Humanity prides itself on the fact that it took five mass extinctions for its distant ancestors to send humans to the top of the food chain.
However, human beings should remember that on the earth, survival is a kind of luck for living things, which was true on the earth in the past, and it is everywhere in the current cold epidemic, rather than a low-hanging fruit, and the slightest turmoil in the earth's environment may make the biosphere fall into the end of the world.
Time back to the Ordovician 440 million years ago, when human ancestors were still playing in the mud in erie Garden, north of the super gondwana continent, the first biological purge in earth's history quietly arrived. The extinction was divided into two acts, the first was the rapid sea retreat caused by the super glacier event, and the second was the global ocean hypoxia event caused by the rapid sea advance, and the two acts were staged in turn, which together led to the tragic extinction of 85% of the earth's organisms.
In this so-called second mass extinction of the earth, the second ocean hypoxia event is only a continuation of the first super glacier event, and the continental glacier that lasted for nearly two million years is the original culprit of this extinction, so what triggered the trigger of the super ice age at the end of the Ordovician?
Volcanic activity
The extensive development of large-scale volcanic rock deposits in the strata near the Ordovician-Silurian boundary, as well as the anomalies of pyrite sulfur isotopes in the strata, suggest that one of the culprits of this extinction is the old friend of the well-known volcano.
In the billions of years of earth's evolutionary history, volcanoes have loved and killed the earth, and volcanoes have helped the earth get rid of hundreds of millions of years of ice, and have also killed hundreds of millions of people on the earth several times.
At the end of the Ordovician period, the collision of the continent of Lauia near the equator and the Baltic continent intensified, forming a series of mountains including the Appalachian Mountains, accompanied by a large number of volcanic eruptions.
This eruption may not have been as spectacular as the one at the end of the Permian, but when the material from the sustained eruption entered the stratosphere, everything changed.
At the end of the Ordovician , " stratospheric volcanic eruptions " transported large amounts of sulfur dioxide , hydrogen sulfide and other volcanic materials to the stratosphere and formed a sulfate-based aerosol layer.
Compared with the unpredictable troposphere, which is full of wind, rain, thunder and lightning, the second stratosphere of the Earth's atmosphere is more stable, so the formed aerosol layer can stay for a long time.
During this period, the sulfate aerosol layer plays a crucial role in global climate change and the heat balance of the Earth system, and stratospheric sulfate aerosols reflect solar radiation with shorter wavelengths while absorbing long-band solar radiation, resulting in a decrease in the Earth's surface temperature.
That is, the sulfate aerosol layer heats itself up, cooling the earth, causing the earth's temperature to drop.
In addition to the sulphides spewed by volcanoes directly cooling the Earth, volcanic activity also transports large amounts of magma to the surface, which cools to form new silicate rocks, which, when weathered, absorb CO2 from the air, form carbonates and are buried.
In fact, in nature, the silicate rock weathering absorption of carbon dioxide chemical reactions in the atmosphere has been occurring, the overall content of carbon dioxide in the global atmosphere is about 800 billion tons, and the annual consumption of carbon dioxide through silicate weathering is 138 to 169 million tons.
This chemical reaction occurs very slowly and seems to have little effect, but after the silicate rock weathering fixes the carbon dioxide, it transports it to the ocean and becomes carbonate deposits, which can be sequestered for a long time. This carbon is often buried for more than a million years, and the impact of this process on climate change is on a 10,000-year scale.
Especially when volcanoes transport basalt magma deep in the earth's crust to the surface and replace granite, alkaline basalt weathers much faster than acid granite, and basalt removes CO2 from the atmosphere at a much faster rate than it did before volcanic activity, leaving the earth to lose its small cotton jacket and shift from a greenhouse to an ice chamber.
Prior to late Ordovician glaciation, the concentration of carbon dioxide in the atmosphere was 7,000 ppm, and the drop in carbon dioxide concentration at the end of the Ordovician period plummeted to 4,400 ppm, which seems to confirm this decline.
Therefore, the sulphide aerogel caused by volcanic eruptions and the weathering of silicate rocks can jointly lead to a drop in the earth's temperature, allowing glaciers to spread to low latitudes, a large amount of water vapor condenses into glaciers to seal up, the global sea level drops sharply, many of the original oceans become land, ecosystems collapse, and the Ordovician mass extinction begins.
Gamma-ray bursts
In addition to the expansion of glaciers caused by the earth's own changes, scientists have also set their sights on the universe, and gamma-ray bursts are also one of the hypotheses of ordovician glacier expansion.
In the evolution of the Earth for more than four billion years, the Earth has been subject to the interstellar environment all the time, as small as a meteorite to the spiral of the Milky Way. Normally, the earth's warmth and cold depend on the energy released by the sun, but when a gamma-ray burst occurs, the sun's light appears so dim.
Gamma-ray bursts are the most violent explosions in the universe, and when huge star fuels run out and collapse, or when two neighboring dense stars merge, they release huge amounts of energy in a short period of time, that is, gamma-ray bursts, which release energy in a few minutes equivalent to the sum of trillions of years of sunlight.
At the end of the Ordovician, gamma-ray bursts occurred on supernovae less than 6,000 light-years from Earth, and the explosion of just a few tens of seconds directly reduced the ozone in The Earth's atmosphere by nearly half, the plankton responsible for photosynthesis on the planet's surface was exposed to extreme ultraviolet radiation and burned to death, the base layer of the marine food chain collapsed, and famine began to spread everywhere.
Gamma-ray bursts can also shatter the Earth's atmosphere, under the action of strong radiation, ozone molecules will react with nitrogen to form nitrogen dioxide, which is a dark aerosol that can block more than half of the solar radiation, after losing the sunlight, the Earth's temperature drops rapidly, glaciers quickly form, and the first act of the Ordovician mass extinction opens the curtain.
However, the gamma-ray burst hypothesis is questionable, and there is no clear evidence that gamma-ray bursts occurred at the end of the Ordovician period.
What is the real reason for the arrival of the first act of the mass extinction at the end of the Ordovician, perhaps only by riding on a time machine and traveling back to that fiery era, the various phenomena of the Earth's climate and environmental changes are recorded in the strata and fossils, and the reasons behind these phenomena are always confusing and reverie.
Fortunately, those once overlords of the earth sighed, mourned, and went to eternal decay, and the flowers of the world would bloom, and the magnificent and immortal things would come one after another.
参考资料：Hu, D., Li, M., Zhang, X. et al. Large mass-independent sulphur isotope anomalies link stratospheric volcanism to the Late Ordovician mass extinction. Nat Commun 11, 2297 (2020). https://doi.org/10.1038/s41467-020-16228-2
Melott, A., Lieberman, B., Laird, C., Martin, L., Medvedev, M., Thomas, B., . . . Jackman, C. (2004). Did a gamma-ray burst initiate the late Ordovician mass extinction? International Journal of Astrobiology, 3(1), 55-61. doi:10.1017/S1473550404001910
https://en.wikipedia.org/wiki/The late Ordovician mass extinction #cite_note-FOOTNOTEVrazoCiurca2017235-26
SUN Weidong,LIAO Renqiang. Rapid formation of the ice age led to mass extinction at the end of the Ordovician[J].Chinese Science Bulletin,2020,65(06):431-433.