Ecology And Environment

Causes of Mass Extinction



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A mass extinction event is happening at this very moment. Has been happening, in fact, since Homo sapiens decided to walk out of Oduvai Gorge and spread across every habitable land surface on the planet. From our merciless treatment of habitat, to the chemicals flowing from our cities into the land, air and sea, it seems undeniable: we have become a force of nature. This fact has now been rooted into geological fact, as was made official in a paper by Jan Zalasiewicz in 2008. The Holocene is officially over. Make way for the Anthropocene epoch.

Paleontological evidence suggests that this wouldn't be the first time the planet's biodiversity has taken a major hit. Five other major extinction events have occurred since life began to flourish in the shallow seas of the Cambrian era, approximately 600 million years ago. The late Cambrian and early Ordovician events are thought to have been cause by the onset of glaciations and lowering sea levels, affecting the warm, shallow seas and destroying habitats.

Other events, such as the Devonian approximately 400 million years ago, are less than clear when it comes to causes. This particular event may have been many smaller events spread out over millions of years, showing up as pulses grouped together in a way that has often been interpreted as a singular event. There are several theories for what might have caused this. Global cooling is one, while oceanic volcanism is thought to have merit as well. The natural proliferation of green plants across land surfaces might have pulled carbon dioxide out of the atmosphere, reducing the greenhouse effect. The possibility that an extraterrestrial body impacted during this time has certainly not been crossed out, although it is less likely as a cause.

The most significant event, the end Permian extinction, is known to have wiped out over half of all taxonomic families roughly 250 million years ago, with many groups barely scraping through. Because this event was so sudden, allowing very little time for animals to adapt to the conditions, a bolide (meteor) impact is thought to be unlikely. However, evidence suggests that a 'methane burp', working as a powerful greenhouse gas, may have been released from a large volcanic plateau located in Siberia. This could cause very strong global warming. However, the sheer scale of the event is such that no singular theory alone could explain it. It was at this point in evolutionary history when the synapsids (mammals) split off from the sauropods (reptiles, dinosaurs and later birds), who became the dominant group all the way until the end Cretaceous event.

The popular theory these days for the cause of the Cretaceous event is that a bolide impact occurred somewhere in the Yucatan Peninsula of present-day Mexico. This is well accepted, but it may have just been the final blow. Volcanism in the Deccan Plateau of present-day India would have thrown sulfur oxides, carbon dioxide and ash into the atmosphere over a period of about 800,000 years, resulting in a much slower rate of extinction. This had been the primary theory of the demise of the dinosaurs before Walter and Luis Alvarez provided very strong evidence for their impact theory in the 1980's.

And so we come back to the present day. Saturation of carbon dioxide into the oceans in causing ocean acidification, decimating reef systems. Fish stocks are collapsing. The climate is becoming less predictable every year, with hazardous weather cropping up in places that historically had gone without. Perhaps we, as a species, will recognize our effect on the planet and work toward a more sustainable future. Our impacts, however, have already been deeply etched into the bones of the earth.

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