After three detailed and obsessive episodes on Plate tectonics, we are ready to move towards another chapter and mind blogging event in the History of the earth. The Cryogenian glaciations are also known by the catchy name snowball earth is currently holding the title of the most severe ice ages on the earth.
Annyeong haseyo friends. Welcome to another post of the “History of Earth” series. The previous three posts were based on Plate tectonics including , how plate tectonics had shaped our planet and its effect on natural disasters.
Cryogenian glaciation had sprouted from the most static time of earth’s history and went on a totally opposite way of chaos. The climate which was super static all over the earth for one billion years went totally out of hand in Neoproterozoic. This era is also marked by the rising of the very first complex life on earth, the Ediacaran biota. In many ways, the out-of-control earth systems had awoken earth from the deep slumber of inactivity towards active systems of life and climate.
How the glaciation started?
It is complicated to pinpoint how and what movements triggered the spreading of ice sheets but we all can agree that it started after the breakup of Rodinia. The boring billion from 1.85 billion years to 850 million years ended with Tonian period. It had seen the rise of modern plate tectonic and assemblage of two supercontinents Columbia (Nuna) and Rodinia. Both of these continents were situated near the equator and were surrounded by a mega ocean. Other than these barren land there might have few volcanic islands near the coast due to tectonic movements. We are talking about a time almost 2 billion years ago so obviously, there was nothing on land and sea alike other than microbes. The land was red and dry.
The split of Rodinia
Around 850 million years ago Rodinia split into two pieces (eastern and western) and new coastlines emerged out of the split. Oxygen levels were very low probably a few percent and the atmosphere was dominated by nitrogen and greenhouse gases. The split started around 850 million years ago with few fissures here and there and it came to full motion around 750 million years ago when the planet divided into two parts.
Through the most Tonian period of the Neoproterozoic era, Rodinia was busy splitting into smaller landmasses so the real effect of glaciation set in the Cryogenian period.
New coastlines emerged and it started heavy coastal erosion. Ocean water entered into the rift valley and created a new intercontinental ocean. The powerful ocean waves had begun their constant assault on rocks and turned them into fine grains. These sediments washed into the newly formed ocean and settled into the ocean floor creating sedimentary strata. With the flow of sediments also came an influx of nutrients that was scarce in the ocean. Phosphate, Molybdenum, manganese, and other essential elements washed into the ocean creating a paradise for photosynthetic life to flourish.
The things were about to change now. There are few important parameters to understand how glaciation had begun.
The carbon problem
The carbon cycle got totally interrupted during the Cryogenian period. Firstly, the immense algal bloom around all the coastline had started to suck carbon dioxide out of the atmosphere. The more influx of nutrients had promoted more algal bloom and more carbon dioxide was removed from the atmosphere.
Secondly, Photosynthetic algae were not only sucking carbon dioxide but were also releasing oxygen into the atmosphere. In a simple way, carbon dioxide was decreasing and oxygen was increasing. Oxygen had reacted with methane in the atmosphere and turned it into carbon dioxide giving more fuel to photosynthetic cyanobacteria.
Thirdly, When these photosynthetic bacteria died out they got rapidly buried in sediments. No decaying bacteria had evolved around 750 million years ago so dead photosynthetic life was buried whole taking carbon preserve in them inside the rocks. This carbon was removed from the cycle and more and more carbon dioxide was removed from the surface due to heavy algal bloom. Although, volcanoes were releasing carbon dioxide into the atmosphere continental split was producing more algal bloom due to nutrient influx. Those photosynthetic microbes were taking carbon dioxide more and more and even volcanic eruptions were unable to maintain equilibrium.
Another carbon problem
Yes, folks, there is another carbon problem. Rodinia split apart and countless underwater volcanoes had created under the ocean, a kind of mid-oceanic ridge. These volcanoes had erupted lava and created oceanic floors on both sides. As the supercontinent split more apart more lava sprouted out of volcanoes forming strata of hot lava. Due to this ocean level increased and more portion of land came under weathering. Rock weathering requires carbon dioxide. When more land became exposed to water weathering, more carbon dioxide was used and its concentration in the atmosphere fell.
The faint sun paradox
The continental breakup had pushed earth out of equilibrium into a chaos of climatic instability. Carbon levels plunged downwards and oxygen rose up. Greenhouse effects that depend mainly on carbon dioxide and methane weaken. On top of that, we had another thing to consider. According to research, our sun which too is almost 4.6 billion years old was fainter at Neoproterozoic. Only releasing radiation up to 85% of its present capacity.
