Schematic diagram of a three-stage model for the paleoenvironmental evolution of the Upper Yangtze, China, through the Middle-Late Permian. Credit: Wang et al 2024
Mass extinctions are rapid global declines in terrestrial biodiversity, with five key events identified throughout the planet’s history, the most famous of which occurred around 66 million years ago in the Cretaceous, which ended the reign of the dinosaurs. However, the greatest mass extinction is attributed to the Permian, during which it is estimated that more than 95% of all life on Earth was eradicated.
The cause of this devastating event is still debated, with proponents of a large asteroid impact that caused a plume of dust in the atmosphere, blocking sunlight and generating acid rain, or significant volcanism that released large quantities of CO.2 into the atmosphere and made the oceans toxic to marine life.
New research published in Chemical Geology provides further support for the latter theory. Yu Wang of Nanjing University and colleagues conducted geochemical experiments on sediment samples obtained from the Zhigao Quarry in the upper Yangtze River region of China. Within the samples, scientists identified a significant mercury anomaly, hosted in organic matter, associated with a peak in carbon isotopes.
This negative excursion of the carbon-13 isotope is interpreted to be caused by massive releases of carbon into the atmosphere, likely due to volcanic eruptions, and these events are also a major source of mercury in nature. Indeed, the timing of this excursion coincides with the known volcanic activity of the Emeishan Traps, in Sichuan, a vast area (>250,000 km2) of flood basalts creating a large igneous province.
Trace elements such as molybdenum, uranium and vanadium present in sediment samples are likely to experience reducing and oxidizing conditions, with three notable peaks indicating anoxic conditions when the oceans were depleted of dissolved oxygen. In addition to this, a significant drop in sea level during the Capitanian (around 264 to 259 million years ago) is evidenced by land-derived organic carbon, while lower ratios of Elements cadmium and molybdenum suggest a weakened or seasonal upwelling of cold nutrients. rich waters.
Geochemical data and decreases in calcareous algae and fusulinaceous foraminifera in chert, mudstone, and limestone samples indicate marine anoxia and more pronounced stratification of the water column. Under such conditions, oxygen minimum zones likely developed, with persistent areas of low-oxygen conditions that would have inhibited the survival of organisms.
Field study site in Zhigao Quarry, China, and thin-section slides of sediment samples viewed using a petrographic microscope to identify microscopic marine organisms such as radiolarians, foraminifera and Algae. Credit: Wang et al 2024
The research team proposes a three-step volcanic-climate-ocean model to explain how large-scale volcanism led to a biotic crisis in the oceans during the Capitanian, leading to the ultimate mass extinction at the end of the Permian . During the first stage, the study site was part of a carbonate platform connected to the Paleo-Tethys Ocean, located along the northern margin of the ancient supercontinent Gondwana. This shallow, oxygen-rich marine environment experienced vigorous ocean circulation with nutrient-rich upwellings, ideal for the development of marine life, with evidence of algae, brachiopods and corals in sections from the upper Yangtze.
During phase 2, the appearance of volcanism began at the beginning of the Capitanian, with a significant release of greenhouse gases leading to global warming of 3°C to 5°C. This coincided with sea level rise caused by regional subsidence, thereby exacerbating water column stratification and marine anoxia, as warmer oceans reduced the concentration of dissolved oxygen. While nutrients were brought to the surface by volcanic activity, primary productivity at the ocean surface used most of this oxygen for respiration and decomposition of organic matter, generating minimal zones of oxygen depleted below.
Stage 3 is characterized by a decline in volcanism and a return to shallower marine conditions, with an increase in organic matter transported from land to the ocean, as well as reinvigorated marine circulation that brought back oxic conditions . This allowed marine organisms to recover from hostile anoxic conditions and evolve into new ecological niches.
Scientists are arguably suggesting that we are now in the midst of a sixth mass extinction, and that it is vitally important to continue to explore the role that volcanism could play in tipping the scales towards a new crisis of our marine domain.
More information:
Yu Wang et al, Coupled volcanic activity and marine anoxia in the Upper Yangtze region before the Capitanian mass extinction, Chemical Geology (2023). DOI: 10.1016/j.chemgeo.2023.121838
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