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doi:10.2204/iodp.proc.307.201.2009

Methods

The core section was divided in 5 mm intervals after avoiding identifiable coral pieces. Each subspecimen was powdered and used for geochemical analyses, such as cation (Ca2+, Mg2+, and Sr2+) concentrations, carbonate content, and carbon and oxygen isotopic compositions.

The cation concentrations were measured by atomic absorption (Shimadzu AA-646). A ~50 mg powdered sample was put into a 1.5 mL plastic tube and dissolved first with 0.6 mL of 10% acetic acid and then with 0.6 mL of 20% acetic acid. The reacted solution was centrifuged for 10 min to separate insoluble residue. The sample solution was diluted with 18.2 MΩ Milli-Q ultra-high purity water to the appropriate ionic strength, to which 1 M HNO3 containing 1% La and 0.5% Cs in 10% volume of the sample solution was added. These treatments minimize the interference effect caused by silicic acid and phosphoric acid, which can form refractory ionic compounds with Ca2+ and Mg2+ and result in a measuring error of <10%. Carbonate contents were calculated from cation concentrations and sample weight.

Carbon and oxygen isotope ratios were measured by gas mass spectrometry using a Finnigan delta mass spectrometer attached to a gas separation system (Thermo as Bench). In this system of isotopic analysis, the samples were reacted with purified H3PO4 under 1 atm He in a sealed glass bottle. Reaction temperature and time are 55°C and 3 h, respectively. The reactant CO2 was, together with He, first introduced to the gas chromatograph, and other components, such as H2O, were separated. The separated CO2 was then introduced to the mass spectrometer, which measured isotopic composition. The measured carbon and oxygen isotopic ratios were normalized with a laboratory standard (Solnhofen limestone; δ13C = –0.47‰, δ18O = –5.04‰) calibrated against NBS19 (δ13C = +1.95‰, δ18O = –2.20‰ versus Vienna Peedee belemnite). Repeated measurements of the laboratory standard (n = 22) showed that the reproducibility of δ13C and δ18O was within ±0.20‰ and ±0.27‰, respectively.

The intensively mottled part of the sample was dried at room temperature and hardened with ethanol-diluted epoxy resin (Maruto Eposet). A recommended ratio of ethanol and resin is 2:1 or 1.5:1, and the resin permeates into hollow spaces of the sample. Thin sections made from the hardened samples were observed with an optical microscope for lithified textures.

The insoluble residues left after acid treatment were also observed by scanning electron microscopy (SEM; JEOL JXA-8200).