The Co-Swing of δ¹³C and δ¹⁸O at Neoproterozoic- Cambrian Transition from the Tarim Basin, Northwestern China

Shuichang Zhang^1,2, Xiaomei Wang^1,2, Baomin Zhang¹, and Huajian Wang^1,2
1Research Institute of Petroleum Exploration and Development, Beijing, China
²CNPC Key Laboratory of Petroleum Geochemistry, Beijing, China

Around the end of Neoproterozoic, the world ‘suddenly’ changed from the chilly ‘Snowball Earth’ to a scenario full of new habitators - the Cambrian animals, in a geologically short moment known as the ‘Cambrian Explosion’. However, soon after, at least two striking episodes of mass extinctions occurred in Late Cambrian and Ordovician. These two episodes of extinctions in Cambrian are among the known most severe ones in the history of life. However, the cause of these dramatic changes remains enigma. Here we document our study on a Precambrian-Cambrian (PC-C) succession sampled through a deep drill at Tarim basin of northwestern China. Geochemical measurements of these sediments reveal a remarkable negative δ¹³C and δ¹⁸O excursion just around the PC-C boundary accompanied by large periodic positive trace and major metal anomalies. Then the isotopic signals oscillate with a decreasing magnitude apparently following a power law exponential pattern. We propose the following scenario explaining the global events occurred around the transition at PC-C boundary based on our study from Tarim. By the end of Neoproterozoic Eon, perhaps due to certain catastrophic event(s) induced orbit change, Earth entered a period of Cryogenian-phase. When the inducing force ended, the Earth swung to the opposite – greenhouse phase in a short time. Many dramatic global events occurred during this period of time on Earth, including mass extinction and radiation of species, such as the sudden disappearance of Ediacarane animals and subsequent Cambrian explosion. Lines of global isotopic, chemical, biological and geological evidence mark these changes detectable worldwide. Global upwelling of nutrient elements from bottom water to photic zones generated highly organic enriched sediments including black shale, phosphorous repository, and rich oil-generating source rocks in the greenhouse phases of early Paleozoic sedimentary basins. The dramatic global oscillations and the induced cyclic isotopic events thus mark the great transition from Neoproterozoic to Cambrian. And they all perhaps started with an external inducing force or catastrophic event (s).

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013