotemperature changes are similar to those delineated by some workers on the basis of terrestrial flora. Nannoflora characterized by the cool-water coccolithCoccolithus pelagicus predominate at the equator throughout most of the Paleocene, are confined mostly to high latitudes in the Eocene, and generally remain above mid-latitudes during the Oligocene. Although this migration could indicate that the Paleocene was the coolest epoch in the Early Cenozoic, and the Oligocene the warmest, we suggest instead that this shift ofC. pelagicus to higher latitudes is a result of evolution in its ecology. The implications of paleobiogeography to high-latitude biostratigraphy are discussed and theacme horizon as a time-stratigraphic concept is informally introduced. The following new taxa are described:Fasciculithus rotundus, Neochiastozygus imbriei andThoracosphaera atlantica. The following taxa are recombined:Cyclococcolithus protoannulus, Cyclolithella bramlettei Dictyococcites hesslandii, andNannotetrina alata.