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doi:10.2204/iodp.proc.303306.216.2013

Methods

Sediment samples from Site U1313 were wet sieved using a >63 µm sieve and sonicated for a few seconds to dislodge clay and other fine particles adhered to foraminifers. The washed residue (>63 µm) was dried and then sieved again at >150 µm size fraction for foraminiferal and lithic fragment counts (Bond et al., 1993; Bond and Lotti, 1995). To obtain abundance counts of 300–600 foraminifers per gram of sediment, the dried residue (>150 µm fraction) was split using a microsplitter. A number of splits was used in determining the relative abundance of each species based on estimated total foraminifers per sample. Using a binocular microscope, various species of planktonic foraminifers were identified and counted, including N. pachyderma (s), N. incompta, G. bulloides, and G. inflata (Bé and Tolderlund, 1971; Kennett and Srinivasan, 1983), and were used to estimate their relative abundances, which are relevant to ascertain various climate events and boundaries. In addition to foraminifers, lithic fragments were counted and normalized per gram of dry sediment and reported as IRD/g. IRD petrology such as detrital carbonate grains, quartz, feldspar, and so on were assessed to identify a Heinrich iceberg-rafting event (H11) (Heinrich, 1988; Bond et al., 1993).

Stable oxygen isotope ratios were determined on G. bulloides in the 150–250 µm size fraction. Approximately 30 specimens of G. bulloides were picked to ensure enough weight for mass spectrometer analysis and for future use in replicating isotopic analysis. Each sample was analyzed for δ¹⁸O relative to National Bureau of Standards (NBS) 18, 19, and 20 standards using an automated carbonate Kiel extraction device coupled to a Finnigan Delta IV Plus stable isotope ratio mass spectrometer (Rashid et al., 2012). Samples were acidified under vacuum with 100% ortho-phosphoric acid. Resulting CO₂ was cryogenically purified and delivered to the mass spectrometer. Approximately 10% of all samples were run in duplicate. The standard deviation of repeated measurements of an internal standard was ±0.06‰ for δ¹⁸O. The age model of Site U1313 was constructed by matching the curve of G. bulloides δ¹⁸O with those of the global benthic foraminiferal δ¹⁸O stack (Lisiecki and Raymo, 2005). We used 20 control points, in which high and low G. bulloides δ¹⁸O values were correlated to those of the high and low δ¹⁸O values of the global benthic stack.

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