The history of the Antarctic ice sheet, an important component of global climate history, is preserved in sediments of the Southern Ocean. This history can be most directly inferred by examination of sediments near the continent, where glacial and glaciomarine deposits dominate. Dating these sediments, however, has often been very difficult due to generally poor microfossil preservation, reworking, and hiatuses. One of the major goals of Ocean Drilling Program Leg 178 was to recover this glacial record, at least partially, in sediments with sufficient microfossils for good biochronology. For this purpose, several holes were drilled during Leg 178 near the Antarctic Peninsula from fairly deep water deposits at bathyal depths to very shallow water deposits near the coast (Fig. F1).
Antarctic Neogene radiolarian faunas (compared to most other coeval Antarctic planktonic microfossils) are relatively diverse and evolve rapidly but are still not very well known taxonomically—many, perhaps the majority, of species have not yet been described. Despite this taxonomic "deficit," radiolarians are, along with diatoms, the main source of biostratigraphic data in Neogene Southern Ocean sediments. Previous studies have established a zonation scheme that has been applicable throughout the Southern Ocean and that is calibrated by means of paleomagnetics to the standard geologic timescale (Chen, 1975; Lazarus, 1990, 1992; Abelmann, 1990, 1992; Caulet, 1991) (Fig. F2). This zonation is, however, of only moderate resolution, and calibrations are sometimes questionable because of uncertainties in age models used. Calibration is a particular problem in the late Miocene to earliest Pliocene, where hiatuses are unusually common in Southern Ocean records. Recent work by Nishimura et al. (1997) on Holocene material also suggests that in very nearshore regions, full oceanic radiolarian assemblages are not present. Instead, a depauperate assemblage is found that is dominated by species not found in coeval oceanic Southern Ocean assemblages. As previous Neogene biostratigraphic work has been mainly from pelagic sections, it has not been established whether the standard Neogene radiolarian zonation can be applied in nearshore regions as well.
Radiolarians from Leg 178 sites were initially studied by A. Weinheimer (Shipboard Scientific Party, 1999). Because of poor breakdown of samples in the shipboard laboratory, only very general information was obtainable. Her results suggested that reasonably well preserved radiolarians were present in most sites, but older assemblages (early Pliocene and late Miocene) were recovered only in Hole 1095B (Fig. F3). This hole was drilled into a continental rise glaciomarine sediment drift near the Antarctic Peninsula.
The present study reports the late Miocene-earliest Pliocene (~4-10 Ma) radiolarian biostratigraphy from Hole 1095B, with the goals of dating the section for use in Antarctic ice sheet studies; testing the viability of standard zonal schemes in a nonpelagic, relatively nearshore setting; and better calibration of individual events to the paleomagnetic polarity timescale.