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doi:10.2204/iodp.proc.303306.206.2009

Results

Site U1313

All 111 sediment samples studied yielded sufficient numbers of calcareous nannofossils for biostratigraphy. Calcareous nannofossil preservation is moderate in upper Pliocene samples, moderate to good in lower Pleistocene samples, and good in middle Pleistocene samples (Table T1).

A total of 13 biostratigraphic datums are recognized in the Pleistocene section of Site U1313. The most recent nannofossil event, the base of Emiliania huxleyi acme (0.082–0.063 Ma) is found in Sample 306-U1313B-1H-3, 12–13 cm (3.29 mcd). This zone was first defined by Gartner and Emiliani (1976) and represents the change in dominance from gephyrocapsids to E. huxleyi. The last occurrence (LO) of Helicosphaera inversa occurs in Sample 306-U1313C-2H-2, 12–13 cm (4.75 mcd). The first occurrence (FO) of E. huxleyi (0.289 Ma), which marks the base of Zone NN21 of Martini (1971), is present in Sample 306-U1313B-2H-5, 2–3 cm (13.68 mcd). The LO of Pseudoemiliania lacunosa (0.440 Ma), which defines the top of Zone NN19, as well as the FO of H. inversa, is found in Sample 306-U1313C-3H-6, 15–16 cm (21.29 mcd). The LO of R. asanoi (0.905 Ma) occurs in Sample 306-U1313C-5H-5, 90–91 cm (42.64 mcd). This event represents the last consistent occurrence of this species, and occurrences above are attributed to reworking or the difficulty of accurately determining the >6.5 µm size distinction using an eyepiece micrometer. The recurrence of medium Gephyrocapsa (>4 µm; 1.007 Ma) occurs in Sample 306-U1313C-5H-7, 90–91 cm (45.64 mcd). The FO of R. asanoi (1.136 Ma) is found in Sample 306-U1313B-6H-2, 16–17 cm (51.91 mcd). The LO of large Gephyrocapsa spp. (1.255 Ma) occurs in Sample 306-U1313B-6H-6, 16–17 cm (57.91 mcd). The LO of Helicosphaera sellii (1.256 Ma) is found in Sample 306-U1313B-7H-3, 91–92 cm (65.20 mcd). The FO of large Gephyrocapsa spp. (1.560 Ma) occurs in Sample 306-U1313C-8H-4, 17–18 cm (72.70 mcd). The LO of Calcidiscus macintyrei (1.607 Ma) occurs in Sample 306-U1313B-9H-2, 17–18 cm (78.80 mcd). The FOs of medium (>3 µm) G. oceanica and G. caribbeanica (1.689 Ma) occur in Sample 306-U1313B-9H-5, 77–78 cm (89.24 mcd). This event is ~117 k.y. younger than the Pliocene/Pleistocene boundary.

Four Pliocene biostratigraphic datums are recognized in the studied section. Discoaster brouweri (1.926 Ma), the biostratigraphic datum for the latest Pliocene and ~112,000 k.y. older than the Pliocene/Pleistocene boundary, occurs in samples deeper than 306-U1313C-10H-4, 62–63 cm (93.83 mcd). It is difficult to define the true LO of D. brouweri because this species is quite rare in the upper Pliocene samples and some occurrences may be reworked. Rare specimens of medium Gephyrocapsa spp. (>4 µm) co-occur with D. brouweri in the three samples between 306-U1313C-11H-3, 62–63 cm, and 306-U1313B-10H-5, 62–63 cm (102.73–99.83 mcd). This pattern of very rare Gephyrocapsa spp. (>4 µm) occurrences below the Pliocene/Pleistocene boundary is consistent with results from Site U1312 (see the “Site U1312” chapter) and reports of similar occurrences from the Blake Ridge, northwest Atlantic (Okada, 2000). Thus the FOs of G. oceanica and G. caribbeanica are placed at the base of consistent occurrences of these species.

The LO of Discoaster pentaradiatus occurs in Sample 306-U1313C-12H-5, 27–28 cm (116.27 mcd). No reworked specimens were found above this horizon, and D. pentaradiatus was consistently present below. The LO of Discoaster surculus is questionably placed in Sample 306-U1313B-12H-5, 27–28 cm (120.21 mcd), based on a broken specimen. Consistent occurrences of D. surculus are present below this horizon. The last biostratigraphic datum in the examined section is the LO of Discoaster tamalis, which is found in Sample 306-U1313B-13H-6, 27–28 cm (131.78 mcd). D. tamalis is generally consistently present below this horizon, although it is not found in every sample. The next biostratigraphic datums are the LOs of Sphenolithus spp. and R. pseudoumbilicus. Both are sporadically reworked in the upper Pliocene and Pleistocene sediments of Site U1313, but the lack of consistent occurrences of Sphenolithus spp. in the last samples examined indicate that those sediments must be younger than the LO of Sphenolithus spp. (3.52–3.56 Ma).

Site U1314

All 60 samples examined contain abundant calcareous nannofossils (Table T2). Calcareous nannofossil preservation is moderate to good in middle Pleistocene samples and good in upper Pleistocene samples.

The youngest biostratigraphic event, the base of the E. huxleyi acme (0.082–0.063 Ma), occurs in Sample 306-U1314B-2H-2, 77–79 cm (7.53 mcd). The LO of H. inversa is found in Sample 306-U1314B-2H-5, 77–79 cm (12.03 mcd). The FO of E. huxleyi (0.289 Ma) occurs in Sample 306-U1314B-4H-3, 127–129 cm (28.58 mcd). This event defines the base of Zone NN21. The LO of P. lacunosa (0.440 Ma), which defines the top of Zone NN19, occurs in Sample 306-U1314C-4H-7, 24–25 cm (36.05 mcd). The rare, sporadic occurrences of H. inversa made it impossible to identify the FO of this species in this hole. The LO of R. asanoi (0.905 Ma) occurs in Sample 306-U1314C-8H-6, 24–25 cm (75.15 mcd), and is the last biostratigraphic event in the samples examined from this site.

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