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Paleontology and biostratigraphy

Paleontological investigations carried out during the expedition focused primarily on calcareous nannofossils and planktonic and benthic foraminifers. Preliminary biostratigraphic and paleontological determinations in Holes U1378B, U1379C, U1380A, and U1381A are based on calcareous nannofossil and planktonic foraminiferal datums and bioevents.

Calcareous nannofossils

Sediment sample processing

Calcareous nannofossil assemblages were examined and described using smear slides made from core catcher samples. If the core catcher sample was barren, at least one additional sample from the particular section of core was examined. A standard smear slide preparation was used for immediate biostratigraphic examination. For this process, a small portion of the raw material was placed directly onto a glass coverslip. A drop of distilled water was added, and a flat-sided toothpick was used to spread the sediment over the coverslip, which was then dried rapidly on a hot plate. With sediment-side down, the coverslip was mounted onto a glass microscope slide with Norland optical adhesive. All samples were examined by light microscopy using phase contrast, brightfield, and cross-polarized light under magnifications of 400× and 1000×.

Relative abundances were determined using the method of Hay (1970) as follows:

  • H = highly abundant (>100 specimens per field of view).

  • V = very abundant (11–100 specimens per field of view).

  • A = abundant (1–10 specimens per field of view).

  • C = common (1 specimen per 2–10 fields of view).

  • F = few (1 specimen per 11–100 fields of view).

  • R = rare (1 specimen per 101–1000 fields of view).

The following basic criteria were used to qualitatively provide a measure of preservation of the nannofossil assemblage:

  • G = good (no dissolution or overgrowth).

  • M = moderate (slight dissolution and/or overgrowth).

  • P = poor (considerable dissolution and/or overgrowth).

Standard zonations by Martini (1971), Bukry (1973, 1975), and Okada and Bukry (1980) were utilized during the study to evaluate nannofossil age datums. The zonal scheme of Martini (1971) was selected for the range-distribution chart. These datums were correlated to the geomagnetically derived chronologic timescale of Berggren et al. (1995a, 1995b) for the Pleistocene and Pliocene. Although not utilized in this study, we acknowledge the June 2009 ratification of the redefinition of the base of the Pleistocene (see Ogg et al., 2008, and recent revisions), which pushes the Pliocene/Pleistocene boundary back from 1.806 to 2.588 Ma.


Sediment sample processing

Core catcher samples were processed following routine methods for the study of foraminifers. Core catcher samples first were dried and weighed. Dry sediment was washed with distilled water over a 63 µm wire mesh sieve. Indurated samples were soaked in water for a short time prior to washing to promote disaggregation of the sediment. Once sieved, all samples were dried on filter paper in a low-temperature oven at ~60°C, weighed, and subsequently examined under a binocular microscope. To minimize contamination of foraminifers between samples, the sieve was placed into a sonicator for several minutes and thoroughly checked between samples to enable identification of contaminants from previous samples. Species identification for planktonic foraminifers were generally made on the >63 µm size fractions. Planktonic and benthic foraminiferal abundance data were based on counts of ~50 specimens from the >63 µm size fractions where possible. The preservation, abundance, and zonal assignment for selected samples were entered through DESClogik before uploading into the LIMS database.

The preservation status of planktonic and benthic foraminifers was estimated as follows:

  • VG = very good (no evidence of overgrowth, dissolution, or abrasion).

  • G = good (little evidence of overgrowth, dissolution, or abrasion).

  • M = moderate (calcite overgrowth, dissolution, or abrasion are common but minor).

  • P = poor (substantial overgrowth, dissolution, or fragmentation).

Planktonic foraminiferal zonal scheme and taxonomy

The planktonic foraminifer zonal scheme used here follows Blow (1969). This scheme is widely employed in tropical regions. Incorporated into this zonation are the modifications proposed by Kennett and Srinivasan (1983). Age calibration of planktonic foraminiferal datums follows that of Wade et al. (2011).

Benthic foraminiferal taxonomy and paleodepth determination

Taxonomic assignments follow Jones (1994). The generic classification of Loeblich and Tappan (1988) was used. The depth distribution of benthic foraminifers is based on recent ecological data of Bandy and Arnal (1957), Crouch and Poag (1987), Heinz et al. (2008), and Smith (1963, 1964) reported from the Pacific off Central America.

Relative percentages of benthic to planktonic tests are also available to evaluate relative changes in paleobathymetry (Ingle et al., 1980). Relative percentages of benthic to planktonic tests were determined by counting specimens on the picking tray during Expedition 334.