So, we all unanimously agree what Earth looks like today – it’s a blue orb with liquid water and a protective atmosphere that helps to sustain life (of course we mean no disrespect for the conspiracy theorists who believe in Flat Earth theory). Let us rewind approximately 650 million years back in time. If really we could get our hands on time travel, the view of Earth ~650 million years ago (mya) would have left us in complete dismay. What we would be looking at would be planet completely blanketed with ice.
In essence we would be looking at a frozen planet that today we call home and for millions of years, other life forms have called it home. This is known as the Snowball Earth – a theory which states that about 650 mya, Earth was either completely frozen or was almost completely frozen. According to the theory, back in those days, our planet was covered by a single sheet of ice extending from pole to pole. Scientists however think that Snowball Earth was not a single incident and that it happened multiple times with the duration of each event varying.
Some experts say that prior to Snowball Earth, there was at least one more global-scale glaciation somewhere between 2,400 and 2,100 mya. This global glaciation has a different name – Huronian Glaciation. Those who support the global glaciation theory (that is those who support Snowball Earth and Huronian Glaciation) say that Huronian Glaciation triggered what is known as Great Oxygenation Event – the event when oxygen appeared in Earth’s atmosphere for the very first time. Similarly, the proponents of global glaciation also say that Snowball Earth resulted in multi-cellular organism evolution on this planet.
Now that we have a faint idea about Snowball Earth, let us try and answer some of the commonest questions asked about this theory. In the process, we will learn a few interesting concepts. You ready? Of course you are!
Question: What is Snowball Earth?
Answer: Snowball Earth refers to climatic condition of planet Earth when:
- Glacial ices extended from one pole to another.
- Global temperatures dropped to nearly -50 degrees Celsius or -122 degrees Fahrenheit.
- Temperatures at equator stayed at nearly -20 degrees Celsius or -68 degrees Fahrenheit. This is the temperature that we face today in Antarctica.
- Even the water bodies froze there were not oceanic activities that could moderate fluctuations in temperatures that are usually caused by cycles of day and night.
- The whole of earth turned into a solid surface made of ice.
Question: Who came up with the theory of Snowball Earth and how?
Answer: The credit for the theory goes to Joe Kirschvink from Caltech, Pasadena. Actually he coined the term. He was actually working with the works of Jim Kasting, Hal Marshall and James Walker – three well-know atmospheric scientists. Basically, Joe was studying Marinoan Glaciation (Snowball Earth) and found that the glaciation had a character of self-reversing and ended up connecting runaway ice-albedo with the glaciation event. A runaway ice-albedo is a process in which formation of ice covers due to cooling leads to reduced solar energy absorption because ice is known to reflect away sunlight instead of absorbing. The reduction in absorbed solar energy lead to further cooling and hence, increase in the ice cover, which eventually lead to a complete ice blanket on earth. The theory was radical and people were having hard time accepting it. It was only in 1998, when a paper with Paul Hoffman from Harvard University as lead author was published that the Snowball Earth received a significant attention.
In the paper Hoffman actually said that the 3rd prediction made by Kirschvink (ultra-greenhouse aftermaths) actually explains cap-carbonates which are found in post-glacial periods. One of the most characteristic features of the Snowball Earth is the presence of cap-carbonates.
Question: So, what the hell are these cap-carbonates?
Answer: Cap-carbonates are actually rocks. They are continuously layered and are either dolostone or limestone or both. They have very distinctive texture and are results of glacial meltdowns. Their thickness range between 3 and 30 meters. Cap-carbonates are usually the result of chemical precipitation and are often found in areas that are absolutely devoid of carbonate rocks. Cap-carbonates have distinctive features that vary very significantly from standard carbonates.
Not only the chemical compositions of cap-carbonates differ from standard carbonates, they also have weird-looking sedimentary structures that often look like gigantic ripples. They are also often characterized by tubular structures and microbial mounds.
Scientists have not reached a unanimous agreement on how the cap-carbonates formed after Snowball Earth but it is usually argued that after deglaciation, the excess CO2 that was present in atmosphere was dissolved in water from glacial ice meltdown, resulting in formation of carbonic acid. The resulting carbonic acid would have then poured down as acid rain, reacting with carbonate rocks and silicate to release calcium in high volumes. The released calcium was then washed down into oceans, forming sedimentary carbonate rocks with distinctive textures. The Snowball Earth was proposed in the first place because of the discovery of an abiotic cap-carbonate.
