The Late Ordovician to early Silurian was an important period that witnessed the first of “big five” (~445 million years ago), and it was also the most important interval of source rocks development in South China that containing the main commercial shale gas source/reservoir of China. What caused the mass extinction? Is there any internal relationship between the source rocks, the mass extinction event and environmental evolution? Answers to these questions will enrich our understanding of life evolution on the planet, and it also provide insights into the development mechanism and exploration stratagem of unconventional shale oil and gas. Previous studies have shown that there are close relationships between the volcanic activities, mass extinction and the related environmental evolution, and source rocks development. However, much more work needs to be done to reveal the less studied and contentious mechanisms of them. During the Late Ordovician, tremendous volcanisms occurred worldwide, triggered by the orogeny activities in South China and other parts of the world. The orogeny nearby marine setting was an ideal place that can preserve the volcanism records, and therefore they were the keys to reveal the mechanism of volcanism’s impact on the life evolution and the global environmental change during this mysterious period.
Recently, the research group of Professor Hu discovered a typical marine section that across the Ordovician/Silurian transition, with numerous interbedded volcanic ash layers (more than 100 layers) in Zhenjiang, Jiangsu Province. Based on paleontological identification, zircon U-Pb dating and element and isotope geochemical analysis of volcanic ashes, they found that the volcanic activities lasted at least 22 million years (449.3 ± 3.6 Ma to 427.6 ± 4.1 Ma) and therefore became the first to propose that the initial time of the Late Ordovician mass extinction in South China was 445.14 Ma. This time is very close to the reference time (445.2 ± 1.4 Ma) given by the International Commission on Stratigraphy, and it also provides a relative accurate time frame for the global comparison of the Late Ordovician mass extinction events and environmental evolution. In addition, geochemical analysis indicates that the volcanic eruption has two different sources up section. The early eruption was dominated by relative basic parent rocks, and containing much mantle components. The late eruption was dominated by relative acidic parent rocks, and involving a large number of crustal components. This finding well explains and furtherly supports the hypothesis that volcanism caused the sharp decrease in marine strontium isotopes during the Late Ordovician (Figure 1), and it also provides a new perspective for the controversial studies of the early Paleozoic tectonic background in South China. The spatial coupling of volcanism and source rock indicating volcanic materials could promote primary productivity, and volcanic input would be an important factor for the development of source rocks.
Figure 1. Global volcanic eruption and environmental evolution during the Ordovician-Silurian transition
This research was published in Earth and Planetary Science Letters (2019, v518) titled by "Duration, evolution, and implications of volcanic activity across the Ordovician-Silurian transition in the lower Yangtze region, South China." Yang Shengchao, grade 2016 Ph.D. candidate from the college, is the first author of this paper, and Professor Hu Wenxuan is the corresponding author. The research was jointly funded by the Key Projects of the National Natural Science Foundation of China (grant number 41830425) and the National Science and Technology Major Project of China(grant number 2016ZX05002-006-005).
Link: https://doi.org/10.1016/j.epsl.2019.04.020
