The Snowball Earth hypothesis attempts to explain a number of phenomena noted in the geological record by proposing that an ice age that took place in the Neoproterozoic was so severe that the Earth's oceans froze over completely, with only heat from the Earth's planetary core causing some liquid water to persist under ice more than two kilometers thick. The general hypothesis has been around for several decades. Joseph Kirschvink, Professor of Geology at the California Institute of Technology coined the term "Snowball Earth" in 1992. The hypothesis has since been reformulated and championed by Paul F. Hoffman, Sturgis Hooper Professor of Geology at Harvard University and his colleague Daniel P. Schrag.
|(millions of years ago)|
Since the 1960s, it has been hypothesized that the Earth's continents were subjected to severe glacial action between about 750 million and 580 million years ago, so much so that the period is named the Cryogenian Period. Later, paleontologist W. Brian Harland pointed out that glacial till deposits of this period can be found on all continents, and first proposed that the Earth must have been in an ice age at this time. The problem is that the evidence-bearing deposits are found on all continents; but even during the worst of the ice age just past, no evidence of ice has been found in equatorial continents except on the higher parts of the highest mountain ranges. The then-new theory of plate tectonics made the oddly placed glacial discontinuities and deposits of glacial till even more enigmatic: studies of the magnetic orientations of the rocks of the late Proterozoic period showed that the continents were clustered around the equator during at least the start of the corresponding time around 750 Mya— in one of the earliest of the configurations known as supercontinents. This equatorial clustering and collision of continents about 750 mya has been named Rodinia; it being near the equator, rather than near the poles as might have been expected, taken together with thermal evidence of a severe ice age 750 to 635 mya (the dating suggested by the widespread geologic deposits) is what has led to the Snowball Earth theory.
The Snowball Earth theory argues from the documented locations of glacial till dropped by these glaciers, to suggest that the Earth must have completely frozen over. The mechanism by which it did so is still mysterious. One suggestion is that normally, as the ice spread, it would cover some of the land, and so slow the carbon dioxide absorption, and so increase the greenhouse effect, as volcanoes continue to emit carbon dioxide, and the ice spread would stop; but with all the continents clustered along the equator, this would not happen until the freezing process had run away. Once frozen, the condition would tend to stabilize: a frozen earth has a high albedo, reflecting more of the sun's radiation, and a frozen earth, with reduced evaporation, has a very dry atmosphere, water vapor being one of the greenhouse gases. A "Snowball Earth" would have a blindingly clear blue sky above its reflective surface.
The mechanism by which the Earth would unfreeze — as it must have done if it froze—would leave distinctive traces, which are the subject of ongoing research.
White Earth is a name given to a theoretical equilibrium found in computer climate simulations whereby the model Earth undergoes complete glaciation. While this seems to have originally been considered a degenerate case by the time James Gleick wrote his history of chaos theory Chaos: Making A New Science, it was not dismissed in his book but simply restated as something that probably just had not happened yet. The current evidence for the Snowball Earth would seem to back that theory and its computer models.
Geological formations which "Snowball Earth" proponents point to as evidence of the hypothesis are iron-rich rocks like taconite deposits and carbonate cap rocks. The association of the Snowball Earth event with the Cambrian Explosion (the sudden appearance of multicellular lifeforms between 570 and 530 million years ago) is also of great interest.
Lack of photosynthesizers
There are two stable isotopes of carbon in sea water: carbon-12 (C-12) and carbon-13 (C-13). Because biochemical processes tend to preferentially incorporate the lighter C-12 isotope, there is a tendency for ocean-dwelling photosynthesizers, both protists and algae, to be very slightly depleted in the rare heavier C-13, relative to the abundance found in the primary volcanic sources of the Earth's carbon. Therefore, an ocean with photosynthetic life has more C-12 sequestered as organic remains which leave a higher C-13 content in the ocean water. The carbonate rocks precipitating from this ocean water will have a slightly higher concentration of C-13 than would be the case for a dead ocean. During the proposed period of Snowball Earth, there are variations in the concentration of C-13 that are rapid and extreme compared to normal modern variations. This is consistent with a deep freeze that killed off most or nearly all photosynthetic life in the water. The last isotopes are submerged beneath the ocean water sediment.
In the Earth's oxygen rich (now nearly 21% by volume) atmosphere, iron naturally rusts, forming banded sediments known as banded iron formations. Since non-oxidized iron-rich rock deposits can only form in the absence of that ubiquitous atmospheric oxygen, and since these subject deposits are seen at the supposed time of the worst glaciations, presence of non-oxidized iron deposits laid down in the Cryogenian period lends strength to the Snowball Earth theory. The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps 20 times the volume of oxygen present in the modern atmosphere, and virtually all of it results from iron dissolved in water then subjected to oxygen, which precipitates out of the solution. Banded iron beds significantly are considered to be Precambrian sedimentary rocks and are rare in Phanerozoic strata.
