Late Miocene to Early Pliocene Radiolarians from Glaciomarine Drift Deposits, ODP Leg 178, Hole 1095B (Bellinghausen Basin, Antarctic Ocean)

Abstract

Early Pliocene to middle late Miocene hemipelagic and distal turbidite sediments from Hole 1095B, near the Antarctic Peninsula, yield moderately abundant, moderately well preserved radiolarian faunas and other biosiliceous material (diatoms, silicoflagellates, and sponge spicules). Preservation characteristics, however, vary strongly even between closely related samples, and there are many intervals of poor preservation. In the 140to 460-meters below seafloor interval studied, it was possible to identify the following standard Southern Ocean radiolarian zones: Upsilon, Tau, Amphymenium challengerae, Acrosphaera? labrata, Siphonosphaera vesuvius, and upper Acrosphaera australis (total age range ~4–10 Ma). Some normally common radiolarian groups, such as actinommids, are unusually rare in the studied material, and the relative ranges of several individual species, such as Acrosphaera labrata vs. A. australis, appear to be somewhat anomalous. These observations imply that the ranges of taxa in this section may be somewhat diachronous, due to either local ecologic factors and/or the highly variable preservation of the faunas. Thus, the ages of events reported are probably only approximate, although they are still useful for constraining the age of sediments in this section. 1Lazarus, D.B., 2001. Late Miocene to early Pliocene radiolarians from glaciomarine drift deposits, ODP Leg 178, Hole 1095B (Bellinghausen Basin, Antarctic Ocean). In Barker, P.F., Camerlenghi, A., Acton, G.D., and Ramsay, A.T.S. (Eds.), Proc. ODP, Sci. Results, 178, 1–22 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Museum fur Naturkunde, Invalidenstrasse 43, 10115 Berlin, Federal Republic of Germany. david.Lazarus@rz.hu-berlin.de Initial receipt: 22 August 2000 Acceptance: 8 May 2001 Web publication: 25 July 2001 Ms 178SR-218 D. LAZARUS RADIOLARIANS FROM GLACIOMARINE DRIFT DEPOSITS 2 INTRODUCTION AND PREVIOUS WORK 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.