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doi:10.2204/iodp.proc.320321.102.2010

Biostratigraphy

Preliminary age assignments were based on biostratigraphic analyses of calcareous nannofossils, planktonic foraminifers, radiolarians, and diatoms. Paleodepth interpretations were based on benthic foraminifers. The biostratigraphy is tied to the geomagnetic polarity timescale (GPTS) used for Expedition 320/321, which is based upon a composite of several timescales (Table T6) (Cande and Kent, 1995; Lourens et al., 2004; Pälike et al., 2006b). Planktonic foraminifer, calcareous nannofossil, radiolarian, and diatom bioevents for the middle Eocene to Pleistocene were recalibrated to Lourens et al. (2004) and Pälike et al. (2006b) (Tables T2, T3, T4, T5). Neogene calcareous bioevents are reported to a 10 k.y. resolution, Paleogene calcareous bioevents are reported to a 100 k.y. resolution, and biosiliceous bioevents are reported to a 10 k.y. resolution.

Calcareous nannofossil datum depths were determined by examining core catcher samples and, where appropriate, additional section samples (sampling spacing of 1.5–0.25 m). Planktonic and benthic foraminifers, radiolarians, and diatoms were examined in core catcher samples and additional section samples (sampling spacing of ~2–4 m). Preservation, abundance, and zonal assignment for selected samples and for each microfossil group were initially recorded and saved in Microsoft Excel and subsequently entered through DESCLogik into the LIMS database. Biostratigraphic analyses were focused on Hole A, allowing greater sampling density. Samples were taken from the composite splice to provide more complete stratigraphic coverage, where appropriate.

Calcareous nannofossils

Calcareous nannofossil zonal scheme and taxonomy

The zonal scheme of Martini (1971), zonal code numbers NP and NN, was used for Cenozoic calcareous nannofossil biostratigraphy. These zonations represent a general framework for the biostratigraphic classification of mid- to low-latitude nannofossil assemblages and are presented in Figure F2. Top Sphenolithus ciperoensis is used for recognizing the base of Zone NN1, following Perch-Nielsen (1985). The base of Zone NN2 (base of Discoaster druggii) cannot be differentiated because of the rare and sporadic occurrences of the marker fossil. Ages and calibration sources for calcareous nannofossil datums are presented in Table T2. The age estimates presented are all adjusted to the Expedition 320/321 timescale (see "Reconstruction of geologic age during Expedition 320/321"). Nannofossil taxonomy follows Bown (1998, 2005) and Perch-Nielsen (1985), where full taxonomic lists can be found. A taxonomic list of nannofossil species from Table T2 is given in BIOSTRAT in "Supplementary material."

Methods of study for calcareous nannofossils

Calcareous nannofossils were examined in smear slides using standard light microscope techniques under crossed polarizers, transmitted light, and phase contrast at 1000× magnification.

Total calcareous nannofossil abundance within the sediment is recorded as follows:

D = dominant (>90% of sediment particles).

A = abundant (>50%–90% of sediment particles).

C = common (>10%–50% of sediment particles).

F = few (1%–10% of sediment particles).

R = rare (<1% of sediment particles).

B = barren (none present).

Abundance of individual calcareous nannofossil taxa is recorded as follows:

D = dominant (>100 specimens per field of view).

A = abundant (>10–100 specimens per field of view).

C = common (>1–10 specimens per field of view).

F = few (1 specimen per 1–10 fields of view).

R = rare (<1 specimen per 10 fields of view).

Preservation of the calcareous nannofossils is recorded as follows:

G = good (little or no evidence of dissolution and/or recrystallization, primary morphological characteristics only slightly altered, and specimens were identifiable to the species level).

M = moderate (specimens exhibit some etching and/or recrystallization, primary morphological characteristics somewhat altered; however, most specimens were identifiable to the species level).

P = poor (specimens were severely etched or overgrown, primary morphological characteristics largely destroyed, fragmentation has occurred, and specimens often could not be identified at the species and/or generic level).

