| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1996: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 1995: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1994: ¥800,000 (Direct Cost: ¥800,000)
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| Research Abstract |
Very recently, a sound sulfur isotopic oriterion for recognizing the ocean oxic-anoxic history has been proposed by the present investigator. It is based on information on microbial sulfur geochemistry obtained from some classical and latest examinations in natural and laboratory systems. The basis is the remarkable sulfur isotopic bimodalism in sedimentary and/or diagenetic sulfides recognized in the worldwide stratigraphic record. This bimodalism can most reasonably be accounted for by a drastic chage of environmental redox condition associated with discrimination of microbial habitat. In an aerobe-free anoxic basin where the sulfate reduction exclusively occurs because of the absence of any effective oxidants, the extent of 34S/32S fractionation of sulfide, with respect to contemporaneous seawater sulfate, shold be restricted within the currently confirmed range of kinetic isotope effect during dissimilatory sulfate reduction by anaerobic sulfur-reducing bacteria, that is, in the ra
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nge of -25 <double plus> 10 *. In an ordinary oxic basin where aerobes are relatively predominant, however, the extent of fractionation is expected to more or less exceed this range to give the values typically in the range of -50 <double plus> 10 * as generally observed in the modern oceanic sediments, since the initial sulfide once produced is reoxidized by aerobic sulfur-oxidizing bacteria or by other oxidants such as dissolved free oxygen and then incorporated in a sebsequent redox cycle during sedimentary and burial diagenetic processes. Thus, the isotopic composition of sedimentary and/or diagenetic sulfide provides a sound geochemical criterion for recognizing and reconstructing the oceanic oxic-anoxic hi
Applying this principle, an attempt has been made to evaluate paleoceanic redox settings of some important geologic events in Phanerozoic time. The results reveal that the mass extinctions and the stratabound type mineralizations in the world are almost always associated intimately with intense oceanic anoxia. This would provide a new additional constraint on modeling the earth history. Less
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