A remarkable increase of the animal genera and a subsequent mass extinction in the late Early Cambrian are well known as the "Cambrian explosion" and the "Botomian-Toyonian crisis." A composite global curve of the carbon isotope ratios for inorganic carbon (delta C-13(carb)) shows multiple fluctuations during the evolution events, and it indicates significant changes of the oceanic carbon cycle at that time. This study reveals a new continuous isotopic chemostratigraphy for inorganic carbon (delta C-13(carb)) from the bottom of the Shipai to the base of the Shilongdong formations in Three Gorges area, South China. This section covers the Canglangpuian to the Longwangmiaoian stages in the Lower Cambrian. The delta C-13(carb) variation exhibits three negative excursions: a remarlcably negative excursion down to ca. -12%. in the middle Canglangpuian stage, a negative excursion to ca. -1.0 parts per thousand in the upper Canglangpuian stage, and a negative excursion to ca. -1.0%. in the Longwangmiaoian stage, respectively. The largest negative delta C-13(carb), excursion and a positive excursion before the excursion are definitely consistent with the delta C-13(carb) negative shift (AECE) during the mass extinction and the delta C-13(carb) positive values (MICE) during the increase of animal genera, respectively. However, the minimum values of the negative shifts among South China, Siberia, and Canada sections are different from each other. The positive delta C-13(carb) excursion at the bottom of the Canglangpuian stage indicates that primary productivities and organic carbon burial were enhanced. A sea level rise in the Qiongzhusian to bottom of the Canglangpuian stages in South China corresponds to the Sinsk transgression event in Siberia and Canada. A eutrophication due to higher continental weathering during the transgression after the long-term retrogression enhanced the high primary production and consequently promoted the significant increase of animal diversity.
On the other hand, deposition of laminated black shales without bioturbation signatures and a decline of trilobite diversity are observed during the negative delta C-13(carb) excursion in the Canglangpuian stage, indicating that the shallow water environment became anoxic at that time. The negative delta C-13(carb) shift indicates an influx of abundant (CO2)-C-12 due to oxidation of organic carbons in seawater. The difference of the minimum values among sections implies the local difference in size of the organic carbon reservoirs and extent of the degradation of the carbons. The largest delta C-13 anomaly in South China suggests the presence of the largest OCPs due to higher activity of primary production and high degree of oxidation of the OCPs because of higher activity of animals. The coincidence of the timing of the negative VC excursions in the Canglangpuian stage among the sections indicates a global event, and suggests that the onset was caused by increase of oxygen contents of seawater and atmosphere. Abundant oxygen yielded by the increased primary productivity in the Atclabanian and the Qiongzhusian stages caused onset of the oxidation of OCP, and possibly led to the shallow water anoxia and the mass extinction of benthic animals in the Botomian and the Canglangpuian stage.

Carbon isotope ratios; Early Cambrian; Botomian-Toyonian crisis; Cambrian explosion

Northwest University; Chang'an University