Proponents of the theory point out that oxygen in the Earth's atmosphere is not naturally stable, and must receive continuous maintenance (replenishment) from the biosphere as it is constantly leached out of the atmosphere in a wide variety of chemical reactions, particularly iron and silicon. A tremendous glaciation would curtail plant life on Earth, thus letting the atmospheric oxygen be drastically depleted and perhaps even disappear, and thus allow (non-oxidized) iron-rich rocks to form. Detractors argue that this kind of glaciation would have made life extinct entirely, which did not happen. Proponents counter that it may have been possible for reservoirs of anaerobic and low-oxygen life powered by deep oceanic hydrothermal vents to have survived such an event within Earth's deep oceans and crust. Alternatively, deep ocean regions distant from the supercontinent Rodinia or its remnants as it broke apart and drifted on the tectonic plates may have allowed for some small regions of open water preserving small quantities of aerobic life (Contrary to the normal sense of aerobic, in this case, aerobic dependancy would be CO2 for consumption by plants during photosynthesis generating trace amounts of oxygen sufficient to sustain the aerobic (usual oxygen dependent sense) needs of the organisms during the dark of night). Another place where life might have survived would be in nunatak areas in the tropics where daytime tropical sun heated bare rock and made small temporary melt pools which would soon freeze as sunset got near.
Carbonate cap rocks
The carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, but would be able to accumulate due to the opposite of the effect mentioned earlier as a possible mechanism triggering the freeze in the first place; if the Earth was completely covered with ice, silicate rocks would not be exposed to erosion, and carbon dioxide would not then be removed from the atmosphere. That is, the biggest remover of carbon-dioxide from the atmosphere is the atmospheric weathering process wherein the CO2 combines with the overwhelmingly silicate based rocks to form dusts and sands and expose new rock face to further attack by the atmosphere— this is a significant process as can be easily seen by observing older headstones in the local cemetery. In a Snowball Earth, essentially all rock would eventually become locked up and covered by ice and snow leading to a rapid carbon-dioxide buildup.
Eventually enough CO2 would accumulate, perhaps after an era of increased volcanic activity (a prodigious producer of this greenhouse gas), that the oceans around the equator would finally melt, which would produce a band of open ice-free water, much darker than the highly reflective ice, which would absorb more energy from the sun. This would in turn heat the Earth more, melting more water to absorb more light, and so on. Concurrently, the abundance of CO2 would provide plenty of food to feed a cyanobacterial population explosion, resulting in a relatively rapid reoxygenation of the atmosphere to feed the following Cambrian Explosion with the new multicellular lifeforms. This positive feedback loop would melt the ice in geological short order, perhaps less than 1000 years; replenishment of atmospheric oxygen and depletion of the CO2 levels would take more thousands of years.
However, the carbon dioxide levels would still be two orders of magnitude higher than usual. Rain would wash CO2 out of the atmosphere as a weak solution of carbonic acid, which would turn exposed silicate rock to carbonate rock, which would then erode easily, wash into the ocean and form deep layers of carbonate sedimentary rock. Thick layers of exactly this abiotic carbonate sediment can be found on top of the glacial till that first suggested the Snowball Earth.
Eventually the carbon dioxide level would get so low that the Earth would freeze over again. This cycle went on until Rodinia had dispersed so much that the Earth's land was no longer strung out along the equator and the primary cause of Snowball Earth would no longer operate.
Proponents of this theory also point out that the frozen period may have ended only a few million years or so before the beginning of the Cambrian Explosion, at the beginning of the Ediacaran period. While not evidence per se, they consider the apparent sudden appearance of multicellular life suggestive of the removal of some great environmental stress holding life back, and propose that the cause of this stress was the deep freeze.
Survival of life through the frozen periods
There are several possible refugia in which microbial life may have survived through each of the frozen periods:-
- Around deep-sea hydrothermal vents.
- As eggs and dormant cells and spores deep-frozen into ice right through the frozen period.
- In nunatak areas in the tropics where tropical sun or volcanic heat heated exposed rock and made small temporary daytime patches where ice melted.
- Under the ice layer, in chemolithotrophic (mineral-metabolizing) ecosystems theoretically resembling those in existence in modern glacier beds, high-alpine and arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of volcanism or geothermal activity.
- In pockets of liquid water within and under the ice caps, similar to Lake Vostok in Antarctica. In theory, this system may resemble microbial communities living in the perennially frozen lakes of the Antarctic dry valleys.
Other Snowball Earths
Another Snowball Earth has also been proposed for the first known ice age, 2.3 billion years ago. There the proposed mechanism is the first appearance of atmospheric oxygen, which would have absorbed any methane in the air. As methane is a powerful greenhouse gas, and as the Sun was notably weaker at the time, temperatures plunged. The evidence here is weaker, but a layer of iron-rich rock can also be found from this time.
One competing theory to explain the presence of ice on the equatorial continents was that the Earth's axial tilt was quite high, in the vicinity of 60°, which would place the Earth's land in high "latitudes". An even less severe possibility would be that it was merely the Earth's magnetic pole that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depends on the magnetic and rotational poles being relatively similar (to be fair, there is some evidence to believe that this is the case). In either of these two situations, the freeze over would be limited to relatively small areas, as is the case today, and severe changes to the Earth's climate are not necessary.