Is First Mass Extinction Primarily Caused by Volcanic Eruptions?

What is Mass Extinction?

The fossil record reveals that something strange, occurred millions of years ago. From terrestrial megafauna to microscopic aquatic creatures, a vast spectrum of animals died unexpectedly. This loss of a significant number of animals is referred to as a Mass Extinction, also known as a Biotic Crisis or Extinction Event. Older species fade away as new species adapt to fill ever-changing ecological niches. However, the pace of extinction is not constant. The extinction of the dinosaurs was one of five major mass extinction events that wiped out millions of species. 75 to more than 90% of all species on Earth have vanished in a geological blink of an eye during mass extinctions at least a handful of times in the last 500 million years.


Marine life during Ordovician period.
image source: wikipedia
Marine life during Ordovician period.
     Marine life during Ordovician period, image source: wikipedia


Despite the fact that mass extinctions are disastrous events, they allow new forms of life to arise on the earth. The most well-studied mass extinction, which occurred around 66 million years ago and marked the transition between the Cretaceous and Palaeogene periods, wiped off non-avian dinosaurs, allowing mammals and birds to swiftly diversify and adapt.

There have been five mass extinction events in Earth’s history: the Late Ordovician Mass Extinction (450-440 million years ago), the Late Devonian Mass Extinction (375-360 million years ago), the Late Permian Mass Extinction (252 million years ago), the Late Triassic Mass Extinction (201.3 million years ago), and the Late Cretaceous Mass Extinction (66 million years ago). The second largest Phanerozoic mass extinction occurred at the Ordovician-Silurian (O-S) boundary known as the Late Ordovician Mass Extinction (LOME).

Late Ordovician Mass Extinction:

The Ordovician period, which lasted 485.5 million years and ended 443.8 million years ago, was a period of significant transition for life on Earth. Species diversity flourished throughout a 40.1-million-year span, but when the period came to a close, the first known mass extinction on Earth happened in a time when organisms like corals and shelled brachiopods populated the world's shallow-water regimes but hadn't yet ventured onto land. After first appearing approximately 3.7 billion years ago, life was beginning to spread and diversity. However, some 440 million years ago, a climate shift led sea temperatures to rise, resulting in the extinction of the bulk of marine life.

The Late Ordovician (end-Ordovician) Period was characterised by significant climatic, environmental, and biological changes. Studies show that a long period of cooling began in the Katian Stage (453.0–445.2 Ma) or possibly earlier and culminated in an extreme glaciation on the North African coast of Gondwana and peri-Gondwanan Europe during the Hirnantian Stage (445.2–443.8 Ma).

The ascent of North America's Appalachian Mountains may have initiated the ice onslaught. The large-scale weathering of these newly raised rocks removed carbon dioxide from the atmosphere, causing the planet to become significantly cooler. A quick beginning of mass glaciation enveloped the southern supercontinent, Gondwana, at the end of the Ordovician period; glaciation on this scale locked away large percentages of the world's water. As a result, global sea levels dropped hundreds of feet, robbing many species of their essential habitats, breaking food chains, and reducing reproductive success. The habitats of creatures living in shallow seas would have drastically cooled and shrunk, suffering a huge blow. In chemically adverse seas, whatever life survived was slowly recovered: When Sea levels began to rise again, marine oxygen levels dropped, allowing dissolved hazardous metals to accumulate more readily in ocean waters.

glaciers
image source: pexels.com


The Hirnantian Stage witnessed the second-worst known mass extinction event, the Late Ordovician mass extinction (LOME), which was believed to have wiped off 49–61 per cent of genera and nearly 85 per cent of marine species globally. The event had the greatest impact on marine organisms, affecting all major taxonomic groups and resulting in the extinction of one-third of all brachiopod and bryozoan families, as well as numerous bivalves, corals, eel-like creatures known as conodonts, echinoderms, graptolites, and trilobites.

Marine life during Ordovician period.
image source: wikipedia

The first pulse of extinction has been related to global cooling, habitat loss, and sea-level fall caused by glaciation, but the second pulse of extinction has been linked to sea-level rise and the expansion of oceanic anoxia as a result of deglaciation during the Late Hirnantian. Several lines of evidence, however, imply that anoxia, volcanism, or a mix of the two may be linked to the LOME.



Marine extinction intensity during the Phanerozoic: The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the traditional "Big Five" extinction events and the more recently recognized Capitanian mass extinction event.
Marine extinction intensity during the Phanerozoic: The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the traditional "Big Five" extinction events and the more recently recognized Capitanian mass extinction event. image credit: wikipedia

The cause of the first of the "Big Five" major mass extinction events remains unknown. Many theories have been proposed. One idea proposes that the cooling process was triggered by the creation of the Appalachian Mountains in North America; large-scale weathering of these mountainous silicate rocks is linked to the removal of greenhouse gas carbon dioxide from the atmosphere.

Not all scientists agree with this, however. Alternative theories suggest that toxic metal may have dissolved into ocean waters during a period of oxygen depletion, wiping out marine life. Other scientists suggest that a gamma-ray burst from a supernova ripped an enormous hole in the ozone layer, allowing deadly ultraviolet radiation to kill life below.

Volcanic Activity
image credit: pexels.com


While environmental degradation and oceanic changes linked with the Hirnantian glacial have been widely highlighted as probable extinction triggers, new evidence for a massive igneous province eruption during that time has cast doubt on that theory. The timing of Hg anomalies with the extinction horizon in the continents suggests that extensive and widespread volcanism had a global climatic and ecological impact and was a primary trigger for the prolonged and synergetic deterioration of the Late Ordovician environment, including climate change, ocean acidification, and anoxia, resulting in the second most severe Phanerozoic biodiversity crisis - Late Ordovician mass extinction (LOME). 


Late Ordovician Mass Extinction Summary


https://www.academia.edu/download/67266673/928c3214547d66457fbb4ae3fb2b0650fdea.pdf

https://www.sciencedirect.com/science/article/pii/S0921818120302654

https://www.sciencedirect.com/science/article/pii/S0012821X19302249




Post-graduated/post-doc position at University of Milan, Italy

Two Tenure-Track Faculty Positions (Climate Change & Global Sustainability), Graduate Institute of Environmental Education, National Taiwan Normal University, Taipei, Taiwan.

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