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doi:10.2204/iodp.proc.344.102.2013

Paleontology and biostratigraphy

Paleontologic investigations carried out during Expedition 344 focused primarily on calcareous nannofossils, radiolarians, and benthic foraminifers. Preliminary biostratigraphic determinations were based on nannofossil and radiolarian assemblages.

Calcareous nannofossils

Calcareous nannofossil assemblages were examined and described from smear slides made from core catcher samples. Standard smear slide techniques were utilized for immediate biostratigraphic examination. In order to process a sample, a small portion of sediment was placed directly on a glass coverslip. A drop of distilled water was added, and the sediment was evenly spread across the coverslip using a flat-sided toothpick. The coarser grained fraction was removed during this process. The coverslip was then dried on a hot plate. After drying, it was mounted onto a glass microscope slide with Norland optical adhesive (Number 61) and placed under a UV lightbulb until the adhesive hardened. All samples were examined using a Zeiss Axiophot light microscope with an oil immersion lens. Phase contrast, brightfield, and cross-polarized light, all under magnifications of 400×, 630×, and 1000×, were used. Photomicrographs were taken using a Spot RTS system with Image Capture and Spot software.

Relative abundances of calcareous nannofossils were determined using the criteria defined below:

• 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 (no specimen).

Species abundances were determined using the criteria defined below:

• D = dominant (>100 specimens per field of view at 1000× magnification).
• A = abundant (10–100 specimens per field of view at 1000× magnification).
• C = common (1–10 specimens per field of view at 1000× magnification).
• Freq = frequent (1 specimen per 1–10 fields of view at 1000× magnification).
• R = rare (<1 specimen per 10 fields of view at 1000× magnification).

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

• G = good (little or no evidence of dissolution and/or recrystallization; primary morphological characteristics only slightly altered; all specimens identifiable at the species level).
• M = moderate (some etching and/or recrystallization; primary morphological characteristics partially altered; most specimens not identifiable at the species level).
• P = poor (specimens severely etched or overgrown; primary morphological characteristics largely destroyed; fragmentation evident; most specimens not identifiable at the species and/or generic level).

The standard nannofossil zonations of Martini (1971), Bukry (1973, 1975), and Okada and Bukry (1980) were utilized during the study to evaluate nannofossil age datums. The website Nannotax (www.nannotax.org/) was consulted to find updated nannofossil genera and species ranges. The zonal scheme of Martini (1971) was selected for the range-distribution chart, and this zonal scheme was correlated to the Gradstein et al. (2012) geological timescale. Determinations on the degree of preservation and group and species abundances were recorded in DESClogik and uploaded to the LIMS database.

Radiolarians

Radiolarian assemblages were examined and described from core catcher samples using smear slides. Core catcher samples were first soaked in 10% HCl to dissolve the calcium carbonate component of the sample (determined through the reactivity of the mixture). The residue was sieved over a 63 µm mesh, soaked in H₂O₂ (hydrogen peroxide) to remove any organic material present, and sieved a second time. A portion of the residue was placed directly onto a glass coverslip and spread evenly across the surface using a flat-sided toothpick. The coverslip was then dried on a hot plate. After drying, the coverslip was mounted onto a glass microscope slide with Norland optical adhesive (Number 61) and placed under a UV lightbulb until hardened. Samples were examined using a Zeiss Axioscope. Phase contrast, brightfield, and cross-polarized light, all under magnifications of 10× and 100×, coupled with a photomicrograph camera system were used.

Relative abundances of radiolarian assemblages were determined using the method below:

• A = abundant (>200 specimens per slide traverse).
• C = common (50–200 specimens per slide traverse).
• F = few (1–49 specimens per slide traverse).
• N = none (0 specimens per slide traverse).

The following criteria were used to categorize relative abundances of radiolarian species per sample:

• A = abundant (>30 specimens per slide traverse).
• C = common (>6–30 specimens per slide traverse).
• F = few (>1–5 specimens per slide traverse).

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

• VG = very good (no evidence of overgrowth, dissolution, or abrasion).
• G = good (little evidence of overgrowth, dissolution, or abrasion).
• M = moderate (common calcite overgrowth, dissolution, or abrasion).
• P = poor (substantial overgrowth or infilling, dissolution, or fragmentation).

Sanfilippo and Nigrini (1998), Nigrini and Moore (1979), Takahashi (1991), and Riedel and Sanfilippo (1970) were used for taxonomic identification. Low-latitude zonation was assigned based on Sanfilippo and Nigrini (1998) and Sanfilippo et al. (1985). These datums were correlated to the Gradstein et al. (2012) geological timescale.

Benthic foraminifers

Core catcher samples were processed following routine methods for the study of foraminifers. Core catcher samples were first sieved with tap water over 125 and 63 µm mesh sieves, and the residue was dried on filter paper in a low-temperature oven at ~60°C. If the samples were indurated, they were first soaked in water and then in H₂O₂ (3%–5%) to further disaggregate the sediment before sieving took place. To minimize contamination of foraminifers between samples, the sieves were placed in an ultrasonic bath for several minutes and thoroughly checked between each sample to prevent contamination.

Species identifications for benthic foraminifers were made on the >125 µm size fraction and examined under a binocular microscope. Where possible, at least 150 specimens were picked, identified, and counted to determine benthic foraminiferal relative abundances. Planktonic foraminifers were not identified to species level but were used in planktonic to benthic ratio (P/[P + B]) determinations for each sample to evaluate relative changes in paleobathymetry (Ingle et al., 1980). Determinations on the degree of preservation and species counts were entered into DESClogik and uploaded into the LIMS database.

Relative abundances of benthic foraminifers were determined using the criteria defined below:

• D = dominant (>30% of the >125 µm size fraction).
• A = abundant (20%–30% of the >125 µm size fraction).
• C = common (10%–20% of the >125 µm size fraction).
• F = few (5%–10% of the >125 µm size fraction).
• R = rare (1%–5% of the >125 µm size fraction)
• P = present (<1% of the >125 µm size fraction).

The preservation status of benthic foraminifers was estimated as follows:

• G = good (little or no evidence of dissolution and/or secondary overgrowth of calcite; diagnostic characters fully preserved).
• M = moderate (dissolution and/or secondary overgrowth present; partially altered primary morphological characteristics; nearly all specimens can be identified at the species level).
• P = poor (severe dissolution, fragmentation, and/or secondary overgrowth with primary features largely destroyed; many specimens cannot be identified at the species level and/or generic level).

Benthic foraminiferal taxonomy and paleodepth determination

Genera were assigned following Loeblich and Tappan (1988), and the standard foraminifer literature was consulted for species identification. Ecological and paleobathymetric interpretations are based on a compilation of ecological data, including, but not limited to, the equatorial Pacific, such as Bandy and Arnal (1957), Crouch and Poag (1987), Heinz et al. (2008), and Smith (1963, 1964).

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