Eight sites were drilled on the Kerguelen Plateau and Broken Ridge during Ocean Drilling Program (ODP) Leg 183 from December 1998 to February 1999. Although the primary objective of the cruise was focused on the igneous history of the province, the overlying Cretaceous and Cenozoic sediments were also recovered in the course of operations. Several studies on these sediment sections are aimed at building on the regional biostratigraphic and paleoceanographic databases and adding information to that derived from Deep Sea Drilling Project (DSDP) Leg 26 and ODP Legs 119, 120, and 121 in the same area.
Hole 1138A was drilled on the Central Kerguelen Plateau (53°33´S, 75°58´E) in a water depth of 1141.4 m. The site is located south of the modern Polar Frontal Zone (PFZ) and ~180 km southeast of Heard Island (Fig. F1). Hole 1138A penetrated ~700 m of upper Neogene through Upper Cretaceous sediment and ~143 m of Cretaceous basement to a maximum depth of 842.70 meters below seafloor (mbsf). The sediment directly overlying subaerially erupted lava flows is assigned a Turonian age (~90 Ma) from nannofossil and planktonic foraminifer biostratigraphy (Coffin, Frey, Wallace, et al., 2000). The entire section in Hole 1138A was drilled with a rotary core barrel system and, therefore, only marginal recovery was achieved in some intervals. However, considering that the soft oozes of this section were rotary drilled, recovery is surprisingly high in Cores 183-1138A-13R through 1R (~77%), compared to ~49% for the entire sedimentary section. Most cores, though, show signs of drilling disturbance (Coffin, Frey, Wallace, et al., 2000).
The sediments recovered in Hole 1138A are divided into nine primary lithologic units and subunits (Coffin, Frey, Wallace, et al., 2000). The Neogene section ranges from Core 183-1138A-32R through 1R and includes lithostratigraphic Unit I, Subunits IIA and IIB, and the upper part of Subunit IIIA (Fig. F2). Unit I (0.0-112.0 mbsf) consists of foraminifer-bearing diatom clay and ooze. Subunits IIA and IIB (112.0-265.9 mbsf) are composed of foraminifer-bearing nannofossil clay and ooze. Several tephra horizons and intervals containing dispersed ash were also identified within Units I and II. Subunit IIIA (265.9-496.40 mbsf) is a foraminifer-bearing nannofossil chalk.
This report provides biostratigraphic information to supplement and expand on the diatom data originally presented in the shipboard report for Hole 1138A (Coffin, Frey, Wallace, et al., 2000). In addition to a biostratigraphic survey of diatoms, this study is also aimed at documenting Neogene tephra layers recovered in the hole and providing a summary of age-depth information. The age model derived here will assist in the limited paleoceanographic studies currently in progress, and, ultimately, these data will also aid in planning future drilling on the Central Kerguelen Plateau. Additional diatom results from Leg 183 are reported by Arney et al. (this volume) for Hole 1140A, located on the Northern Kerguelen Plateau. Their zonal scheme and taxonomy, however, differ slightly from those presented here (see "Systematic Paleontology," for general notes on the taxonomy applied to diatom assemblages from Hole 1138A).
The nearest comparable section to Hole 1138A previously drilled on the Central Kerguelen Plateau is ODP Hole 747A (Leg 120), located 150 km southeast of Site 1138 (54°49´S, 76°48´E) in a water depth of 1695.0 m (Fig. F1). Diatom biostratigraphy for Hole 747A is reported in Harwood and Maruyama (1992). The diatom record and biochronology of these two holes are compared below.