The Snowball Earth Hypothesis states that, at least three times in Earth history, the entire planet was covered by ice. Despite some controversy, the hypothesis offers a fascinating example of the complex balancing act that takes place between different parts of the Earth system and its overall sensitivity. The history of the idea provides a valuable insight into how the scientific method unfolds and the way that theory, observation, and experiment give rise to a holistic overview of the planet’s behaviour.
These events occurred at ~2.4-2.1 billion years ago (the Huronian glaciation), ~717-640 million years ago (the Sturtian), and ~650-630 million years ago (the Marinoan). Evidence is spurious given how long ago they were (the geologic record becomes less reliable with time due to lack of preservation and the alteration of materials via. plate tectonics), however it is well-established that there were unprecedented glaciations covering the majority of the Earth’s surface. This is shown by field observations and reconstructions of palaeolatitude, often reliant on elongated magnetic minerals that align themselves with the Earth’s magnetic field. Those field observations came much later in the Snowball Earth story though.
Early Models:
The story has its origins in scientists working with early climate models and tweaking different variables such as atmospheric CO2 and aerosol concentrations. These kinds of models were also used to develop the concept of a nuclear winter, which became a key element of the cold war, as neither side could use their nuclear weapons without destroying the entire planet, thereby locking the USA and Soviet Union in a stalemate, or a state of “mutually assured destruction”. Early climate models, such as the General Circulation Model pioneered by Syukuro Manabe, applied energy balance equations (those that combine conservation laws with fluid dynamic equations concerning fluxes of moving material through some volume) to unravel climate response to changes in energy e.g. from the Sun or by changing some variable of interest.
One variable of interest is the Earth’s albedo. This is the ability of the planet's surface to reflect solar radiation back into space. A high albedo means a high amount of the Sun’s energy that reaches Earth is reflected back, resulting in surface cooling. Different types of surface have a different capacity to either reflect light or absorb light. Ice, being a white surface, is great at reflecting more light so if ice sheets grow, more solar energy gets reflected back. This, in turn, causes more cooling and more ice to grow in a self reinforcing cycle. Climate models explored by Soviet researcher Mikhail Budyko implied that, if ice were to advance to a certain point of latitude, this feedback loop would sustain itself until a complete Snowball Earth state is achieved.
This work provided a hypothetical framework for what later became known as the Snowball Earth hypothesis. His model, however, showed no mechanism by which the planet could escape such an extreme climate and so it was concluded that, while possible in theory, the planet had never actually experienced a Snowball state. Additionally, field observations from this time could not account for the movement of continents over geologic timescales. The theory of continental drift/plate tectonics itself was very new while this research was taking place so why debate continued throughout the 1970s and 80s, there was little direct evidence that such an event ever occurred. Even though tectonic theory was making major advances, the ability to reconstruct where the continents were millions of years ago was yet to be seen.
Snowball Earth Hypothesis is Born:
In 1987, Cal. Tech researcher Joseph Kirschvink and undergraduate student Dawn Sumner were intrigued by a glacial rock sample collected by palaeontologist Bruce Runnegar. The sample, from the Flinders ranges in South Australia, was part of the Elatina Formation, known to contain evidence pointing to a glacial origin. Curiously, this sample (and others from the same formation) were also shown to have been deposited in equatorial latitudes on the basis of their magnetic properties.
We understand the low-latitude formation of these rocks because of palaemagnetic data, where elongated magnetic minerals in rocks align themselves in the direction of the magnetic field when deposited and are free to move. When surrounding sediments are cemented to form rock, the direction of the magnetic minerals is locked in, which provides an indicator of the latitudes they are deposited in (the magnetic field changes with latitude due to the Earth’s shape and internal structure).
The magnetic signatures associated with the sample reignited interest in the idea of a global glaciation. Repeated tests by Sumner, Kirschvink, and other independent researchers were carried out, all concluding an equatorial origin for the sample.
