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Methods and materials

A total of 111 samples from Site U1313 and 66 samples from Site U1314 were included in this study. The sampling interval between samples was ~1.5 mcd. The meters composite depth scale was constructed by correlating cores from multiple holes drilled at one site using closely spaced measurements of physical properties (see the “Site U1312–U1315 methods” chapter). Sediment samples were prepared using standard smear slide techniques (e.g., Bown and Young, 1998). Calcareous nannofossils were examined at 1500× magnification under a Nikon E600 polarizing light microscope or at 1250× magnification on a Zeiss Axioskop 2 polarizing light microscope. Preservation of nannofossils was recorded as follows:

  • G = good: little or no evidence of dissolution and/or overgrowth, little or no alteration of primary morphological features, and specimens are identifiable to the species level;

  • M = moderate: minor dissolution or crystal overgrowth observed, some alteration of primary morphological features, but most specimens are identifiable to the species level; and

  • P = poor: strong dissolution or crystal overgrowth, significant alteration of primary morphological features, and many specimens are unidentifiable at the species and/or generic level.

Semiquantitative data were collected by identifying and counting at least 300 upper photic zone specimens in a varying number of fields of view per sample. These data are normalized to 100%; reworked species were not included in this calculation. Lower photic zone species Florisphaera profunda was counted separately in the same fields of view when encountered, and its relative abundance within the total coccolithophore flora was calculated. After completion of the initial examination, samples were scanned for rare taxa.

Results are correlated to the calcareous nannofossil biostratigraphic zonation of Martini (1971). Absolute ages for datums are assigned based on the astrobiochronology of Raffi et al. (2006) whenever possible. Calcareous nannofossil species considered in this paper are listed in the “Appendix,” where they are arranged alphabetically by generic epithets. Bibliographic references for these taxa can be found in Perch-Nielsen (1985), Bown (1998), and Sáez et al. (2003).

Pliocene and Pleistocene calcareous nannoflora contain abundant coccoliths of Gephyrocapsa and Reticulofenestra. Intensity of calcification of the central area of coccoliths produced by these genera varies greatly. Because taxonomic concepts differ among authors, particularly for gephyrocapsids (see Flores et al., 1999, or Raffi, 2002, for a synthesis), it is important to define species concepts; in this study, we generally follow the gephyrocapsid species concepts described in Flores et al., 1999. Coccoliths of Gephyrocapsa with well-calcified (mostly closed) central areas are identified as Gephyrocapsa caribbeanica. Coccoliths with less calcified (more open) central areas and a bridge angle >30° are identified as Gephyrocapsa oceanica. Coccoliths of Gephyrocapsa with a less calcified central area and with a low-angle bridge (<30°) are identified as Gephyrocapsa muellerae. Noelaerhabdaceae coccoliths without a bridge are identified as Reticulofenestra. Specimens with heavily calcified (closed) central areas are assigned to Reticulofenestra productella, and those with less calcified (open) central areas are identified as Reticulofenestra spp.

Coccoliths of Gephyrocapsa and Reticulofenestra from Pliocene and Pleistocene sediments show great size variation, and the size ranges of some have been used as biostratigraphic datums (e.g., Raffi et al., 1993; de Kaenel et al., 1999; Maiorano and Marino, 2004). To provide objective information of biostratigraphic size variation of Gephyrocapsa and Reticulofenestra in the Pliocene and Pleistocene sediments of this study, the length of coccoliths of these two genera was estimated using an eyepiece micrometer at 1 µm intervals for most specimens between 2 and 7 µm in diameter. Under the light microscope, it is difficult to identify small specimens (<3 µm) to species level. Following Okada (2000) we separate small gephyrocapsids (<3 µm) into two size categories. All coccoliths <2 µm with a bridge are grouped as Gephyrocapsa spp. (<2 µm). Coccoliths 2–3 µm in length are divided into two groups: those with a closed central area as G. caribbeanica s.l. (2–3 µm) and those with a more open central area as Gephyrocapsa spp. (2–3 µm). López-Otálvaro et al. (2008) separated small placoliths (2.5–3 µm) with a closed central area as a small morphotype of G. caribbeanica for the purposes of coccolith carbonate calculations. We found we could reliably separate small (2–3 µm) gephyrocapsids with a closed central area from other specimens and so follow that definition. Small coccoliths without a bridge are separated into two size categories: very small placoliths (<2 µm) and Reticulofenestra spp. (2–3 µm).

The short-term occurrence of Reticulofenestra asanoi in the middle Pleistocene is a useful biostratigraphic event. In this study, specimens of Reticulofenestra >6.5 µm with a less calcified central area are assigned to R. asanoi, consistent with the definition used for the astrobiochronology of Raffi et al. (2006), which is based on Wei (1993) and Raffi (2002). The mid-Pliocene extinction of Reticulofenestra pseudoumbilicus is also an important biostratigraphic datum but is easily misidentified depending on the size definition used for this species. A recent study by Gibbs et al. (2005) found a synchronous extinction of R. pseudoumbilicus >7 µm at 3.82–3.81 Ma, which is used in the astrobiochronology of Raffi et al. (2006). Therefore, for the purposes of this study, R. pseudoumbilicus includes specimens >7 µm.