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doi:10.2204/iodp.proc.318.202.2016

Methods

Sample preparation and foraminifer identification

Sediment samples from Hole U1361A were weighed and then dried in an oven at 50ºC for 24 h. The dried samples were soaked in a “Calgon solution” (5.5 g of Fisher Scientific laboratory grade sodium metaphosphate (NaPO₃)₆ mixed with 1 L H₂O) to help break down the sediment. The slurry was decanted into a 63 µm sieve and gently washed with water to flush out silt- and clay-sized particles. The sediment was then separated into 425–355, 355–250, 250–125, and 125–63 µm size fractions by dry sieving. Planktonic and benthic specimens >63 µm were picked under a reflected light binocular microscope with a fine paintbrush and separated by species. Visual inspection of the foraminiferal tests indicated excellent preservation, often with a glassy appearance. The following species were picked for isotopic analysis: Oridorsalis umbonatus, Gyroidinoides soldani, Epistominella exigua, Cibicidoides wuellerstorfi, Cibicidoides bradyi, and Globigerina bulloides. Specimens were placed in double distilled water and sonicated for 5–10 s using a Bransonic ultrasonic cleaner prior to stable isotope analysis.

Stable isotopes

The cleaned benthic and planktonic foraminifers were first sent to the stable isotope laboratory at Lamont-Doherty Earth Observatory (USA) for δ¹⁸O and δ¹³C analysis. A VG Optima isotope-ratio mass spectrometer equipped with a multiprep carbonate preparation module with long-term precision of ±0.07‰ for δ¹⁸O and ±0.05‰ for δ¹³C was used to collect stable isotope data. Foraminifers were also analyzed at the Stable Isotope Laboratory at Rutgers University (USA) using a Micromass Optima mass spectrometer with an attached multiprep device with analytical error (1σ) of ±0.08‰ for δ¹⁸O and ±0.05‰ for δ¹³C.

Foraminifers were reacted in 100% phosphoric acid at 90ºC for 10 min, and the evolved CO₂ was collected in a liquid nitrogen cold finger. Ratios are reported in standard delta notation in parts per thousand (per mil; ‰) d = [(R_sample/R_standard) – 1] × 1000 where R = ¹⁸O/¹⁶O or ¹³C/¹²C, relative to Vienna-Pee Dee Belemnite (δ¹⁸O and δ¹³C _VPDB) through the analysis of an in-house laboratory standard (RGF1). This standard is routinely calibrated to National Bureau of Standards NBS-19 to ensure consistency in reported values.

Age model

The age model for Hole U1361A was constructed using the geomagnetic Chron C5n.2n (o), at 11.04 Ma and the biostratigraphic datum the first occurrence of Denticulopsis dimorpha s.l. at 12.4 Ma (Fig. F2). This resulted in an age range of 11.0–12.0 Ma for the study interval between 311.60 and 349.99 mbsf. In addition, the carbonate preservation event was correlated to a similar preservation event at Ocean Drilling Program Site 689 in the Weddell Sea (DeCesare, 2014), which has a robust magnetostratigraphic age model (Speiß, 1990). This enabled a more precise dating of the event to 11.9–10.5 Ma (DeCesare, 2014).

Calcium carbonate weight percent and methane concentration

For discussion of calcium carbonate weight percent and methane concentration, see the “Methods” chapter (Expedition 318 Scientists, 2011a).

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