Late Pliocene Diatoms in a Diatomite from Prydz Bay, East Antarctica

Abstract

Very well-preserved Pliocene diatoms from a diatomite unit interbedded within glacial sediments at Ocean Drilling Program Site 742 in Prydz Bay, Antarctica are documented and illustrated. The presence of Thalassiosira kolbei, T torokina, Actinocyclus actinochilus, A. karstenii and the absence of Nitzschia interfrigidaria. T insigna and T vulnifica in Sample 119-742A-15R-4, 44-46cm constrain its age to ca. 2.2-1.8 Ma (late Pliocene). Diatoms associated with sea ice constitute 35% of the Pliocene diatom assemblage, compared with 71% of the modem sediment assemblage at the site, suggesting that sea ice was present during the late Pliocene period of deposition of the sample, although it probably was not the significant feature it is today. Thalassiosira ellitipora (Donahue) Fenner is described and illustrated in detail and is validly published. An expanded description and numerous illustrations are also presented for T. torokina Brady. INTRODUCTION During Ocean Drilling Program Leg 119 on the continental shelf in Prydz Bay, Antarctica, a 60-cm-thick unit of distinctive greenish gray to gray diatomite (lithologic Subunit IIIB) was recovered within a thick (300+m) sequence of coarse-grained glacial diamictite in Core 15R of Hole 742A at 67?32.98'S, 75?24.27'E and a water depth of 426.2m (text-fig. 1) (Shipboard Scientific Party 1989). The abundant Pliocene diatoms found in this diatomite were apparently deposited during a period of enhanced biologic productivity in a distal glaciomarine setting, possibly during an interglacial period warmer than today (Shipboard Scientific Party 1989). The presence of well-preserved Pliocene diatoms within otherwise glacial sediments that typify the Antarctic shelf is relatively rare and offers an opportunity to study a nearshore environment of East Antarctica during an interval of the late Pliocene. A study of the diatoms of this Pliocene diatomite may provide evidence that bears on the hypothesis of Webb et al. (1984) and Webb and Harwood (1991) that calls for large scale deglaciation of East Antarctica during parts of the early and late Pliocene. In addition, the well-preserved nature of the diatoms in this deposit allows their study under the scanning electron microscope(SEM) and may contribute to solving some of the numerous problems of taxonomic confusion that currently plagues the study of Pliocene Antarctic diatoms (Harwood and Maruyama 1992). MATERIALS AND METHODS Sample 119-742A-15R-4, 44-46cm was collected from the 60cm-thick diatomite unit. The sample was processed for light microscopy (LM) and SEM by the Van der Werff (1955) hydrogen peroxide method. Strew slides were mounted in Hyrax for LM with individual species ringed for identification and photographic reference. Examination of taxa was made using a Leitz Ortholux II and an Olympus IMT at magnifications of x500 and x1250. SEM examination and photography was made using a Cambridge 250 mark II SEM at the U.S. Geological Survey (Menlo Park) and a Hitachi S520 at the California Academy of Sciences. Diatom terminology follows that of Ross et al. (1979). Sample material (CAS number 613278, ODP 119-742A-15R-4, 44-46cm) and slides (CAS numbers 217016-217018, 217022, 217026, 217027, 217029) are deposited at the California Academy of Sciences. TEXT-FIGURE 1 Location of ODP Site 742 in Prydz Bay, East Antarctica, and other sites cores by ODP Leg 119. Contours in meters. micropaleontology, vol. 42, no. 3, pp. 285-302, text-figure 1, plates 1-8, table 1, 1996 285 This content downloaded from 157.55.39.138 on Tue, 19 Apr 2016 06:51:08 UTC All use subject to http://about.jstor.org/terms Albert D. Mahood and John A. Barron: Late Pliocene Diatoms in a Diatomite from Prydz Bay, East Antarctica TABLE 1 Tabulation of diatom taxa in Pliocene Sample ODP-119-742A-15R-4, 44-46cm and their relative percentage compared with the relative percentage of taxa recorded in surface sediment at the site by Stockwell et al. (1991). * = extinct; ( ) = possible decendent.