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doi:10.2204/iodp.proc.320321.208.2013

Methods

Sample processing

All samples were cleaned with 18 MΩ deionized water to remove salts that precipitate from interstitial water. Samples were then freeze-dried. After drying, the samples were split into two aliquots, one of ~0.5 g for stable isotope analyses and one of 2–5 g for future biostratigraphic and carbonate dissolution proxy analyses. Samples from the first aliquot were powdered and homogenized in glass mortars.

Geochemistry

Stable isotope analyses were conducted at the Stable Isotope Laboratory at the University of California, Santa Cruz (UCSC), USA. Initially, small portions of all samples were reacted “online” in orthophosphoric acid at 90°C to generate CO₂ and H₂O. After separating the water and noncondensable gases, CO₂ was introduced into a Fisons Prism III dual-inlet isotope ratio mass spectrometer (IRMS).

Selected samples across the three sites have a very large range in carbonate content, including some with probably <0.5% CaCO₃. Early on, it became clear that some samples could not be analyzed for stable isotopes using the current setup for the Prism IRMS at UCSC. For these samples, a different configuration was used that allowed a much greater sample size. Solid samples (as much as 20 mg) were placed in an individual-vial acid-drop ThermoScientific Kiel IV carbonate device. Samples were reacted in orthophosphoric acid at 75°C to generate CO₂ and H₂O. After separating the water and noncondensable gases, CO₂ was introduced into a ThermoScientific MAT-253 dual-inlet IRMS. Importantly, several samples were analyzed by both methods (Table T1) to assess results from the two approaches.

During all analytical runs, a calibrated in-house standard (Carrera marble) was used to correct for drift in stable isotope values. Two National Bureau of Standards (NBS)-19 limestone samples were also analyzed in each run to monitor accuracy and precision. Based on these replicate analyses, precision is better than ±0.05‰. However, this does not extend to the samples with very low carbonate content. Corrected delta values from the instruments were then expressed relative to Vienna Peedee belemnite for δ¹³C and δ¹⁸O.

The Kiel-MAT 253 configuration generates a measurement of CaCO₃ equivalent weight in the sample. This can be divided by the total sample weight to calculate carbonate content (Table T1). We have not yet evaluated the accuracy of these values.

In theory, carbonate content can also be generated using the Prism configuration because measurements on the pressure transducer on the automated carbonate preparation line relate to calcium carbonate mass. These measurements can be calibrated to mass and then divided by the sample weight. However, with these particular samples, the accuracy and precision were poor, and we do not report the data.

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