Carbon isotope composition and comparison of Lower Triassic marine carbonate rocks from Southern Longmenxia section in Guang’an, Sichuan Basin

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• 作者：SiJing Huang ; KeKe Huang ; YiJiang Zhong ; XiaoNing Li…
• 关键词：Southern Longmenxia section ; Early Triassic ; Carbon isotopic composition ; Global events ; Smithian ; Spathian boundary event
• 刊名：Science China Earth Sciences
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：60
• 期：1
• 页码：80-94
• 全文大小：
• 刊物主题：Earth Sciences, general;
• 出版者：Science China Press
• ISSN：1869-1897
• 卷排序：60

文摘

The Early Triassic was a period of ecological restoration for the earth’s system after the end-Permian mass extinction. Geochemical records linked to the variations in marine ecosystems during this period have attracted the interest of geologists for many years. Based on petrographic analysis of samples and evaluations of their reliability as proxies for original seawater, this study investigated the carbon and oxygen isotopic compositions of 350 carbonate rock samples from the Lower Triassic (and adjacent strata) in the southern Longmenxia section of Guang’an, eastern Sichuan Basin. The results indicate that the Triassic carbonate rocks from the southern Longmenxia section favorably preserved the original seawater’s δ13C signal. Furthermore, carbon and oxygen isotopic compositions are found to be poorly correlated, with a determination coefficient as low as 0.0205 and only 44 rock samples show results of Mn/Sr>2 and/or δ18O<-6.5‰. The complete carbon isotopic curve for the Lower Triassic is established using the data from the remaining 306 samples with Mn/Sr<2 and/or δ18O>-6.5‰. This curve presents favorable comparability on a global scale, specifically in the δ13C minima near the Permian-Triassic boundary, at the top of the Jia1 and within the the Jia3, as well as in the δ13C maxima at the tops of the Ye1 and Ye4, at the base of the Jia2 and at the top of the Ye3. The peaks and troughs corresponding to these maxima and minima all reflect global signals. By comparing these results to previous research on coeval carbon isotopic curves established within the chronostratigraphic framework, the ages of these critical evolution points are determined. The results show that the Ye1 roughly corresponds to the Griesbachian substage; the Ye2 through Ye4 correspond to the Dienerian substage; Jia1 corresponds to the Smithian substage; from the Jia2 to the lower part of the Jia4 correspond to the Spathian substage; and the middle and upper parts of the Jia4 belongs to the Aegean Substage of the Middle Triassic. Around the boundary between the Jia1 and Jia2 (which represents the Smithian-Spathian boundary (SSB)), the value of δ13C increases rapidly from -0.911‰ to 3.679‰. The span during which the seawater’s carbon isotope experiences this drastic change may be less than 36 kyr. The oxygen isotope, which is more sensitive to sedimentary environmental changes, exhibits changes prior to the carbon isotope near the SSB, indicating a significant increase in the salinity of the seawater before a sharp rise in the carbon isotope; this event leads to the formation of evaporites and dolomites.