Previous   |    Next

doi:10.2204/iodp.proc.318.109.2011

Biostratigraphy

Samples 318-U1361A-1H-CC through 41X-CC (1.5–386.31 mbsf) were analyzed for microfossils. Siliceous microfossils (diatoms, radiolarians, silicoflagellates, and sponge spicules), palynomorphs (e.g., dinoflagellate cysts [dinocysts] and sporomorphs), planktonic and benthic foraminifers, and calcareous nannofossils were recorded in Hole U1361A. In addition, Samples 318-U1361B-1H-CC (2.64 mbsf) and 2H-CC (12.00 mbsf) were analyzed for diatoms.

The abundance data of siliceous microfossils, palynomorphs, calcareous nannofossils, and foraminifers are summarized in Figure F10. Diatoms and radiolarians are present throughout the recovered intervals in variable preservation (Table T2), whereas calcareous nannofossils are abundant in discrete intervals (calcareous nannofossil oozes) in Hole U1361A (Table T3; see “Lithostratigraphy”). Palynomorphs are present in all ten samples processed for palynology, with half of them yielding trace to few non–age diagnostic dinocysts with poor to moderate preservation, whereas well-preserved, likely reworked sporomorphs (Table T4) were found in each sample. Planktonic and benthic foraminifers are either absent or occur in low abundance except in two samples, 318-U1361A-2H-CC (19.04 mbsf) and 34X-CC (321.07 mbsf) where they are diverse and relatively abundant (Table T5).

Diatoms and radiolarians provide good biostratigraphic control for Hole U1361A, resolving an uppermost middle Miocene through uppermost Pleistocene sedimentary succession with no major breaks in sedimentation (Fig. F4). Magnetostratigraphy is in good agreement with biostratigraphy (see “Paleomagnetism”). All index events (biostratigraphy and magnetostratigraphy) are compiled in Table T6.

Siliceous microfossils

The sediments recovered from Site U1361 contain well-preserved and abundant biogenic silica dominated by diatoms and radiolarians, with variable abundances of silicoflagellates and sponge spicules (Table T2; Fig. F10). Diatoms and radiolarians provide the primary biostratigraphic control, resolving an uppermost middle Miocene through uppermost Pleistocene sedimentary succession, which appears to be complete. Diatom assemblages are dominated by typical Neogene Southern Ocean open-water forms 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 U1361. Moderate reworking of middle Miocene sediments into younger strata is exemplified by trace occurrences of the index diatoms Actinocyclus ingens var. nodus and A. ingens var. ovalis (Table T2). Fluctuations in the abundance of marine benthic and tychopelagic taxa such as Cocconeis spp., Diploneis spp., Paralia sulcata, stephanopyxids, and Trinacria excavata may indicate episodic transport of shelfal material to the drill site.

Diatoms

Diatoms are few to abundant above 349.99 mbsf (Sample 318-U1361A-37X-CC) but rare to few below this level to the bottom of Hole U1361A (386.31 mbsf) (Table T2). In Hole U1361B they are few to common (Table T2). A clear succession of index species (e.g., Cody et al., 2008) was recognized throughout Hole U1361A (Fig. F4; Table T6) and in the two cores recovered from Hole U1361B. Samples 318-U1361A-1H-CC through 4H-CC (9.03–36.53 mbsf) are assigned to the Pleistocene, Samples 5H-CC through 13H-CC (47.42 to ~122.37 mbsf) to the Pliocene, and Samples 14H-CC through 41X-CC (~132.57–386.31 mbsf) to the late middle to late Miocene.

Pleistocene

Seven diatom datums were recognized in Samples 318-U1361A-1H-CC through 4H-CC (9.03–36.53 mbsf) (Table T6):

• Last occurrence (LO) of

• Actinocyclus ingens (0.54 Ma),

• Thalassiosira elliptipora (0.68 Ma),

• Thalassiosira fasciculata (0.89 Ma),

• Fragilariopsis barronii (1.24 Ma), and

• Rouxia antarctica (1.50 Ma) and

• First occurrence (FO) of

• Fragilariopsis separanda (1.41 Ma) and

• Fragilariopsis obliquecostata (1.70 Ma).

