Ishiga-diagram as a geologic time scale

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Ishiga-diagram displays sedimentary cycles of bedded cherts and is useful for evaluation of change of primary production and oceanic condition. Examination of this diagram has been carried out in the Carboniferous to Jurassic bedded cherts sequence, where detailed radiolarian and conodonts biostratigraphic age was examined previously. The sedimentary cycles consists of first order cycle of ca. 1 Ma, and this is a combination of 5 second order cycles. The second order cycle is ca. 200 kys, and form a peak of productivity.Mode of change in productivity varies in ages, and significantly rhythmic cycles appeared in Upper Triassic to Lower Jurassic. The decrease in productivity shown in the Ishiga-diagram well coincides to the geologic boundary. Thus primary production can be linked to depression or distinction of the other macro-fauna.This research was expanded to the geochemical examination of the geologic boundary, such as Permian/Triassic and geologic event e.g. Toarcian anoxic event. This is because these events are marked by appearance of organic black shales, which show depression of radiolarian productivity and paleooceanic environmental change. Although the productivity of bedded cherts in the Uppermost Permian gradually became relatively higher than the Middle and Lower Permian, cherts color change into black and includes significantly high contents of organic carbon. The Middle and Upper Permian boundary was also geochemically examined and that was characterized by anoxic basin formation in marginal basin. This might be caused by eutrophication by supposed enrichment of river-born nutrient supply by active volcanism of the continental margin.Ishiga-diagram is useful for evaluation of the oceanic condition change, and geochemistry of the sediments both pelagic and terrigenous may be fundamental for systematic examination of the earth environment.