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doi:10.2204/iodp.proc.318.107.2011

Biostratigraphy

Core catcher samples from all holes and selected core samples from Holes U1359A and U1359D, as well as selected samples from Holes U1359B and U1359C, were analyzed for diatoms, silicoflagellates, ebridians, actiniscidians, chrysophyte cysts, and sponge spicules (Tables T3, T4, T5, T6). All core catcher samples from Holes U1359A, U1359B, and U1359D were examined for radiolarians (Table T7). Fourteen samples were examined for calcareous nannofossils from Hole U1359B (Table T8). Thirteen samples from Hole U1359A and seventeen samples from Hole U1359D were processed for palynology (Table T9). Selected core catcher samples from Holes U1359A, U1359B, and U1359D were examined for foraminifers (Table T10).

The abundance data for each of these microfossil groups are summarized in Figure F26. Siliceous microfossils are present throughout the recovered intervals in variable preservation. Diatoms are present in all samples examined except for Sample 318-U1359A-3H-3, 82 cm (13.92 mbsf), whereas radiolarians occur in all samples studied. Rare and poorly preserved calcareous nannofossils occur in Holes U1359A, U1359B, and U1359C but are common and well preserved in Hole U1359D (Fig. F26). Palynomorphs are present in most core catcher samples processed for palynology. Planktonic and benthic foraminifers occur sporadically in core catcher samples from all holes; planktonic foraminifers are common to abundant in Samples 318-U1359A-4H-CC (28.56 mbsf) and 5H-CC (29.34 mbsf) and 318-U1359D-24R-CC (371.86 mbsf).

An upper middle Miocene through upper Pleistocene sedimentary succession was recovered at Site U1359. Integrated diatom, radiolarian, foraminifer, and magnetostratigraphic data indicate a late Pliocene to early Pleistocene hiatus (~2.5–1.5 Ma) and a lower to mid upper Miocene condensed interval (~9.8–7 Ma). All index events (biostratigraphy and magnetostratigraphy) are compiled in Table T11. The age-depth models for individual holes are presented in Figure F27 and combined for all four holes in Figure F7 (in meters below seafloor) and in Figure F28 (in meters composite depth).

Siliceous microfossils

Recovered sediments at Site U1359 contain well-preserved biogenic silica dominated by diatoms and radiolarians, with variable abundances of silicoflagellates, ebridians, chrysophyte cysts, and sponge spicules. Diatoms and radiolarians provide the primary age control in holes drilled at Site U1359. Assemblages are dominated by typical Neogene Southern Ocean open-water taxa with variable abundances of benthic, neritic, and sea ice–associated diatom taxa, indicating a high-nutrient, high-productivity sea ice–influenced setting throughout the depositional history of Site U1359.

Diatoms

Combined holes at Site U1359 revealed a clear succession of index species (e.g., Cody et al., 2008) (Figs. F7, F27, F28; Table T11). Samples 318-U1359A-1H-1, 2 cm, through 5H-CC (0.02–29.34 mbsf) and 318-U1359C-1H-1, 40.0 cm, through 5H-1, 35.0 cm (0.40–36.05 mbsf), are assigned to the Pleistocene; Samples 318-U1359A-6H-2, 132.5 cm, through 22X-CC (41.42–190.26 mbsf), 318-U1359B-5H-2, 131.0 cm, through 17H-1, 35 cm (39.01–150.55 mbsf), and 318-U1359C-5H-2, 140.0 cm, through 16H-3, 35 cm (38.60–143.55 mbsf), are assigned to the Pliocene; and Samples 318-U1359B-20H-4, 25 cm, through 28X-CC (183.45–243.11 mbsf), and 318-U1359D-4R-5, 105 cm, through 48R-CC (178.35–596.31 mbsf) are assigned to the middle Miocene.

Pleistocene

Four diatom datums were recognized in Samples 318-U1359A-1H-1, 2 cm, through 5H-CC (0.02–29.34 mbsf) and 318-U1359C-1H-1, 40.0 cm, through 5H-1, 35.0 cm (0.40–36.05 mbsf) (Tables T3, T5, T11):

• Last occurrences (LOs) of

• Actinocyclus ingens (0.54 Ma),

• Thalassiosira elliptipora (0.68 Ma),

• Fragilariopsis barronii (1.24 Ma), and

• Rouxia antarctica (1.50 Ma).

