Carbon isotope geochemistry and its geological significance in the Jixian system of the Qishan section, Ordos Basin

Abstract

Carbon isotopes are important criteria for determining the underground diagenetic evolution and the evolution characteristics of the Earth's surface environment. Because the Mesoproterozoic was between the two great oxidation events, the changes in inorganic carbon isotopes were relatively gentle, making high-resolution sections of inorganic and organic carbon particularly important for global comparisons. To better constrain the isotopic variation and origins of the Mesoproterozoic, in this study, the Qishan section in the Ordos Basin of China was selected for investigation of its sedimentology, petrology, and geochemistry. The results show that the carbon isotope variations of the Jixian Formation carbonates in the Qishan section were generally not significant, with an average value close to 0‰. Similar characteristics were observed in the Luonan section, the Jixian section and the Jixian Group section (a combined section located at the north margin of North China) in the Yanliao Basin. This indicates that the carbon pool in the seawater had not undergone large-scale changes, and that the aquatic environment remained relatively consistent. A short-term negative excursion that appeared in the L2 member of the Longjiayuan Formation, at the same time may be closely related to greater organic carbon retention caused by a short-term event of biological death. It was not ruled out that carbonate minerals converted by 12C-rich produced by diagenetic evolution may had an impact on carbon isotopes during the rock evolution process.