Earth: 720-360 Million Years Ago

In the previous post, we discussed the initiation of Earth’s atmosphere, and the first formation of life. We learned how microscopic life managed to extremely change the climate by altering the amount of greenhouse gases in the atmosphere. As a result, Earth entered its first ice age known as the Huronian, which was followed by a greenhouse period, or warm period, extending from 2.1 billion to 720 million years ago. Warmer periods such as this will be discussed later. For now, we are going to move on to the end of this greenhouse period, which came with the start of another extreme ice age.

-Cryogenian Period: 720 – 635 Million Years Ago 

Up next in Earth’s geologic/climate timeline, we have the Cryogenian ice age. This period is also characterized by extreme cold events that likely caused ice to extend to the equator for about 65 million years of the period’s 85-million-year span. Like the Huronian ice ages, scientists are unsure whether the entire “snowball Earth” was ice, or if it was more slush-like.  

Evidence of the once-volcanic province that may have kickstarted the Cryogenian Period. Sills, or intrusions of volcanic material into older rock layers, cut across older, sand-colored rock. The bands in the lighter rock result from the coastline rising after the glaciers that weighed down the coast retreated. (Mike Beauregard)

Additionally, according to climate.gov, this period was broken up into two glaciations… 

Sturtian (720-660 million years ago): The longer period of ice formation. 

Marinoan (640-635 million years ago): The Marinoan glaciation dominated for the remainder of the Cryogenian’s 15 million years.  

However, it is in-between these glaciations when things get interesting. It is believed that intense volcanic and tectonic activity released tremendous amounts of carbon dioxide into the atmosphere through outgassing. Since most of the world was covered in ice, there was little precipitation and no exposed rock to absorb the carbon dioxide from the atmosphere. This brought rising temperatures as well as an end to each glacial period in the Cryogenian. More importantly, it began something new.  

A study from 2019 shows that during this time, when Earth became warmer between glaciations, life in the oceans, particularly algae, flourished. More specifically, algae abundances increased by a factor of about 100 to 1,000. The study concluded that on the surface, rocks and the nutrients within them were grinded by the moving glaciers, eventually washed away with the melting ice, and collected in the oceans. The increased availability of nutrients in the oceans likely caused this surge in ocean life, and ultimately led to the evolution of more complex plant and animal life. 

But how did Earth break the icy grip of the Cryogenian ice age?

The Ediacaran Period: 635 – 541 Million Years Ago 

The Ediacaran Period begins with the rapid retreat of the extensive ice associated with the Marinoan glaciation. As the continents continued to collide and drift, volcanic activity also continued. As previously mentioned, since the world was covered in ice, rainfall became rare. Thus, greenhouse gases could not be taken out of the atmosphere by weathering rock, and temperatures climbed once again. When large areas of ice melt as a result of this warming, there is a positive feedback that constitutes an increase in greenhouse gases and temperatures. (climate.gov)  

Learn how scientists use rock layers for evidence of extreme warming that began the Ediacaran period. (snowballearth.org)

Toward the end of the Ediacaran, this evidence suggests that temperatures may have reached higher than 90°F, compared to today’s average of less than 60°F. Ediacaran life took a hit at this point, paving the way for the next step in life and climate evolution.

According to climate.gov, it is worth mentioning that no amount of climate change in the last 500 million years has been as extensive as the Huronian and Cryogenian ice ages. Several more ice ages have occurred and we will talk briefly about a few of them as they come up. On the other hand, let’s turn our attention back to Earth’s warmer periods, like the end of the Ediacaran, or the beginning of the Paleozoic Era, when temperatures on Earth were much hotter than they are today.  

(More about life in the Ediacaran)

The Paleozoic Era: 541 – 252 Million Years Ago 

Temperatures remained elevated for about 180 million more years after the start of the Paleozoic, with the exception of the Andean-Saharan ice age, occurring some 460 – 430 million years ago. This era begins with an event that changed life forever.

The Cambrian Explosion: 541 Million Years Ago

This event is known as the Cambrian Explosion, which was quite literally an explosion of new life on Earth. (natural history.si) Although there was no plant or animal life on land just yet, the ancestors of many plants, clams, crabs, spiders, worms and insects are known to have flourished in the oceans. This event was an extremely important point in the evolution of life and yet another turning point for the atmosphere. 

One fossilized creature, a trilobite, dating back to Cambrian times.

At this point, around 485 million years ago, temperatures were very hot, but they began dropping as the supercontinent Gondwana was shifted to the south pole where much of it was submerged under water. Additionally, the area north of the tropics was almost completely ocean. As a result, ocean currents changed and Gondwana began accumulating ice at the south pole.

This website allows you to view Earth millions of years ago. 540 million years ago, a view from the north pole shows almost entirely ocean. Movement of the continents over millions of years is easily viewable.
Andean-Saharan Ice Age: 460 – 430 Million Years Ago

As the south pole began accumulating ice, Earth was sent into its next ice age. This ice age, called the Andean-Saharan ice age, chilled the Earth between 460 and 430 million years ago and is characterized by a decrease in sea levels, a drop in greenhouse gases and a loss of marine life. One of the first forms of plant life, mosses and liverworts, evolved and survived through the ice age. According to one study, they may have even helped cause it by taking carbon dioxide out of the atmosphere over millions of years.

The Devonian Period: 419 – 360 Million Years Ago

Speaking of plant life… What better way to introduce the Devonian Period? By the beginning of this period, glaciers of Gondwana have receded as the continent began drifting northward. Additionally, colonization of land by plants no more than a few centimeters tall was already under way. These first plants did not have leaves or roots, but the evolution of plant life was prepared to take over. Eventually, xylem, a component of wood, was created and by the end of the Devonian, most of the plants had true leaves and roots. Some of the trees grew up to almost 100 feet tall as the world’s first forests were formed. Many new species of fish were also spreading in the seas.

The Archaeopteris, a tree, grew over 30 meters tall during the Devonian. Pictured above is a fossilized branch with leaves.

For most of the Devonian, the world was hot, with carbon dioxide concentrations of about 4,000ppm (What does that mean?). Today, concentrations are at about 412 ppm. While this was a hot, flourishing, successful world for life, at this point, you could probably imagine that it didn’t last long. Of course, we should know there must be some event that will dramatically change this new world. We’ll discuss what happened during the remainder of the Paleozoic Era in the next post.

-Conclusion 

During this post, we briefly discussed a couple hundred-million years’ worth of extreme climate change beginning 720 million years ago. Through this time, we saw the Earth and its inhabitants transform from a snowball to a hot, flourishing world. We saw how the movement of continents, weathering of rock, evolution of life, and changing of ocean currents can cause these huge climate swings over periods of millions of years.

As we approach the present, one thing we should know for sure is that a dropping temperature is usually accompanied by a loss of greenhouse gases over time. It is when these greenhouse gases accumulate, that we can expect an increase in global temperatures. We should also realize that these large changes in climate are occurring over the course of millions of years.  

For more information on how past climates are reconstructed, check out this website.

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More posts coming soon…

-Andrew Bower

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