Kyushu University and JAMSTEC reveal that extinction of marine life during the Late Triassic 'Carnian Pluvial Episode' was caused by anoxia at the end of volcanic activity

A research group led by Assistant Professor Yuki Tomimatsu, Assistant Professor Honami Sato, and Professor Tetsuji Onoue, all of the Graduate School of Science at Kyushu University, together with Acting Group Leader Tatsuo Nozaki of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and Associate Professor Yutaro Takaya of the Graduate School of Engineering at The University of Tokyo, in collaboration with Kobe University and Waseda University, has revealed that large-scale volcanic activity triggered the Carnian Pluvial Episode (CPE). They also revealed the relationship between the environmental changes that occurred at that time and the extinction of marine life. The research group determined this by analyzing strata deposited in five regions in Japan during the CPE, using a multi-collector-inductively coupled plasma-mass spectrometer (ICP-MS) installed at JAMSTEC. The results were published in the September 28 issue of Scientific Reports.

The Triassic period, spanning from approximately 251.9 million to 201.3 million years ago, is known to have had a generally hot and dry climate. However, the Carnian, which divides the Triassic into smaller sub-periods, had a long extreme rainfall event, known as the 'Carnian Pluvial Episode,' which lasted for approximately 2 million years. Recent studies have indicated that the cause of this event may have been volcanic activity in the Wrangellia flood basalts of the northwestern edge of present-day North America and basalts distributed in Japan and Russia.

To clarify the cause of the CPE and its impact on marine life, the research group analyzed cherts (hard, dense siliceous sedimentary rocks composed mainly of silicon dioxide) that were deposited at the time on the floor of the Panthalassa Ocean in five areas in Japan (Saiki City, Oita Prefecture; Nantan City, Kyoto Prefecture; Yamagata City, Gifu Prefecture; Shimohei District, Iwate Prefecture; and Sakahogi Town, Gifu Prefecture).

Analysis using a multi-collector ICP-MS revealed low osmium isotope ratios, characteristic of Earth's interior mantle material at the early Carnian, in the cherts of all regions studied. This indicates that large amounts of osmium derived from large-scale volcanic activity were supplied to the oceans.

All of the studied areas showed traces of volcanic activity, indicating that large-scale volcanic activity had occurred, with the large amounts of magma generated by the massive mantle plume of the Panthalassa Ocean forming a large igneous rock zone over a vast area. Furthermore, elements such as vanadium and uranium tend to concentrate in sediments in hypoxic to anoxic environments, and by examining changes in the concentrations of such elements in the strata of the study areas, it was revealed that the deep-ocean floor was anoxic at the end of volcanic activity. This anoxia may have extended to the shallow water areas of the ocean at that time.

Fossils from the cherts also revealed that the diversity of conodonts (primitive vertebrates that originated in the Cambrian and lived for ∼300 million years) declined significantly during the same period of anoxia, and most species that lived in the early Carnian became extinct. These results indicate that the oceanic anoxia that occurred at the end of large-scale volcanic activity may have led to the extinction of marine life, including conodonts.

Tomimatsu commented, "These research findings revealed evidence of widespread volcanic activity and oceanic anoxia, which led to mass extinctions, across Carnian strata found throughout various regions in Japan. However, while evidence of oceanic anoxia is consistent, the exact cause remains unclear. Interestingly, some species managed to escape extinction and witnessed an increase in diversity despite the impact. Moving forward, we plan to further investigate these issues, aiming to unravel the complete picture of the Carnian Pluvial Episode, a significant turning point in the history of life on Earth."

Journal Information
Publication: Scientific Reports
Title: Pelagic responses to oceanic anoxia during the Carnian Pluvial Episode (Late Triassic) in Panthalassa Ocean
DOI: 10.1038/s41598-023-43525-9