Science

Ozone depletion triggered mass extinction during the Age of Fishes

Ozone depletion triggered mass...
Life on Earth is ordinarily protected from harmful solar emitted UV radiation by our planet's thick atmospheric shell
Life on Earth is ordinarily protected from harmful solar emitted UV radiation by our planet's thick atmospheric shell
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An image of a normal spore (left), compared to one that has grown abnormally due to ultraviolet radiation exposure (right)
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An image of a normal spore (left), compared to one that has grown abnormally due to ultraviolet radiation exposure (right)
Life on Earth is ordinarily protected from harmful solar emitted UV radiation by our planet's thick atmospheric shell
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Life on Earth is ordinarily protected from harmful solar emitted UV radiation by our planet's thick atmospheric shell

A new study has revealed that a cataclysmic disruption of Earth’s protective ozone layer may have allowed damaging levels of ultraviolet (UV) radiation to saturate the Earth 359 million years ago, triggering a global mass extinction. The researchers behind the study warn that the sudden warming process that caused the ozone layer to weaken could occur again in the future, as our planet continues to heat up as a result of climate change.

On a dark night, the stars that pepper the night sky are a beautiful sight. But their twinkling visage belies their violent tendencies. Many stars are prone to lashing out at orbiting planets with vast quantities of radiation, rendering the unlikely scenario of life emerging on those worlds all the more unlikely.

Thankfully, since the onset of recorded history, every creature to walk Earth’s surface or swim its oceans has been protected from our Sun’s potentially damaging radiation by our planet’s dense atmosphere. More specifically, a region of Earth’s stratosphere known as the ozone layer acts as an effective shield, warding away the harmful UV radiation pouring from our star.

However, this does not mean that the evolution of life on Earth has progressed without challenge. The geological records of Earth have revealed evidence of periodic, planet spanning mass extinction events, each of which has claimed the lives of countless species, and altered the very path of evolution.

The most famous mass extinction event is without doubt the asteroid impact that was arguably responsible for wiping out the dinosaurs some 66 million years ago. However, others have been triggered by global volcanic eruptions, which degraded Earth’s surface environment and altered the chemical composition of the atmosphere.

Now, a team of scientists has discovered evidence to suggest that life on Earth has not always been protected from the dangers posed by our parent star. More specifically, the evidence suggests that a collapse of ancient Earth’s ozone layer may have allowed a burst of UV radiation to engulf the planet, altering the environment and killing off many ocean species.

The mass extinction examined by the scientists is thought to have occurred roughly 359 million years ago as Earth was emerging from an ice age near the end of the Devonian period. This part of our Blue Marble’s history is colloquially referred to as the Age of Fishes.

If you were to travel back in time and view our planet from orbit, it would appear completely unrecognizable. During the Age of Fishes, Earth’s surface was dominated by two enormous supercontinents. The southernmost of the two is referred to now as the continent of Gondwana, and would one day fracture and drift apart to become modern day South America, Australia, Africa, India, China and Antarctica.

The second landmass – known by some as the Old Red Sandstone Continent – was also located in ancient Earth’s southern hemisphere, albeit closer to the equator, and accounted for modern day Europe, Iceland and Northern America.

Though at this time our distant ancestors – the tetrapods – were busy evolving rudimentary limbs, life was confined largely to the oceans, in which impressive species of armoured sharks such as the Titanichthys reigned.

The team collected samples from modern day Greenland – in what was once an ancient lake bed in the dry interior of the northernmost supercontinent – and from the Andean Mountains in Bolivia, which would have been located at the edge of a melting ice sheet. This allowed the scientists to compare extinction data taken from near the equator to that harvested from the south polar region of ancient Earth.

Once transported to a laboratory setting, the team set about dissolving the rock samples in hydrofluoric acid in order to gain access to the well-preserved microscopic plant spores contained within that dated back to the extinction event.

Many of the spores were found to have taken on a darker pigmentation than would have been expected, and had grown unusual spines on their surfaces. According to the team, the change in color is likely a protection response to an excess of UV radiation. This wavelength of radiation could also have damaged the spores’ DNA, triggering the growth of the unusual spines.

But then how did such a damaging amount of UV radiation reach Earth’s surface? The team believe that the ozone disruption was a natural part of Earth’s climate cycle, rather than an aberration stemming from a bout of powerful volcanic eruptions.

