Solar Radiation Exacerbates a Bad Situation
Published in The EarthSphere Blog. Feature Image: Our Sun Flaring UV Radiation (by ArcheanArt, based on public domain photos from NASA)
Prologue
We started tracing the collapse of the Late Devonian environment in our last Forgotten Origins article. The stunning success of plant life on Earth had repercussions. Devonian seas were inundated with organic matter and nutrients, and widespread anoxia developed in the world’s shallow oceans, extinguishing much of the existing life there.
Last time: Devonian Life Sinks into Mass Extinction
Ozone
Life is finicky. A single oxygen molecule doesn’t do the trick for efficient cellular respiration. But put two of these molecules together, and you have free oxygen, an essential ingredient for life in Earth’s biosphere. Two is the magic number. Adding a third oxygen molecule creates a gas called ozone, which accumulates in the stratosphere between 7 to 30 miles above Earth’s surface.
The positioning of the ozone layer is fortuitous for humans since high concentrations of the triple oxygen molecule are harmful to our lungs. But the ozone layer serves another life-preserving purpose. Stratospheric ozone forms from the interaction between free oxygen (O2) and solar ultraviolet (UV) radiation, and ultimately, the ozone layer protects the biosphere from the Sun’s harmful UV radiation.
Without the ozone layer, both plant and animal life would suffer from the effects of excessive UV radiation. One of the nastier consequences of increased UV exposure is higher rates of genetic damage.
Researchers at the University of Southampton found something interesting while studying pollen cell walls from the Late Devonian. The samples used in their study came from rocks in East Greenland that span the boundary between the Devonian and the subsequent Carboniferous Period. Their work uncovered a high incidence of genetic damage in plant spores at the end of the Devonian, about 359 million years ago. The damage they observed was consistent with deformities caused by excessive exposure to UV radiation.
Radiation Poisoning
Plant spore DNA is particularly sensitive to UV radiation during the plant’s developmental period before its protective sporopollenin walls form. Excessive radiation at this phase of their life cycle causes genetic damage.
The East Greenland samples provided interesting insights into life at the end of the Devonian. Increasingly damaged spores preceded a notable decline in spore diversity, indicating the extinction of many plant species. These events were tied directly to the disappearance of large, stemmed plants in the overlying rock strata.
The researchers theorized that the end of the Devonian coincided with a loss of Earth’s ozone layer, and the resulting genetic damage caused a collapse of the Late Devonian forest ecosystems.
The cause for the ozone problem is postulated as rising continental temperatures that allowed more convective water transport into the upper atmosphere. The water vapor carried chemical compounds, including chlorine, which is produced naturally by some plants and algae. When the chlorine reached the stratosphere, it broke down the ozone, thus allowing damaging ultraviolet radiation to reach the Earth’s surface. Essentially, the problem was a weakened ozone layer caused by climate warming.
The last stages of the Devonian coincided with the onset of glaciation. The glacial episode was short-lived, followed by warming that extended into the early Carboniferous, where estimated global temperatures were 20 degrees C, significantly warmer than today. This scenario is consistent with the theory that rising continental temperatures may have affected Late Devonian ozone levels.
All of the Above — Cascading Events
We know mass extinctions are caused by rapidly changing environmental conditions. Still, because of the lengthy 24-million-year decline encompassed by the Late Devonian extinction, its specific causes are more speculation than fact. But there are various bits of evidence hinting at possible causes, and a possible answer is all of the above — the perfect storm.
A classic perfect storm happened in 1991, and the movie version was produced in 2000. The meteorological backstory details a set of unusual circumstances that allowed a nor’ easter to absorb a hurricane and evolve into a superstorm. This storm resulted in thirteen deaths and thirty-foot waves pounding the east coast of the USA, causing around a half-billion dollars of damage in today’s money. Cascading natural events led to the perfect storm.
Perhaps the end of the Devonian was a combination of cascading global events. The expansive growth of continental forests caused frequent over-fertilization in the oceans and subsequent widespread marine anoxia. The combination of plant and algae growth also pumped more natural chlorine into the atmosphere, weakening the ozone layer. The global collapse of the Devonian forest ecosystems from ozone depletion then created a massive influx of organic material into the oceans, further prolonging the anoxia.
We don’t know the answer, but 70–80 percent of Earth’s species disappeared. Extinction events are associated with dramatic climate change, and the Anthropocene extinction is no exception. We cannot see all of the environmental connections, and a series of cascading events can lead to unexpected disasters. Rising CO2 levels lead to higher temperatures, leading to Arctic thawing, Antarctic melting, rising sea levels, and ocean acidification that kills off the base of the marine food web. But we can already see this chain of events. What is out there that we can’t yet see?
(Next up — Carboniferous Coal Beds)
ArcheanArt
Sources:
What is Ozone? (Source: EPA)
Prehistoric Climate Change Damaged the Ozone Layer and Led to a Mass Extinction (By John Marshall; National Interest)
UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism (by John E. A. Marshall, Jon Lakin, Ian Troth, and Sarah M. Wallace-Johnson; Science Advances)
