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

Abstract

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 87Sr/86Sr values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative δ13C 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 87Sr/86Sr 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.