The succession of diatom species (and radiolarians) indicates the Pliocene/Pleistocene boundary interval (1.81 Ma) is missing in a ~1 m.y. hiatus between ~2.5 and 1.5 Ma (Figs. F7, F28).

Pliocene

Fifteen diatom datums were recognized in Samples 318-U1359A-6H-2, 132.5 cm, through 22X-CC (41.42–190.26 mbsf), 318-U1359B-5H-2, 131.0 cm, through 17H-1, 35 cm (39.01–150.55 mbsf), and 318-U1359C-5H-2, 140.0 cm, through 16H-3, 35 cm (38.60–143.55 mbsf) (Tables T3, T4, T5, T11):

• LOs of

• Thalassiosira vulnifica (2.17 Ma),

• Actinocyclus fasciculatus (2.16 Ma),

• Fragilariopsis interfrigidaria (2.43 Ma),

• Thalassiosira insigna (2.48 Ma),

• Thalassiosira inura (2.54 Ma),

• Rouxia diploneides (2.62 Ma), and

• Fragilariopsis clementia (4.56 Ma) and

• First occurrences (FOs) of

• Fragilariopsis kerguelensis (2.20 Ma),

• T. vulnifica (3.15 Ma),

• T. insigna (3.25 Ma),

• F. interfrigidaria (4.06 Ma),

• F. barronii (4.40 Ma),

• Thalassiosira complicata (4.68 Ma),

• T. inura (4.74 Ma), and

• Shionodiscus oestrupii (4.88 Ma).

The major discrepancy between magnetostratigraphic (see “Paleomagnetism”) and diatom data between ~5.5 and 4.5 Ma (Figs. F7, F28) highlights an interval where using the mid-point absolute age value for diatom events from the average range model of Cody et al. (2008) is not appropriate and, therefore, where the biochronology or biostratigraphic methodologies of Expedition 318 may be improved. Since the magnetostratigraphy at Site U1359 is supported by radiolarian datums in this interval, we suggest that recalibration of the pertinent diatom datums to the local magnetostratigraphy is necessary in this case.

Miocene

Fourteen diatom datums were recognized in Samples 318-U1359B-20H-4, 25 cm, through 28X-CC (183.45–243.11 mbsf), and 318-U1359D-4R-5, 105 cm, through 48R-CC (178.35–596.31 mbsf) (Tables T4, T6, T11):

• Last abundant occurrence (LAO) of Denticulopsis simonsenii (8.63 Ma);

• LOs of

• Denticulopsis ovata (9.64 Ma),

• Denticulopsis dimorpha var. areolata (10.25 Ma),

• D. dimorpha (10.28 Ma),

• Nitzschia denticuloides (11.72 Ma),

• Actinocyclus ingens var. nodus (12.56 Ma), and

• Crucidenticula nicobarica (12.62 Ma); and

• FOs of

• Thalassiosira oliverana var. sparsa (8.53 Ma),

• D. dimorpha var. areolata (10.31 Ma),

• D. ovata (11.09 Ma),

• D. dimorpha sensu lato (12.50 Ma),

• Fragilariopsis claviceps (12.68 Ma),

• Denticulopsis praedimorpha sensu lato (12.97 Ma), and

• N. denticuloides (13.49 Ma).

Diatom biostratigraphy is not sufficiently well resolved to define the Miocene/Pliocene boundary. Instead, the boundary is defined by magnetostratigraphy (see “Paleomagnetism”) at ~155 mbsf in Hole U1359B (Figs. F7, F27).

Silicoflagellates, ebridians, chrysophyte cysts, and sponge spicules

Silicoflagellates and sponge spicules occur sporadically in trace to rare abundance throughout the recovered intervals, whereas ebridians were observed in Samples 318-U1359B-12H-CC (112.55 mbsf), 13H-CC (120.30 mbsf), 16H-CC (146.35 mbsf), and 20H-CC (188.12 mbsf) and 318-U1359C-5H-1, 35 cm (36.05 mbsf), and 5H-2, 140 cm (38.60 mbsf). Chrysophyte cysts were only rarely noted (Tables T3, T4, T5, T6).

