31. Further Investigations of Shipboard vs. Shore-based Spectral Data: Implications for Interpreting Leg 164 Sediment Composition

Diffuse reflectance spectra measured during shipboard core description with a hand-held Minolta spectrophotometer are reliable data for interpreting sediment composition. Factor analysis of these spectra indicates that only Glad Cling Wrap, a polyethylene food wrap, should be used to cover wet cores during measurement because it introduces substantially less noise into the spectra than other brands of plastic food wrap. This validates our previous recommendation that only Glad Cling Wrap should be used to cover wet core surfaces before spectral analysis on board ship. Factor analysis also indicates that the red end of the spectrum was a little noisy; however, it is unclear if this noise was produced by the Glad Cling Wrap, by the Minolta instrument, or by the sediment being analyzed when wet (water tends to affect the red end of the spectrum more than the violet). Comparison of spectra taken directly from wet cores on board ship with the Minolta CM-2002 to those taken from dried, prepared samples with our shore-based Perkin-Elmer Lambda 6 spectrophotometer indicates that water mutes the spectral signal. Although factor analysis of the Perkin-Elmer data produced five factors, four of those factors are the same as the factors extracted from the wet core data with the Minolta. This suggests that the water content in the wet cores was not high enough to seriously hinder interpretation of sediment composition. These four common factors are interpreted as carbonate, chlorite or glauconite, organic matter, and iron oxides (probably a mixture of hematite and goethite). The presence of hematite can be attributed to hematite-rich sediment derived from the Canadian Maritime provinces, which has been transported southward by the Western Boundary Undercurrent (WBUC) along North American continental margin. Based on the distribution of hematite down the Leg 164 holes, the WBUC apparently has washed over the Leg 164 sites on the crest of the Blake Ridge only during the Holocene and very latest Pleistocene.