CARBON AND OXYGEN ISOTOPE CHEMOSTRATIGRAPHY OF THE UPPER EARLY CAMBRIAN SECTION, THREE GORGES AREA, SOUTH CHINA: SHALLOW WATER ANOXIA DURING THE CANGLANGPUAN CRISIS

Many paleontological studies show animals drastically evolved in the Early Cambrian, called as “Cambrian Explosion”. Especially, the most remarkable diversity of genera occurred in the Atdabanian stage. In contrast, the generic diversity declined and animals were extinct in the late Botomian stage, known as the Botomian-Toyonian crisis. This crisis is thought due to a global transgression event, “Sinsk event”. However, the global relationship between those environmental changes such as the Sinsk event and the decrease of animal genera is still obscure, because the elucidation of secular change of carbonate carbon isotope ratio (d13Ccarb) is only reported in Siberia and Canada. In South China, it is proved that animals highly evolved through the Qionzhusian stage, a Chinese stage in the Early Cambrian, and a mass extinction subsequently occurred in the Canglangpuan stage. It is estimated that the Qionzhusian stage is equivalent for the Atdabanian stage and the Canglangpuan stage corresponds to the Botomian and the Toyonian stage. Although the mass extinction in the Canglangpuan stage seems corresponding to the Botomian-Toyonian crisis, exact relationship between them is still controversial.

In this study, we have performed scientific drilling, covering the Quionzhusian and the Canglangpuan stages in the Three Gorge area, South China. We conducted carbonate carbon and oxygen isotope analyses and found one negative carbon isotope excursion through this section ca. -12 permil. The extremely negative values in South China are different from those in other sections. Hence, the d13Ccarb excursion in this study would reflect the local change of the carbon cycle in South China. Those negative values are obviously smaller than those of incoming carbon, ca. -5 permil. It indicates that isotopicaly light carbons flew into the ocean around South China. Since black shales deposit during that negative excursion, light carbons of deep water would invade to shallow water when South China sea was isolated and stratified. Shallow water anoxia derived from that invasion probably caused the mass extinction in the Canglangpuan stage. The d13Ccarb chemostratigraphy in this study shows that South China was isolated at that time. However, the invasion of deep water is probably related to the global transgression during the Sinsk event.