The fainter sun couldn’t keep up with radiation and warmth and when carbon level fell (including carbon dioxide and methane) it was unable to withhold the greenhouse effect. Temperatures dropped drastically and ice started to form from the poles. Ice had higher albedo and it reflects most of the sunlight back into the atmosphere. The greenhouse effect became so weak that it couldn’t hold reflected light and temperatures dropped even more.
The ocean only reflects 10% of sunlight and absorbs the majority of it. Thick sheets of ice spread over oceans and lowered their albedo. Ice started from poles spread across the planet and some estimated suggest that it also covered the equator encasing earth under miles of thick ice. Blue planet turned into white planet more like Europa, Jovian moon. Photosynthetic microbes died out in heavy numbers some only surviving became they were near the volcanic islands, only somewhat warm places on an otherwise icy planet.
The Yoyo climate cycle
The field observations and paleomagnetic studies have given us countless prove of global ice age events where glacial disposal has found in equatorial regions. Glacial rocks and tillies have been found on every continent and evidence suggests that the equatorial ocean was all but almost frozen in the Cryogenian period.
The first ice age
We know that algal bloom happened because sediments deposited around that time have heavier carbon isotope C13. When photosynthetic microbes increased in numbers they have absorbed a lighter C12 isotope for photosynthesis and heavier ones were left for weathering process.
Algal bloom had taken up most carbon dioxide and released oxygen into the atmosphere. This oxygen had reacted with methane and turned in into more carbon dioxide. Other than photosynthesis, carbon dioxide had helped in rock weathering forming thick sediments. Carbon levels plunged and oxygen levels had risen up. Ice that started forming on poles reflected most of the sunlight. A weak greenhouse was unable to conceal the warmth and ice started to spread towards the equator. When the earth was under full-fledged ice, oceans were unable to soak of radiation, in turn, all photosynthetic life except a few near equator and volcanic island died out.
Inter glacial hothouse period
Earth was under the ice for millions of years but it is bound to change. Plate tectonics and volcanic eruptions had again released carbon dioxide into the atmosphere. As earth including land and ocean were under thick ice sheets no weathering and photosynthesis could happen. Due to this carbon levels increased and greenhouse started to build up. A stronger greenhouse was able to withhold solar radiation resulting in higher temperature. Ice started to melt and the oceans cleared up to soak their share of solar heat. It took a while for life to bounce back and rock weathering to start but till then carbon levels were too high in the atmosphere. This incredible greenhouse on earth has transferred the planet into a hothouse.
Again back to the ice
Two ice ages have happened during the Cryogenian period. Hothouse earth was all set for promoting a surge of life. Increased carbon dioxide, when mixed with rain, fell as acid on land and ocean. This acid rain had increased weathering turning solid rock into clay. Weathered rocks flowed into the ocean supplying nutrients for photosynthetic bacteria. High carbon dioxide and ample nutrients worked as catalyzers and photosynthetic life surged all over the globe.
The ice age repeated itself when photosynthesis increased beyond level. Life started to suck carbon dioxide out and more oxygen was pumped back into the atmosphere. Rock weathering also increased. More carbon was removed and the greenhouse effect plunged down. Again ice sheets formed from poles and equator encasing earth in another long spell of cold.
This yoyo climate effect happened perhaps three including two major ice ages. Climate rolled from one extreme to another extreme bring ice and fire on earth.
The effect of boring billion
Cryogenian is the second period of the Neoproterozoic era that started 750 million years ago with the breakup of Rodinia. Tonian the first period did not have earth-changing events and it too began with rather static the boring billion. The whole billion years had pretty stable atmospheric and oceanic conditions. We had a pretty stable Rock cycle. However, boring billion from 1.85 billion years to 850 million years ago had set the stage for modern plate tectonics the most unique quality of the earth.
The boring billion was very stable in respect to climate. Everything was very uneventful. Although we have evidence of two minor ice ages before however, nothing like snowball earth has ever happened on earth. There is an intuitional reason for this.
The rise of modern plate tectonics
Before boring billion plate tectonic was very shallow more like a think layer of ice floating on water can be easily broken. These shallow plates were not affecting the earth’s interior and vice versa. Mineral cycle was very primary as well as recycle of volatiles (carbon, nitrogen, sulfur) was of the shallower levels. This changed when plate tectonic upgraded to a modern level. Strong and deep subduction zones started shaking the upper mantle. Magma traveled all the way down from the lower mantel to have erupted from powerful, huge volcanoes bringing exotic volatiles and minerals never seen on earth before.
Modern tectonism brought effect more like when you try to spread cold butter on bread. Crust became thicker and its movement had created interesting features on land as well ocean floor. Volcanism increased and materials from the earth’s depth were involved in surface cycles. Stick and stable climates of previous eons became active and chaotic. Little imbalance was enough to bring the whole system down and that’s what happened during the Cryogenian period.
