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doi:10.2204/iodp.proc.314315316.122.2009

Biostratigraphy

Calcareous nannofossils

Calcareous nannofossil biostratigraphic classification of sedimentary sequences recovered during Expedition 315 follows the recent review by Raffi et al. (2006), who astrobiochronologically calibrated Pleistocene to late Oligocene calcareous nannofossil datum events. Astrochronological age estimates for the Neogene rely on the geologic timescale developed by the International Commission on Stratigraphy (ICS) in 2004 (Lourens et al., 2004). Included biohorizons are based on standard nannofossil zonations established by Martini (1971) and Okada and Bukry (1980) with zonal modifications by Young (1999). Size-defined species of the genera Gephyrocapsa and Reticulofenestra were placed into size categories as proposed by Young (1999) instead of referring to species names, which vary widely among different authors. Consequently, in the late Pleistocene the abundance change between Gephyrocapsa spp. (>3.5 µm) and Emiliania huxleyi was used instead of the crossover between Gephyrocapsa caribbeanica and E. huxleyi.

Biostratigraphic event zonal markers for the Cenozoic are shown in Figure F16, along with events defining zonal boundaries and additional biohorizons. In Table T5, well-dated nannofossil datums used during Expedition 315 are listed.

Methods

For nannofossil analyses, the core catcher sections of each core were sampled. Samples from other sections were sometimes included when nannofossils were not abundant in core catcher material. Preparation of smear slides for light microscope examination followed standard procedures. Taxon identification was carried out under plane- and cross-polarized light using a Zeiss Axio Imager.A1m microscope at 1250× magnification. Abundance, preservation, and zonal data for each sample investigated were recorded in the J-CORES database. To determine species abundance, specimens in 10–20 fields of view were counted, depending on overall nannofossil abundance. Moreover, the slide was scanned along its entire long axis for additional taxa. The following scale was used to estimate the relative abundances of individual taxa present in each sample:

• B = barren (none).
• R = rare (<0.1%).
• F = few (0.1% to <1%).
• C = common (1% to <10%).
• A = abundant (10%–50%).
• D = dominant (>50%).

Assessments of calcareous nannofossil preservation were based on the following criteria:

• P = poor, severe dissolution, fragmentation and/or overgrowth has occurred; primary features may have been destroyed, and many specimens can not be identified at the species level.
• M = moderate, dissolution and/or overgrowth are evident; besides frequently broken nannofossils, the number of delicate forms is reduced.
• G = good, little or no evidence of dissolution and/or overgrowth; diagnostic characteristics are preserved and nearly all species (~95%) can be identified.
• VG = very good, no evidence of dissolution and/or overgrowth; diagnostic characteristics are preserved and all specimens can be identified.

Some of the additional samples from core sections were only scanned for biostratigraphic key species without recording the abundances of the entire assemblage. These samples were not included in the range charts (see the “Expedition 314 Site C0001” chapter) but may be listed in the table of nannofossil events (Table T5).

Planktonic foraminifers

The planktonic foraminifer zonation of Blow (1969) and astronomically calibrated biohorizons of Neogene planktonic foraminifers compiled by the ICS in 2004 (Lourens et al., 2004) were applied for this expedition. In addition, useful biohorizons were employed from literature in the field and converted in age to the current geomagnetic polarity timescale (GPTS).

The last occurrence (LO) of Globigerinoides ruber rosa was located at 0.12 Ma in the Indian and Pacific Oceans by Thompson et al. (1979) and confirmed by others (i.e., Li et al., 2005, at ODP Site 1143, South China Sea). The LOs of Neogloboquadrina asanoi and Globoquadrina dehiscens and the first occurrence (FO) of Globoconella inflata modern form were correlated with geomagnetic polarities at ODP Sites 1150 and 1151 off northeast Japan (Motoyama et al., 2004). The coiling direction change of Pulleniatina spp. from sinistral to dextral has been reported just above Chron C2n (Olduvai) in the Boso Peninsula of central Honshu, Japan (Oda, 1977). The FOs of Truncorotalia crassaformis hessi, Truncorotalia tosaensis, and Pulleniatina primalis; the first consistent occurrence of Neogloboquadrina acostaensis; and the LO of Paragloborotalia mayeri were compiled by Berggren et al. (1995) and converted in age to the current GPTS. These biozones and biohorizons are shown in Table T6 and Figure F16.

Methods

About 10 cm³ of sediment from core catcher sections was collected for foraminifer analyses. Soft sediment samples were disaggregated using hydrogen peroxide solution. Firm mudstone samples were treated by the sodium tetraphenylborate method (Hanken, 1979). After samples were macerated, each sample was wet-sieved through a screen (63 µm opening). Planktonic foraminiferal specimens >125 µm were taken from the dried residues. Semiquantitative estimates of species were made of the relative abundance as follows:

• + = present (<4% or species from samples yielding <100 total individuals).
• R = rare (4% to <8%).
• C = common (8% to 16%).
• A = abundant (>16%).

Preservation of each sample was recorded by the following criteria:

• P = poor, dissolution of surface structure and fragmentation are observed; most individuals cannot be identified at the species level.
• M = moderate, dissolution and fragmentation are commonly evident; some individuals are hard to identify.
• G = good, no dissolution; fragmentation of individuals has slightly occurred.

Abundance and preservation data for each sample investigated were uploaded into the J-CORES database.

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