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doi:10.2204/iodp.proc.329.106.2011

Paleontology and biostratigraphy

At Site U1368, we recovered a ~16 m section of pelagic clay, clay-bearing nannofossil ooze with foraminifers, and lithic sand deposited over basaltic basement 13.4–13.6 Ma in age (Gradstein et al., 2004). Four samples covering the different lithologic units (Figure F30; see also “Lithostratigraphy”) were examined for paleontological and biostratigraphic purposes using the coarse (>63 µm) sediment size fraction. The coarse sediment size fraction of the samples taken from the calcareous ooze interval is dominated by planktonic foraminifers, with lesser amounts of benthic foraminifers and ostracods. The silt and clay size fractions (<63 µm) were examined by smear slide samples taken at a higher resolution to characterize the lithologic units (see “Lithostratigraphy”). The biogenic component of the fine fraction (<63 µm) is dominated by calcareous nannofossils.

The preservation and abundance of calcareous microfossils varies with depth. They are well-preserved and abundant in samples from the middle part of the stratigraphic section (329-U1368D-1H-CC [7.47 mbsf] and 329-U1368B-2H-4, 25–27 cm [10.25 mbsf]) (Fig. F14A) and poorly preserved and scarce in the uppermost sample at 3 mbsf (Sample 329-U1368B-1H-3, 0–2 cm) and in the lowermost sample at 16.08 mbsf (Sample 329-U1368C-2H-CC). The lowermost sample comes from a lithic black sand layer composed of strongly magnetic altered basaltic grains (see “Lithostratigraphy”). In this layer, most foraminifers are very recrystallized and altered (Fig. F14B). The decrease in preservation near the top is probably due to sinking of the aging seafloor relative to the lysocline. The decrease in carbonate preservation is also reflected in the deepwater environment by the near absence of benthic foraminifers and ostracods (Tables T3, T4). Only one specimen of the benthic foraminifer genus Epistominella was found in Sample 329-U1368B-1H-3, 0–2 cm. See the “Appendix” for a list of foraminifer taxa.

Planktonic foraminifers

Biostratigraphic assignments for Site U1368 are derived from planktonic foraminifers. The planktonic foraminiferal assemblage near the sediment/basalt interface (at the base of Core 329-U1368C-2H; ~16.1 mbsf) is characterized by Sphaeroidinellopsis disjuncta, Globigerinita naparimaensis, Praeorbulina glomerosa, Praeorbulina sicana, Praeorbulina transitoria, Sphaeroidinellopsis kochi, and Sphaeroidinellopsis subdehiscens. Other taxa such as Catapsydrax dissimilis, Globigerina woodi, Globigerinita uvula, Globigerinella praesiphonifera, Globigerinoides trilobus, Globoquadrina binaiensis, Globoquadrina deshiscens, Orbulina universa, and Paragloborotalia mayeri occur in lesser abundances (Table T5). The assemblage composition allows us to assign the base of the sedimentary record at Site U1368 to planktonic foraminiferal Zone M9/M8 (~13–14 Ma) based on the last occurrence (LO) datums of C. dissimilis, G. naparimaensis, G. binaiensis, Globorotalia archeomenardii, P. mayeri, and P. sicana and the first occurrence (FO) datums of S. subdehiscens and P. glomerosa (Pearson and Chaisson, 1997; Shackleton et al., 1999; Wade et al., 2011). The biostratigraphic age assignment agrees with Gradstein et al.’s (2004) tectonic and paleomagnetic age for the oceanic crust at this location.

Sample 329-U1368B-2H-4, 25–27 cm (10.25 mbsf), contains the FO and LO of Fohsella fohsi and the FO of Globoturborotalita nepenthes in the record. The FO of F. fohsi is placed between 12.7 and 13.74 Ma by Berggren et al. (1995) and Wade et al. (2011), respectively, and the LO is placed between ~11.7 Ma (Chaisson and Pearson, 1997; Wade et al., 2011) and 11. 9 Ma (Berggren et al., 1995). The FO of G. nepenthes has been set between 11.55 and 11.8 Ma (Berggren et al., 1995; Wade et al., 2011). Based on these datums, we can assign Sample 329-U1368B-2H-4, 25–27 cm, to planktonic foraminiferal Zones M11–M10.

Sample 329-U1368D-1H-CC (7.47 mbsf) yielded the FO of Globorotalia crassaformis, Globoconella inflata, Globorotalia pseudomiocenica, and Globorotalia tosaensis, which allows us to assign this sample to Zones PL2–PL1 (Pearson and Chaisson, 1997; Wade et al., 2011). The common occurrence of G. nepenthes and Dentoglobigerina altispira in the uppermost sample (329-U1368B-1H-3, 0–2 cm) indicates that Zone PL2 extends to at least 3 mbsf. This interpretation is supported by the lack of typical late Pliocene–Pleistocene taxa in this part of the record. Shore-based examination at a higher resolution will be carried out on samples from Site U1368 to refine the foraminifer biostratigraphy at this site.

Benthic foraminifers

More than 17 species of benthic foraminifers were observed in the four samples examined from Site U1368 (Table T3). Overall, the assemblage was of relatively high diversity in the samples from ~7.5 and 10.25 mbsf. The uppermost sample was affected by dissolution, and only one specimen of the genus Epistominella was found. Overall, the benthic foraminifer taxa resemble those found at Site U1367 and include Anomalinoides globosus, Buliminella parvula, Cassidulina subglobosa, Cibicidoides mundulus, Cibicidoides wuellerstorfi, Epistominella sp., Gyroidina sp., Gyroidinoides lamarckiana, Gyroidinoides soldanii, Karreriella chapapotensis, Lagena spp., Nodosaria sp. A, Nodosaria sp. B, Nuttallides umbonifer, Oridorsalis umbonatus, Polymorphina spp., and Siphonodosaria spinata (Table T3).

Ostracods

Seven ostracod genera were observed at Site U1368 (Table T4). As with the benthic foraminifers, maximum diversity was observed in Samples 329-U1368D-1H-CC (7.47 mbsf) and 329-U1368B-2H-4, 25–27 cm (10.25 mbsf). No ostracods were found in the upper- or lowermost samples. Ostracods were identified to the generic level and consist of genera that have worldwide distribution and are common in Cenozoic deep-sea sediment (Whatley and Ayress, 1988; Ayress et al., 1997; Dingle and Lord, 1990; Cronin et al. 2002; Alvarez Zarikian, 2009; Alvarez Zarikian et al., 2009). The assemblage is dominated by Krithe, followed by Poseidonamicus, Henryhowella, Bradleya, Eucythere, and Pelecocythere.

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