The abundances of trace elements in sedimentary rocks have been employed to elucidate the sources and formation environment of the sediments. For example, it is suggested that a cerium anomaly found in the rare earth element (REE) patterns of chert reflects its depositional environment, that is, whether it was deposited in the marginal sea, the continental shelf, or the deep-ocean floor (Shimizu and Masuda, 1977). Murray (1994) developed a similar method in chert and employed a diagram on which he plotted the ratio of La and Ce against the ratio Al2O3/(Al2O3+Fe2O3). The MnO/TiO2 ratio can be another indicator of the depositional environment (Sugisaki et al., 1982; Yamamoto, 1983; Sugisaki, 1984; Sugitani, 1996). To use these abundance data (Al, Ti, Mn, Fe, La, and Ce) as geochemical signatures, it is necessary to carefully study various factors, such as diageneis, that can alter the initial signature. In particular, redox-sensitive elements can be mobilized during early diagenesis, altering the initial geochemical signatures. However, there have been few studies on the direct characterization of the oxidation states of elements in sediments using proper physico-chemical methods (except for iron, which has been studied by Mössbauer spectroscopy).
Sediments recovered from ODP Leg 191 Site 1179 (41.4°N, 159.6°E; water depth = 5565 m) located on the abyssal seafloor northwest of Shatsky Rise, ~1600 km east of Japan (Fig. F1), are mainly siliceous ooze and clay overlaying cherty rocks (Kanazawa, Sager, Escutia, et al., 2001). We studied the oxidation states of Mn and Ce by X-ray absorption near-edge structure (XANES) on samples throughout the sediment core. It is considered that these elements are immobile in their oxidized forms (Mn[IV] and Ce[IV]), whereas they may be more mobile in their reduced forms (Mn[II] and Ce[III]). Therefore, we compared the depth profiles of abundances and oxidation states of these elements in the sediment core at Site 1179. The direct determination of the oxidation states of these elements was used to explain the variation of abundances of Mn and Ce in the sediments. Other discrimination diagrams based on major element abundances are employed to compare the information obtained from the MnO/TiO2 ratio and REE signature.