However, the attempted explanation for cap-carbonate formation has some problems that are listed below:
- Excessive atmospheric CO2 should have turned ocean waters acidic enough to actually dissolve carbonates.
- In certain areas, cap-carbonates are way too thick. Such thickness cannot be reasonably achieved with Snowball Earth deglaciation which was relatively quick.
- Furthermore, cap-carbonates are absent in several sequences of glacial events from nearly the same period.
Question: What really lead to Snowball Earth?
Answer: Experts argue that greenhouse gases were reduced significantly from the atmosphere because of rock weathering that took place due to tectonic activities. This, according to experts, happened when our Sun was not as bright as it is now. The rock weathering managed to reduce the amount of methane (CH4) and carbon dioxide (CO2) – the two primary greenhouse gases. As the greenhouse gasses dropped significantly, global temperatures dropped that led to the formation of ice sheets. Now as ice sheets formed, they started reflecting about solar energy. This lead to further decrease in temperature, eventually leading to self-sustaining glaciations that reached the equator from both poles, resulting in Snowball Earth.
Explaining further, the total carbon dioxide present in our oceans and atmosphere remains in a balance. The total CO2 supply is always maintained by volcanoes as well as metamorphic emanations. Excess CO2 is taken out of equation by a unique chain of events. Excess atmospheric CO2 is removed in form of acid rain when atmospheric moisture dissolves CO2 to form carbonic acid, which then falls down as rain. The acid rain then reacts with silicate rocks creating Ca2+ and HCO3– ions (calcium and bicarbonate icons). These ions are then washed into the oceans by rivers. In the oceans, organic matter and calcifying organisms such as algae and cyanobacteria make use of these ions to precipitate calcium carbonate (CaCO3), that is limestone. This whole process is termed as silicate weathering. Silicate weathering is slow where climate is dry and cold but is quick where climate is wet and hot. According to scientists, it was during the Cryogenian Period that lasted between 800 and 635 mya, the tropical continents were wet and hot. This led to quick silicate weathering and this in turn led to drop in global CO2 levels, which eventually led to Snowball Earth.
But what about methane (CH4)? The process of methane depletion was fairly simple as per the experts. It is a well-known fact that methane is a far stronger (30 times to be precise) greenhouse gas compared to carbon dioxide. However, there is a problem. Methane is unstable and quickly reacts with oxygen in a process called oxidation. The problem back in Cryogenian Period and earlier was that there was a massive amount of methane is atmosphere with very little oxygen (as low as just 1% of present atmospheric level or PAL). Back in those day, sun was less bright (please note that sun’s luminosity has been increasing at about 6% every 1 billion year since the day our Solar System was created) and hence, earth received less of solar energy. This was however compensated by massive amounts of atmospheric methane that trapped solar energy and kept earth warm. It is believed that during those days, a biological revolution took place in form of oxygenic photosynthesis – a process in which oxygen is released in atmosphere (basically it is photosynthesis that we have been taught in schools). This lead to increase in oxygen in atmosphere and subsequent decrease in methane, which resulted in a fall in greenhouse effect and the global temperature dropped.
However, there is a problem! Fossil records show that oxygenic photosynthesis existed long before. In fact it existed at least 500 million years before the Huronian Glaciation, also known as Makganyene Snowball Earth that happened somewhere between 2.4 and 2.2 billion years ago. Separate geochemical data suggests that oxygen levels increased above 1% PAL of oxygen during the Huronian Glaciation. If so then, the current hypothesis about methane depletion in Snowball Earth or Marinoan Glaciation is debatable. In fact, currently it is actively debated.
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Question: Exactly how many global glaciations took place?
Answer: If we go by definition of global glaciation then all global glaciations are snowball earths. However, we actually use the term Snowball Earth to indicate Marinoan Glaciation because the term ‘Snowball Earth’ was specifically coined for describing Marinoan Glaciation. Apart from the Marinoan Glaciation (or famously dubbed as Snowball Earth), there were two more global glaciations:
- Makganyene Snowball Earth or Huronian Glaciation which ended somewhere in 2.2 billion years ago.
- Sturtian Snowball Earth that ended somewhere in 710 million years ago.
The Snowball Earth or the Marinoan Glaciation was the last global glaciation that ended somewhere in 635 million years ago.
Okay, this article is not complete yet. We will continue with other questions in our second part to make sure that you are not bored to death in a single article!