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During Expedition 308, calcareous nannofossils and foraminifers were examined for age dating of samples taken primarily from core catchers. Chronobiostratigraphy was mainly based on the zonation scheme of Berggren et al. (1995) (Fig. F5). As most drilling activities were on late Quaternary sequences, local species datums and zonations based on species abundance such as those of Kennett and Huddlestun (1972) were also used. Ages for assemblage zonal boundaries were adjusted to marine isotope stages (MIS) given in Bassinot et al. (1994).

Calcareous nannofossils

Zonation and taxonomic concept

We have referred primarily to the zonation and code of Hine and Weaver (1998) (Fig. F6) for dating the cored sediments. We followed taxonomic concepts summarized in Perch-Nielsen (1985).


Standard smear slides were prepared for all samples using Norland optical adhesive as a mounting medium.

Assemblages of calcareous nannofossils were studied from smear slides prepared for each core catcher sample and a few samples from selected cores. In the latter case, hemipelagic and pelagic beds were sampled in order to avoid effects of resedimentation resulting from turbidites and mass transport deposits (MTDs), and special attention was given to lithologic changes. Examination was done exclusively with a transmitted light microscope, and a magnification of 1000× was used to estimate relative abundances.

We used Bugwin (version 2002.3.8) software for collecting and analyzing nannofossil data. BugCam software, specifically designed by the University of Florida, was used for taxa identification. All species were counted within 10 fields of view. Abundance of individual species is reported on the basis of 100 fields of view. On collecting data the species distribution charts were created and analyzed.

Individual species abundances were determined as follows:

  • Rare = 1–3 specimens/100 fields of view (FOV).
  • Sparse = 4–10 specimens/100 FOV.
  • Frequent = 11–25 specimens/100 FOV.
  • Frequent–common = 26–100 specimens/100 FOV.
  • Common = 101–500 specimens/100 FOV.
  • Abundant = 501–1000 specimens/100 FOV.
  • Very abundant = 1001–10,000 specimens/100 FOV.
  • Ooze = >10,000 specimens/100 FOV.

The total abundance of calcareous nannofossils for each sample was estimated as follows:

  • B = barren.
  • R = rare (<100 specimens/100 FOV [half of a traverse]).
  • F = frequent (100–500 specimens/100 FOV).
  • C = common (501–1000 specimens/100 FOV).
  • A = abundant (1001–10,000 specimens/100 FOV).
  • W = very abundant (>10,000 specimens/100 FOV).

The preservation of calcareous nannofossils was classified as good, moderate, or poor. These categories represent subjective impressions with the following definitions:

  • P = poor (strong and severe dissolution, overgrowth, and/or fragmentation has occurred; primary diagnostic features may have been destroyed, impossible to identify many specimens at the species level).
  • M = moderate (dissolution and/or overgrowth are evident; a significant proportion [as much as 25%] of the specimens cannot be confidently identified at the species level with absolute certainty).
  • G = good (evidence of dissolution and/or overgrowth is very little; diagnostic characteristics are preserved nearly in all nannofossils; ~95% of all specimens or more can be identified).



Samples of ~40 cm3 were soaked in a Calgon solution and washed through a 63 µm sieve. Samples were rinsed and then dried under a heat lamp or in an oven at 60°C.

Preservation characteristics were divided into four categories:

  • P = poor (almost all specimens were dissolved or broken and fragments dominated).
  • M = moderate (30%–60% of specimens showed dissolved or broken chambers).
  • G = good (61%–90% of specimens were well preserved and unbroken).
  • VG = very good (>90% of specimens were well preserved and unbroken).
Planktonic foraminifers

The >125 µm size fraction was examined for planktonic foraminifers and the 63–125 µm size fraction was studied for zonal markers if these were absent in the larger size fractions.

The abundance of planktonic foraminifers is defined as follows:

  • B = barren.
  • T = trace (1 or 2 specimens).
  • VR = very rare (3–10 specimens).
  • R = rare (11–30 specimens).
  • F = frequent (31–100 specimens).
  • C = common (101–500 specimens).
  • A = abundant (>500 specimens).

We mainly referred to Bolli and Saunders (1985) for planktonic foraminifer taxonomy. The zonal scheme and species datum levels of Berggren et al. (1995) were used for dating major stratigraphic boundaries (Fig. F5). The planktonic foraminifer assemblage zones based on Globorotalia menardii for subdividing the late Quaternary in the Gulf of Mexico region (Kennett and Huddlestun, 1972; Martin et al., 1990) (Fig. F7) were also attempted when time allowed. Ages for these assemblage zone boundaries were modified according to the timescale of Bassinot et al. (1994).

Benthic foraminifers

Benthic foraminifer relative abundance was defined as follows:

  • B = barren.
  • R = rare (<1% of the fauna).
  • F = frequent (1%–5% of the fauna).
  • C = common (6%–20% of the fauna).
  • A = abundant (>20% of the fauna).

Benthic foraminifers were examined from the >63 µm size fraction. Paleobathymetry estimates were primarily based on Poag (1981) and van Morkhoven et al. (1986). Bathymetric zones were defined as follows:

  • Neritic = 0–200 m.
  • Upper bathyal = 200–600 m.
  • Middle bathyal = 600–1000 m.
  • Lower bathyal = 1000–2000 m.
  • Abyssal = >2000 m.