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早三叠世作为二叠纪末集群灭绝后地球系统的生态恢复时期,与古海洋生态变化有关的各种地球化学记录为地质学家长期高度关注.在岩石学研究和样品对海水代表性评估的基础上,测试了四川盆地东部广安龙门峡南剖面下三叠统(包括相邻地层)350个碳酸盐岩的碳氧同位素组成.研究表明:龙门峡南剖面下三叠统碳酸盐岩对海水碳同位素组成具有良好的保存性,碳氧同位素组成之间缺乏相关性,二者间的确定系数只有0.0205,350个样品中只有44个Mn/Sr>2和/或δ~(18)O<-6.5‰;根据306个Mn/Sr<2和/或δ~(18)O>-6.5‰的样品建立了完整的下三叠统碳同位素曲线,曲线具有非常好的全球可对比性,二叠,三叠界线附近、嘉一段顶部和三段内部的δ~(13)C极小值,夜郎组一段顶部、四段顶部和嘉陵江组二段底部、四段下部的δ~(13)C极大值,以及代表这些极大值的峰和代表这些极小值的谷所反映的都是全球信号;与全球其他地方年代地层框架内同期碳同位素曲线的对比结果表明,剖面夜郎组一段可大致与Griesbachian亚阶对应,夜郎组二,四段可大致与Dienerian亚阶对应,嘉陵江组一段大致与Smithian亚阶对应,嘉陵江组二-三段以及四段下部大致与Spathian亚阶对应,嘉陵江组四段的中上部则应归于中三叠世的Aegean亚阶;代表Smithian-Spathian亚阶界线(SSB)的嘉一、嘉二段界线附近δ~(13)C从-0.911‰急剧上升至3.679‰,海水碳同位素如此剧烈的变化耗时可能小于36kyr,对沉积环境反映更为敏感的氧同位素在SSB附近先于碳同位素变化,反映海水在碳同位素急剧升高之前,盐度已显著增加,并导致了蒸发岩和白云岩的形成,该问题的更深入研究还需要考虑白云石-水和方解石-水系统、白云石-CO_2和方解石-CO_2系统白云石和方解石形成过程中同位素分馏效应的差别.
In the period of ecological restoration of the earth system after the end of Permian extinction in the Early Triassic, various geochemical records related to paleoceanic ecological changes have long been of great concern to geologists. In the study of petrology and the assessment of representativeness of seawater samples , The carbon and oxygen isotopic compositions of 350 carbonate rocks in the Lower Triassic (including the adjacent strata) of the Longmenxia southern section of Guang’an in the eastern Sichuan Basin were tested. The results show that the Lower Triassic carbonate rocks The seawater carbon isotope composition is well preserved and the correlation between carbon and oxygen isotope composition is lacking. The determination coefficient between the two is only 0.0205 and only 44 of Mn / Sr> 2 and / or δ ~ (18) O <-6.5 ‰. A complete Lower Triassic carbon isotope curve was established based on 306 samples with Mn / Sr <2 and / or δ ~ (18) O> -6.5 ‰. The curves have very good global comparability, The δ ~ (13) C minima inside the top and third sections of the Jiayi section, the top of the first section of the Yelang Formation, the top of the fourth section and the bottom of the second section of the Jialingjiang Formation, ) C max, as well as the peaks representing these maxima and the valleys representing these minima The results show that the first section of the Yelang Formation can be roughly corresponding to the Griesbachian suborder, and the second and fourth sections of the Yelang Formation can be roughly corresponding to the Dienerian suborder, The first section of the Jialingjiang Formation corresponds roughly to the Smithian suborder, the second and third sections of the Jialingjiang Formation and the lower part of the fourth section correspond roughly to the Spathian suborder, and the middle and upper parts of the fourth member of the Jialingjiang Formation belong to the Aegean suborder of the Middle Triassic. On behalf of the Smithian-Spathian Δ 13 C near the boundary of Jianyi and Jia 2 of the sub-boundary line (SSB) increased sharply from -0.911 ‰ to 3.679 ‰, so the time-consuming change of seawater carbon isotopes may be less than 36 kyr, reflecting more the sedimentary environment Sensitive oxygen isotopes precede changes in carbon isotopes near the SSB, indicating that salinity has increased significantly before the seawater sharply increases in carbon isotopes and has led to the formation of evaporites and dolomites. A more in-depth study of this issue needs to be considered Differences in isotopic fractionation effects between dolomite and calcite formation in the stone-water and calcite-water systems, dolomite-CO 2 and calcite-CO 2 systems.