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doi:10.2204/iodp.proc.322.104.2010

Biostratigraphy

Preliminary shipboard analysis of the core catcher and additional samples from Cores 322-C0012A-1R through 52R reveals assemblages of calcareous nannofossils and planktonic foraminifers (Tables T7, T8). The nannofossil and planktonic foraminifer events recognized in Hole C0012A are reported in Tables T9 and T10. Biostratigraphic datums were mainly derived from calcareous nannofossils (Fig. F32; Table T9). According to these datum events, the composite of Hole C0012A has a Pleistocene age in Core 322-C0012A-1R and an age range of early Pliocene to early Miocene below Core 2R.

Floral and faunal assemblages of planktonic organisms provide some insight into paleoceanographic conditions. Species abundance is generally common to rare. The sediments throughout the sequence contain warm-water genera such as the calcareous nannofossil genus Discoaster.

Calcareous nannofossils

Preliminary examination of all core catcher samples from Hole C0012A obtains moderately to poorly preserved nannofossils, which occur in common abundance in most of the samples; species diversity is comparatively high. However, severe dissolution occurred, leading to barren or very poor occurrence of coccoliths in some intervals (Table T7). Most of the zonal markers of Martini's zonation (1971) and Raffi et al. (2006) have been identified in the sedimentary sequence. The nannofossils that occurred in Hole C0012A are listed in Table T7, and the correlations between the hole and the biostratigraphy are shown in Figure F32.

The youngest assemblages in this hole belong to the lower part of the Ceratolithus cristatus Zone NN13 and are present in Samples 322-C0012A-2R-4, 15 cm, through 4R-CC, 14–19 cm, where Amaurolithus spp. occurred together. The first occurrence (FO) of C. cristatus, which defines the bottom of Zone NN13, is observed between Samples 322-C0012A-4R-CC, 14–19 cm, and 5R-CC, 17.5–22.5 cm.

Discoaster quinqueramus and Discoaster berggrenii, which indicate upper Miocene Zone NN11 (5.59–8.52 Ma), were found in Samples 322-C0012A-5R-CC, 17.5–22.5 cm, through 17R-CC, 41–43 cm. The paracme end (Raffi et al., 2006) of Reticulofenestra pseudoumbilicus was recorded between Samples 322-C0012A-7R-5, 40–42 cm, and 8R-4, 45 cm. The bottom of Zone NN11, marked by the FO of D. berggrenii, was observed between Samples 322-C0012A-17R-1, 41–43 cm, and 17R-CC, 10.5–15.5 cm. The paracme beginning (Raffi et al., 2006) of Reticulofenestra pseudoumbilicus (8.78 Ma) was also identified between Samples 322-C0012A-17R-CC, 10.5–15.5 cm, and 18R-1, 40–41 cm. Zones NN9 and NN8 are defined by the presence of Discoaster hamatus and Catinaster coalitus, which were not found in core catcher and additional samples. Therefore, nannofossils indicate a Zone NN9–NN8 hiatus or condensed section in this hole. Samples 322-C0012A-21R-3, 7 cm, through 26R-CC, 4–9 cm, are characterized by the occurrence of Discoaster kugleri and Coccolithus miopelagicus, which reveals that the intervals are assigned to Zone NN7 (between 10.88 and 11.90 Ma). The last occurrence (LO) of Cyclicargolithus floridanus indicates the upper part of Zone NN6 (12.037 Ma), and it was found between Samples 322-C0012A-26R-CC, 4–9 cm, and 27R-3, 67 cm. Therefore, the middle Miocene/upper Miocene boundary is situated between Samples 322-C0012A-21R-3, 7 cm, and 26R-CC, 4–9 cm. Sphenolithus heteromorphus occurred continuously in Samples 322-C0012A-40R-5, 16 cm, through 48R-CC, 11–16 cm, and disappeared at the Zone NN5/NN6 boundary (13.532–13.654 Ma) in the middle part of the middle Miocene, which lies between Samples 40R-2, 61 cm, and 40R-5, 16 cm. The assemblages in Samples 322-C0012A-52R-1, 67 cm, through 52R-CC, 7–12 cm, are characterized by the occurrences of Helicosphaera ampliaperta, Discoaster druggii, Sphenolithus disbelemnos, and Sphenolithus dissimilis and the absence of S. heteromorphus and Sphenolithus belemnos, which places these samples in the upper part of Zone NN2 (18.921–20.393 Ma). Zones NN4 (H. ampliaperta Zone) and NN3 (S. belemnos Zone) are missing. However, a middle–lower Miocene hiatus or condensed section occurs at this site. Therefore, Unit VI is characterized as early Miocene from 18.921 to 20.393 Ma, which corresponds to the deposition of sediments from 528.67 to 530.325 m CSF.

