Sulfur-isotope study on the mass extinction boundaries

系统的全岩硫化物硫同位素数据来自横跨日本赤部天津丸的天津丸和日本丹巴地体的佐山的二叠系/三叠系(P/Tr)界线上的硅质岩为主的连续表层沉积序列。P/Tr界线的特征是产出硅质页岩，伴生碳质黑色泥岩，在外观上与世界各地分布的白垩系/第三系(K/T)界线粘土岩相似。所观察到的数据清楚地表明了显著的双峰型。在整个中二叠世，相对于CDT的34S/32S比值普遍较低(约-39%至-25%)。从上二叠统下部开始，持续到下三叠统(约-20%~-2%)，出现了显著的上升，并出现了短暂而剧烈的负移，下降到与希德尔二叠世大致相同的同位素水平，只是在可疑的P/Tr界线上(约-41%至-23%)。有趣的是，跨越P/Tr边界的同位素漂移模式与最近在日本北加岛卡瓦鲁普发现的跨越K/T边界的模式形成了鲜明的对比。高-34S/32S组的表观分馏程度为-25SY+-10%，与目前确认的细菌异化硫酸盐还原的动力学同位素效应范围相当；低-34S/32S组的分馏程度大多超过这一范围，其值一般在-45SY+-10%的范围内，这与目前在海洋沉积物中普遍观察到的值相似。目前的数据将为大体上停滞、缺氧的层状海洋的发育提供强有力的证据，该海洋可能在晚二叠世早期开始形成，并持续到早三叠世，并为其在可疑的P/Tr界线上爆发暂时的大规模混合提供了强有力的证据。这样一位海洋缺氧历史学家

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