Radiolarians

Radiolarians occur in all samples examined from Holes U1359A, U1359B, and U1359D with good to moderate preservation. No material from Hole U1359C was examined. Radiolarian-based age constraints on the individual samples examined are summarized in Table T7.

Calcareous nannofossils

Rare and poorly preserved calcareous nannofossils occur in Holes U1359A, U1359B, and U1359C; however, they are common and moderately to well preserved in Hole U1359D (Fig. F26; Table T8). A diatom-bearing nannofossil ooze occurs within the lowermost part of Core 318-U1359D-24R and the uppermost part of Core 25R, and nannofossil-bearing sediments were observed within Sample 14R-CC and Cores 23R through 29R (see “Lithostratigraphy”). A more detailed analysis of selected samples (Table T8) permitted identification of a low diversity calcareous nannofossil assemblage, mainly consisting of Reticulofenestra pseudoumbilicus, Reticulofenestra minuta, and Reticulofenestra minutula (whereby closed central area morphotypes are dominant; e.g., Reticulofenestra gelida, Dictyococcites antarcticus, and Dictyococcites productus) characteristic of the late Miocene–early Pliocene from high latitudes. No standard biozonal markers were found.

Palynology

Thirty samples were analyzed for palynology from core catcher material from Holes U1359A and U1359D. With the exception of Sample 318-U1359D-6R-CC (200.32 mbsf), all samples proved palynologically productive. The palynological associations are strongly dominated by reworked sporomorphs, with the exception of the material studied between Samples 318-U1359D-12R-CC and 40R-CC (253.66–520.60 mbsf), which is dominated by dinocysts. All palynological data from Site U1359 are presented in Table T9.

Dinocysts

Dinocysts occur in trace to common abundances in Samples 318-U1359A-6H-CC (47.73 mbsf), 8H-CC (66.60 mbsf), 9H-CC (75.80 mbsf), 15H-CC (133.70 mbsf), and 18X-CC (153.96 mbsf) and in all samples from Hole U1359D except Sample 318-U1359D-6R-CC (200.32 mbsf) (Fig. F26; Table T9). The overall preservation of dinocysts is poor to moderate but generally improves downhole. Dinocyst assemblages are overwhelmingly dominated by protoperidinioid (heterotrophic) dinocysts. Within these assemblages, specimens of Brigantedinium spp. and Selenopemphix nephroides are most abundant; specimens attributable to Lejeunecysta spp. are only present in trace amounts. Specimens of Selenopemphix spp. found in samples from Holes U1359A and U1359D are remarkably small. This phenomenon is consistent with the coeval record obtained at Site U1356A (Cores 318-U1356A-1R through 14R; 4.60–130.70 mbsf). In contrast, the Selenopemphix spp. specimens from Oligocene strata at Sites U1356 and U1360 are approximately twice the size of middle and late Miocene specimens. Although more data are needed to substantiate this observation, the size differences of Selenopemphix spp. specimens may eventually provide a convenient stratigraphic marker for the middle to late Miocene.

Besides protoperidinioid dinocysts, Samples 318-U1359A-6H-CC (47.73 mbsf), 8H-CC (66.60 mbsf), and 18X-CC (153.96 mbsf) and 318-U1359D-35R-CC (474.91 mbsf) through 45R-CC (567.44 mbsf) yield Impagidinium spp., which are characteristic of oceanic, oligotrophic settings. In addition, several samples, particularly from Hole U1359D (Table T9), contain numerous specimens of a previously unknown dinocyst taxon, here informally termed “Impagidinium brown.” Morphologically, this taxon appears to be attributable to the genus Impagidinium. However, its conspicuous brown color is unknown from any Impagidinium species and strongly reminiscent of protoperidinioid cysts. An in-depth characterization and formal description of this taxon will be carried out during postexpedition research. Several samples also contain specimens of Vozzhennikovia spp. and Enneadocysta spp.; these are considered to be reworked from the Eocene (Table T9). Sample 318-U1359A-16R-CC (295.95 mbsf) contains specimens of Habibacysta tectata, which suggests a maximum age of 14.0 Ma for that sample (see Tables T3 and T4 in the “Methods” chapter for references).

