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doi:10.2204/iodp.proc.341.205.2019

Results

Calcareous nannofossil assemblage

A complete list of qualitative total nannofossil abundance, relative abundance, and fossil preservation is presented in Table T1.

Nannofossils were barren to present in rare abundance from the top of the section to ~140 m CCSF-A (Fig. F2A). Below this depth the frequency of barren intervals increased. Two main intervals were observed that contain few, if any, nannofossils: 142–168 m and 248–262 m CCSF-A. Fossil preservation was poor to moderate, with only six samples containing fossils that were moderate–good to good (Fig. F2B). The number of fossils counted on each slide decreased with nannofossil abundance and preservation (Fig. F2C).

The dominant taxa were Coccolithus pelagicus and Gephyrocapsa spp., which included G. muellerae, Geophyrocapsa spp. < 3 µm, Gephyrocapsa/Emiliania spp. < 3 µm, and Gephyrocapsa spp. > 3 µm (Fig. F2D, F2E). The abundance of Gephyrocapsa/Emiliania spp. < 3 µm and Gephyrocapsa spp. > 3 µm were influenced by the preservation of the sample and the loss of the central bar with dissolution. The abundance patterns of Coccolithus and Gephyrocapsa co-varied, likely the result of a closed-sum effect. Coccolithus pelagicus was most abundant in the upper 96 m CCSF-A. Below this depth, the assemblage was dominated by Gephyrocapsa spp., which composed up to 95% of the individuals.

Other taxa included in the counts were Pontosphaera spp. and Cruciplacolithus neohelis.

Reworked specimens made up an average of 7 specimens per 100 fossils and peaked at 42 individuals in Sample 341-U1418C-24H-4W, 31–35 cm (Fig. F2F). These reworked fossils included Reticulofenestra dictyoda, R. bisecta, R. daviesii, R. umbilicus, Chiasmolithus oamaruensis, Isthmolithus recurvus, Cyclicargolithus floridanus, Pontosphaera pulchra, and Toweius spp. (Plates P1, P2). This reworked assemblage indicated an age of Eocene–Oligocene (56–23 Ma) and was consistent with fossils found in dredge samples from deep-sea outcropping beds along the continental margin, southeast of Site U1418 (Plafker et al., 1985). The outcrops provide a likely source for the reworked specimens.

Bulk carbonate

Measured CaCO₃ ranged 0.03–9.56 wt% with an average concentration of 2.77 wt% (Table T2). A total of 26 replicate samples were measured with an average resulting difference of ±0.31%.

There were slight trends in CaCO₃ of the sediments downcore (Fig. F3). The uppermost ~100 m CCSF-A contained values averaging 2.94 wt%; values gradually decreased downcore to ~2 wt%. At ~100 m CCSF-A, carbonate values increased to ~3 wt% and decreased to ~1.5 wt% at ~186 m CCSF-A. Below ~186 m CCSF-A, carbonate values increased again and the base of the measured section (~186–266.2 m CCSF-A) had an average CaCO₃ concentration of ~3 wt%. These trends in CaCO₃ were broadly matched by the abundance of Gephyrocapsa spp. (Fig. F2).

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