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Methods and materials

Slide preparation

Sediment samples (~5 g dry weight) were placed in a beaker with 10% H2O2 and disaggregated by warming on a hot plate for a few hours. Calcareous components were dissolved by adding a 10% solution of hydrochloric acid. The solution was then washed through a 40 µm sieve. Strewn slides were prepared by pipetting the residue onto a microscope coverslip, which was then dried on a hot plate. Canada Balsam, applied to the coverslip, was used as a mounting medium. The coverslip was then inverted, placed on the slide, and indurated by placing the slide back on the hot plate for a few minutes.

Radiolarian taxonomy and zonal schemes

The main references for the taxonomy of radiolarians studied here were Nigrini and Lombari (1984), Sanfilippo et al. (1985), Lombari and Lazarus (1988), Lazarus (1990, 1992), Abelmann (1992), Nigrini and Sanfilippo (2001), Vigour and Lazarus (2002), and Nigrini et al. (2006). All radiolarian semiquantitative abundance data produced for this study are shown in Table T1.

Site U1371 is located at an intermediate latitude, and therefore the radiolarian biostratigraphic scheme developed for low/equatorial latitudes (Sanfilippo et al., 1985) as well as the age calibration of the key taxa used to establish such a zonation is not applicable, as most of the key taxa are not present.

Both high- and middle-latitude marker taxa have been recognized in this study, thus requiring the adoption of two different zonal schemes for their age interpretation and leading to two different age models for Hole U1371D. However, the two proposed age models do not differ substantially.

The middle-latitude radiolarian zonal scheme used here is the one described in Nigrini and Sanfilippo (2001), and its zonal definitions, and key marker species are shown in SMT1.XLS in RADIOLAR in “Supplementary material.”

The high-latitude (Southern Ocean) radiolarian zones introduced by Lazarus (1990, 1992), Abelmann (1992), and Takemura (1992) were also applied to Hole U1371D samples. This composite zonation is shown in SMF1.JPG in RADIOLAR in “Supplementary material.” The middle Miocene to Pleistocene zonation of Lazarus (1990) was refined by Lazarus (1992) using sediment recovered during Ocean Drilling Program Legs 119 and 120 (Kerguelen Plateau). The early to middle Miocene age zonation of Abelmann (1992) is based on radiolarian studies from Leg 120. The studied sediment was not old enough to enable the recognition of any of the biozones defined in Takemura (1992).

Age estimates for radiolarian datums from the Quaternary to the Miocene (Table T2) are based on their calibration to magnetostratigraphy according to Hays and Opdyke (1967), Gersonde et al. (1990), Barron et al. (1991), Caulet (1991), Abelmann (1992), Lazarus (1992), Harwood et al. (1992), Shackleton et al. (1995), and Kamikuri et al. (2004).

The ages of biostratigraphically useful radiolarian datums from both the middle- and high-latitude zonations (Fig. F4) define the age model for Hole U1371D (Fig. F3).

Microscope examination and radiolarian abundance conventions

Radiolarian biostratigraphy was based on examination of the suite of samples listed in Table T1. Each slide was scanned throughout following traverses at 100× magnification.

The following conventions/abundance schemes were used for preservation, overall radiolarian abundance, and relative abundance of individual species.

Radiolarian preservation:

  • G = good (most specimens complete, minor dissolution, recrystallization, or breakage)
  • M = moderate (minor dissolution, some recrystallization or breakage)
  • P = poor (strong dissolution, recrystallization, or breakage)
  • B = barren (no radiolarians observed)

Overall radiolarian abundance:

  • A = abundant (>100 specimens per slide traverse)
  • C = common (51–100 specimens per slide traverse)
  • F = few (10–50 specimens per slide traverse)
  • R = rare (<10 specimens per slide traverse)
  • B = barren (no radiolarians in sample)

Abundance of individual species (relative to the total assemblage):

  • A = abundant (>10% of the total assemblage or >20 per slide)
  • C = common (5%–10% of the total assemblage or 11–20 per slide)
  • F = few (<5% of the total assemblage or 5–10 per slide)
  • R = rare (1–4 specimens per slide)
  • X = absent (looked for, but not observed in sample)
  • Rw = species presumably reworked