Carbon, oxygen and strontium isotopic and elemental characteristics of the Cambrian Longwangmiao Formation in South China: Paleoenvironmental significance and implications for carbon isotope excursions

The biosphere experienced episodic perturbations during the Neoproterozoic-early Cambrian, accompanied by major fluctuations of the carbon cycle. However, compared to the Ediacaran-early Cambrian, geochemical records of the Cambrian Toyonian Stage (Longwangmiao), which contains the significant Redlichiid-Olenellid Extinction Carbon Isotope Excursion (ROECE), are scarce. To address this data gap, we conducted measurements of carbon, oxygen and strontium isotopes and element contents of a continuous series of carbonate samples from the Longwangmiao Formation of Well HS2, located in the central Sichuan Basin of South China. Our specific aims were to reconstruct the paleoenvironment and to determine the origin of the ROECE. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. The Sr-87/Sr-86 values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative delta C-13 excursion in the late Toyonian is globally associated with the ROECE, which is widely thought to have resulted from extinctions caused by a transgression. However, a transgression is not shown in the Sr-87/Sr-86 profile of Well HS2, which we attribute to local karst formation and erosion caused by frequent regional regressions. The interrelationships among several paleoenvironmental proxies show that continental weathering was climatically controlled, which affected the supply of terrigenous detritus, and that the carbonate platform was semi-restricted. Furthermore, profiles of paleoenvironmental proxies indicate that the environment was warm and humid, favoring high biological productivity and diversity during Interval I (early Toyonian), and that it gradually became colder and drier during Interval II (middle Toyonian). We suggest that by the time of Interval III (late Toyonian), the adverse effects of a cold and arid environment were superimposed on the transgression, triggering the ROECE. Our results provide a reference and possible interpretation for other carbon isotope excursion events. (C) 2022 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Automated summary

This study provides detailed geochemical research on the Longwangmiao Formation in the central Sichuan Basin of South China, revealing the paleoenvironmental changes and the origin of carbon isotope excursion events during this period. The results indicate that continental weathering is climatically controlled and was influenced by local karst formation and regional regressions during transgressions. The study also highlights the warm and humid paleoenvironment during the early Toyonian, gradually transitioning to colder and drier conditions in the middle Toyonian.