With the support of the National Natural Science Foundation of China, Professor Shen Jiaheng of the School of Earth and Space Sciences of Peking University made important progress in the study of mass extinction at the end of the Permian. The results of the study, titled “Early and late phases of the Permian–Triassic mass extinction marked by different atmospheric CO2 regimes”, were published online in the internationally renowned journal Nature Geoscience (IF: 21.531) on October 3, 2022. Professor Shen Jiaheng is the first and corresponding author of the research work, and the co-organizers are China University of Geosciences (Wuhan), Texas A&M and Harvard University. Full-text links:https://www.nature.com/articles/s41561-022-01034-w
Has the Earth entered the “sixth mass extinction”? Are humans experiencing rising carbon dioxide concentrations, rapid global warming, and deteriorating ecological environments? To answer this question, it is not enough to rely on the short-scale data that we humans have observed. Studying the “Deep Time” record, the mass extinction event at the end of the Permian, can help us seek answers.
The mass extinction at the end of the Permian, which occurred about 250 million years ago, was the largest mass extinction in the evolutionary history of life on Earth, with about 90% of marine species and 70% of terrestrial species dying, a rare geological mutation event in which marine and terrestrial ecosystems faced collapse at the same time. Large-scale volcanic activity in Siberia is believed to be the “culprit” of this event, and then a series of climatic environmental changes (global warming, ocean hypoxia, acidification, hypercapnia, etc.) have caused the destruction of organisms. However, these climatic, environmental and biological evolutionary mechanisms have yet to be further precisely determined. In response to this problem, the study provides a direct geological record of climate change and the evolution of the paleomarine primary productivity community structure by performing assay and analysis of the Guangyuan Shangsi section sample, and further explains the different mechanisms of the extinction of the two acts of organisms in this period in combination with the model.
In response to climate change in this period, researchers at Jiaheng Shen quantitatively reconstructed the atmospheric carbon dioxide concentration in this period using the biomarker compound monomer carbon isotope of ancient chlorophyll (Figures 1 and 2). The reconstruction results show the extinction of the first act with the lowest pCO2; Subsequently, pCO2 rose rapidly and entered a period of slow rise, and this high pCO2 continued until the extinction of the second act of the Early Triassic and reached its highest value; After that, pCO2 begins to decline slowly. This reconstruction details the different pCO2 characteristics of the two acts at the time of the extinction. Biomarker compound monomer nitrogen isotopes were used to quantitatively reconstruct the evolution of marine primary productivity community structures (bacteria and eukaryotes) during this period (Figures 1 and 2). The reconstruction results show that at the time of the extinction of the first act, the marine primary production community was dominated by eukaryotes; In the second act of extinction, cyanobacteria occupy the marine primary production community with an absolute advantage of ~100%.
In conjunction with the Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir Model (LOSCAR), the authors reshaped the response mechanisms for climate and carbon cycle perturbations during this period. The low pCO2 at the time of the first act extinction is due to the large-scale basalt eruption in early Siberia (before the first act extinction ~300 kyr) that produced a large amount of fresh weatherable material rather than a strong exhaust effect. The enhancement of pulsed weathering capacity at this stage can effectively inhibit the accumulation of carbon dioxide in the atmosphere, and bring a large number of nutrient salts and minerals to the ocean, increase the alkalinity of the ocean, and buffer the ocean carbonate balance system and the pH value of seawater; The high import of nutrient salts leads to eukaryotic eutrophication in the ocean, which is accompanied by high-productivity exports leading to a brief and severe hypoxic environment in the ocean, leading to biological extinction.
Subsequently, the continuous action of prolonged Siberian volcanic activity depletes the surface weatherable material, causing the silicate weathering feedback to fail and lose the ability to regulate atmospheric carbon dioxide. This tipping point (the first act of extinction) marks the beginning of the accumulation of carbon dioxide in the atmosphere and the failure of ocean buffering capacity; At the same time, nutrient fluxes have decreased dramatically, leading to a decline in productivity and an alleviation of ocean hypoxia. After this tipping point, carbon dioxide rises rapidly, the climate warms, and ocean stratification intensifies; At the same time, the nutrient-poor environment of the ocean has prompted cyanobacteria to completely replace eukaryotic algae with absolute superiority. As a result, the collapse of production communities at the bottom of the marine food chain and the weakening of productivity export fluxes eventually led to persistently high atmospheric carbon dioxide concentrations in the early triad and ocean acidification, triggering the extinction of Act II. The LOSCAR model also quantitatively verifies that the long-term high pCO2 of the Early Triassic was not only caused by volcanic activity, but was fundamentally caused by changes in the characteristic structure of marine ecosystems (Figure 3). In addition, the authors used the LOSCAR model to reconstruct the possible carbon emission scenarios for this period: the total emissions were about 5,000 PgC (without considering early Siberian volcanism), and the emission time concentration occurred during the P-Tr boundary period and lasted about 200 kyr. In addition, the composition of possible carbon sources during this period: 40% from mantle sources of volcanic eruptions and 60% from large amounts of lighter carbon sources released by high-temperature magma intrusions into the rich organic layer.
Based on the above, the study proposes that the two-act mass extinction of organisms at the end of the Permian has fundamentally different characteristics of extinction mechanisms. The first act is characterized by eutrophication and hypoxia leading to the extinction of habitat loss, while the second act is the extinction of extreme heat, hypercapnia, and the collapse of the food web. This result helps explain why the mass extinction at the end of the Permian was the most extinct in Earth’s history.
Figure 1: Whole rock carbon, nitrogen isotopes and biomarker monomer carbon and nitrogen isotopes were studied and analyzed
Figure 2: Evolution of atmospheric carbon dioxide concentrations and paleomarine primary productivity community structures during the late Permian mass extinction based on monomer isotope reconstruction of biomarker compounds
Figure 3: LOSCAR simulation results
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