Although rotary drilled, the Neogene section in Hole 1138A provides insight into the geologic and paleoceanographic history of the Central Kerguelen Plateau. A moderately well constrained Neogene age-depth model for the hole is derived from 36 diatom datums and 5 nannofossil and planktonic formanifer datums, as well as magnetostratigraphic constraint. Four "outlying" planktonic foraminifer and nannofossil datums are inconsistent with the age model defined by the other datums. Five 40Ar-39Ar ages from tephra horizons provide further age constraint and are in good agreement with the biostratigraphic age interpretations. Previous paleomagnetic calibrations of the LO of N. denticuloides are directly confirmed in Hole 1138A by a 40Ar-39Ar age determination of 11.45 ± 0.22 Ma for a tephra horizon overlying this datum.
An important feature of the Neogene strata recovered at Site 1138 is the presence of dispersed tephra and several discrete tephra horizons. Ash material at this site is interpreted to have originated from Heard Island and deposited from both primary ash fall and distal turbidity plumes (Coffin, Frey, Wallace, et al., 2000). Neogene tephras at Site 1138 are predominantly trachytic to rhyolitic in composition and document the nature of late Cenozoic evolution of Kerguelen volcanism. Within the resolution of the age information presented here, the middle Miocene tephra layers documented at Sites 747 and 1138A are correlative.
Hole 1138A contains an expanded lower Pliocene-upper Pleistocene section (~105 m) relative to most other drill cores recovered on the Kerguelen Plateau. In comparison, only ~29 m of Pliocene-Pleistocene section was recovered in nearby Hole 747A. In addition to high sedimentation rates, the preservation of both carbonate and siliceous microfossils is a distinctive feature of upper Miocene to Pleistocene sediments in Hole 1138A. Furthermore, a detailed investigation of late Neogene shifts in the PFZ is possible at this site through future piston coring operations.