FLUCTUATIONS IN THE COMPOSITION OF LATE MIOCENE CALCAREOUS NANNOFOSSIL ASSEMBLAGES AS A RESPONSE TO ORBITAL FORCING

Fluctuations in abundance between the two calcareous nannofossil species Coccolithus pelagicus and Reticulofenestra pseudoumbilica are observed in the upper Miocene sediments recovered from Deep Sea Drilling Project hole 552A. A record of these fluctuations (Cp/Rp) is established for a time interval of similar to 1.35 m.y. (from 4.9 to 6.25 Ma) and is shown to yield periodicities which correspond to Milankovitch periodicities. A change in regime occurred at 5.72 Ma, as indicated by a shift from a series dominated by the 20-kyr cycle to a series dominated by the 100-kyr cycle. The Cp/Rp is compared with the isotopic delta O-18 and delta C-13 records from benthic and planktonic foraminifera. It is shown that a good correlation between the Cp/Rp and the isotopic records occurs only at times of major global changes. This suggests that the Cp/Rp, which reflects water masses movements, yields a complex paleoclimatic/paleoceanographic message which can help improving our understanding of the late Miocene history.