The Snowball Earth Hypothesis
The concept of Earth being in an extreme climatic state which resulted in the covering of the Earth’s surface and oceans in glaciers and ice sheets about 600 million years ago in the Neoproterozoic era, which lasted for about ten (10) million years or more, is considered to be the Snowball Earth hypothesis ( Hoffman and Schrag, 2000). This theory suggests that an uncontrollable positive feedback of the ice-albedo caused the complete freezing of the Earth including the tropical and equatorial latitudes (Allen and Etienne, 2008).
The ending of this extreme state and the climatic reversal is said to be brought about by high levels of carbon dioxide emitted into the Earth’s atmosphere from volcanoes over long periods of time. In support of the Snowball Earth theory, there has been evidence in the deposition and composition of rocks, the alignment of minerals in rocks to the magnetic field, and the composition of the oceans from the Neoproterozoic era.
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However, there have been several factors and evidence such as sedimentary rock deposition and the fossils of several microscopic organisms which indicated that during the event of this extreme glaciation, there were some oceans that remained unfrozen. Based on W. Brian Harland’s conclusion from the magnetic orientation of the mineral grains in the glacial rocks which are predated to the Neoproterozoic era, the relatively horizontally alignment with the magnetic field was due to their close proximity to the equator.
During the Neoproterozoic era all the continents were clustered together near the equator. Because of the position of continents near the equator, the ice albedo feedback increased tremendously causing the rapid decrease in the surface temperatures as was explained by Mikhail Budyko by using equations based on the interactions of the solar radiation with the Earth to control climate (Hoffman and Schrag, 2000).
The cold temperatures sustained and stabilized the snow and ice over the Earth’s surfaces resulting in freezing of the Earth’s surfaces including equatorial glaciation at sea level (Baum, Crowley, Hyde and Peltier, 2000). This catastrophe would have terminated all life on Earth; however fossil records predating to the time of Snowball Earth implies that there was life of microscopic organisms which are the ancestors of the present forms of organisms.
It was discovered and concluded that most of the surviving micro-organisms were extremophiles and many others were supported by volcanic activities which permitted for suitable living conditions within open water (Hoffman and Schrag 2000). Climate modeling and research among several individuals have shown that the isolation and the pressures exerted by the extreme during and post conditions of a Snowball Earth may have led to their rapid development in form and thereby the evolution of metazoans (Baum, Crowley, Hyde and Peltier, 2000. nd Hoffman and Schrag, 2000). Joseph Kirchvink also proposed a theory based on iron deposits found mixed within glacial deposits. He deduced that the iron was expelled from the ocean floor when it was deprived of oxygen by ice cover but once the ice melted and oxygen was again mixed with the sea water, the iron would be forced to precipitate out with the debris. During the Neoproterozoic era, it was perceived that there were many advances and recessions of large-scale glaciers across the Earth’s surface.
The evidence is seen in the formation of rocks from deposits of dirt and debris that remained from the periodic melting of the glaciers and those that dropped out from floating ice. Above the layer of these rocks are other layers of rocks composed chiefly of calcium carbonate and magnesium carbonate minerals which are evidential to the immense increase in temperature of the Earth’s climate after the glaciation period (Hoffman and Schrag 2000). These rock formations were found continentally further emphasizing that glaciation occurred globally in advancing and retreating phases.
The glacigenic sediments typically constitutes of diamictites, however, diamictites are also deposited from other sources of mass flow which does not involve glacial activities. Therefore other components of glacial influenced sedimentation were used to determine whether particular sedimentary depositions are results of Cryogenian succession in the Neoproterozoic era. Laminated dropstones, which are also common component of Cryogenian succession, were therefore also used to determine glacigenic sedimentation.
Alternating sedimentary layers of dropstones and rare discoveries of grooved and striated bedding surfaces found provided further evidence of glacial advancement-retraction and movement from a fluctuating ice margin (Allen and Etienne, 2008). The preceding discoveries of microscopic organisms, the stratigraphy of interleaving glacimarine diamictites and equatorial glaciation at sea level in the Neoproterozoic era is evidence that there were unfrozen oceans. The early microscopic organisms needed an area of shallow water whose climate remained habitable in order to survive.
The location in which they were found therefore could have solely contained ice-free continental margins to fit the requirement of such an area (Baum, Crowley, Hyde and Peltier, 2000). This was concluded after much research, modeling and studying of the events of a Snowball Earth after it was proposed by several researchers who interpreted carbon-isotope data as being a result of the ceasing of biological productivity of oceans for million of years during the Neoproterozoic era (Baum, Crowley, Hyde and Peltier, 2000).
There was also evidence of rippled sandstone deposition on sea-beds and mudstone deposition from turbid columns from the ice front with no direct glacial influence. The rippled sandstones was within reach of surface wind-generated waves distinguished by specific features associated with ripples formed by wave activity in open, relatively shallow water bodies. These water bodies would have also had to be large enough for the generation of these periodical waves strong enough to leave such inflicting marks (Allen and Etienne, 2008).
The end of the Snowball Earth was said to be brought about by volcanic activities over long periods of time. Joseph L. Kirchvink emphasized that volcanoes were continuously built by plate tectonics during the Snowball Earth which supplied the atmosphere with carbon dioxide. The continuous accumulation of carbon with no water to take it out of the atmosphere resulted in the tremendously high levels of carbon dioxide which heated up the planet, replacing the high albedo ice with low albedo seawater thus reversing the positive feedback and ending Snowball Earth (Hoffman and Schrag 2000).
This concept was also confirmed by several other researchers who also indicated that along with the water not being available, there was negligence of terrestrial weathering of silicate rocks which would have drawn down the carbon dioxide (Allen and Etienne, 2008). Modeling and stimulation of a Snowball Earth, revealed that as a consequence of the green house effect, a rapid precipitation of calcium carbonate caused by chemical erosion of rocks by carbonic acid produced by the washing of carbon dioxide out of the atmosphere produced cap carbonates.
These cap carbonates which are found in the strata of the Neoproterozoic era contain an unfamiliar correlation in ratio of two carbon isotopes; carbon 12 and carbon 13. This pattern is observed worldwide indicating that this was a phenomenon of a Snowball Earth. Preceding the glacial deposits, the amount of carbon 13 is equivalent to that which is sourced from volcanoes which is about 1 percent carbon 13 and the remainder is carbon 12. This indicates the reduction in biological life in ice covered oceans as compared to the higher percentage of carbon 13 to carbon 12 if there was a greater degree of life in the ocean. The decrease in the amount of carbon 13 continued through the cap carbonates following the layers of glacial deposits and only gradually increased several hundred meters above which indicated the recovery of life at the end of the intense green house effect period (Hoffman and Schrag 2000).
In reviewing the evidence to defy the Snowball Earth Hypothesis and the counteraction and new evidence to support it, there could have been such an event as Snowball Earth with oceans containing ice caps or sheets but remained unfrozen below the surface.
Allen, A. P. & Etienne, J. L. (2008). Sedimentary challenge to Snowball Earth. Nature Geoscience. 1 817-825 Baum, S. K. , Crowley, J. T. , Hyde, W. T. & Peltier, W. R. (2000). Neoproterozoic ‘snowball Earth’ simulations with a coupled climate/ice-sheet model. Nature. 405 425-429 Hoffman, P. F. & Schrag, D. P. (2000). Snowball Earth. Scientific American. 68-75