Carbon Isotope Negative Excursion of the Sturtian Glacial Sediments in the Nanhua Basin and its Geological Significance

Key words: 
• Sturtian Glaciation /
• Tiesi’ao Formation /
• carbon isotope /
• Nanhua Basin

Abstract: The Neoproterozoic Sturtian Glaciation (~ 717-660 Ma) developed widely across the world, which was well recorded in the Nanhua Basin, but the study of carbon cycling during this period has been lacked. To understand the characteristics of carbon isotope compositions during the Sturtian Glaciation, this study focuses on the drillcore ZK2115 in Songtao area, eastern Guizhou Province, and the high-resolution organic and inorganic carbon isotopes (δ13Corg and δ13Ccarb) of the syn-Sturtian Tiesi’ao Formation are analyzed. The results show that the values of δ13Ccarb vary between -9.29 ‰ and -3.37 ‰ (mean -7.24 ‰), indicating the genitive excursion. The values of δ13Corg vary between -33.63 ‰ and -23.35 ‰ (mean -29.29 ‰). Besides, there is a positive correlation for the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by the diagenesis, and can be used to reflect the original carbon isotope signals of the syn-Sturtian Nanhua Basin. Combined with the low TOC contents (mean 0.25%) of the Tiesi’ao Formation, showing that the photosynthesis still existed under the extreme glacial conditions with low rates, and the contents of the generated organic matter are low. The research of carbon isotope compositions in this study can be used to explore the carbon cycling during the Sturtian Glaciation and can also provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic condition.