Previous   |   Next

doi:10.2204/iodp.proc.311.210.2009

Sampling and analytical methods

Sampling of authigenic carbonates was realized onboard the JOIDES Resolution; they occur as light-colored cements, small indurated concretions, or hard rock pieces embedded in the carbonate-poor sediments (Blanc-Valleron et al.). The carbonate content (in weight percent of total sediment) of the 80 selected samples varies between 9 and 89 wt%, with 74 samples containing >25 wt% of carbonate.

The mineralogical composition of samples was systematically determined using X-ray diffraction (XRD) (Blanc-Valleron et al.). The same powder prepared for XRD was used for the isotopic analyses, but it was rinsed with distilled water to eliminate soluble salts that often produce parasite gas during acid attack. Because of the complex mixing of different carbonate minerals in a few samples, it was impossible to separate these phases for isotopic measurements, which thus correspond to the composition of bulk carbonate.

The oxygen and carbon isotope compositions of carbonates are expressed in the conventional δ notation defined as

δ = [(Rs/Rr) – 1] × 1000,

where R = ¹⁸O/¹⁶O or ¹³C/¹²C, respectively, in the sample (Rs) and in the reference (Rr). The reference for δ¹⁸O and δ¹³C is Vienna Peedee belemnite (VPDB) (Craig, 1957). The CO₂ gas extracted from the carbonate by digestion with 100% phosphoric acid at 25°C (24 h for calcium carbonate and 72 h to 1 week for dolomite, siderite, and rhodochrosite and composite mixtures of various carbonate phases) was analyzed with a triple collector mass spectrometer (VG Sira 9). The analytical precision was 0.01‰ for both δ¹⁸O and δ¹³C; the reproducibility was generally better than 0.1‰ for δ¹⁸O and δ¹³C.

The oxygen isotopic compositions of calcite and dolomite precipitated in isotopic equilibrium with the bottom seawater may be evaluated using the equations of O'Neil et al. (1969) and Fritz and Smith (1970), respectively. These calculated theoretical δ¹⁸O values vary between 2.96‰ and 3.35‰ for calcite and between 5.97‰ and 6.35‰ for dolomite and may thus be compared to the δ¹⁸O values measured in the authigenic carbonates.

Top of page   |   Previous   |   Next