| Research Abstract |
Systematic sulfur isotope data for whole-rock sulfides have been obtained from the chert-dominated, continuous, pelaqic sedimentary sequences spanning the Permian/Triassic (P/Tr) boundary at Tenjinmaru in the chichibu Terrane and at Sasayama in the Tanba Terrane in Japan. The P/Tr boundary is characterized by the occurrence of siliceous shales in association with a carbonaceous black mudstone which is similar in appearance to the worldwide distributed Cretaceous/Tertiary (K/T) boundary claystone. The obsarved data ciearly demonstrate a significant bimodalism. The 34S/32S ratios with respect to CDT are generally low throughout the Hiddle Permian (ca.-39 to -25 %.) and show a remarkable increase beginning in the lower Upper Permian and persisting into the Lower Triassic (ca.-20 to -2 %.), with a temporary and drastic negative shift, down to roughly the same isotopic level as in the Hiddle Permian, just at the suspected P/Tr boundary (ca.-41 to -23 %.). Interestingly, the mode of isotopic excursion across the P/Tr boundary is in striking contrast to that across the K/T boundary which was recently disclosed at Kawaruppu in Hokkado, Japan. The apparent extent of fractionation, with rospect to contemporaneous seawater sulfate, in the high-34S/32S grouplies within the range of -25(〕SY.+-.〔)10 %, which is quantitatively equivalent to the currently confirmed range of kinetic isotope effect during bacterial dissimilatory sulfate reduction, and that in the low-34S/32S group mostly exceeds this range, giving the values typically in the range of -45(〕SY.+-.〔)10 %, which is similar to what is generally observced in the present-day oceanic sediments. The present data would provide strong evidence for the development of a largely stagnant, anoxic stratified ocean, which presumably began to form in the early Late Permian and persisted into the Early Triassic, and for the outbreak of its temporary massive mixing just at the suspected P/Tr boundary. Such an oceanic oxic-anoxic histor
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