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doi:10.2204/iodp.proc.318.108.2011

Biostratigraphy

Cores 318-U1360A-1R through 5R (0–42.31 mbsf) contain siliceous microfossils (e.g., diatoms) (Table T2), whereas organic-walled microfossils (e.g., dinoflagellate cysts) are preserved in Cores 318-U1360A-1R through 6R (0–53.78 mbsf) (Table T3). Foraminifers were encountered only in Sample 318-U1360A-1R-CC (0.54 mbsf). Calcareous nannofossils are not present in Hole U1360A. Dinocysts and diatoms provide age control for Hole U1360A and indicate that Core 318-U1360A-1R (0–0.54 mbsf) is of latest Pleistocene age, comprising a matrix with intraclasts of late Eocene to early Oligocene age. Samples 318-U1360A-3R-1, 8 cm, through 6R-CC (23.38–53.78 mbsf) are of early Oligocene (<33.6 Ma) age. Sample 318-U1360A-2R-CC (14.30–14.52 mbsf) contains no age-diagnostic diatoms but does contain typical heavily silicified Paleogene taxa, and because it falls within lithostratigraphic Unit II, the top of which is at 14.3 mbsf, and is assigned to early Oligocene. This implies a hiatus between Samples 318-U1360A-1R-CC and 2R-CC, lasting from early Oligocene to latest Pleistocene.

Siliceous microfossils

Samples from Core 318-U1360A-1R (0–0.54 mbsf) contain abundant to common well-preserved diatoms, whereas Cores 2R through 5R (14.3–42.31 mbsf) contain trace to few diatoms (Table T2). Radiolarians are only preserved in Sample 318-U1360A-1R-CC (0.54 mbsf); Samples 6R-1, 81 cm (52.01 mbsf), and 6R-CC (53.78 mbsf) are barren of radiolarians.

Diatoms, ebridians, silicoflagellates, chrysophyte cysts, and sponge spicules

Thirteen samples from Hole U1360A were analyzed for diatoms, ebridians, silicoflagellates, chrysophyte cysts, and sponge spicules. All of these siliceous microfossil components were encountered in Hole U1360A. Silica-selective processing was carried out to concentrate the biosiliceous component for those samples in which analysis of smear slides indicated low biosilica content. Diatoms suggest the in situ sedimentary succession is uppermost Pleistocene for Core 318-U1360A-1R (0–0.54 mbsf) (Cody et al., 2008) and Oligocene for Sample 318-U1360A-3R-1, 8 cm, through Core 5R (below 23.38 mbsf) (Scherer et al., 2000; Olney et al., 2007); dinocysts further constrain this lower section to the lower Oligocene (see below).

A characteristic latest Pleistocene Southern Ocean diatom flora (i.e., a mix of sea ice–associated and open-water taxa) is encountered in Samples 318-U1360A-1R-1, 9 cm (0.09 mbsf), 1R-1, 19–20 cm (0.20 mbsf), and 1R-CC (0.54 mbsf), whereas a mixed latest Pleistocene/late Eocene to early Oligocene flora is found in Sample 1R-1, 17 cm (0.17 mbsf). The latter sample was taken from an intraclast (rather than the sedimentary matrix), accounting for the floral differences between this sample and the other three samples in Core 318-U1360A-1R (Table T2). Samples 318-U1360-3R-1, 8 cm, through 5R-CC (23.38–42.31 mbsf) contain typical Oligocene Antarctic shallow-water siliceous microfossils similar to those recorded in the Cape Roberts Project CRP-2/2A drill core (Scherer et al., 2000; Olney et al., 2007).

Core 318-U1360A-1R (0–0.54 mbsf): uppermost Pleistocene

Samples taken from the sedimentary matrix of Core 318-U1360A-1R are characterized by trace to abundant, well-preserved latest Pleistocene index diatom taxa (e.g., Eucampia antarctica, Fragilariopsis curta, Fragilariopsis kerguelensis, Fragilariopsis rhombica, Fragilariopsis separanda, Thalassiosira antarctica, and Thalassiosira lentiginosa). The presence of T. antarctica (last occurrence [LO] at 0.61 Ma) constrains this core to the last 610 k.y., although the absence of Actinocyclus ingens (LO at 0.54 Ma) may indicate a slightly younger age (i.e., the last 540 k.y.). Trace occurrences of the Paleogene diatom Pyxilla sp. and the Pliocene index diatoms Shionodiscus tetraoestrupii var. reimeri and Thalassiosira torokina in Sample 318-U1360A-1R-CC indicate reworking of older material into the sedimentary matrix.

