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doi:10.2204/iodp.proc.333.102.2012

Biostratigraphy

Without a micropaleontologist on board during Expedition 333, a preliminary calcareous nannofossil investigation was completed by shore-based analysis during and after the cruise by sending samples from the ship. Similar in study procedure to Expedition 322, preliminary ages were assigned to sedimentary strata primarily based on core catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. The biostratigraphic events, zones, and subzones for nannofossils we used are summarized in Fig. F15. The Pliocene/Pleistocene boundary has been formally located just above the top of the Olduvai (C2n) magnetic polarity subchronozone (Aguirre and Pasini, 1985) and just below the lowest occurrence of medium Gephyrocapsa caribbeanica (≥3.5 µm but <5.5 µm) (Takayama and Sato, 1993–1995; Raffi et al., 2006). We used the lowest occurrence of medium G. caribbeanica (≥3.5 µm but <5.5 µm) to mark the Pliocene/Pleistocene boundary. The Miocene/Pliocene boundary has not been formally defined yet in terms of nannofossils. We tentatively used the last occurrence (LO) horizon of Discoaster quinqueramus for the boundary.

Details of the methods are described as the following (zonal scheme and nannofossil biohorizons are shown in Figure F15 and Table T5):

• The calcareous nannofossil zonal scheme established by Martini (1971) and Okada and Bukry (1980) and modified by Young (1998) was used for lower Miocene–Quaternary sequences.

• The calcareous nannofossil biostratigraphic classification of sedimentary sequences follows the review by Raffi et al. (2006).

• In addition, the upper Pliocene–Quaternary biohorizons originally defined by Sato and Takayama (1992) and modified by Raffi et al. (2006) were used for more detailed correlations.

• Astronomically tuned age estimates for the lower Miocene–Quaternary rely on the geological timescale developed by the International Commission on Stratigraphy in 2004 (Lourens et al., 2004).

• A size-defined species of the genera Reticulofenestra was placed into size categories as proposed by Young (1998).

• For the gephyrocapsids, we adopted the concept of Raffi et al. (2006), and morphological terminology used here is summarized in Perch-Nielsen (1985) and Takayama and Sato (1987). Accordingly, Gephyrocapsa is divided into four major groups by maximum coccolith length:

• Small Gephyrocapsa (<3.5 µm),

• Medium Gephyrocapsa (G. caribbeanica and Gephyrocapsa oceanica; ≥3.5 but <5.5 µm),

• Gephyrocapsa sp. 3 (Gephyrocapsa parallela; ≥4 µm but <5.5 µm), and

• Large Gephyrocapsa (G. caribbeanica and G. oceanica; ≥5.5 µm).

Methods

For nannofossil analyses, the core catcher sections of each core were sampled. Samples from other sections were sometimes included where nannofossils were not abundant in core catcher material, and many such samples were targeted specifically in burrows. Standard smear slide methods were utilized for all samples using optical adhesive as a mounting medium. We followed the taxonomic concepts summarized in Takayama and Sato (1987).

Calcareous nannofossil preservation was based on the following:

• G = good (little or no evidence of dissolution and/or overgrowth).

• M = moderate (minor dissolution or crystal overgrowth observed).

• P = poor (strong dissolution or crystal overgrowth, many specimens unidentifiable).

Total abundance of calcareous nannofossils for each sample was estimated as

• A = very abundant (>50 specimens per field of view [FOV]).

• C = common (>10–50 specimens per FOV).

• F = few (>1–10 specimens per FOV).

• R = rare (1 specimen per 2 or more FOVs).

Nannofossil abundances of individual species were recorded as

• A = abundant (1–10 specimens per FOV).

• C = common (1 specimen per 2–10 FOVs).

• R = rare (1 specimen per >10 FOVs).

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