How CO2 starvation Caused the Greatest Extinction Event
13 hours ago Guest Blogger 54 Comments
By Jim Steele
Around 400 million years ago during the Devonian, carbon dioxide concentrations were over 2000 ppm, 5 times higher than today’s level (graphic A). That allowed evolving land plants to rapidly spread across the land. Plant species diversified and increased so rapidly it was called the Devonian Explosion. Marine species likewise multiplied enabling greater fish speciation, so the Devonian was called the Age of Fishes. However, by the end of the Devonian, the increase in photosynthesizing plants had greatly reduced CO2 concentrations to near dangerous levels.
During the following geologic period known as the Carboniferous, great forests of primitive Lycopod trees now covered the earth’s wetlands. Trees buried in the swamps were slow to decompose, creating some of the earth’s greatest coal deposits (graphic G). That further sequestered CO2. Some research suggests CO2 levels fell as low as 150 ppm, plant starvation levels. That led to the Carboniferous Rainforest Collapse around 305 million years ago (graphic B). Low CO2 levels also correlated with a die-off of ocean algae known as the “Phytoplankton Blackout” (graphic D). This dramatic collapse in primary production on land and in the sea, disrupted the earth’s Permian food web and set in motion a series of long-drawn-out extinctions known as “Dead Clades Walking”.
The vigorous photosynthesis of the Carboniferous had also generated the earth’s greatest levels of oxygen (graphic C). Compared to our atmospheric oxygen of 21%, oxygen levels reached 30 to 35%. High levels of oxygen allowed giant arthropods to evolve (graphic E). It also enabled amphibian ancestors, that breathed by absorbing oxygen through their skin, to better colonize the land. However, as forest species and phytoplankton went extinct, oxygen levels plummeted (graphic C). As a result, the giant arthropods, as well as primitive amphibians like Euryops, (graphic F) adapted to more abundant oxygen, were the first to go extinct by the early Permian around 295 million years ago. Falling oxygen concentrations also reduced land animals’ ability to use high altitude ecosystems.
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