One sample was taken from an intraclast in Core 318-U1360A-1R (Sample 318-U1360A-1R-1, 17 cm) (Table T2) that contains a mix of the latest Pleistocene diatom assemblage encountered in the matrix of Core 318-U1360A-1R (see above) plus moderately to well-preserved late Eocene to early Oligocene siliceous microfossils similar to those encountered in Samples 318-U1360A-3R-1, 8 cm, through 5R-CC (see below). The late Eocene to early Oligocene age constraint is based on one specimen of Kisseleviella gaster (Olney et al., 2005) and is therefore rather tentative, although the assemblage associations provide supporting age evidence. In particular, Synedropsis cheethamii, a sea ice–associated diatom from lower Oligocene sediments of the Ross Sea (Olney et al., 2009), occurs in trace abundance alongside other small, lightly silicified pennates such as Kannoa hastata and Grammatophora sp. (Table T2), as well as heavily silicified taxa such as Pyxilla sp. and Hemiaulus sp. These associations are discussed below in context of Samples 318-U1360A-3R-1, 8 cm, through 5R-CC. However, the finding is important because it shows that not only do some of the intraclasts in Core 318-U1360A-1R contain moderately to well-preserved siliceous microfossils but that they also capture intriguing insights into Oligocene paleoenvironments at Site U1360.

Samples 318-U1360A-2R-CC through 5R-CC (14.30–42.31 mbsf): early Oligocene

Sample 318-U1360A-2R-CC did not yield age-diagnostic microfossils but contains typical heavily silicified Paleogene taxa such as Hemiaulus spp., Pyxilla spp., and Stephanopyxis spp. We assign this sample to the Oligocene based on these assemblage associations and lack of latest Pleistocene diatoms present in Core 318-U1360A-1R. Samples 318-U1360A-3R-1, 8 cm, through 5R-CC contain typical Oligocene Antarctic shallow water–shelf siliceous microfossils in trace to few abundance and moderate to good preservation. In Sample 318-U1360A-3R-1, 8 cm (23.38 mbsf), heavily silicified relatively large taxa such as Goniothecium odontella, Hemiaulus sp., Pyxilla sp., and Stephanopyxis sp. are preserved alongside more lightly silicified small pennates such as Fragilaria spp., Grammatophora spp., K. hastata, Kisseleviella spp., small Sceptroneis spp., and S. cheethamii. The paleoecology of these small pennates is largely unknown, but we suggest they could indicate low salinities (or possibly a sea ice environment, see below) because of morphological similarities to living freshwater and coastal taxa (Round et al., 1990; Olney et al., 2009). Low salinities are also indicated by the presence of Cocconeis sp. in Core 318-U1360A-4R (Table T2); further, the presence of the tychopelagic diatom taxa Actinoptychus senarius and Paralia sulcata suggest a shallow (near shore) environment. Importantly, if the identification of S. cheethamii is correct, it may indicate the presence of seasonal sea ice according to Olney et al. (2009), who described this species from lower Oligocene sediments of the Ross Sea. Scanning electron microscope analysis is required to confirm the identification of the small pennate diatoms in Hole U1360A because specific valve features were not resolvable in the light microscope on ship.

Trace occurrences of Kisseleviella cicatricata in Sample 318-U1360A-3R-1, 8 cm (23.38 mbsf) (Table T2), suggests an Oligocene age (Olney et al., 2005). This interpretation is corroborated by the presence of Rocella praenitida, although locally this species may range into the late middle Eocene according to unpublished data from Leg 189 Hole 1172A on the East Tasman Plateau (C.E. Stickley, pers. comm., 2010).

Considering that samples from Core 318-U1360A-3R downhole allow this part of lithostratigraphic Unit II to be assigned an early Oligocene age, we consider that the entire Unit II is of that age, including Sample 318-U1360A-2R-CC.

Radiolarians

Few radiolarians are found in Sample 318-U1360A-1R-CC (0.54 mbsf). The assemblage is dominated by Antarctissa denticulata, suggesting an age no older than late Miocene. No additional age inferences are available from the radiolarian assemblages in Core 318-U1360A-1R. Samples 318-U1360A-2R-CC through 6R-CC are barren of radiolarians.

Palynology

Six samples from Hole U1360A were processed for palynology (Table T3). All samples yield few to common organic-walled dinocysts. Sporomorphs are present in common (Sample 318-U1360A-1R-CC) to trace (Sample 6R-1, 31–33 cm) abundances. Preservation of both sporomorphs and dinocysts is good in all samples.