Foraminifers

Planktonic foraminifer zonal scheme and taxonomy

The tropical planktonic foraminifer zonal scheme for the Paleocene (P zones) follows Olsson et al. (1999). The Eocene and Oligocene (P and O zones, respectively) scheme follows Berggren and Pearson (2005). The Neogene (M, PL, and PT zones) scheme follows Berggren et al. (1995). The planktonic foraminifer zonal scheme used during Expedition 320/321 is illustrated in Figure F4. Ages and calibration sources of planktonic foraminifer datums are from multiple sources (Table T3). The age estimates presented are adjusted to the Expedition 320/321 timescale. Lourens et al. (2004) reports the astronomical calibration for the Neogene and we have incorporated additional recalibrated datums from Berggren et al. (1995), Berggren and Pearson (2005), and recent studies with biomagnetostratigraphy. For a more detailed discussion of the planktonic foraminifer zonation and revised calibrations to the PEAT timescale see Wade et al. (2007). Zone division of the middle Miocene is provided by the fohsellid lineage. However, we are unable to differentiate the base of Zone M9 because of the lack of calibration of Globorotalia (Fohsella) lobata to the astronomical timescale. Therefore, throughout this interval we employ both the M zones of Berggren et al. (1995) and the N zones of Blow (1969) (Fig. F2B). Planktonic foraminifer taxonomic concepts selectively follow those of Kennett and Srinivasan (1983), Bolli and Saunders (1985), Toumarkine and Luterbacher (1985), Loeblich and Tappan (1988), Spezzaferri and Premoli Silva (1991), Chaisson and Leckie (1993), Leckie et al. (1993), Spezzaferri (1994), Pearson (1995), Chaisson and Pearson (1997), Pearson and Chaisson (1997), and Pearson et al. (2006). A taxonomic list of planktonic foraminifer datum species from Table T3 is given in BIOSTRAT in "Supplementary material."

Benthic foraminifer taxonomy and paleodepth determination

Taxonomic assignments follow Tjalsma and Lohmann (1983), van Morkhoven et al. (1986), Miller and Katz (1987), Thomas (1990), Katz and Miller (1991), Kaminski et al. (1993), Jones (1994), Nomura (1995), Holbourn and Henderson (2002), Kuhnt et al. (2002), and Ortiz and Thomas (2006). The generic classification of Loeblich and Tappan (1988) was used and updated in some instances, in particular for uniserial taxa (Hayward, 2002). A taxonomic list of benthic foraminifers recorded during Expedition 320/321 is given in BIOSTRAT in "Supplementary material."

Paleodepth estimates are based on van Morkhoven et al. (1986) using the following categories:

Neritic = <200 m.

Bathyal = 200–2000 m.

Abyssal = >2000 m.

Methods of study for foraminifers

Sediments were washed with distilled/tap water over a 63 µm wire mesh sieve. Indurated samples were soaked in a 3% hydrogen peroxide solution (with a small amount of sodium borate added) prior to washing. All samples were then dried in the sieves or on filter papers in a low-temperature oven (~50°C) and subsequently examined under a binocular light microscope. To minimize contamination of foraminifers between samples, the sieve was placed into a sonicator for several minutes and thoroughly checked or dipped in a dilute solution of methyl blue dye between samples to enable identification of contaminants from previous samples. Species identification for planktonic foraminifers were generally made on the >250 and >150 µm size fractions. The 63–150 µm size fraction was scanned for distinctive taxa. Planktonic foraminifer species distribution and range charts are presented in each site chapter. Benthic foraminifer assemblage composition and paleodepth estimates were based on counts of ~200 specimens from the >250 and >150 µm size fractions, where possible. Relative percentages of benthic to planktonic tests were determined by counting specimens in four adjacent quadrants in three different locations on the picking tray during Expedition 321.

The following abundance categories were estimated from visual examination of the dried sample for planktonic and benthic foraminifers:

D = dominant (>30% of foraminifer assemblage).

A = abundant (>10%–30% of foraminifer assemblage).

F = few (>5% to <10% of foraminifer assemblage).

R = rare (>1% to <5% of foraminifer assemblage).

P = present (<1% of foraminifer assemblage).

B = barren.

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).

Radiolarians

Radiolarian zonal scheme and taxonomy

The taxonomy for radiolarians studied during Expedition 320/321 is taken from Nigrini et al. (2006) and Nigrini and Sanfilippo (2001), with a few species from additional sources as noted in the taxonomic list (see BIOSTRAT in "Supplementary material"). The radiolarian zonal scheme used is described in Sanfilippo and Nigrini (1998) and Nigrini et al. (2006). The species considered are based on the biostratigraphy developed for the tropical Pacific by Nigrini et al. (2006) (Leg 199) and Moore (1995) (Leg 138) (see BIOSTRAT in"Supplementary material;" Fig. F3). These ODP legs were restricted to the equatorial zone of the Pacific and had good paleomagnetic control through much of the recovered sections. All age estimates for the radiolarian datums are based on the material collected during Legs 138 and 199 and conform to the timescale used during Expedition 320/321. This entailed recalibration of ages for individual datums based on the more recent calibration of the paleomagnetic timescale (Lourens et al., 2004; Pälike et al., 2006b). Leg 138 samples have been reviewed, and in some cases datums were shifted to conform with more accurate taxonomic concepts. In addition, new datums from the Leg 138 material have been added to those reported in the Leg 138 Scientific Results volume (Pisias, Mayer, Janecek, Palmer-Julson, and van Andel, 1995).

