1% of the assemblage) throughout the section (Table T1). These specimens are interpreted to have been laterally transported from shallower areas, possibly by the mechanism of turbidity currents described by Coffin, Frey, Wallace, et al. (2000).
Only limited paleoenvironmental information can be inferred from the qualitative diatom abundance data collected in the present study. The diatom assemblages recovered in Hole 1138A are predominantly planktonic in composition. Benthic taxa, such as Cocconeis spp. and Rhabdonema spp., were observed in low abundance (1% of the assemblage) throughout the section (Table T1). These specimens are interpreted to have been laterally transported from shallower areas, possibly by the mechanism of turbidity currents described by Coffin, Frey, Wallace, et al. (2000).
The lower to middle Pliocene sediments of Hole 1138A are rich in biosiliceous material and contain numerous diatom taxa with a modern Subantarctic distribution or a known fossil distribution restricted primarily to the middle latitudes. These taxa include Hemidiscus karstenii, N. reinholdii, Proboscia spp., and, possibly, Azpeitia harwoodii. Similar preservation of lower Pliocene biosiliceous material has been described in many Southern Ocean sections. This observation, in addition to the presence of numerous "warm-water" diatom, radiolarian, and silicoflagellate taxa, has been interpreted as a reflection of early Pliocene surface water warming in the Southern Ocean (e.g., Abelmann et al., 1990; Bohaty and Harwood, 1998). Quantitative documentation of these assemblages on the Kerguelen Plateau is necessary to delineate early to late Pliocene warming and cooling events and related shifts in the position of the PFZ.
Numerous samples from the Pliocene-Pleistocene interval of Hole 1138A contain moderately abundant nannofossil (Fig. F2) and foraminiferal assemblages. Nannofossil assemblages for this time interval are not well documented on the Kerguelen Plateau. Nannofossil abundance and assemblage composition may similarly provide useful paleoenvironmental information regarding shifts in the PFZ. Additionally, alkenones may be present in the nannofossil-bearing intervals and may provide independent estimates of high-latitude surface temperatures during the Pliocene and Pleistocene.
Fragilariopsis curta is a modern diatom taxon that is ecologically associated with sea-ice environments. In Pleistocene-Holocene sediment studies, the quantitative abundance of F. curta is used as a proxy for the relative influence of sea ice (Gersonde and Zielinski, 2000). In Hole 1138A, the FO of F. curta is recorded in the upper Pliocene at 74.04 mbsf. This first occurrence of F. curta at ~3.0 Ma on the Central Kerguelen Plateau most likely represents a late Pliocene expansion of sea-ice influence and cooling. Several modern taxa, such as Asteromphalus parvulus and Fragilariopsis separanda, also have first occurrences in the upper Pliocene and lower Pleistocene in Hole 1138A. These first occurrences, combined with the progressive extinction of several taxa through this interval, represent the initial development of the modern endemic diatom flora in the Southern Ocean. Progressive endemism in Antarctic diatom assemblages has also been associated with regional late Pliocene-early Pleistocene cooling (e.g., Harwood, 1991).
