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!