Sporomorphs

Sporomorphs are present in all samples except Samples 318-U1359D-29R-CC (412.83 mbsf) and 318-U1359D-32R-CC (446.36 mbsf) (Fig. F26; Table T9) and they are remarkably well preserved in general. All sporomorphs encountered are considered to be reworked from older (Paleogene, Mesozoic, and Paleozoic) strata. This is suggested by the dominance of thick-walled spores and the generally dark color (i.e., strong thermal maturity) of sporomorphs. Sporomorph genera identified include Classopollis/Corollina spp., Baculatisporites/Osmundacidites spp., Cicatricosisporites spp., Deltoidospora/Triletes spp., and Ischyosporites spp.

Foraminifers

Thirty-nine samples were examined for foraminifers from Holes U1359A, U1359B, and U1359D (Table T10). Calcareous benthic and planktonic foraminifers are common to abundant in Samples 318-U1359A-4H-CC (28.56 mbsf) and 5H-CC (29.34 mbsf) and 318-U1359D-14R-CC (276.07 mbsf), 24R-CC (371.86 mbsf), and 30R-CC (424.08 mbsf) (Table T10). Relatively diverse assemblages of foraminifers occur in Samples 318-U1359D-14R-CC and 24R-CC. Most other samples examined are barren of foraminifers or limited to a few agglutinated specimens.

The low-diversity planktonic foraminifer assemblages and species identified at Site U1359 are characteristic of sub-Antarctic waters (Berggren, 1992).

Planktonic foraminifers

Samples 318-U1359A-4H-CC (28.56 mbsf) and 5H-CC (29.34 mbsf) are characterized by relatively high abundances of Neogloboquadrina pachyderma and Globigerina bulloides. Neogloboquadrina pachyderma comprises >80% of the total planktonic foraminifer assemblages. Based on these taxa, and using the Subantarctic zonal scheme of Berggren (1992), the upper 29.34 mbsf of Hole U1359A can be assigned to Subantarctic Zone AN7, which ranges from 9.2 Ma to the present (Gradstein et al., 2004).

Neogloboquadrina nympha (LO at 10.1 Ma) and Neogloboquadrina acostaensis (FO at 10.9 Ma) were identified in Samples 318-U1359D-24R-CC (371.86 mbsf) and 27R-CC (391.62 mbsf), constraining the 371.86–391.62 mbsf interval in Hole U1359D to between 10.1 and 10.9 Ma. Long-ranging G. bulloides and Globigerina woodi comprise the other planktonic species in the assemblage.

Globorotalia miozea (FO at 16.6 Ma; Gradstein et al., 2004) was recorded in Sample 318-U1359D-34R-CC (464.36 mbsf). We therefore assign a maximum age of 16.6 Ma to this sample.

Benthic foraminifers

Calcareous benthic foraminifers were observed in Sample 318-U1359A-4H-CC (28.56 mbsf) and sporadically between Samples 318-U1359D-14R-CC (276.07 mbsf) and 34R-CC (464.36 mbsf). In all other samples, foraminifers are either absent or limited to few specimens (<3 per sample) of agglutinated taxa, such as Hyperammina laevigata, Rhabdammina linearis, and Martinotiella nodulosa. Sample 318-U1359A-4H-CC (28.56 mbsf) yielded a single specimen of Melonis sphaeroides, a bathymetric indicator for abyssal depths (van Morkhoven et al., 1986), and several agglutinated specimens of Hyperammina spp., Martinotiella communis, and R. linearis.

Samples 318-U1359D-14R-CC (278.07 mbsf) and 24R-CC (371.86 mbsf) contain relatively diverse benthic foraminifer assemblages. For example, M. sphaeroides and Pullenia bulloides collectively dominate the assemblage in Sample 318-U1359D-14R-CC (276.07 mbsf), contributing 47% and 18% of the total foraminiferal assemblage, respectively, whereas Cibicidoides mundulus and Bolivina spp. occur in lower abundances (<15%).

