Revisiting the carbonate carbon isotopic composition of the lowermost Doushantuo Formation in Yichang, Hubei Province: Implications for searching the original seawater carbon isotopic signal

The Ediacaran Doushantuo Formation in the Yangtze Gorges area of South China records several prominent carbonate carbon isotope (δ13Ccarb) shifts, some of which have also been recognized elsewhere in South China and around the world. However, the extremely negative δ13Ccarb values (< 30‰, VPDB) occurred in cap carbonate, and the δ13Ccarb fluctuations between positive and negative values in the overlying strata of the lower Doushantuo Formation, are exclusive to some specific Ediacaran sections in the Yangtze Gorges area of South China. The prevailing hypothesis attributes these irregular isotope features to the occurrence of diagenetic carbonate minerals, raising concerns about the reliability of utilizing bulk rock inorganic carbon isotope data (δ13Cbulk ) to reveal paleo-oceanic carbon isotope information. To assess the potential impact of diagenesis on δ13Cbulk and to obtain the original δ13C signal of ancient seawater, comprehensive studies including petrologic observations and carbon isotopic measurements were conducted on a drill core of the Ediacaran Doushantuo Formation at Wuhe, Yichang of Hubei Province. The results indicate the presence of diagenetic calcite in the lower Doushantuo Formation dolostones, and limited late-stage diagenetic alteration for the dolomites. These observations highlight that, in contrast to δ13Cbulk, the carbon isotope composition of dolomites (δ13Cdolo) from samples in Member I and lower Member II of the Doushantuo Formation may have higher potential to record contemporaneous seawater signals. To acquire the δ13Cdolo from the analyzed carbonate rocks, the bulk rock powders were reacted with 30% phosphoric acid for 2 hours to remove calcite minerals. The experiment on the mixture of calcite and dolomite powder with known carbon isotopic compositions indicates that calcite can be completely removed after this reaction. The results show that δ13Cdolo values of samples from the lower Member II of the Doushantuo Formation are consistently positive and significantly higher than their δ13Cbulk values, probably reflecting the primary variation of oceanic carbon isotope composition following the termination of the Neoproterozoic global glaciation. © 2024 Geological Society of China. All rights reserved.