Marine geologists have used color, which is the human eye's perception of reflected radiation in the visible region of the electromagnetic spectrum (400-700 nm), to describe marine sediment cores for many years. Sediment color is usually determined visually by comparison to a color chart like the Geological Society of America Rock Color Chart (Goddard et al., 1948), which is a derivative of the Munsell Color Chart. Such color-chart analysis is qualitative and inexact because no two observers have the same color perception. Color also tends to obscure differences in visible spectra because similar colors may result from the mixing of different spectral wavelengths, a condition termed metamerism. Many of the problems related to qualitative color analysis can be overcome by using diffuse-reflectance spectrophotometry, a technique in which light reflected from a sample is collected in a reflectance sphere and compared to light reflected from a pure white standard throughout the wavelength range being analyzed. The quantitative data produced by this technique are reflectance values as a function of wavelength relative to the standard.
Attempts to use reflectance spectra without respect to color to interpret marine cores dates back to the mid-1960s (e.g., Chester and Elderfield, 1966, 1968; Chester and Green, 1968); however, a concerted effort to exploit near-ultraviolet (NUV), visible (VIS), and near-infrared (NIR) spectral reflectance as a marine geological research tool has only recently been undertaken. Deaton (1987) quantified Munsell color-chart chips with a reflectance spectrophotometer in an attempt to help geologists relate color to spectra. Although such analysis of color chips makes the determination of color more precise, it does not alleviate the problems associated with the scientific use of color. Studies by Barranco et al. (1989), Deaton and Balsam (1991), Balsam and Deaton (1991, 1996), Balsam et al. (1998), Herbert et al. (1992), and Mix et al. (1992) have shown that many marine sediment components have distinctive spectral signatures. VIS reflectance spectra have been used to identify the iron oxide and oxyhydroxide minerals hematite and goethite; the clay minerals illite, montmorillonite, and chlorite; calcite; and sediment organic content (Deaton and Balsam, 1991; Balsam and Deaton, 1991; Balsam et al., 1998). Balsam and Deaton (1996) obtained quantitative estimates of carbonate, opal, and organic content by applying regression techniques to NUV/VIS/NIR spectra.
Herbert et al. (1992) used infrared reflectance spectra from a Fourier transform infrared spectrophotometer (FTIR) to quantify the abundances of a number of minerals including calcite, quartz, and various clay minerals. Both the reflectance spectrophotometer used by Balsam and Deaton and the FTIR used by Herbert et al. analyze only one sample at a time, and each sample has to be changed manually. Both machines take ~60 s to analyze a sample; the FTIR actually performs a single analysis in 5 s but uses numerous stacked analyses to remove noise. Mix et al. (1992, 1995) developed a VIS/NIR scanning spectrophotometer that can be deployed at sea. This instrument has been utilized to scan cores aboard the JOIDES Resolution and is capable of analyzing a small area of a core's surface (about a 2-cm circle) in 5 s, then automatically advancing down the core and making subsequent measurements. More recently, compact handheld spectrophotometers, such as the Minolta CM-2002, have become available and are being routinely used to measure color spectra of sediments in the laboratory and at sea without having to take and prepare sediment samples. A Minolta CM-2002 instrument has been routinely used during the past 10 yr during core description and processing during Ocean Drilling Program (ODP) legs aboard the JOIDES Resolution, and the data are commonly used to correlate from core to core, to determine glacial-interglacial climatic cycles, and to determine mineral compositions downcore (e.g., Mix et al., 1992, 1995; Schneider et al., 1995; Balsam et al., 1997, 1998, 1999; Balsam and Damuth 2000; Giosan et al., 2001).
In a series of previous studies based on cores from ODP legs and existing piston cores, we (W. Balsam and J. Damuth) have compared the results of spectral data collected at sea on board the JOIDES Resolution with the handheld Minolta CM-2002 spectrophotometer and our shore-based, research-grade spectrophotometer (PerkinElmer Lambda 6) (Balsam et al., 1997, 1998, 1999; Balsam and Damuth, 2000). Comparison of spectra from wet cores measured aboard ship with the Minolta instrument to spectra measured from comparable core samples using a shore-based PerkinElmer Lambda 6 spectrophotometer showed that although the spectral signal is muted in the percent reflectance curves from the wet shipboard sediments compared to curves generated from dry core samples onshore, both sets of reflectance curves are quite similar when processed using a first-derivative transformation. This observation is further supported by factor analysis of parallel (shipboard vs. shore based) data sets (400-700 nm) produced by the two instruments (Balsam et al., 1997; Balsam and Damuth, 2000).
For the present report we measured and analyzed spectral data from core samples recovered at ODP Sites 1165 and 1167 using our laboratory-grade PerkinElmer Lambda 6 spectrophotometer, referred to above. Site 1165 is from the Wild Drift, a large contourite drift deposit on the continental rise seaward of Prydz Bay, and Site 1167 is from the Prydz Channel Trough Mouth Fan on the continental slope seaward of Prydz Channel (O'Brien, Cooper, Richter, et al., 2001). We did not analyze any cores from Site 1166 on the continental shelf because of the sparse core recovery and the lithified and disturbed nature of much of the recovered sediments.
At Site 1165, Wild Drift, we determined NUV/VIS/NIR spectra for closely spaced (~10 cm) samples for the interval from Sections 188-1165B-1H-1, 0 cm, through 6H-7, 38 cm (0-54.17 meters below seafloor [mbsf]), in Hole 1165B, which is the Pliocene-Pleistocene age interval being studied in detail by the High-Resolution Integrated Stratigraphy Committee (HiRISC) and described in this volume (e.g., Warnke et al., this volume). We also determined calcium carbonate content for all samples in this HiRISC interval. Sample spacing for NUV/VIS/NIR spectral studies below 54 mbsf for holes 1165B and 1165C, as well as throughout Hole 1167A from the Prydz Channel Trough Mouth Fan, was on the order of one sample per core section (i.e., 1-2 m spacing). We did not routinely determine calcium carbonate downhole below 54 mbsf at Site 1165 or for any interval at Site 1167 because spot checks suggested that the carbonate content was at or near zero throughout these sections. Table T1 lists all samples used for this study. The carbonate and spectrophotometer analyses were conducted at the University of Texas at Arlington (UTA).