The Snowball Earth hypothesis made a dramatic comeback into the literature during the early 1990s, this time with field observations and geochemical evidence indicating that Snowball Earth events have in fact been achieved in the past. Kirschvink (who coined the term Snowball Earth in a short 1992 paper), suggested that two main lines of evidence supported the bold claim that the entire planet was frozen over:
Banded iron formations (BIFs) made a return in the geologic record after an extremely long period of absence. Kirschvink interpreted this to be related to oceans being starved of oxygen. Photosynthesis would have been unable to occur due to the ice sheets preventing light needed to facilitate the process and so the recurrence of BIFs is consistent with what would be expected.
Glacial-related sedimentary features associated with units deposited in low latitude regions e.g. dropstones (large rocks transported by glaciers and later deposited to the ocean floor), tillites (comprising material eroded from the advancing glacier), and evaporites. The previously mentioned magnetic data supported the findings.
Abundance of BIFs through geologic time. Source: https://opengeology.org/historicalgeology/case-studies/snowball-earth/
Kirschvink also outlined how a Snowball state can be overcome by volcanic degassing, addressing the key question that arose from Budyko’s work. Volcanic CO2 was able to accumulate in the atmosphere for long enough to eventually cause enough melting to reverse the albedo feedback loop and cause extreme, rapid warming as more and more ice melts. This is supported by the presence of cap carbonates observed above these glacial deposits, indicating a sharp transition from extreme cold to extreme warmth. Cap carbonates form as a result of the reconnecting of atmospheric CO2 with the ocean, resulting in rapid carbonate formation in the water. This ocean-atmosphere exchange would not have been possible when ice sheets covered the ocean. This played a vital role in the termination of the Neoproterozoic Snowball Earth events as CO2 was allowed to gradually accumulate in the atmosphere until a threshold was exceeded. Kirschvink had not only provided evidence for a Snowball Earth but also evidence for the cessation of such an extreme climate.
Later work through the late 1990s and early 2000s by Kirschvink and Harvard researchers Paul Hoffman and Daniel Shrag fleshed out the theory with further field observations in Namibia, adding to those from Australia and Canada. They showed that carbon isotope data, along with evidence that the Earth’s crust was subsiding in response to the sheer mass of the overlaying ice, further supported the theory.
Despite the overwhelming evidence for a Snowball Earth event having occurred, questions still remain. The question of timescales remains difficult to unravel. Why did the first of the two Neoproterozoic Snowball Earths last so much longer than the following event? Additionally, how did life manage to survive through such ecological strain that must have been imposed? The environmental stress would have wiped out all photosynthesising species unless there was some kind of refuge for those species.
Alternative Hypotheses and Future of the Snowball Earth Hypothesis:
An alternative hypothesis known as “Slushball Earth” describes a slightly less dramatic scale of glaciation, where ice sheets were not able to exist near the equator. Photosynthesising life in the oceans could survive in this situation as oases of water were present in low latitudes. Snowball Earth supporters refute this claim by stating that life could have thrived on top of the ice sheets and that a slushball state would not have been able to survive for the millions of years observed. An ocean incapable of drawing down atmospheric CO2 is essential to the hypothesis and this could not be achieved in a slushball state. The extreme shift from icehouse to greenhouse, as cap carbonates record, is consistent with a Snowball state rather than a slushball.
To fully cover the planet in ice would require an extraordinary amount of change to happen and, as is so often the case with dramatic climate change, volcanism appears to be the main culprit in both the onset and eventual demise of these strange events. While there is still controversy and many questions remaining, it is well-substantiated that there were enormous ice-age events. It will be difficult for future studies to satisfy answers to all these questions due to the spurious material to work with and the lack of tools to reconstruct climates so far back in the geologic record, however, it will be interesting to see how future research reconciles the inconsistencies present. Recent work focuses on orbital forcing of the ice, where astronomical cycles related to Earth’s orbit resulting in a waxing and waning of ice sheets would have periodically supplied oxygen to the ocean. This would explain how multicellular life survived such stresses imposed by a global glaciation.
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