Samples 318-U1359D-15R-CC (283.43 mbsf), 17R-CC (300.99 mbsf), and 19R-CC (320.83 mbsf) each contain single specimens of the calcareous foraminifer Epistominella exigua. In Sample 318-U1359D-24R-CC (371.86 mbsf), E. exigua, P. bulloides, and M. sphaeroides comprise 20%, 19%, and 16% of the total benthic assemblage, respectively; C. mundulus and agglutinated species each contribute <10%. This assemblage is similar to that recovered from core top material from deepwater sites between Antarctica and Australia that are associated with cold (–0.2° to +0.4°C) AABW (Corliss, 1979). The preservation of foraminifers within calcareous nannofossil ooze (see “Lithostratigraphy”) (e.g., Sample 318-U1359D-24R-CC; 371.86 mbsf]) is excellent, with many specimens exhibiting a glassy appearance.

Age model and sedimentation rates

In this section, we summarize and integrate the biostratigraphic interpretations inferred from the individual microfossil groups. The resulting integrated age-depth model presented in Table T11 and Figures F7 and F27 also incorporates paleomagnetostratigraphic age constraints presented in “Paleomagnetism.”

An upper middle Miocene through upper Pleistocene sedimentary succession was recovered at Site U1359. A late Pliocene–early Pleistocene ~1 m.y. hiatus (~2.5 to 1.5 Ma) is recognized between Samples 318-U1359A-5H-CC and 6H-2, 132.5 cm (29.34–41.42 mbsf) and between Samples 318-U1359C-5H-1, 35 cm, and 5H-2, 140 cm (36.05–38.60 mbsf). In addition, a condensed interval (low sedimentation rate) is recognized within the lower to mid upper Miocene succession between Samples 318-U1359B-21H-CC and 25X-4, 58 cm (198.15–218.78 mbsf), and between Sample 318-U1359D-7R-CC and 8R-CC (209.40–216.64 mbsf). This period of low sedimentation rate lasted from ~9.8 to 7 Ma. High sedimentation rates are inferred for the strata below this condensed interval to the bottom of Hole U1359D at 596.31 mbsf (see below).

Pleistocene (0.54–1.5 Ma)

Diatoms and radiolarians resolve the sedimentary sequence for the Pleistocene within Holes U1359A (0.02–29.34 mbsf) and U1359C (0.40–36.05 mbsf). The top of Hole U1359A is dated as latest Pleistocene (0.54 Ma), below which we infer continuous sedimentation of ~1 m.y. duration accumulating ~33 m of sediment (to 29.34 mbsf in Hole U1359A and 36.05 mbsf in Hole U1359C) (i.e., average rate = ~33 m/m.y.). Below this, a ~1 m.y. hiatus is inferred for the interval ~2.5–1.5 Ma based on diatom and radiolarian biostratigraphy in corroboration with magnetostratigraphy. This indicates the Pliocene/Pleistocene boundary interval (1.81 Ma; Gradstein et al., 2004) is not preserved at Site U1359.

Pliocene (2.5–5.33 Ma)

Diatoms, radiolarians, and magnetostratigraphy provide primary age control for the Pliocene sedimentary succession recovered in Holes U1359A (41.42–190.26 mbsf), U1359B (39.01 to ~155 mbsf), and U1359C (38.60 to ~155 mbsf). Magnetostratigraphy and radiolarian bioevents are well correlated within this section and suggest a sedimentation rate of ~44 m/m.y. (~126 m of sediment in 2.83 m.y.) (Fig. F7); diatom bioevents in disagreement with this chronostratigraphy should be recalibrated. Pliocene sedimentation therefore continued to be deposited at the same rate as that for the latest Miocene (see below).

The recognition of the Miocene/Pliocene boundary (5.33 Ma) is constrained by magnetostratigraphy (see “Paleomagnetism”), occurring between the termination of Chron C3An.1n and the onset of Chron C3n.4n (i.e., ~155 mbsf in Hole U1359B). Biostratigraphic information cannot provide more detail.

