Carbon-isotope stratigraphy of the Lower Cambrian (Cambrian explosion) in northwestern Yangtze Platform, China: Implications for global correlation

The Archaeocyathid Extinction Carbon Isotope Excursion (AECE) and Mingxinsi Carbon Isotope Excursion (MICE) are major events in the Early Cambrian sedimentary records but controversially heterogeneous in their magnitudes, as well as local and global correlations. This paper presents a high-resolution δ13Ccarb profile from the Shiliu section in the western Yangtze Platform (China) that spans the lower part of the Lower Cambrian. The investigated Lower Tianheban interval (∼ 110m) is dominated by marine carbonates (lime mudstones) alternating occasionally with thinly-bedded black shale and mudstone interbeds. The investigated micritic carbonates were examined through a multi-technique protocol, combining petrographic and geochemical assessments, to evaluate the degree of sample preservation. The δ13Ccarb values (-2.4‰ to 1.8‰) show an insignificant correlation with the δ18Ocarb values (-12.6‰ to -8.7‰, R2=0.012), the Mn/Sr ratios (R2=0.001), and Al concentrations (R2=0.041). This supports the preservation of predominantly at least near-primary δ13Ccarb signatures, which can be utilized to reconstruct a reliable, high-resolution δ13Ccarb profile for chemostratigraphic correlations in the area and beyond. The profile displays a lower positive excursion of ∼ 3‰ overlain immediately by a negative excursion of ∼ 2.3‰. The archaeocyath zonation scheme suggests that the lower excursion is correlated with the global MICE event and the upper excursion with the AECE event. The results provide an alternative option for interpreting the carbon isotope excursions in the Lower Cambrian within the Yangtze Platform as well as other equivalent sections at a global scale.