Following the melting of continental ice caps at the close of the Devonian ice age, the climate would have suddenly grown very warm. This excess of heat above the continents could have interacted with the delicate ozone layer above, destroying chemicals in the atmosphere and disrupting its protective properties. This in turn could have allowed dangerous levels of UV radiation to reach the surface.

The ozone may have existed in this weakened state for a period of several thousand years, during which time the UV radiation wreaked havoc on our world, collapsing forest ecosystems, and wiping out entire species of marine life and surface plants.

The bone-armored giant sharks were killed off, along with countless other species, but other sharks and bony fish were able to survive along with some hardy plants that were able to cling to existence on the supercontinents above.

Our own ancestors – the tetrapods – were likely also affected. The disruption and loss of habitat would have led to evolutionary bottlenecks in fish and tetrapods that shifted the course of our evolution.

According to the team behind the study, we should be alert to the danger of ozone depletion in the future, as our world continues to warm as a result of climate change.

"Current estimates suggest we will reach similar global temperatures to those of 360 million years ago, with the possibility that a similar collapse of the ozone layer could occur again, exposing surface and shallow sea life to deadly radiation,” comments lead researcher Prof. John Marshall, of the University of Southampton's School of Ocean and Earth Science. “This would move us from the current state of climate change, to a climate emergency."

Source: University of Southampton

5 comments
Jose Gros
I'm not that sure raising CO2 levels and thus, temperatures, may reduce the Ozone (O3) layer; the recent, and mostly recovered, reduction in Ozone layer was caused by Fluorocarbons and other elements, not by warming.
An element controlling the amount of ionizing radiation reaching earth surface is Van Allen belts, the planet magnetic field, when polarity of magnetic field reverses, magnetic pole is moving today quite fast, intensity of field reduces, allowing more dangerous radiations arriving.
The fossil and magnetic records may allow knowing if a reversal of magnetic field also had influence in that extinction.
We know now Betelegeuse is about to change in SuperNova, it's quite close to us, and its flare will send us a shower of cosmic rays and other dangerous radiation; after Novation this Red Giant Star will be similar to the Moon in size and brightness.
Should we return to caverns in order of protecting ourselves from these sources of radiation?
Blessings +
buzzclick
We will never truly know just how many mass extinctions there have been in Earth's prehistory. Our existence is but a speck of time, and time is not something we have much of as finite creatures. If just the fluorocarbons from the world's refrigeration systems caused a hole in the ozone layer above Antarctica, then the atmosphere is relatively fragile. But when someone uses the words "sudden changes" in geological history, it could be thousands of years to millions, which makes our 100-year lifespan hardly significant. Collectively though, we grow, consume and breed into one expanding mass that can adversely affect the Earth's creatures and climate in more ways than can at first be evident. From volcanic eruptions to atmospheric anomalies to interstellar events to orchestrating our own demise, it can seem fragile, but since our individual lifetimes are brief we live for the moment and choose to ignore all that demanding "stuff".
JustSaying
More proof of human kind adapting to the every changing environment of natural global climate change.
Karmudjun
Very interesting update on real science. I appreciate this synopsis of several different teams startling findings. It leads one to suspect that they may indeed have an opinion on exactly how the mechanism of ozone depletion occurred at the end of the Denovian Ice Age. I understand the volcanic extinction theories, I also understand the planetary impact (or meteoric impact) theory of extinction. While I may not be 100% sure of either theory, they are much more than plausible in my mind than this event from mere warming. There are many things affecting the atmospheric circulation patterns in our current age of 7 continents. Plus the ocean circulation that has mitigated much of the ocean warming (to date) is very different from that of a two continent planet. So I'd like to read the basic research or attend a few lectures on such when I retire so I can judge the plausibility of the hypothesis. I am sure that there are more than hydrofluorocarbons influencing the composition of the stratosphere and the ionization of the airborne molecules - so how did they (scientists) know that previous life was protected from the UV radiation for example? But you aren't writing the scientific paper so I'm not going to state my opinion on whether or not your synopsis accurately frames the process, whether or not a mere temperature change would affect it, or whether there were other factors involved that make this scenario. I am sure that this theory - to be vetted in a peer reviewed journal - would encompass a more complete picture than a mere rise in global warming gases from the known methane releases that occur when the planet tips past a certain temperature range. Very interesting.
Signguy
A lot of "suggestions" here...