Development of a largely anoxic stratified ocean and its temporary massive mixing at the Permian/Triassic boundary supported by the sulfur isotopic record

Systematic sulfur isotope data for whole-rock sulfides have been obtained from the chert-dominated, continuous, pelagic 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 observed data clearly demonstrate a significant bimodalism. The ³⁴S/³²S ratios with respect to CDT are generally low throughout the Middle Permian (ca.-39 to -25%0) and show a remarkable increase beginning in the lower Upper Permian and persisting into the Lower Triassic (ca.-20 to -2%0), with a temporary and drastic negative shift, down to roughly the same isotopic level as in the Middle Permian, just at the suspected P/Tr boundary (ca.-41 to -23%0). 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 described at Kawaruppu in Hokkado, Japan. The apparent extent of fractionation, with respect to contemporaneous seawater sulfate, in the high ³⁴S/³²S group lies within the range of -25 ± 10%0, 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 ± 10%0, which is similar to what is generally observed 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 lower Upper Permian and persisted into the Lower Triassic, and for a brief episode of its temporary massive mixing just at the suspected P/Tr boundary. Such an oceanic oxic-anoxic history may account to some extent for the relatively high enrichments of chalcophile elements in basal Triassic sediments in the world and add a significant constraint to the current arguments on the cause and consequence of the terminal Permian mass extinction.