Animals Appear After the Thaw
Published in The EarthSphere Blog. Feature Image: Snowball Earth (by ArcheanArt)
Prologue
Previously in the Forgotten Origins series, we traced the rise of eukaryotes, an early step in the development of complex life forms. We also questioned why evolution seemed to stagnate for a billion years after that step forward.
Hot and Cold
Earth has run hot and cold for 4.5 billion years, but a warm planet is the norm. Earth spent about 670 million years of its history in glacial periods, so cold spells account for 15% of its existence. Meaning Earth’s normal state is a hot ice-free planet. Technically, we are currently in a cold spell but rapidly moving back to hot Earth conditions because of Anthropocene warming. We are now emerging from the last ice age, which was quite cold with massive glaciers covering much of the northern hemisphere. But 720 million years ago, our planet plunged into an unimaginable deep freeze during a period called the Cryogenian.
Atmospheric carbon dioxide (CO2) is measured in parts per million (ppm), and currently, Earth averages about 420 ppm of CO2 in its atmosphere. I once had someone comment that CO2 certainly couldn’t make much of a difference in temperature at such low concentrations. After all, even a change of 100 ppm is just a 0.01 percent alteration. They were arguing against the reality of climate change. But small concentrations of CO2 are deceptive because they have an oversized effect on atmospheric heat retention. Perhaps the best way to understand the problem is by pondering what the world would be like if our current 420 ppm level of CO2 dropped to zero.
Global temperatures today average about 15 degrees Celsius (59 Fahrenheit). If we removed all greenhouse gases from our atmosphere, with CO2 being the primary warming component, global average temperatures would plummet to minus 18 degrees C (-0.4 F). Freshwater freezes at zero degrees C, and seawater freezes at -2 degrees C. Without greenhouse gases, Earth would become an ice world.
The Blue Planet
Today Earth is a blue jewel floating in the dark vacuum of space. A stunningly beautiful planet we see in pictures from satellites. We live on a planet with 71 percent of its surface covered by water. The oceans absorb most of the color from the light spectrum and reflect predominately blue wavelength radiation. However, a blue Earth would not be the case without greenhouse gases. Earth would look like a sparkling white diamond suspended against the deep, black background of the Universe. An ice-encrusted planet, shinning like a snowball in space. The Cryogenian is also known as “Snowball Earth.”
The Cryogenian period began 720 million years ago and lasted for 85 million years. The period occupied the middle of the Neoproterozoic Era and was followed by the Ediacaran. Geologists divide the Cryogenian into two phases. The early Sturtian glaciation event from 720 million years before present (BP) to about 660 million years BP was the longest of the two. It was followed by the Marinoan glaciation, which ended 635 million years ago.
The debate about conditions on Earth during the Cryogenian alters between a planet frozen solid from the poles to the equator, to a world where equatorial oceans existed as an icy slush. However, some simulations indicate average equatorial temperatures of -23 Celsius or lower during the Cryogenian, supporting an ice-bound globe. Regardless of whether we discuss a frozen-solid Earth or a slushy one, such conditions were not static, and undoubtedly the climate fluctuated over time.
But these extreme climatic conditions would have stressed young Earth’s biosphere. However, the length of time over which Cryogenian glaciation occurred allowed evolution to cope with the adverse environmental conditions.
Consensus
There is no consensus on the causes of Cryogenian glaciation, but there is general agreement that fluctuations in solar radiation and Earth’s carbon cycle are both critical factors. The onset of icy Cryogenian conditions coincides with the breakup of the Rodinia supercontinent. The planet’s continental plates were welded together into this single landmass that broke apart about 750 million years ago. Some theories hold that the breakup promoted excessive continental weathering, thereby reducing atmospheric CO2 (extrovert assembly). As CO2 levels dropped, the planet cooled.
The normal geological carbon cycle involves the injection of CO2 into the atmosphere from volcanic eruptions and basalt flows. This process is counterbalanced by the chemical weathering of silicate continental rocks, which removes CO2 from the air and converts it to dissolved bicarbonate.The cycle involves a feedback loop whereby increases in atmospheric CO2 drive global warming, and the warmer climate encourages increased chemical weathering, removing the extra CO2. When this process becomes unbalanced, the planet can become excessively hot or cold depending on whether CO2 is being added or removed from the atmosphere.
Rapid planetary cooling promotes more glaciation. The resulting ice and snow have a higher albedo than rock and open water and reflect more sunlight. Less solar radiation is then absorbed by the planet, further contributing to a global drop in temperature. The inference is that chemical weathering and reduced solar radiation conspired to cause Snowball Earth.
The exact causes of Cryogenian glaciation are not certain, but the planet plummeted into a deep freeze. When Earth thawed, life had mysteriously taken a leap forward, and the first simple animals appeared in our oceans, sponges. Sponges are simple, stable creatures with life spans of up to 10,000 years and an ability to adapt to a vast range of environmental conditions. Evolution picked well.
(Next we will examine evidence and speculation about the rise of animal life)
ArcheanWeb
ArcheanArt
Sources:
International Chronostratigraphic Chart (Source: International Commission on Statigraphy)
The Cryogenian Glaciations and the Birth of Animals (Source: Earthly Universe)
CO2, Weathering and Climate (Source: GFS Helmholtz Centre Potsdam)
One of The Supercontinents Is Different from the Others (It’s Rodinia) (Source: Carnegie Science)
