抄録
Geochemical indicators for origins and sedimentary environments of marine sediments and siliceous sedimentary rocks are reviewed. Al2O3/TiO2 and Zr/TiO2 values are useful indicators for sources of detrital materials in sediments, because these values in igneous rocks change systematically (acidic > intermediate > mafic). MnO/TiO2 and Co/TiO2 values and Co-Ni-Zn diagram, on the other hand, are useful for discrimination between pelagic sediments and hemipelagic ones. Pelagic sediments deposited under oxic environment are characterized by high MnO/TiO2 and Co/TiO2 values and enrichment of Co and Ni relative to Zn, compared with hemipelagic sedmiments. In addition to these indicators, stratigraphic variation of MnO and CO2 (derived from carbonate minerals) concentrations can discriminate between pelagic sediments and hemipelagic ones. In hemipelagic sediments where diagenetic remobilization of manganese is active, MnO and CO2 concentrations often show covariation and manganese carbonate bands (rhodochrosite-concentrated layers) are found at depths. It should be also noted that these geochemical indicators are, in some cases, modified by diagenesis and subaerial alteration. Careful application of these indicators to analyses of sedimentary paleoenvironments is required.
