We used the vacuum-gasometric technique of Jones and Kaiteris (1983) to determine weight percent calcium carbonate content. With this technique a small sample, ~0.25 g, is digested in concentrated phosphoric acid under vacuum and the pressure generated by the release of CO2 recorded on a vacuum gauge. Percent CaCO3 is calculated by relating the pressure increase in the sample to the pressure increase in reagent carbonate after correcting for temperature and pressure. The Jones and Kaiteris (1983) technique has an accuracy of about ±1%. As with most techniques used to determine percent carbonate, the Jones and Kaiteris (1983) technique lumps together both polymorphs of CaCO3, calcite and aragonite, as well as carbonate from biogenic and nonbiogenic sources.
Two types of diffuse reflectance spectrophotometers were used to obtain spectral data; a Perkin-Elmer Lambda 6 was used at UTA for laboratory-based measurements, whereas a Minolta CM-2002 was used on the JOIDES Resolution for the shipboard measurements. Reflectance spectrophotometers are designed to scan different wavelengths of light reflected from a sample's surface and record the intensity of that reflected light relative to a white standard (e.g., barium sulfate). The Perkin-Elmer Lambda 6 spectrophotometer we use is equipped with a diffuse reflectance attachment, a reflectance sphere, to allow total reflectance measurements to be made. The Lambda 6 contains two light sources (a tungsten lamp for the VIS and NIR, and a deuterium lamp for the NUV), a moving grating (to separate light into different wavelengths), and a photomultiplier tube (to measure the intensity of light reflected from the sample surface). Data from the spectrophotometer are recorded directly on a floppy disk at 1-nm intervals from 250-850 nm, the analytical range of the Lambda 6 in the reflectance mode. Samples were analyzed using a slit width of 2 nm and scanned at 600 nm/min. The actual time per analysis, including changing samples, is 2-3 min.
The Minolta CM-2002 spectrophotometer is a portable, hand-held, instrument capable of analyzing a sample in 3 s. The spectrophotometer uses a Xenon arc light source that is diffused inside an integrating sphere to provide even illumination of the sample's surface; the measurement area is 8 mm in diameter, although the area illuminated is 11 mm. The CM-2002 uses a double-beam feedback system with two photodiode arrays. Light reflected from the sample's surface enters an optical fiber cable and is transmitted to one array. At the same time, light inside the integrating sphere is transmitted to a second spectral sensor array by an optical fiber cable. By determining both the intensity of the source illuminant and the light reflected from a sample, variations in spectral characteristics and intensity of illumination can be ameliorated. Before the light is measured, it is separated by filters at a 10-nm wavelength pitch. The silicon photodiode spectral sensors in the array then measure the intensity of the light at these 10-nm intervals from 400-700 nm by converting the received light into electrical currents that are proportional to light intensity. The CM-2002 also allows the specular component to be included (SCI setting on the instrument) or excluded (SCE setting). Including the specular component essentially takes into account glare and provides a better estimate of color as seen by the human eye. However, glare does not contribute to the spectrum and, therefore, including the specular component (SCI setting) makes comparison to data from a laboratory spectrophotometer difficult because most of these instruments read only diffuse reflectance. Thus, all our measurements were taken with the SCE setting to produce data consistent with previous spectral studies (Balsam and Deaton, 1991; Balsam et al., 1995). Unlike the Lambda 6, in which the sample must be held vertically, the Minolta CM-2002 can be used in any orientation.
As with previous studies using a Perkin-Elmer Lambda 6 spectrophotometer (Balsam and Deaton, 1991; Deaton and Balsam, 1991), a white BaSO4 plate was used to set the 100% reflectance level. On board the JOIDES Resolution, the Minolta instrument was calibrated with a white ceramic tile cap attachment, the normal method for calibrating this instrument. The cap itself does not set the 100% level; rather, the cap is a ceramic transfer standard that is physically calibrated against a primary BaSO4 standard. The absolute reflectance of this primary BaSO4 standard, as determined at the National Physical Laboratory in the United Kingdom, is used for reference. This primary standard is a pressed BaSO4 plate formed according to ISO 7724/2. The absolute reflectance values relative to the primary BaSO4 standard at 31 wavelengths (400-700 nm at 10-nm intervals) for the ceramic tile cap are stored on the memory card of the Minolta. When a white calibration is performed, the instrument sets the reflectance at each wavelength to its stored value relative to the ISO standard (Balsam et al., 1997). The Minolta CM-2002 used on board the JOIDES Resolution was calibrated using this white tile transfer standard.
For the Perkin-Elmer Lambda 6 spectrophotometer, where the sample has to be held vertically, sample preparation followed the procedures described by Balsam and Deaton (1991). That is, samples were ground to <38µ, made into a thick slurry on a glass microslide with distilled water, and slowly dried either in air or on the warm setting of a hot plate. Shipboard data gathered with the Minolta CM-2002 spectrophotometer were taken directly from a split core surface. Because core surfaces were wet, they were covered with Glad Cling WrapTM, to protect the instrument. No attempt was made to control the degree of water saturation before the shipboard samples were analyzed. Actual water saturation is a function of the original saturation of the sample and the length of time the sample was exposed to the atmosphere before being analyzed.