The nannofossil biostratigraphic framework was provided by the zonal compilation schemes of Martini (1971), with modifications by Gartner (1977), Martini and Müller (1986), and Bukry (1973, 1975) as well as zonal code numbers added and modified by Okada and Bukry (1980). Pospichal et al. (1992), however, illustrated a significant decrease in biostratigraphic resolution in the Martini Cenozoic zonation from the low to high latitudes of the Southern Hemisphere. The greatest loss occurs in the Neogene scheme, where little stratigraphic control can be achieved between 51°S and 65°S.
Wei and Wise (1992) summarized and calibrated to the paleomagnetic timescale a few useful Neogene high-latitude nannofossil datums detectable on the Kerguelen Plateau and elsewhere in the Southern Ocean. These are indicated on Figure F3 alongside the low-latitude zonation of Bukry for comparison. In this figure, the datums have been recalibrated for comparison with the Berggren et al. (1995) timescale (see discussion below). With so few datums at these latitudes, however, most Neogene nannofossil zones have had to be combined into a total of five zones.
High-latitude nannofossil zonations with moderate resolution have been developed for the Oligocene to mid-middle Eocene (Wise, 1983; Wei and Wise, 1990; Wei and Thierstein, 1991), and these have been inserted into Figure F3. Ages for key datum levels have been calibrated in the region of the Kerguelen Plateau against magnetostratigraphy by Wei (1992); these are indicated in bold type in Figure F3, where they have been recalibrated against the Berggren et al. (1995) timescale.
As noted by Wei (1992), biomagnetostratigraphic correlations at several Southern Ocean sites may show considerably different ages relative to those compiled from the mid-latitudes by Berggren et al. (1985, 1995). Where such differences exist, we have, in most instances, chosen to use ages derived from the high-latitude calibrations against the magnetostratigraphy.
As noted above, where such ages differ from those in the lower latitudes, the high-latitude ages are shown in bold type in Figure F3 following the corresponding datum level; similarly, high-latitude biostratigraphic datums are also indicated in bold type. For major differences in age assignment, arrows indicate where on the chart a datum has been repositioned for purposes of this study.