Ozone depletion triggered mass extinction during the Age of Fishes

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