The assemblages are characterized by the occurrence of warmer water species. In particular, the assemblages found in all samples are characterized by high species diversity throughout the hole.

Planktonic foraminifers

Preliminary analysis of all core catcher samples from Cores 322-C0012A-1R through 52R and a few additional samples were examined on board the ship. Planktonic foraminifer fossils occurred at rare (or even barren) to common abundances with good to poor preservation. Hole C0012A sediment samples showed the planktonic foraminifer assemblages are barren to rare in abundance in an almost consecutive number of intervals (Tables T8, T10).

Five biohorizons and datum events were noted in Hole C0012A (Table T8). The first biohorizon occurred in the middle part of Unit I and was recognized by the FO of Globigerinoides conglobatus between Samples 322-C0012A-6R-CC, 10.5–15.5 cm, and 7R-CC, 10–15 cm. The upper part of Unit II is characterized by three biohorizons between Samples 322-C0012A-12R-CC, 15–20 cm, and 13R-CC, 10–15 cm, due to the FO of Globorotalia plesiotumida. The FO of Globoturborotalita nepenthes is situated between Samples 322-C0012A-13R-CC, 11–16 cm, and 14R-CC, 16–21 cm. The LO of Paragloborotalia mayeri between Samples 322-C0012A-17R-CC, 10.5–15.5 cm, and 19R-CC, 10–15 cm, is also situated in Unit II. The fifth and last biohorizon occurred in Unit IV and was recognized by the FO of Orbulina universa between Samples 322-C0012A-36R-CC, 0–1 cm, and 37R-CC, 0–5 cm (Table T8). However, some planktonic foraminifer shells show deformation in their structure, as observed in the core catcher samples from Samples 322-C0012A-38R-CC, 14.5–19.5 cm, 48R-CC, 11–16 cm, 49R-CC, 8.5–13.5 cm, and 52R-CC, 7–12 cm (Fig. F33). For the better explanation of rare availability and correct position of datum events, postcruise studies are required on the additional samples from the core sections.

Sedimentation rates based on biostratigraphy

The changes in sedimentation rate in Hole C0012A, based on calcareous nannofossils, are shown in Figure F34. The sedimentation rate was 1.59 cm/k.y. between 5.56 and 7.12 Ma (86.92 and 111.67 m CSF), which corresponds to the middle part of Unit I. In continuation, the sedimentation rate of 5.58 cm/k.y. between 7.12 and 8.40 Ma (111.67 and 199.02 m CSF) corresponds from the middle part of Unit I to the middle part of Unit II. The lower part of Unit II shows a slow sedimentation rate of 2.65 cm/k.y. from 8.40 to 9.60 Ma (199.02 to 225.89 m CSF). Interestingly, a consistent sedimentation rate of 6.93 cm/k.y. from 10.80 to 14.91 Ma (225.89 to 510.55 m CSF) corresponds to the upper part of Unit III to the lower part of Unit V. However, Unit VI sediment deposition is characterized by the time interval from 18.921 to 20.393 Ma (528.67 to 530.325 m CSF). Hence, error bars have been plotted for all the datums (Fig. F34; Table T9), and the possible error on the oldest sediment from this site is ±0.736 m.y., which may be considered the age of the oldest sediment at this site, between 18.921 and 20.393 Ma.

The sedimentation rates based on calcareous nannofossil datum events indicate a possible time gap of ~2.1 m.y. in Unit III during the late Miocene (LO of Discoaster hamatus, 8.78 Ma, to the FO of Catinaster coalitus, 10.88 Ma) and of ~4 m.y. at the Unit V/VI boundary during the early Miocene (FO of Sphenolithus belemnos, 14.91 Ma, to the FO of Sphenolithus belemnos, 18.921 Ma).

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