Silicoflagellates from Leg 120 on the Kerguelen Plateau, Southeast Indian Ocean

The biostratigraphic distribution and abundance of Eocene to Pleistocene silicoflagellates is documented from Ocean Drilling Program Leg 120 Holes 747A, 748A, 748B, 749B, and 751A on the Central Kerguelen Plateau. Well-preserved silicoflagellates are reported here from the middle Eocene Dictyocha grandis Zone to the Pleistocene Distephanus speculum speculum Zone. Assemblage diversity and abundance is variable, with many intervals either barren of silicoflagellates or containing only limited numbers. INTRODUCTION Twelve holes at five sites on the Central Kerguelen Plateau were drilled by Ocean Drilling Program (ODP) Leg ! 20 during March and April 1988 (Table 1 and Fig. 1). All sites are south of the present Polar Front and lie within the Antarctic water mass. Leg 120 represents the second half of a two-leg latitudinal transect from the Northern Kerguelen Plateau to the Antarctic margin near Prydz Bay. Leg 119 drilled numerous holes north and south of the Leg 120 sites (Fig. 1). This report documents the biostratigraphic occurrence and abundance of silicoflagellates from Holes 747A, 748A, 748B, 749B and 751A on the Kerguelen Plateau. No individual hole spans the entire interval where silicoflagellates occur, middle Eocene to Pleistocene, but collectively most ages are represented in at least two holes (Figs. 2 and 3). In general, Pliocene to Pleistocene sediments are of diatom ooze or foraminifer diatom ooze, with middle Eocene to upper Miocene sediments of nannofossil ooze or chalk. The exception to this is Site 751, where siliceous microfossils are well represented throughout the Miocene nannofossil-rich intervals below the Pliocene-Pleistocene diatom ooze. Near-continuous recovery in several sites from the upper Oligocene to Pleistocene enables detailed biostratigraphic and paleoceanographic studies, although several hiatuses representing 1-4 m.y. (Fig. 3) are clearly present (see biostratigraphic syntheses of Harwood et al., this volume, and Berggren et al., this volume). The lower range of silicoflagellate recovery corresponds with the occurrence of chert in the lower Oligocene and Eocene. Sparse Upper Cretaceous assemblages are noted in Core 120-748C-47R and Section 120-750B-6W-CC. Existing literature on silicoflagellate biostratigraphy from previous Deep Sea Drilling Project (DSDP) and ODP legs to the Southern Ocean provides a wealth of information for taxonomic and biostrati-graphic reference. In sequential order of the legs, they include Bukry (1975a, 1975b), Ciesielski (1975), Perch-Nielsen (1975), Hajos and Stradner (1975), Haq and Riley (1976), Busen and Wise (1977), Shaw and Ciesielski (1983), McCartney and Wise (1990), Ciesielski (1991), and Bukry (1991). Silicoflagellate distributions in Antarctic continental shelf sediments are reported in the Ross Sea Embay1 Wise, S. W. , Jr., Schlich, R. , et al., 1992. Proc. ODP, Sci. Results, 120: College Station, TX (Ocean Drilling Program). 2 University of Maine at Presque Isle, Presque Isle, ME 04769, U.S.A. 3 Department of Geology, 214 Bessey Hall, University of NebraskaLincoln, Lincoln, NE 68588-0340, U.S.A. ment by Ling and White (1979), White (1980), Harwood (1986, 1989), and Harwood et al. (1989), and on the Antarctic Peninsula by Harwood (1988). SAMPLE PREPARATION Samples used in this study are from the same samples used for diatom investigation (Harwood and Maruyama, this volume). Raw samples were prepared as smear slides with a 22 × 50 mm cover slip when diatoms occurred in abundances >50% of the microfossils. Lower values of diatom abundance required chemical treatment with H2O2 and HC1 to concentrate the siliceous fossils, and strewn slides were then prepared after the residues were washed of these chemicals. Where silicoflagellates were abundant, 300 specimens were counted. If silicoflagellates were less abundant, up to an entire slide was examined. The abundance of silicoflagellates varied considerably, with 300 counts reached after only two or three traverses of some slides, whereas other samples were barren or nearly so. To show the general abundance of silicoflagellates, the approximate fraction of the slide examined is recorded in the abundance charts (Tables 2-5). Each tenth of the slide examined represents approximately 1.1 cm; the relative abundance of any two slides can be obtained by dividing the total count by the area of each slide examined. Only those fragments representing more than one half of a silicoflagellate were included in the counts. Because the number of teratoid (aberrant) specimens may be an indication of environmental stress, a separate tally was made of these. Aberrant include specimens with fused spines or struts, two struts attaching to a basal side, or distorted basal rings (see McCartney and Loper, 1989, for a discussion on the basic rules of silicoflagellate form). Because the aberrants are often counted among the specific taxa, the number of abberants is not included in the total counts. Because silicoflagellates typically make up only a small proportion of the siliceous microfossils in a given sediment sample, more time must be spent per slide to obtain useful data. For this reason, the sample interval used in producing Tables 2-5 is larger than that for the diatom work (Harwood and Maruyama, this volume). However, many additional samples received less detailed analysis to show more precisely the boundaries between zones and better clarify the range of important taxa. SILICOFLAGELLATE ZONATION A number of southern high-latitude silicoflagellate zonations have been proposed in the literature (Bukry, 1975b;