Latest middle Miocene and late Miocene (5.33–12.42 Ma)

Uppermost Miocene sediments (above ~198.15 mbsf in Hole U1359B) were deposited at a sedimentation rate of 44 m/m.y. (Fig. F7), defined by magnetostratigraphy and radiolarians. The underlying ~15 m of sediment (197.35–212.8 mbsf in Hole U1359D) of early to mid late Miocene age was deposited within 3.3 m.y. (10–6.7 Ma) according to diatom, radiolarian, and magnetostratigraphic age control. This suggests a reduced average sedimentation rate of just 4.5 m/m.y. and therefore a condensed interval for this part of the record (Fig. F7).

Below the condensed interval, diatoms, radiolarians, and magnetostratigrapy resolve an upper middle to lower upper Miocene stratigraphy between 212.80 and 596.31 mbsf (bottom of Hole U1359D). This ~384 m of sediment was deposited in ~2.5 m.y., giving an average sedimentation rate of 153 m/m.y., the highest for the entire sedimentary succession at Site U1359 (Fig. F7).

Paleoenvironmental interpretation

Pleistocene

Diatom and radiolarian Pleistocene assemblages at Site U1359 are dominated by typical Neogene Southern Ocean open-water taxa with variable abundances of benthic, neritic, and sea ice–associated diatom taxa. This indicates a pelagic, well-ventilated, nutrient-rich, sea ice–influenced setting, corroborated by the presence of heterotrophic-dominated dinocyst assemblages. The preservation of planktonic foraminifers in the Pleistocene indicates that bottom waters were favorable to the preservation of calcium carbonate. Further, pervasive reworked sporomorphs of Paleogene, Mesozoic, and Paleozoic age point to strong erosion in the hinterland.

Pliocene

Pliocene diatom assemblages are also dominated by open-water taxa and variable abundances of benthic, neritic, and sea ice–associated taxa. The dinocyst assemblages predominantly comprise heterotrophic taxa, indicating that, as for the Pleistocene, Pliocene biosiliceous-rich sediments were deposited in a pelagic, well-ventilated setting characterized by high productivity and sea ice influence.

The presence of dinocysts attributed to Impagidinium spp., which are indicative of oceanic settings and relatively warm surface waters, within some intervals (e.g., Samples 318-U1359A-6H-CC [47.73 mbsf], 8H-CC [66.60 mbsf], and 15H-CC [133.70 mbsf]) suggests that transient pulses of warmer surface waters bathed Site U1359. High abundances of sporomorphs reworked from Paleogene, Mesozoic, and Paleozoic strata suggest strong erosion in the hinterland.

The general lack of planktonic and calcareous benthic foraminifers suggests that bottom waters were corrosive to the thin-shelled tests of planktonic foraminifers.

Latest middle Miocene and late Miocene

As in the Pliocene and Pleistocene, Miocene diatom assemblages at Site U1359 mainly include open-water taxa. In addition, a notable increase in the abundance of stephanopyxid specimens occurs within Samples 318-U1359D-47R-CC (589.96 mbsf) and 48R-CC (596.31 mbsf) (Table T6). The stephanopyxid group comprises many robust neritic forms that may be easily swept up into the plankton and survive reworking. This increase may therefore be interpreted as either an indication of shallowing water depths or an increase in reworking of shallower water sediments. The presence of Impagidinium spp. in Samples 318-U1359D-29R-CC (412.83 mbsf) and 48R-CC (596.31 mbsf) indicates an open-water, well-ventilated setting in agreement with the diatom data, but also higher sea surface temperatures. The lack of planktonic and benthic foraminifers below Sample 318-U1359D-34R-CC (464.36 mbsf) to the bottom of Hole U1359D (596.31 mbsf) suggests that late middle Miocene bottom waters were corrosive to calcareous foraminifers except for brief periods (e.g., ~10 m.y., represented by Sample 318-U1359D-14R-CC; 276.07 mbsf) when calcareous benthic foraminifers were preserved.

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