The analytical results are given in Table 2. Principal component analysis has been applied to the results to characterize the geochemistry of major elements. The correlation coefficients between different chemical compositions have been calculated and listed in Table 3, and the chemical compositions were categorized to five groups (Fig. 1) by cluster analysis (refer to Davis, 1973 for the method): (1) Al2O3, TiO2, SiO2, K2O, and Fe2O3; (2) MnO, H2O(-); (3) CaO, LOI; (4) MgO, Na2O; (5) P2O5.
As shown in Figure 2, the ratio of Al/Ti in the studied sediments ranges from 16 to 20, which are the typical values for average shale and upper crust (Nakai et al., 1993; Weber et al., 1996), so the scavenged Al doesn't play a prominent role in influencing the Al amount of sediment as described by Murray and Leinen (1996). Thus, there is no doubt that Al2O3 and TiO2 are typically in terrigenous detritus. As SiO2, K2O, and Fe2O3 correlate well with Al2O3 and TiO2, the implication is that these compositions were mainly of terrigenous origin. Al2O3, SiO2, and K2O exist in the form of silicate and/or aluminosilicate, whereas TiO2 and Fe2O3 are in the form of oxides. Diatoms and spicules have been commonly observed microscopically in the studied sediments; therefore, SiO2 is also partly biogenic origin. Mn is in the same group with H2O(-), indicating its hydrogenous origin (Matsumoto et al., 1985), though it is not derived only from ferromanganese oxyhydroxides. The close correlation between CaO and LOI indicates that most CaO is hosted by carbonates of either biogenic or authigenic origin. Though the P2O5 contents in sediments are not so high (generally less than 0.3%), part of the CaO is bounded by phosphate. It should be noted that part of LOI is from H2O(+) and the oxidation of sulfide, although the amount produced is relatively small compared with carbonates, according to XRD analytical results on mineral compositions (Matsumoto, Lu, and Watanabe, unpubl. data). The existing status is somewhat complicated to explain for the group of MgO and Na2O, because the correlation coefficient between them is not high, though they are in the same group. Probably they coexist in the authigenic clay minerals as montmorillonite, with Na2O also found in plagioclase, and chlorite and MgO found in carbonates (dolomite and/or high-Mg calcite), which have been identified in the studied sediments by XRD analyses. P2O5 is in the phosphate minerals.
As shown in Table 2, the contents of hydrogenous MnO (generally <0.15%), and P2O5 (mainly <0.3%) are very low, and only a small part of Na2O and MgO are in authigenic clay minerals, according to the XRD analytical results on mineral compositions (Matsumoto, Lu, and Watanabe, unpubl. data); whereas the terrigenous components (group [1]) and carbonates (group [3]) compose more than 99% of the studied sediments. Thus, the studied sediments from Site 997 are mainly composed of carbonates and terrigenous materials.