10. COMPARISON OF SHIPBOARD VS. SHORE-BASED SPECTRAL DATA FROM AMAZON FAN CORES: IMPLICATIONS FOR INTERPRETING SEDIMENT COMPOSITION1

William L. Balsam,2 John E. Damuth,2 and Ralph R. Schneider3

ABSTRACT

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.

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. balsam@uta.edu and jedamuth@utarlg.uta.edu
3Fachbereich Geowissenschaften, Universitšt Bremen, Postfach 330440, D-28334 Bremen, Federal Republic of Germany.