In this manner, boring billion have brought the global ice age directly or indirectly.
Snowball or slush ball?
For a long time, scientists had believed that the earth was frozen completely from poles to the equator. Recently many scientists believe that in spite of blood-freezing cold equator was warm enough to be iceless.
To understand recent climate trends in wake of climate change scientists have developed many sophisticated climate models. They are using them to understand the sensitive effects of tiny changes. When researchers have used these models to understand Cryogenian climate results have shown that even at height of the glaciation equator didn’t completely freeze. There are pieces of evidence of ocean current, moving water in the equator. Perhaps the equator was covered with a thin sheet of ice capable of dismantling it. At this same point, researchers think that photosynthetic life was able to survive under a thin sheet of ice. When ice episodes ended it bounced back and spread across the globe.
On the basis of arguments, scientists believe that earth was more like a slush ball instead of a solid snowball. The Equator was more like temperate regions with relatively cold water and very thin ice sheets.
Read why researchers think that snowball might have been slush ball.
Why Cryogenian glaciations are so unique?
There are two ways to establish why Cryogenian glaciations were so unique. Firstly, Plate tectonics was in part responsible for dramatic changes. Secondly, although the sun was fainter before the internal heat from the earth was stronger to prevent ice formation. At the time of Cryogenian, the period sun was only radiating up to 85% of its current capacity. Before that, it was only 70% and 65% powerful. This weaker sun was not a major source of heat on earth but still, the earth was unfrozen. Since the earth had formed it is releasing heat out to space. The heat had formed due to the collision of planetesimals and radioactivity. It was keeping the earth alive and working. In earlier years when it was stronger, it had worked as a heat source for the earth. It had kept the earth warm in spite of having a faint sun.
Earth had seen two smaller ice ages in Archean eon which happened due to excess greenhouse gas. The constantly erupting volcanoes had filled the atmosphere with carbon dioxide, sulfur, and methane that encircled the globe like a thick blanket. This had covered the earth and blocked the sunlight from reaching the surface bringing a mini ice age.
The Cryogenian period is so unique is because never before and not after it at least yet something like this has happened. Ice was present till the equator even if it was not frozen. Secondly, the ice ages brought a beautiful present as they ended the dry spell of complex life on earth.
The rise of complex life
When ice ages ended in the Ediacaran period oxygen levels were high, high enough to push life towards complexity. With almost 15% oxygen level soft creatures more like jelly and worms originated in oxygen-saturated oceans. The oldest fossils of multicellular life have found from the Ediacaran period. Scientists have found fossil evidence in the Ediacaran Hills of western Australia. Firstly, scientists had dismissed them because it was an established fact that multicellular life had originated in the Cambrian period 541 million years ago. But, now Ediacaran biota is an established evidence-backed theory.
Another thing to remember is that oxygen levels were 15 % in the atmosphere that means the ozone layer was already in motion. When oxygen was released into the atmosphere it had first formed ozone. The excess oxygen had saturated the atmosphere after it. A protecting ozone layer had blocked ultraviolet rays making complexity possible on earth.
refer to the following post did snowball earth make animals? where the author is trying to establish a connection between glaciation and the Cambrian explosion.
- A different take on Cryogenian glaciation by PBS eons how volcanoes froze the earth (twice).
- An interesting documentary on two catastrophes- Snowball earth and The Permian extinction.
- if you want a short and detailed explanation of the snowball catastrophe does refer to it.
- Read this post on The story of snowball earth explained by famed geologist Paul Hoffman, the pioneer in snowball earth research.
- One whole website dedicated to snowball earth events if you want to go into more depth.
The complex multicellular life had spiced up an otherwise plain story of the earth. From Ediacaran especially from Cambrian, the equation complexity had kickstarted and it is still working on towards more uniqueness and diversity. Stay tuned for the next blog post on Ediacaran biota and the new complexity on earth. Until then do the Revelation.
Disclaimer: I do have written 3 blog posts on plate tectonics before as I thought it is quite an important and detailed topic. Modern plate tectonics started to take shape in boring billion which comprises most of the Mesoproterozoic era and the last period of the Paleoproterozoic era. After the end of the Mesoproterozoic era, tectonic systems were more like today’s standard. After the end boring billion Cryogenian glaciations. Although according to the timeline I should have written about boring billion first but I am still searching for enough satisfactory material for this important time period. I will update a blog post on it as soon as I will get enough material. So, according to the timeline after the oxidation event boring billion is the next great event then comes snowball earth.