The Collapse of the Devonian Ecosystem: Unraveling Extinction
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Chapter 1: Prologue to Extinction
In our previous discussion on the decline of the Late Devonian environment, we examined the remarkable proliferation of plant life on Earth and its consequences. The Devonian oceans became saturated with organic matter and nutrients, leading to widespread anoxia in shallow marine areas and the extinction of much existing life.
As we previously noted, the Devonian era's biological diversity was on the verge of a catastrophic downturn.
Section 1.1: The Role of Ozone
Life on Earth is incredibly delicate. While a single oxygen molecule isn't sufficient for effective cellular respiration, two combine to form free oxygen, which is vital for life. When a third oxygen molecule joins, it produces ozone, which accumulates in the stratosphere about 7 to 30 miles above the planet's surface.
The ozone layer is crucial for humans; high levels of this triple oxygen molecule can be harmful to our lungs. Moreover, it serves an essential protective function, forming through the interaction of free oxygen (O2) and solar ultraviolet (UV) radiation, safeguarding the biosphere from the Sun's damaging UV rays. Without this layer, both flora and fauna would be adversely affected by excessive UV radiation, leading to increased genetic damage.
Section 1.2: Genetic Damage in the Late Devonian
Researchers from the University of Southampton made a fascinating discovery while investigating the pollen cell walls from the Late Devonian period. Their analysis of samples from East Greenland, which span the boundary between the Devonian and the Carboniferous Period, revealed a significant occurrence of genetic damage in plant spores dating back to around 359 million years ago. This damage aligns with the deformations caused by excessive UV exposure.
The first video titled "Late Devonian Mass Extinction Event ~360 Million Years Ago | GEO GIRL" delves into the catastrophic events during this period, shedding light on the extinction mechanisms at play.
Subsection 1.2.1: Radiation Effects on Plant Life
Plant DNA is especially vulnerable to UV radiation during its early developmental stages before the protective sporopollenin walls are formed. Excessive radiation during this critical phase results in genetic damage.
The findings from East Greenland suggested that the increasing damage to spores preceded a significant decline in spore diversity, marking the extinction of numerous plant species. This trend was directly linked to the disappearance of larger stemmed plants found in the overlying rock layers.
Chapter 2: The Ozone Crisis
Researchers speculated that the end of the Devonian era was associated with a depletion of Earth's ozone layer, which in turn led to genetic damage and the collapse of Late Devonian forest ecosystems. The proposed cause of this ozone depletion was rising continental temperatures, which facilitated greater convective water transport into the upper atmosphere. This water vapor contained various chemical compounds, including chlorine—naturally produced by certain plants and algae. Once in the stratosphere, chlorine contributed to the breakdown of ozone, allowing harmful UV radiation to penetrate the Earth's surface. Thus, climate warming led to a weakened ozone layer.
The second video titled "What Was Life Like During The Devonian Period?" explores the conditions of life during this significant era, providing context for the environmental changes that precipitated extinction.
Section 2.1: Cascading Catastrophes
Mass extinctions are often the result of rapid environmental changes. However, given the prolonged 24-million-year decline leading to the Late Devonian extinction, the specific causes remain largely speculative. Various pieces of evidence suggest that multiple factors contributed to this extinction event, hinting at a "perfect storm" scenario.
A historical example of such a storm occurred in 1991, culminating in a superstorm that caused significant destruction. This illustrates how a series of natural events can converge to create a catastrophic outcome.
Perhaps the end of the Devonian was a result of a combination of global events: the extensive growth of continental forests led to frequent over-fertilization of oceans and subsequent widespread marine anoxia. The increased vegetation also released more natural chlorine into the atmosphere, further depleting the ozone layer. The ultimate collapse of the Devonian forest ecosystems due to ozone depletion resulted in a massive influx of organic material into the oceans, exacerbating the anoxic conditions.
Despite the uncertainty surrounding these events, it is clear that 70-80% of Earth's species disappeared during this period. Extinction events are closely tied to significant climate shifts, and the current Anthropocene extinction is no exception. The complex interplay of environmental factors can lead to unforeseen disasters. Rising CO2 levels, for example, result in elevated temperatures, Arctic thawing, Antarctic melting, rising sea levels, and ocean acidification, all of which threaten the foundational elements of the marine food web.
(Next up — The Emergence of Carboniferous Coal Beds)
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)