Methods of study for radiolarians

Samples were disaggregated by warming in a solution of 10% H₂O₂. After effervescence subsided, calcareous components were dissolved by adding a 10% solution of hydrochloric acid. The solution was then sieved through a 63 µm sieve. A strewn slide was prepared by pipetting the microfossils onto a microscope slide, which was then covered with a 22 mm × 40 mm glass coverslip before the water was evaporated. To avoid crushing the radiolarians with the coverslip, in some cases adhesive tape with an 18 mm × 35 mm rectangular hole was placed on the microscope slide.

Abundance estimates of the radiolarian assemblage are qualitative estimates of the concentration of radiolarians in individual sediment samples, with categories as follows:

A = abundant.

C = common.

R = rare.

B = barren.

Abundance of individual radiolarian species was recorded as follows:

C = common (>100 specimens per slide).

F = few (>10–100 specimens per slide).

R = rare (<10 specimens per slide).

VR = very rare (1–2 specimens per slide).

(blank) = not found or not recorded.

Preservation of the radiolarian assemblage was recorded as follows:

G = good (majority of specimens complete, with minor dissolution, recrystallization, and/or breakage).

M = moderate (minor but common dissolution, with a small amount of breakage of specimens).

P = poor (strong dissolution, recrystallization, or breakage; many specimens unidentifiable).

Mixing of older radiolarian microfossils into younger sections frequently occurs in the tropical Pacific sediments. The amount of admixed older specimens in a sample was estimated as

(blank) = no mixing of older specimens detected.

1 = 1–3 reworked specimens were detected per slide.

2 = 3–10 reworked specimens were detected per slide.

3 = >10 reworked specimens were detected per slide.

Diatoms

Diatom zonal scheme and taxonomy

The taxonomy for the diatoms studied during Expedition 320/321 is taken largely from Burckle (1972), Akiba (1986), Akiba and Yanagisawa (1986), Baldauf and Iwai (1995), Barron (1981, 1985a, 2006), and Barron et al. (2004). The diatom zonal scheme used here mainly follows biostratigraphic studies by Baldauf and Iwai (1995) (Leg 138), Barron (1983, 1985a, 1985b, 2006), and Barron et al. (2004) (Leg 199), with additional information from Akiba (1986), Akiba and Yanagisawa (1986), Barron (1981, 1983), Barron and Gladenkov (1995), Burckle (1972, 1978), Burckle and Trainer (1979), Burckle et al. (1982), and Fenner (1985) (Table T5; Fig. F4). Legs 138 and 199 were restricted to the equatorial zone of the Pacific and had good paleomagnetic control through much of the recovered sections. Therefore, age estimates for diatom datums from Leg 138 and 199 data are adjusted to the timescale used during Expedition 320/321. A taxonomic list of diatom species in Table T5 is given in BIOSTRAT in "Supplementary material."

Methods of study for diatoms

Samples examined from Expedition 320 were prepared using H₂O₂ and HCl as detailed in Baldauf and Iwai (1995). Slides for examination were prepared with optical adhesive similar to the procedures discussed below for Expedition 321. Strewn slides were prepared for samples examined from Expedition 321 by placing a small amount of raw sediment onto a slide and allowing the water to evaporate by heating on a hot plate for ~5 min. About 1–2 drops of optical adhesive were applied to the dry slide, which was then covered with a 22 mm × 40 mm glass coverslip. The adhesive was solidified by placing the slide under ultraviolet light for ~10 min. Strewn slides were scanned at a maximum magnification of 1250× for stratigraphic markers and other common taxa.

Abundance estimates of the diatom assemblage are qualitative estimates of the concentration of diatoms in individual sediment samples, with categories as follows:

D = dominant (>90% of sediment particles).

A = abundant (>50%–90% of sediment particles).

C = common (>10%–50% of sediment particles).

F = few (1%–10% of sediment particles).

R = rare (<1% of sediments particles).

B = barren (none present).

Abundance of individual diatom species was recorded as follows:

D = dominant (>50 valves per counted transect).

A = abundant (>20–50 valves per counted transect).

C = common (>10–20 valves per counted transect).

F = few (>1–10 valves per counted transect).

R = rare (≤1 valve per counted transect).

Preservation of the diatom assemblage was recorded as follows:

G = good (majority of specimens complete with minor dissolution and/or breakage and no significant enlargement of the areolae or dissolution of the frustules rim detected, the sample generally has a high diatoms per gram concentration).

M = moderate (minor but common areolae enlargement and dissolution of the frustule rim with a considerable amount of breakage of specimens).

P = poor (strong dissolution or breakage, some specimens unidentifiable, strong dissolution of the frustule rim and areolae enlargement the sample generally has lower diatoms per gram concentration).

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