Mineralogical and geochemical study of piston cores collected from the Shichitou-Iwojima ridge area

Two piston cores collected from the Shichitou-Iwojima ridge area have been studied from mineralogical and geochemical points of view by means of X-ray diffraction (XRD), analytical transmission electron microscopy (ATEM), and X-ray fluorescence (XRF). XRD analysis shows that illite and chlorite are the major clay minerals, whereas smectite and kaolinite are small amount of the clay mineral assemblage except some tuff layers. The vertical homogeneity in clay mineral composition and the morphology and chemistry of clay minerals suggest that clay minerals in the present cores are detrital origin. However, the saponite contained in core St. 16 tends to be more abundant than that in core St. 19. Most of the saponite contained in core St. 16 was transported from volcanic province on land but some may have been transported from the neighbouring ridge and formed in situ-submarine. In this case the saponite was transported by the currents or water masses except the Kuroshio ocean current. The composition of clay-sized minerals is 64% clay minerals versus 36% non-clay minerals in core St. 16 and 63% clay minerals versus 37% non-clay minerals in core St. 19, respectively. Geochemical parameters showed that the influences of clastic minerals and the biological carbonate calcium were strong in the lower part (deeper than 130cm from the top of the core, >25Ka), particularly in the lowest part of core St. 16, whereas the influence of clastic minerals tended to decrease toward the upper part and the biological carbonate calcium showed an opposite trend. Geochemical parameters of core St. 19 exhibited a positive relationship between the intensity of the organic material productivity and clastic minerals around 14 Ka and the intensity tended to decrease toward the younger age. The authors pointed out that the variability of wind activity and biological productivity during the past 25, 000 years was attributed to climatic change.