We examined microbialites deposited near the Permian–Triassic boundary (PTB) at the Xiejiacao section of South China, including the size and morphology of pyrite framboids, carbonate carbon isotopes (δC), carbonate associated sulfur isotope (δS), major and trace elements, and total organic carbon (TOC) concentrations. The microbialite unit is much thicker than the PTB beds in deeper water sections, providing more detailed geochemical records from that time. A prominent negative δC excursion was recognized in association with the second phase of the Permian–Triassic mass extinction (PTME), comparable with that of several other shallow platform facies sections worldwide. δS stratigraphic profiles also show a negative excursion, with minimum values occurring slightly later than those of δC. The minimum peak can be calibrated to the middle part of the conodont Zone, corresponding to the second phase of PTME. Over all, the carbon and sulfur isotopes demonstrate a coupled long-term relationship with large negative excursions during the Permian–Triassic transition followed by gradual recovery in the lowest Triassic, represented an extremely low concentration of sulfate in the ocean during this period. In contrast, the CS isotopes are strongly decoupled within the microbialite unit, which could be linked with the oxygen-poor conditions and microbial sulfate reduction (MSR: Sulfate reducing microorganisms facilitated the precipitation of sulfide and carbonate, such as SO + 2CHO → HS + 2HCO). The second phase of PTME was calibrated to the top of the microbialite unit, which was marked by the second δC-δS negative excursions. The distinct marine oxygen deficiency was indicated by the small pyrite framboids, relatively high values of U (>1) and Ce/Ce* (>0.8). Combined with the previous biostratigraphic work, we propose that continuous volcanic activity induced intensified marine anoxia, leading to biotic mortality in the second phase of the PTME. Overall, our isotopic and geochemical data, integrated with results of previous studies, provided a new explanation for the carbon and sulfur isotope variations and additional evidences into the concurrent environmental changes during the Permian–Triassic transition.

中文翻译：

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浅台地背景下二叠纪-三叠纪边界的碳硫同位素及主量和微量元素变化（华南谢家槽）

我们检查了华南谢家槽剖面二叠纪-三叠纪边界（PTB）附近沉积的微生物岩，包括黄铁矿草莓状体的尺寸和形态、碳酸盐碳同位素（δ13C）、碳酸盐伴生硫同位素（δS）、主量元素和微量元素、和总有机碳（TOC）浓度。微生物岩单元比深水部分的 PTB 层厚得多，提供了当时更详细的地球化学记录。显着的 δ13C 负偏移被认为与二叠纪-三叠纪大规模灭绝（PTME）的第二阶段有关，与世界范围内其他几个浅台地相部分的情况相当。 δS 地层剖面也显示出负偏移，其最小值出现的时间略晚于 δC 的最小值。最小峰值可以校准到牙形刺区的中部，对应于PTME的第二阶段。总而言之，碳和硫同位素表现出与二叠纪-三叠纪过渡期间的大负偏移存在耦合的长期关系，随后在最低三叠纪逐渐恢复，代表了这一时期海洋中硫酸盐浓度极低。相比之下，CS同位素在微生物岩单元内强烈解耦，这可能与缺氧条件和微生物硫酸盐还原有关（MSR：硫酸盐还原微生物促进了硫化物和碳酸盐的沉淀，例如SO + 2CHO → HS + 2HCO)。 PTME 的第二相被校准到微生物岩单元的顶部，其标志是第二次 δ13C-δS 负偏移。较小的黄铁矿片、相对较高的 U (>1) 和 Ce/Ce* (>0.8) 值表明了明显的海洋缺氧。结合之前的生物地层学工作，我们认为持续的火山活动导致海洋缺氧加剧，导致PTME第二阶段的生物死亡。总的来说，我们的同位素和地球化学数据与之前的研究结果相结合，为碳和硫同位素的变化提供了新的解释，并为二叠纪-三叠纪过渡期间同时发生的环境变化提供了额外的证据。