Organic-walled dinocysts

Dinocyst assemblages within all samples consist of abundant Brigantedinium spp., Selenopemphix spp., and Lejeunecysta sp. A (“large”). The identification of Forma T sensu Goodman and Ford (1983) in Samples 318-U1360A-1R-CC through 6R-CC (0.54–53.78 mbsf) suggests the entire cored succession in Hole U1360A is early Oligocene (<33.6 Ma) in age (see “Biostratigraphy” in the “Site U1356” chapter). Lejeunecysta sp. A (“large”) was found consistently, in agreement with lower Oligocene records at Site U1356. This indicates that Lejeunecysta sp. A (“large”) is a reliable index species for the early Oligocene in this region. Diatom investigations indicate that the sediments in Sample 318-U1360A-1R-1, 17 cm (0.17 mbsf), comprise an uppermost Pleistocene matrix with intraclasts of late Eocene to early Oligocene age. We thus consider that the early Oligocene dinocysts found in Sample 318-U1360A-1R-CC (0.54 mbsf) may originate from the intraclasts. No dinocyst index species for the latest Pleistocene were recorded within Sample 318-U1360A-1R-CC.

Sample 318-U1360A-6R-CC (53.78 mbsf) contains relatively high abundances of Spinidinium macmurdoense, Spinidinium schellenbergii, and Vozzhennikovia spp. Although the specimens are well preserved and intact, they may be reworked from older strata.

Sporomorphs in the samples investigated comprise few specimens of Nothofagus and saccate pollen; these may be reworked from Eocene strata and/or represent contemporaneous vegetation in the hinterland (cf. Prebble et al., 2006). Other palynofacies components were found in trace amounts only.

Foraminifers

Shipboard examination of core catcher samples identified well-preserved planktonic and benthic foraminifers in Sample 318-U1360A-1R-CC (0.54 mbsf) only; Samples 2R-CC through 6R-CC are barren of foraminifers.

Planktonic foraminifers

Planktonic foraminifer assemblages in Sample 318-U1360A-1R-CC (0.54 mbsf) comprise a monospecific assemblage of the extant foraminifer Neogloboquadrina pachyderma (first occurrence at 9.4 Ma; Chron C4Ar.1n) (Berggren et al., 1995; Gradstein et al., 2004). Based on this taxon, Sample 318-U1360A-1R-CC is assigned to Subantarctic Zone AN7, which ranges from the middle late Miocene to present, concurring with a latest Pleistocene diatom-based age for Core 318-U1360A-1R.

Benthic foraminifers

Sample 318-U1360A-1R-CC (0.54 mbsf) contains a low-diversity calcareous benthic foraminifer assemblage, with Globocassidulina subglobosa as the dominant species (40% of the total assemblage). Rare specimens of Trifarina angulosa and Cibicidoides mundulus were also found. Since they are well preserved, we argue that the benthic foraminifers in Sample 318-U1360A-1R-CC (0.54 mbsf) originate from the uppermost Pleistocene matrix, rather than the intraclast material.

Paleoenvironmental interpretation

Latest Pleistocene

The latest Pleistocene diatom assemblages are a mix of sea ice–associated and open-water taxa, implying that the shelf setting at Site U1360 was influenced by seasonal sea ice. Benthic foraminifer assemblages comprising high abundances of G. subglobosa are typically associated with Antarctic Bottom Water (Corliss, 1979). Because Antarctic Bottom Water formation is linked to seasonal sea ice formation, this corroborates a seasonally sea ice–influenced environment.

Early Oligocene

The early Oligocene diatom assemblages contain shallow water–shelf taxa, indicating low-salinity conditions and likely seasonal sea ice. The dominance of protoperidinioid dinocysts indicates high surface water nutrient levels and possibly sea ice because these taxa are heterotrophic. Sporomorphs represent reworking from older strata and/or contemporaneous vegetation in the hinterland.

Summary

Sediments in Core 318-U1360A-1R (0–0.54 mbsf) comprise an uppermost Pleistocene matrix with intraclasts of late Eocene to early Oligocene age. Diatoms indicate that during the latest Pleistocene, the shelf setting of Site U1360 was influenced by seasonal sea ice. Combined microfossil analyses suggest the interval below Sample 318-U1360A-3R-1, 8 cm (23.38 mbsf), is early Oligocene (<33.6 Ma) in age. Diatom assemblage associations and lithostratigraphy suggest all of the strata below 14.3 mbsf is early Oligocene in age. The early Oligocene microfossils indicate a shallow-water shelfal environment with low salinities and high nutrient levels, likely driven by seasonal sea ice. Sporomorphs may represent reworking from older strata and/or contemporaneous vegetation in the hinterland.

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