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doi:10.2204/iodp.proc.318.103.2011

Biostratigraphy

Unconsolidated gravels and coarse sands were recovered from Hole U1355A with minor amounts of diatom-bearing silty clay and clasts of diatom-bearing silty clays to silt-rich diatom ooze. Selected clay-rich clasts from Cores 318-U1355A-1R through 4R, as well as the top (0–1 cm) of Section 1R-1, were analyzed for microfossil content. The results are summarized in Table T2.

Siliceous microfossils

Diatoms, silicoflagellates, and sponge spicules

Diatoms, silicoflagellates, and sponge spicules occur throughout the clay-rich clasts and core-top material analyzed in Hole U1355A. Diatoms are common to abundant in Core 1R (including the core top) and present at trace to common abundances in Cores 2R through 4R.

The assemblage in Sample 318-U1355A-1R-1, 0–1 cm, is well preserved and comprises typical Antarctic diatom species including notably Actinocyclus curvatulus (and Actinocyclus spp.), Azpeitia tabularis, Dactyliosolen antarcticus, Eucampia antarctica, Fragilariopsis curta, F. kerguelensis, Fragilariopsis rhombica, Fragilariopsis ritscheri, Fragilariopsis separanda, Shionodiscus gracilis, Thalassiosira antarctica, T. lentiginosa, Thalassiosira trifulta, and Thalassiothrix antarctica. Trace occurrences of A. ingens and Denticulopsis spp. indicate reworked Miocene material. Sample 318-U1355A-1R-1, 0–1 cm, is assigned to the Holocene based on these assemblage characteristics and the absence of common late Pleistocene and older index species.

The diatom assemblages in Samples 318-U1355A-1R-1, 70 cm, through 4R-CC (Table T2) are moderately to well preserved and dominated by F. kerguelensis and T. lentiginosa with trace to few occurrences of A. ingens (reworked or more likely in situ toward the top of its range at 0.5–0.57 Ma according to the average range model of Cody et al., 2008). Other species occurring at trace to common abundance include Actinocyclus actinochilus (and Actinocyclus spp.), Asteromphalus parvulus, A. tabularis, Chaetoceros resting spores, D. antarcticus, E. antarctica, F. curta, Fragilariopsis cylindrus, Fragilariopsis obliquecostata, F. rhombica, F. ritscheri, F. separanda, Fragilariopsis sublinearis, Fragilariopsis vanheurckii, Paralia sulcata, Proboscia spp., Rhizosolenia spp., Rouxia spp., S. gracilis, Stellarima microtrias, Stephanopyxis spp., T. antarctica, Thalassiosira oliverana, Thalassiosira tumida, T. antarctica, Trichotoxon reinboldii, the silicoflagellate Distephanus speculum, and sponge spicules. Trace to few occurrences of A. ingens var. ovalis, Denticulopsis spp., Thalassiosira elliptipora, Thalassiosira inura, Thalassiosira torokina, and Thalassiosira vulnifica indicate reworked Miocene and Pliocene material.

Cores 318-U1355A-2R through 4R contain a similar floral assemblage to that recovered in Core 1R but with more extensive reworking of Pliocene (T. inura), Miocene (Denticulopsis spp. and A. ingens var. ovalis), and probably older material (e.g., stephanopyxids and the ebridian Pseudammodochium lingii).

A middle Pleistocene–Holocene age is assigned to Samples 318-U1355A-1R-1, 70 cm, through 4R-CC based on the consistent and abundant presence of A. ingens and the absence of in situ index species for the early Pleistocene or older.

Radiolarians

A typical Antarctic Holocene radiolarian assemblage characterized by Antarctissa denticulata, Antarctissa strelkovi, Spongotrochus glacialis, and Cycladophora davisiana is present in Sample 318-U1355A-1R-1, 0–1 cm. Sample 2R-1, 149–150 cm, is barren of radiolarians, whereas Samples 3R-CC and 4R-CC contain only very rare specimens. The assemblage is similar to that recovered in Sample 1R-1, 0–1 cm, but includes some reworked specimens such as Prunopyle titian and Cycladophora spongothorax. These radiolarian assemblages suggest an age younger than latest Miocene.

Palynology

Four samples from Hole U1355A were processed for palynology: Samples 318-U1355A-1R-1, 0–1 cm; 1R-1, 68.5–71.5 cm; 1R-1, 71.5–73.5 cm; and 4R-1, 34–35 cm. Palynomorphs are found in trace amounts and preservation is moderate to poor.

The palynological associations contain both marine (primarily dinoflagellate cysts, or dinocysts) and terrestrial (sporomorphs and phytoclasts) components. Other marine palynomorphs include acritarchs, copepod eggs, and foraminifer test linings.

Most dinocysts are reworked from the Eocene, as indicated by the occurrence of, for example, Enneadocysta dictyostila, Phthanoperidinium comatum, Phthanoperidinium stockmansii, Spinidinium macmurdoense, Vozzhennikovia apertura, Vozzhennikovia netrona, and Vozzhennikovia stickleyae. Findings of Deflandrea sp. A of Brinkhuis et al. (2003) and Turbiosphaera filosa further constrain this reworking to the latest Eocene. All of the above-mentioned species are known to reach high abundances in uppermost Eocene strata of the Tasman region (e.g., Sluijs et al., 2003). In addition to the markers for the late Eocene, species characteristic of the Holocene were also identified, namely Brigantedinium spp. and Impagidinium pallidum. The latter occurs in relatively high abundances in all samples, and the general transparency of the specimens strongly suggests that they are in situ, thus reflecting a Holocene age. No dinocysts with stratigraphic ranges limited to the Oligocene, Miocene, or Pliocene were recorded. Hence, the dinocysts exhibit a bimodal age distribution, with a major component of likely late Eocene origin reworked into Holocene material.

Terrigenous input is indicated by sporomorphs and phytoclasts; representatives of both groups occur in trace amounts in all samples. The sporomorphs comprise both pollen (bisaccates, Classopollis, and Nothofagus) and spores (Baculatisporites, Deltoidospora, and Lycopodiumsporites). Findings particularly of Classopollis and Nothofagus suggest reworking from older (Paleogene and Mesozoic) strata, which is only partially consistent with the dinocyst-based age information; they indicate the presence of at least one additional (i.e., pre-Eocene) host-rock source.

Foraminifers

Foraminifers were recovered only in the uppermost section of Hole U1355A (Sample 318-U1355A-1R-1, 0–1 cm). All core catcher samples are barren of foraminifers. In Sample 318-U1355A-1R-1, 0–1 cm, a planktonic foraminifer assemblage characteristic of the high-latitude Southern Ocean was identified. This low-diversity assemblage consists mainly of N. pachyderma (84%), Globigerina bulloides (10%), and Globigerina uvula (2%). Preservation is good, with no evidence of reworking. Benthic foraminifers include Oridorsalis umbonatus, Globocassidulina subglobosa, Astoculus credidus, and Nonion spp.

Summary

Sample 318-U1355A-1R-1, 0–1 cm, and clay-rich clasts from Cores 318-U1355A-1R through 4R yield diatoms, silicoflagellates, radiolarians, sponge spicules, dinoflagellate cysts and sporomorphs, and planktonic foraminifers. Diatoms are the most abundant group and the most useful for age determination; they suggest a middle Pleistocene–Holocene age for Hole U1355A. This age assessment is supported by the other investigated microfossil groups. In addition, diatoms and radiolarians indicate Pliocene and Miocene reworking, whereas palynology indicates moderate to strong reworking of late and middle Eocene material. Pollen and spores also indicate Mesozoic reworking.

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