Semiquantitative data of calcareous nannofossil species abundance were collected for 31 samples from 380.00 to 696.61 m below sea floor (mbsf) in the CIROS-1 core from the western McMurdo Sound (Ross Sea). This core has provided the most complete record of Paleogene glacial history of Antarctica. Only half of the samples yielded calcareous nannofossils and species diversity is generally low. However, several samples contain diverse assemblages and nannofossil zonal markers. Isthmolithus recurvus was found between 406.57 and 681.16 mbsf. This species has an age range of ~35-39 Ma in the mid to high latitudes, as calibrated previously by magnetostratigraphy at a number of DSDP/ODP sites. The interval from 406.57 to 681.16 mbsf can be assigned to the late Eocene-earliest Oligocene (~35-39 Ma), and the sample at 391.85 mbsf, which still contains Reticulofenestra hillae and Reticulofenestra umbilica but does not contain I. recurvus, is identified as early Oligocene age (~33-34 Ma). This represents an age refinement of the previous nannofossil biostratigraphy, where the interval from 385.77 to 690.40 mbsf was dated as middle Eocene-early Oligocene. Comparison of the nannofossil assemblages in the CIROS-1 core with those in a similar glaciomarine sequence recovered in Prydz Bay (East Antarctica) and those at deep-sea ODP Sites 738 and 744, where lowermost Oligocene ice-rafted debris (IRD) were found, suggests that the nannofossils in the CIROS-1 core samples examined are in situ. The semiquantitative nannofossil data also suggest that Isthmolithus recurvus and Reticulofenestra davies¸ are most tolerant of cold waters within the late Eocene-early Oligocene nannofloras
