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doi:10.2204/iodp.proc.338.106.2014

Biostratigraphy

Preliminary biostratigraphy for Hole C0021B is based on shore-based examination of calcareous nannofossils and planktonic foraminifers. Ages constrained by calcareous nannofossils and planktonic foraminifers are generally consistent; both nannofossils and foraminifers indicate that sediment at the bottom of the hole at 195 mbsf is ~1.3 Ma in age. Biostratigraphic age datums are generally in stratigraphic order; however, calcareous nannofossils indicate age reversals and thus the presence of older sediment mixed with younger sediment at ~109 mbsf and within the interval between 146.48 and 175.17 mbsf, which are the intervals of MTDs A and B (see “Lithology”).

Calcareous nannofossils

Calcareous nannofossils are abundantly observed, and their preservation is generally good. A total of 15 samples recovered from 338-C0021B-1H-CC (5.88 mbsf) through 14T-CC (194.48 mbsf) in Hole C0021B were examined (Table T5).

Sample 338-C0021B-1H-CC (5.88 mbsf), which is the uppermost sample examined, contains abundant Emiliania huxleyi (Table T5). Therefore, the interval above this sample is correlated with calcareous nannofossil Zone NN21, corresponding to ages younger than 0.291 Ma (see Fig. F15 in the “Methods” chapter [Strasser et al., 2014a]). The last occurrence (LO) of Pseudoemiliania lacunosa (0.436 Ma) is placed between Samples 338-C0021B-1H-CC and 2H-CC (89.48 mbsf) because of the presence of P. lacunosa below the latter. This event coincides with the base of Zone NN20. Reentrance (RE) of medium Gephyrocapsa spp. (≥4 µm and with a parallel bar oriented to the short axis of a specimen) without Reticulofenestra asanoi indicates that Samples 338-C0021B-2H-CC (89.48 mbsf) through 7H-CC (136.98 mbsf) correspond to 0.436–0.903 Ma.

Sample 338-C0021B-5H-1WR (108.98 mbsf) is older (1.24–1.34 Ma) than the samples above and below based on the presence of Gephyrocapsa spp. (>5.5 µm) (Table T5), indicating that there is a chronostratigraphic discontinuity between 108.5 and 118 mbsf. This interval correlates to MTD A (94.16–116.75 mbsf), suggesting that older sediment was eroded and mixed during MTD emplacement. Moreover, there are also significant stratigraphic discontinuities between Samples 338-C0021B-8H-CC (146.48 mbsf) and 12H-CC (175.17 mbsf). This interval in total corresponds to the age between the LO of P. lacunosa (0.436 Ma) and the RE of medium Gephyrocapsa spp. (≥4 µm) (1.04 Ma), but older sediments are apparently frequently intercalated. This interval corresponds to MTD B (133.76–176.16 mbsf). The lower part of the examined sequences (Samples 338-C0021B-13T-CC [183.75 mbsf] and 14T-CC [194.48 mbsf]) are placed between the LO of Gephyrocapsa spp. (>5.5 µm) (1.24 Ma) and the LO of Helicosphaera sellii (1.34 Ma).

Planktonic foraminifers

Planktonic foraminifers are abundant through the 15 samples examined, and their preservation is generally good (Table T6). The LO of Truncorotalia tosaensis (0.61 Ma) is placed between Samples 338-C0021B-3H-CC (98.98 mbsf) and 4H-CC (108.48 mbsf). In addition, the LO of Globoturborotalita obliquus (1.30 Ma) is placed between Samples 13T-CC (183.75 mbsf) and 14T-CC (194.48 mbsf).

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