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Sediments from Holes U1358A and U1358B contain siliceous and organic microfossils. Calcareous nannofossils and foraminifers were not encountered at Site U1358. Diatoms provide tentative age control throughout the section. Pliocene sediments (between Samples 318-U1358B-2R-CC and 4R-CC; 9.32–28.62 mbsf) are overlain by uppermost Pleistocene to Holocene sediments (Sample 318-U1358A-1R-CC, 1.10 mbsf; and between Samples 318-U1358B-1R-1, 0 cm, and 1R-CC; 0–1.10 mbsf). Dinocysts and radiolarians were encountered in trace amounts only and provide no further age constraints. Diatom assemblages are a mix of sea ice–associated and open-water taxa. This suggests a high-nutrient environmental setting influenced by seasonal sea ice. High abundances of reworked Mesozoic and/or Paleozoic sporomorphs indicate sediment input from the hinterland.

Siliceous microfossils

Samples 318-U1358A-1R-CC (1.10 mbsf) and 318-U1358B-1R-1, 0 cm (0 mbsf), and 1R-CC (0.42 mbsf) contain poor to well-preserved common to abundant diatoms. The diamict matrix of Cores 318-U1358B-2R through 4R (9.32–28.62 mbsf) (see “Lithostratigraphy”) contains trace to rare occurrences of diatoms (Table T2), radiolarians, actiniscidians, silicoflagellates, and sponge spicules.


One sample from Hole U1358A and seven samples from Hole U1358B were analyzed for diatoms (Table T2). A characteristic Neogene, high-nutrient, seasonally sea ice–influenced Southern Ocean diatom flora was encountered in all samples. In terms of valve fragmentation, preservation is moderate to poor in Samples 318-U1358A-1R-CC (1.10 mbsf) and 318-U1358B-1R-1, 0 cm (0 mbsf), and poor in all other samples (0.42–28.62 mbsf). In terms of dissolution, diatom preservation is good in all samples analyzed (Table T2).

Samples 318-U1358A-1R-CC (1.10 mbsf) and 318-U1358B-1R-1, 0 cm (0 mbsf), and 1R-CC (0.42 mbsf) contain common to abundant latest Pleistocene to Holocene diatoms characterized by rare to common occurrences of Chaetoceros resting spores, Eucampia antarctica, Fragilariopsis curta, Fragilariopsis kerguelensis, Fragilariopsis rhombica, Fragilariopsis ritscheri, Fragilariopsis separanda, Shionodiscus gracilis var. gracilis, Thalassiosira antarctica, and Thalassiosira lentiginosa (Table T2). We assign these samples to the latest Pleistocene (<0.61 Ma) based on the presence of the index species T. antarctica (first occurrence [FO] 0.61 Ma) in association with other latest Pleistocene to Holocene index taxa (Table T3). The absence of Actinocyclus ingens (last occurrence [LO] at 0.54 Ma) may further constrain these samples to within the last ~540 k.y.

Samples 318-U1358B-2R-CC through 3R-CC (9.32–21.54 mbsf) contain trace to rare abundances of the Pliocene index taxa Thalassiosira insigna and Thalassiosira inura, the Miocene–Pleistocene index species Thalassiosira torokina, and the long-ranging middle Miocene to late Pleistocene index species A. ingens vars. (Tables T2, T3). We tentatively assign these samples to the mid-Pliocene based on the presence of these species and the absence of Pleistocene index taxa listed above. These data imply a mid-Pliocene to latest Pleistocene hiatus or condensed interval between Samples 318-U1358B-1R-CC and 2R-CC (Table T3). Such a hiatus concurs with physical property data, which show changes in magnetic susceptibility and color in the middle of Section 318-U1358B-2R-2 (see “Physical properties”). The FO of T. insigna (3.25 Ma) and LO of T. inura (2.54 Ma) within Samples 318-U1358B-2R-CC and 3R-CC constrain the age of these samples to 3.25–2.54 Ma. The FO of T. inura (4.74 Ma) and T. torokina (7.23 Ma) and the absence of T. lentiginosa (FO at 3.99 Ma) and F. curta (FO at 3.56 Ma) indicate an older age for Core 318-U1358B-4R (Table T3). However, because diatoms below Core 318-U1358-2R are heavily fragmented and are in trace to rare abundance, further refinement of the biostratigraphy is difficult. Trace occurrences of radiolarians indicate an age younger than Miocene and therefore do not provide any further insight.


One sample (318-U1358B-3R-CC; 21.54 mbsf) was processed for palynology at Site U1358. The sample yielded trace abundances of cysts of heterotrophic dinoflagellates (Protoperidinium spp.) in moderate preservation. These dinocysts provide no age constraints. Palynological data are provided in Table T4.

Sporomorphs are abundant in Sample 318-U1358B-3R-CC, and the assemblage mainly consists of well-preserved (saccate) pollen and spores. The dominance of thick-walled and/or chemically particularly resistant taxa (as indicated by the strong representation of spores and bisaccates), the high degree of thermal maturity (as indicated by the dark exine colors), and the lack of Nothofagus pollen suggests that the sporomorphs in Sample 318-U1358B-3R-CC are reworked from Mesozoic and/or Paleozoic strata.

Other palynofacies components (i.e., black and brown phytoclasts) are present in trace amounts only.

Age model and sedimentation rates

The spliced succession is divided into two intervals: the uppermost 1.10 m (Samples 318-U1358A-1R-CC and 318-U1358B-1R-1, 0 cm, and 1R-CC) is assigned to the latest Pleistocene to Holocene (<0.54/0.61 Ma) based on diatom biostratigraphy, whereas sediments below this level are no younger than mid-Pliocene (>2.54 Ma) and no older than Miocene. A condensed interval or a hiatus is inferred between Samples 318-U1358B-1R-CC and 2R-CC (0.42 and 9.32 mbsf, respectively), between ~2.54 and 0.54/0.61 Ma. Sedimentation rates are not calculated because our age assignments are largely tentative.

Paleoenvironmental interpretation

Diatom assemblages are a mix of sea ice–associated and open-water taxa. This suggests a high-nutrient environmental setting influenced by seasonal sea ice. Palynological associations are a mix of reworked and in situ palynomorphs. The in situ protoperidinioid dinocysts confirm a nutrient-rich environment. High abundance of reworked microfossils indicates a significant input from the hinterland.