Southern Ocean Pleistocene calcareous nannofossil events: calibration withisotope and geomagnetic stratigraphies

Several cores recovered from the northern belt of the Southern Ocean were a nalysed to study the Pleistocene calcareous nannofossil records. Calcareous nannofossil events previously described in medium and low latitudes were i dentified and calibrated with the oxygen isotope and geomagnetic time scale s. Although sedimentation rates, hiatuses and degree of calcareous nannofos sil preservation sometimes prevent the identification and/or accurate calib ration of some of these events, a useful stratigraphic framework was obtain ed. The possibility of using these calibrated events From high to low latit udes facilitates correlations and should facilitate isotope event identific ation in a region with low temperature, where calcareous plankton stratigra phies are in general restricted. In general, Pleistocene southern high lati tude calcareous nannofossil events show synchronism with those observed in warm and temperate surficial waters. Small discrepancies in the assigned ag es are sometimes related to low sampling resolution due to low sedimentatio n rates. The first occurrence (FO) of Emiliania huxleyi and the last occurr ence (LO) of Pseudoemiliania lacunosa are observed in Marine Isotope Stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed close to the MIS 4/5 boundary. MIS 6 is characterised by an increase in G. muellerae and MIS 7 features a dramat ic decrease in the proportion of Gephyrocapsa caribbeanica. This latter spe cies began to increase its proportions from the MIS 13/14 boundary to MIS13 , showing diachronism between the different sites. The LO of Reticulofenest ra asanoi is observed at MIS 22, confirming this event as a global synchron ous reference datum. By contrast, the FO of R. asanoi occurs at MIS 35 and is diachronous with the existing data from other oceanic regions. A re-entr y of medium sized Gephyrocapsa (3-5 mum maximum diameters) can be identifie d in some cores close to MIS 25; although the low abundance of this taxon p revents an accurate calibration, it may be concluded that this event is dia chronous as compared with the existing low-latitude data. The LO of large m orphotypes of Gephyrocapsa is well correlated with MIS 37, showing synchron ism with other oceanic regions, whereas the FO of this species is not well calibrated due to the absence of age-control points. (C) 2000 Elsevier Scie nce B.V. All rights reserved.