The presence of typical Antarctic faunas and consistently good preservation throughout the studied interval suggests that Site 1138 was always located within the Antarctic radiolarian province and thus south of the Polar Front (sensu Lazarus and Caulet, 1994) during the late Miocene interval studied. This is in accord with results from Site 747, also on the Central Kerguelen Plateau but located to the south (Lazarus, 1992). The sporadic and seemingly uncorrelated distribution of other coarse-fraction components, such as sponge spicules and lithic fragments, suggests that dissolution, winnowing, or other bottom processes have not affected the assemblages to a major degree.
Whereas all standard zones (other than the Tau) are identifiable at Site 1138, taxonomic uncertainties near the end of species' ranges make the precise placement of zonal boundaries somewhat imprecise. Site 1138 thus appears to represent an intermediate condition between more southerly Site 747, where Lazarus (1992) reported a well developed zonal pattern, and Site 737 on the northern edge of Kerguelen Plateau, where both Caulet (1991) and Lazarus (1992) encountered difficulties in identifying taxa and applying the standard zonation.
Faunal changes between Cores 183-1138A-12R and 13R, the apparent absence of the Tau Zone, and the presence in Sample 183-1138A-12R-3, 20 cm, of a mixed assemblage from different time intervals of late Miocene and early Pliocene age, all indicate an interval of missing sediment and sediment mixing at the boundary between lithologic Units I and II. The presence of small amounts of nannofossil clay in the lowermost (base of Core 183-1138A-12R) part of lithologic Unit I, which is otherwise of a biosiliceous nature, supports our interpretation of lithologic mixing across the temporal discontinuity. The duration of the discontinuity according to current age calibrations for the radiolarian events is at a minimum 4.6-6.1 Ma. Drilling disturbance is also possible for the mixed assemblage (Sample 183-1138A-12R-3, 20 cm) interval, as it is near the base of the recovered section in Core 183-1138A-12R. Given that Core 183-1138A-12R recovery was significantly incomplete, it cannot be determined from our data alone if the discontinuity originally was a continuous interval of sedimentation, an interval of no accumulation, or an erosional hiatus.
By contrast, the lithologic shift between nannofossil ooze in Core 183-1138A-17R and nannofossil clay in Core 16R does not seem to correspond to any significant change in the radiolarian assemblages or other coarse-fraction components, nor, within the studied interval, do we note any evidence for additional discontinuities in sedimentation.