Pliocene

Eleven diatom datums were recognized in Samples 318-U1361A-5H-CC through 13H-CC (47.42 to ~122.57 mbsf) (Table T6):

• LO of

• Thalassiosira insigna (2.48 Ma),

• Fragilariopsis weaveri (2.49 Ma), and

• Thalassiosira inura (2.54 Ma) and

• FO of

• Thalassiosira elliptipora (2.03 Ma),

• Fragilariopsis kerguelensis (2.20 Ma),

• Thalassiosira vulnifica (3.15 Ma),

• T. insigna (3.25 Ma),

• F. weaveri (3.53 Ma),

• F. barronii (4.40 Ma),

• T. inura (4.74 Ma), and

• Shionodiscus oestrupii (4.88 Ma).

Miocene

Twelve diatom datums were recognized in Samples 318-U1361A-14H-CC through 41X-CC (~132.57–386.31 mbsf) (Table T6):

• LO of

• Thalassiosira miocenica (5.91 Ma),

• Fragilariopsis donahuensis (5.98 Ma),

• Thalassiosira mahoodii (6.85 Ma; Winter and Iwai, 2002),

• Denticulopsis ovata (9.64 Ma), and

• Denticulopsis dimorpha var. areolata (10.25 Ma);

• Last abundant occurrence (LAO) of

• Denticulopsis simonsenii (8.63 Ma) and

• D. dimorpha s.l. (10.78 Ma); and

• FO of

• Thalassiosira oliverana (6.18 Ma),

• Thalassiosira torokina (7.23 Ma),

• T. miocenica (7.29 Ma),

• T. mahoodii (8.47 Ma; Winter and Iwai, 2002), and

• D. dimorpha s.l. (12.50 Ma).

Silicoflagellates and sponge spicules

Silicoflagellates, sponge spicules, and radiolarian fragments occur sporadically in trace abundances in Hole U1361A (Table T2), and none were recorded in samples from Hole U1361B.

Radiolarians

A typical Neogene Southern Ocean radiolarian fauna is preserved in samples from Holes U1361A and U1361B. Specimens are well preserved to moderately well preserved, and abundance varies throughout the hole (Fig. F10).

The FOs of Helotholus vema (4.2 Ma) and Acrosphaera? labrata (7.8 Ma) fall within the interval between Samples 318-U1361A-11H-CC and 12H-CC (104.26–111.32 mbsf) and between Samples 20X-CC and 22X-CC (184.41–205.21 mbsf), respectively. Samples 24X-CC (219.12 mbsf) and 25X-CC (234.82 mbsf) contain only few radiolarian specimens; therefore, the LO of Cycladophora spongothorax (9.1 Ma) is constrained to the interval between Samples 23X-CC and 26X-CC (215.85–244.24 mbsf). The evolutionary transition of Acrosphaera murrayana to Acrosphaera australis (10.4 Ma) occurs within the interval between Samples 30X-CC and 31X-CC (273.55–283.09 mbsf) (Table T6).

Calcareous nannofossils

Hole U1361A contains nannofossil ooze beds and nannofossil-rich/-bearing silty clays (see “Lithostratigraphy”). Nannofossils are particularly abundant and well preserved between 313.60 and 341.62 mbsf (Fig. F10). A nannofossil-bearing clay occurs at 269.53 mbsf, and limestone (containing calcareous nannofossils) occurs between 371.67 and 374.97 mbsf (see “Lithostratigraphy”). Some of these calcareous beds are micritic and contain only trace amounts of nannofossils (see “Lithostratigraphy”). For the 313.60–341.62 mbsf interval, calcareous nannofossils are very abundant to common (Table T3). The assemblage is characterized by low diversity, mainly comprising 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

Ten core catcher samples from Hole U1361A were processed for palynology. All samples yielded palynomorphs. The palynological data are presented in Table T4.

