A total of 72 core samples were processed for phytoliths from Sites 1165 (38 samples) and 1166 (34 samples) (Fig. F1) (O'Brien, Cooper, Richter, et al., 2001). Site 1165 is located at 64°22.8´S, 67°13.1´E on the continental rise offshore Prydz Bay in 3537 m water depth. A combination of two holes (Holes 1165B and 1165C) yielded a 999-m-thick, relatively continuous (recovery = 69.3%) sedimentary record from early Miocene (~21.8 Ma) to Pleistocene. Site 1166 is located at 67°41.8´S, 74°47.2´E on the continental shelf in 475 m water depth, and because of poor recovery (18.6%) yielded 71 m of core from Hole 1166A down to 381 meters below seafloor (mbsf). The age of Hole 1166A ranges from Late Cretaceous at the base to Holocene at the seafloor, with many disconformities throughout. The aim of this study is to describe phytolith assemblages from samples throughout the cores from Sites 1165 and 1166 and provide an interpretation, in combination with the terrestrial palynomorph record (Macphail and Truswell, "Palynology of Site 1166, Prydz Bay, East Antarctica" and Macphail and Truswell, "Palynology of Neogene Slope and Rise Deposits from ODP Sites 1165 and 1167, East Antarctica," both this volume), of vegetation composition onshore throughout the period of deposition. The reconstruction of vegetation communities through time using both proxies of source vegetation can also lead to broad interpretations about regional climate change. Although phytoliths are absent to rare throughout samples studied from both Sites 1165 and 1166, the sparse record complements and supplements the terrestrial palynomorph record, particularly with evidence of grass during the Miocene (Table T1).
Phytoliths are plant microfossils of solid opal silica deposited within and between the cells of most living plants. When extracted from soils, they are commonly thought to represent a decay-in-place record of the overlying source vegetation, although it has been recognized that several factors contribute to the assemblage composition, including fire, herbivory and eolian, edaphic, and fluvial transport (Fredlund and Tieszen, 1994). Erosion of soils, either by wind or water, and subsequent transport and deposition of terrigenous material offshore yields a phytolith history in the stratigraphic record that reflects the vegetation history in the hinterland. Phytoliths have previously been recovered in low but persistent numbers from the Cape Roberts Project (CRP) cores off the southern Victoria Land coast, Antarctica (early Oligocene-early Miocene age) (Thorn, 2001), and in low numbers from Ocean Drilling Program (ODP) cores, for example, from middle Miocene-Pleistocene (Locker and Martini, 1986) and Pliocene-Quaternary sediments (Bukry, 1979). Phytoliths have also been reported from Antarctic terrestrial rocks at Mts. Feather and Crean as far back as the late Devonian (Carter, 1999).
Phytolith processing laboratory methods followed those of Piperno (1988) and Hart (1988), as refined by Carter (1998a, 1998b). Lithologies processed were no coarser than fine sand. Other siliceous microfossils present in the sample in addition to phytoliths were extracted simultaneously. Biogenic opal residues were permanently mounted on glass microscope slides in Canada balsam and viewed under a transmitted light stereomicroscope using oil immersion. One slide was prepared per sample and scanned at 400x magnification, with photographs taken by digital camera at 1000x.
The low phytolith abundances observed could be due to many factors. The presence of a sparse but persistent flora in the CRP cores occurs within 16 km of the current shoreline but probably only a few kilometers from the paleoshoreline (Barrett, 2001). In the Prydz Bay cores, ~200-400 km from the current shoreline, phytoliths are rare to absent, suggesting that phytoliths may not travel very far offshore. Additional taphonomic factors may include low phytolith production by the source vegetation, dissolution of phytoliths in highly alkaline soils, low quantities of terrigenous material reaching the paleoshoreline, and substantial dilution of terrigenous material during offshore transport and deposition. Phytoliths and other siliceous microfossils can also be subject to silica diagenesis within the marine sedimentary record, as has occurred below ~600 mbsf at Site 1165. A similar loss of most siliceous microfossils has affected all except three samples (1.29, 142.32, and 148.11 mbsf) at Site 1166 (Table T1).