Marine geologists have used color to describe marine sediment cores for many years. Color is the human eye's perception of reflected radiation in the visible region of the electromagnetic spectrum (400-700 nm). 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. However, such color-chart analysis is 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 data produced by this technique are reflectance as a function of wavelength relative to a 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 (1991, 1996) and the FTIR used by Herbert et al. (1992) analyze only one sample at a time and each sample then has to be changed manually. Both machines take about 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 on board 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, hand-held spectrophotometers, such as the Minolta CM-2002, which was used in the present study (see below) have become available. During the past 5 years, a Minolta CM-2002 hand-held spectrophotometer has also been routinely used to measure color spectra of sediments during routine core description and processing on several Ocean Drilling Program (ODP) legs (e.g. Schneider et al., 1995), including Leg 164.
Leg 155 was the first ODP leg where sediment cores were analyzed by both a hand-held spectrophotometer (Minolta CM-2002) on board the JOIDES Resolution and a shore-based, research-grade spectrophotometer (Perkin-Elmer Lambda 6) at the University of Texas at Arlington (Balsam et al., 1997). Comparable databases were assembled using both of these shipboard and shore-based instruments to address the following objectives: (1) determine if measurements obtained with both the shore-based and shipboard instruments are comparable, and, if not, how and why do they differ; (2) improve the techniques used to gather data on board ship with the Minolta spectrophotometer to provide data comparable to that of the shore-based spectrophotometer; and (3) determine information about the composition and mineralogy of the terrigenous Amazon Fan sediment using spectral analysis. Comparison of spectra from wet cores measured on board ship during Leg 155 with the Minolta instrument to spectra measured from comparable Leg 155 core samples using a shore-based Perkin-Elmer 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). The Amazon-Fan sediments analyzed represented a rigorous test of the spectral technique because most of the sediments are dark and show little variation in the visible region of the electromagnetic spectrum (Schneider et al., 1995). However, Balsam et al. (1997) demonstrated that it is possible to extract compositional information from these spectra. This study also led us to make important recommendations for using the Minolta CM-2002 spectrophotometer at sea, including the use of only one specific brand of clear polyethylene plastic food wrap, Glad Cling WrapTM, to cover wet cores during measurement so as not to distort the spectra of the sediments, and setting the Minolta instrument to exclude the specular component (SCE setting) for all measurements (Balsam et al., 1997).
Participation during ODP Leg 164 to the Blake Ridge on the continental margin off the Carolinas, U.S.A. (Fig. 1) allowed us to collect additional shipboard spectral data with the Minolta 2002 and to continue our investigations of the usefulness of shipboard spectral analysis for determining sediment composition, mineralogy, and stratigraphy. The basic objectives of our investigations of Leg 164 sediments are (1) to establish a lithostratigraphic framework for the interpretation of chemical and mineralogical changes in Blake Ridge sediments using downhole changes in carbonate content, supplemented by mineralogic markers such as hematite, which reflects transport of sediment (e.g., brick-red lutite) from the Cabot Straits southward to the Blake Ridge by the WBUC (Barranco et al., 1989); and (2) continue to compare shipboard vs. shore-based spectrophotometer measurements and techniques begun during Leg 155 studies (Balsam et al., 1997) to document the similarities and differences of the data extracted by each method to provide guidelines for optimizing spectral readings taken on board the JOIDES Resolution and other ships so that maximum information is obtained. In addition, shipboard analyses are restricted to measuring wet cores; however, the effects of various states of wetness on spectra have not been investigated. Therefore, we investigated the effects of wetness on spectral readings taken on samples from Leg 164 sediments, as well as other piston cores, in controlled wetness experiments (Balsam et al., 1998). We are also presently conducting a study (Balsam et al., unpubl. data) to evaluate the increasingly popular technique of using sediment lightness measured down cores as a proxy for carbonate content (e.g. Bond et al., 1992; Hagelberg et al., 1994; Cortijo et al., 1995) by comparing actual carbonate values from selected Leg 164 cores and other piston cores with gray-scale values measured from photographs and spectra values measured with the both the Minolta and laboratory spectrophotometers.