Dinocysts

Only five samples from Hole U1361A yielded dinocysts, with abundances varying between trace and few. Dinocyst preservation is poor to moderate. Dinocyst assemblages within these samples consist mainly of protoperidinioid taxa (Brigantedinium spp. and Selenopemphix spp.). A taxon herein referred to as “Impagidinium brown” (see “Biostratigraphy” in the “Site U1359” chapter for details) occurs in trace amounts. No dinocyst index species were recorded. The dominance of protoperidinioid (i.e., heterotrophic) taxa suggests high sea-surface productivity, probably associated with sea ice ecosystems similar to the modern Southern Ocean (compare e.g., Esper and Zonneveld, 2007).

The palynomorph signal from cores from Site U1361 correlates well with those of Site U1359, with increasing dinocyst abundance and preservation in the Miocene and presence of “Impagidinium brown” in lower upper and middle Miocene sediments (Table T4; see “Biostratigraphy” in the “Site U1359” chapter).

Sporomorphs

Sporomorphs are abundant in all samples investigated from Hole U1361A, and the assemblage mainly consists of well-preserved spores and (saccate) pollen. The sporomorphs are reworked from Paleogene, Mesozoic, and/or Paleozoic strata. Sporomorph genera identified include Baculatisporites/Osmundacidites spp., Cicatricosisporites spp., Classopollis/Corollina spp., Deltoidospora/Triletes spp., Ischyosporites spp., Lycopodiacidites spp., and Nothofagus sp. Other palynofacies components were found in trace amounts only.

Foraminifers

Twenty-five samples from Hole U1361A were examined for foraminifers (Table T5). Foraminifers are either absent or occur in low abundances and low diversity except in two samples: 318-U1361A-2H-CC (19.04 mbsf) and 34X-CC (321.07 mbsf). The washed residue from Sample 2H-CC contains >90% planktonic foraminifers. Sample 34X-CC (321.07 mbsf) contains a relatively rich benthic foraminifer assemblage, with few poorly preserved planktonic foraminifers.

Planktonic foraminifers

Sample 318-U1361A-2H-CC (19.04 mbsf) contains abundant planktonic foraminifers, including Neogloboquadrina pachyderma and Globigerina bulloides. Neogloboquadrina pachyderma comprises >80% of the assemblage in this sample. It also occurs sporadically between Samples 318-U1361A-3H-CC and 22X-CC (28.26–205.21 mbsf). Based on the presence of N. pachyderma and using the sub-Antarctic zonal scheme of Berggren (1992), the upper 205.21 mbsf of Site U1361 can be assigned to sub-Antarctic Zone AN7, which ranges from 9.2 Ma to the present (Gradstein et al., 2004). A few planktonic foraminifers were also recorded from Sample 318-U1361A-34X-CC (321.07 mbsf). However, it was not possible to determine the specimens because of their poor preservation. Specimens of Neogloboquadrina nympha (FO at 13.4 Ma; LO 10.1 Ma) were identified in Sample 318-U1361A-35X-CC (331.03 mbsf), constraining the age of this sample to between 13.4 and 10.1 Ma.

Benthic foraminifers

Benthic foraminifers are either absent or in low abundance at Site U1361, except for Samples 318-U1361A-2H-CC (19.04 mbsf) and 34X-CC (321.07 mbsf). Sample 2H-CC (19.04 mbsf) contains a diverse assemblage of benthic foraminifers dominated by Ordorsalis tener (70%). This assemblage is similar to those recovered from core-top material from deepwater sites between Antarctica and Australia that are associated with cold (–0.2° to +0.4°C) Antarctic Bottom Water (Corliss, 1979). Sample 318-U1361A-34X-CC (321.07 mbsf) contains a diverse assemblage of benthic foraminifers comprising Cibicidoides bradyi (31%), Planulina renzi (12%), Laticarina pauperata (12%), and the agglutinated species Bolivinopis rosula (19%). This benthic foraminifer assemblage is similar to benthic foraminifer assemblages of late Miocene age recovered from cores drilled at Kerguelen Plateau (Mackensen, 1992).

