10. Comparison of Shipboard vs. Shore-based Spectral Data from Amazon Fan Cores: Implications for Interpreting Sediment Composition

Reflectance spectrophotometry potentially provides a rapid method of investigating the changing characteristics and composition of marine sediments. Recognizing this, the Ocean Drilling Program has permanently deployed a commercially available, handheld spectrophotometer, the Minolta CM-2002, aboard the JOIDES Resolution. The present study evaluated shipboard spectral data obtained from Amazon Fan cores during Leg 155 using the Minolta instrument. These data were compared to spectra measured from comparable Leg 155 core samples using a shore-based, research-grade, Perkin-Elmer Lambda 6 spectrophotometer. The spectral signal is muted in the percent-reflectance curves from the wet sediments analyzed aboard ship when compared to percent-reflectance curves generated from dry core samples on shore. However, when both sets of reflectance curves are processed using a first-derivative transformation, shipboard and shore-based analyses are quite similar and suggest that useful, accurate spectral data can be obtained from wet core sediments at sea. This observation is further supported by factor analysis of parallel (shipboard vs. shore-based) data sets (400–700 nm) produced by the two instruments. The same four factors are present in both data sets, but do not necessarily explain a similar amount of variance. These factors are related to hematite plus goethite, clay minerals, organic matter, and carbonate content. The shore-based spectral data set was subjected to further factor analysis to determine if additional compositional information could be extracted using the increased accuracy and extended wavelength range (250–850 nm) of the laboratory instrument. This analysis produced one additional factor, probably chlorite; separated hematite and goethite into two factors; and clarified the nature of the clay mineral factor as probably a combination of illite and montmorillonite. By calculating factor scores, semiquantitative estimates of concentrations of variations of these sediment components were calculated downhole for several Leg 155 sites. The Amazon Fan sediments represent 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. Nevertheless, it was possible to extract compositional information from the spectra. Our investigations also lead us to make the following important recommendations for using the Minolta CM-2002 spectrophotometer at sea. First, if wet core surfaces must be covered with plastic film during spectral measurements to protect the instrument, only Glad Cling Wrap (a brand of clear plastic food wrap) should be used because this brand does not significantly distort the spectra of the sediments. Second, when calibrating the Minolta instrument, the white calibration cap should not be covered by the clear plastic wrap used to cover the cores. Third, the Minolta instrument should be set to exclude the specular component (SCE setting) for all measurements, and the optional granular-materials cover for the instrument (CM-A40) should not be used. These recommendations will ensure that the spectral data can be compared directly to data generated by laboratory-grade spectrophotometers. INTRODUCTION AND PREVIOUS WORK 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 (VIS, 400–700 nm). Sediment color is usually determined 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 VIS spectra because similar colors may result from the mixing of different spectral wavelengths. Many of the problems related to 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 1Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program). 2Department of Geology, P.O. Box 19049, University of Texas at Arlington, Arlington, TX 76019, U.S.A. [email protected] and [email protected] 3Fachbereich Geowissenschaften, Universität Bremen, Postfach 330440, D-28334 Bremen, Federal Republic of Germany. Previous Chapter Table of C 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 date back to the mid-1960s (Chester and Elderfield, 1966, 1968; Chester and Green, 1968); however, a concerted effort to exploit near ultraviolet, visible, and near infrared (NUV/ VIS/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), 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, 1996). Infrared reflectance spectra identified using a Fourier Transform Infrared Spectrophotometer (FTIR) were utilized to quantify the abundances of a number of minerals including calcite, quartz, and different clay minerals (Herbert et al., 1992).