Meishan: The End Permian Marine Extinction

Marine biodiversity crashed ~251.9 million years ago and ended the Paleozoic Era. Paleozoic ocean floors were dominated by invertebrates known as brachiopods (lamp shells [think Roman oil lamp shapes]), bryozoans (moss animals), rugose and tabulate corals, and stalked echinoderms (crinoids [left] and blastoids). At this time, occurring over a time interval of between 60,000 and 120,000 years, the extinction of these, and other, invertebrate groups exceeded 90% of known fossil marine organisms. Their disappearance from the fossil record has been termed the “Mother of Mass Extinctions” because this series of events marks the greatest loss in the planet’s biodiversity. The Mesozoic recovery in biodiversity set the stage for our current marine lifeforms and ocean ecosystems. Today, modern oceans are dominated by mollusc groups (bivalves, gastropods, cephalopods, etc.), scleractinian corals, echinoderms (sea stars [aka starfish but a misnomer], brittle stars, sea urchins), and crustaceans. Data on which the extinction event is well documented is found in the fossil record of a limestone succession cropping out in Meishan, China. This succession of rocks is designated as both a GSSP (Global Boundary Stratotype Section and Point) and UNESCO World Geoheritage site.

Besides the invertebrate fossil record preserved at Meishan, the rocks have been extensively studied using a multi-disciplinary approach by many international teams of scientists. Geochemical analyses of elements locked in these rocks have documented increasing concentrations and both positive-and-negative shifts in carbon (C), oxygen (O), calcium (Ca), strontium (Sr), sulfur (S), zinc (Zn), and mercury (Hg), amongst other proxies. When these trends are interpreted, they signal a deterioration in environmental conditions that led to the extinction crisis. Critical documented factors that affected biodiversity include a rapid 10° C rise in temperature, decreasing oxygen and increasing sulfur (euxinia), and ocean acidification. The illustration from Sial et al.(2021; identifies three phases (I, II, III) of carbon, mercury, and sulfur perturbations in these rocks during the end of the Permian.

During the late Permian, the sediments now exposed at the Meishan were deposited in the Tethys ocean located at a paleolatitude of ~ 20˚N. Carbonate (limestone) and carbonate-mud (shale) sediments accumulated in an intra-platform depression associated with a rift block. Over the time interval during which the extinction event occurred, the depositional environment changed from a shallow-water carbonate platform (think Bermuda, today) to a deeper offshore setting (think adjacent ocean depths). Interbedded with these carbonates are more than 30 volcanic ash beds that have yielded eight high-precision CA-ID-TIMS geochronological dates. These are coupled with detailed rock magnetic sampling (right) that has yielded a magnetostratigraphy tied to the age estimates. The extinction interval began at 251.94 Ma and ended at 251.88 Ma in a Normal polarity magnetozone.

The Meishan geoheritage site (31.0808333°, 119.7061111°) is ~2 km from the small town of the same name in Changxing County, Zhejiang Province, southeastern China. The limestone/shale succession is exposed in a series of quarries that run west to east. Three geologic formations can be seen in these rocks. The late Permian Changhsingian Formation is overlain by the Yinkeng Formation (uppermost Changhsingian to lowermostTriassic) in which a distinct ash bed (Bed 25) is situated just below the Permian–Triassic Boundary (PTB). A U-Pb age for this bed was reported by Shen et al. (2011) to be 252.28 ± 0.08 Ma. The museum at the geoheritage site consists of exhibits that show the evolution of life on the planet, exceptionally preserved fossils, minerals of China, and Chinese scientists who have studied the Meishan sections. Based on their past and continuing investigations, the rocks exposed across the Meishan sections are recognized as the premier location for world-wide correlation of the PTB.