Age model and sedimentation rates

Here we summarize and integrate the biostratigraphic interpretations inferred from the individual microfossil groups. The resulting integrated age-depth model presented in Figure F4 and Table T6 also incorporates magnetostratigraphic age constraints presented in “Paleomagnetism.” An upper middle Miocene (12.50 Ma) through uppermost Pleistocene (0.54 Ma) sedimentary succession was recovered from Hole U1361A. Sedimentation seems to be continuous throughout the record based on shipboard stratigraphic control. We divide the following sections according to sedimentation rate changes (Fig. F4).

Late Miocene to Pleistocene (6.73–0.54 Ma)

Diatoms and magnetostratigraphy resolve the sedimentary sequence for the Pleistocene in Hole U1361A (9.03–36.53 mbsf). The top of Hole U1361A is dated as latest Pleistocene (0.54 Ma). The Pliocene/Pleistocene boundary (1.81 Ma) is constrained by magnetostratigraphy and occurs at ~33.58 mbsf, within Chron C2n. Further refinement of diatom biostratigraphy around this boundary interval is necessary (Fig. F4). Diatoms and magnetostratigraphy provide age control for the Pliocene sedimentary succession recovered from Hole U1361A (47.42–122.57 mbsf), with additional age constraints provided by radiolarians. Magnetostratigraphy and diatom bioevents are well correlated within this section. The Miocene/Pliocene boundary (5.33 Ma) is constrained by magnetostratigraphy and occurs at ~134 mbsf, between the termination of Chron C3An.1n and the onset of Chron C3n.4n (Table T6). Sedimentation at Site U1361 appears to be continuous from the late Miocene through the Pleistocene at a variable rate, with lowest accumulation (~14 m/m.y.) in the late Miocene and highest accumulation (as much as 38 m/m.y.) in the early Pliocene. This ~154 m long interval spans 6.19 m.y., yielding an average sedimentation rate of 25 m/m.y. (Fig. F4).

Late Miocene to middle Miocene (12.50–6.73 Ma)

Miocene sediments from 153.8 to 314.64 mbsf were deposited with an average sedimentation rate of 37 m/m.y. (Fig. F4), given the magnetostratigraphic and biostratigraphic constraints. A change in sedimentation rates is inferred at 314.64 mbsf; average sedimentation rates within the section below this datum within latest middle to early late Miocene strata are ~69 m/m.y. (Fig. F4).

Paleoenvironmental interpretation

Pleistocene

Diatom assemblages from cores at Site U1361 are typical Neogene Southern Ocean open-water taxa with variable abundances of benthic, neritic, and sea ice–associated diatoms, indicating a high-nutrient, high-productivity, seasonally sea ice–influenced setting throughout the late Neogene depositional history at Site U1361. Moderate reworking of middle Miocene taxa into younger strata suggests continuous input of proximal sediments. High abundances of reworked sporomorphs within the palynological associations indicate continuous strong erosion in the hinterland. In situ dinocysts are absent.

Pliocene

As in the Pleistocene, Pliocene diatom assemblages are dominated by open-water taxa and variable abundances of benthic, neritic, and sea ice–associated taxa. Dinocyst assemblages are poor but mainly comprise heterotrophic taxa. This indicates that Pliocene biosiliceous-rich sediments were deposited in a pelagic, well-ventilated setting characterized by sea ice–influenced high productivity. High abundances of sporomorphs reworked from Paleogene, Mesozoic, and Paleozoic strata suggest continuous strong erosion in the hinterland.

Latest middle Miocene and late Miocene

Miocene diatom assemblages at Site U1361 are indicative of productive, seasonally variable open-marine conditions. Fluctuations in the abundance of marine benthic and tychopelagic taxa such as Cocconeis spp., Diploneis spp., P. sulcata, stephanopyxids, and T. excavata may indicate episodes of transportation of shelfal material to the drill site.

The presence of well-preserved calcareous nannofossils and benthic foraminifers suggests that the depositional setting was favorable for calcite preservation (i.e., not corrosive) for brief intervals in the Miocene. The presence of reworked Mesozoic–Paleozoic sporomorphs within the palynological associations suggests ongoing erosion in the hinterland.

Top of page   |    Previous   |    Next