A positive magnesium isotopic excursion associated with stabilization of metastable carbonates in the shallow Yangtze Sea, South China: Implications for the Hirnantian Isotopic Carbon Excursion

The Ordovician–Silurian transition (OST) was a period marked by pivotal changes in Earth’s paleoenvironments and a large-magnitude positive carbon isotopic excursion, the Hirnantian Isotopic Carbon Excursion (HICE). The OST sediments in the Yangtze region of South China accommodate vast shale gas resources, and tracking the HICE plays an important role in revealing the untapped resources. However, the driving mechanism behind the HICE in shallow Yangtze seawater remains elusive, hindering paleoenvironment reconstruction and energy exploration in this region. An OST limestone section was recently identified in the Wuke region, South China Block, providing an ideal opportunity to investigate the mechanism driving the HICE in the shallow Yangtze Sea. The OST of the Wuke section was classified into four stages, based on coupled C-Mg isotopic analysis. Stages I, II, III, and IV correspond to the P. sinensis, D. mirus–T. typicus, lowermost D. mirus–M. extraordinarius, and M. persculptus graptolite zones, respectively. An ~1.5‰ positive δ26Mg excursion occurs from the lowermost part of Stage II through the end of Stage III. Furthermore, binary mixing analysis of the δ26Mg and Ca/Mg ratios of carbonate fractions shows that the “limestone endmembers” during these two stages are more enriched in 26Mg than those of Stages I and IV. This feature may be attributed to the stabilization of metastable carbonates, as calcites with elevated δ26Mg can form through transformation from metastable species such as aragonite and amorphous calcium carbonate (ACC). Furthermore, the positive δ13C excursion starts at the base of the D. mirus graptolite zone and continues through the end of the M. extraordinarius zone. This suggests that the HICE may have initiated as early as the Late Katian in the shallow Yangtze Sea, and the earlier onset of the carbon isotopic excursion was likely partially caused by the stabilization of metastable carbonates. The results of this study imply that stabilization of metastable carbonates is one potential factor responsible for the HICE in the shallow Yangtze Sea.