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Home , Snowball Earth

The ’s Worst Disaster By Robert Kopp and Joseph Kirschvink

Published in Japanese translation in Miracle Planet II: The Evolution of Our World. Tokyo: NHK (Japan Broadcasting Corporation), 2004.

Scientists, environmentalists, and the a reaction utilized by those who heat their wiser members of the political class worry homes with natural gas. today about global . Will rising Two and a half billion years ago, tides plunge Tokyo, London, and New York however, most researchers think the beneath the ocean’s waves? Will meltwater was essentially free of . pouring off of North America shift the concentrations could build up to circulation of the North Atlantic Ocean and levels a thousand times higher than at present. plunge Europe into an Age? Yet, as High levels of methane, combined with high worrisome as these prospects are, the Earth has levels of , kept the planet toasty. faced far greater climatic catastrophes in the was only single-celled. Because past. The greatest among these was the most single-celled organisms make poor Snowball Earth event, which fossils, we do not know how closely the 2.3 billion years ago smothered the planet with ancient bacteria resembled their modern a blanket of ice for tens of millions of years. descendants. Roger Summons from MIT and At the beginning of the Jochen Brocks from Harvard have found in 2.7 Paleoproterozoic Era, 2.5 billion years ago, the billion year old Australian rocks organic Earth was a far different place than it is today. molecules, hopanes and steranes, that look like The cores of the ancient continents – the they may have been produced by Canadian Shield of North America, the and . Pilsbara Craton of Australia, the Kalahari Cyanobacteria, once known as blue- Craton of southern Africa – had just begun to green , are bacteria that act like plants -- form. they use energy from light to transform water Though the was only four-fifths of into oxygen; indeed, they are relatives of the its present brightness, the planet was warmer; chloroplasts that perform the reaction in plants. some studies of the oxygen of ancient Today, they are the main producers of certain and phosphate minerals suggest that it types of molecules called hopanols. The may have been 60ºC warmer – too hot for eukaryotes, the domain of life that includes plant or animal life. Only once does the plants, animals, and yeast, produce sterols in a record an interval before 2.5 billion years ago process that now require oxygen. when, as today, there were on the Cyanobacterial hopanols and eukaryotic sterols Earth. transform into hopanes and steranes over Climate modelers believe the geologic time. Yet we do not know for certain explanation of this apparent paradox rests in that cyanobacteria and eukaryotes, as opposed the composition of the atmosphere. In today’s to now-extinct ancestors or cousins, were atmosphere, the concentration of methane, a responsible for the ancient organic molecules. potent , is about 2 parts per Shortly after 2.5 billion years ago, the million. It is kept low because 20% of our global climate began to change. Glaciers began atmosphere is oxygen. Once sparked by light to advance and retreat. The first three or heat, methane and oxygen react rapidly glaciations of this time period are recorded in together to produce carbon dioxide and water – rocks from the Huronian Supergroup of central Canada. Glaciers grind up rocks into powder as

1 they scrape across the land surface,.When they magnetic signatures left in ancient rocks, reach the oceans they break off into icebergs provides the key to addressing the question. that slowly melt and drop powder and stones The Earth is a giant magnet. Liquid onto the ocean floor. The jumbled remains of circulating in the Earth’s core produces the collapse – pebbles, cobbles, and boulders, the planet’s . The field, which called clasts, embedded in fine-grained on average through time aligns with the axis material called the matrix – form a type of about which the Earth spins, protects the planet called a . Glaciers from solar radiation and allows people to are not the only agents that can form navigate using a simple magnetic compass. At , but glacial transport leaves the magnetic , the field is parallel to the distinctive signatures, including scratches in surface of the Earth; at the poles, it is vertical, multiple directions caused as rocks carried in a which is why compasses are not very useful in scrap across the country rock. Three the Arctic. major Huronian diamictites tell geologists that Certain minerals are magnetic. the Earth in Huronian times had cooled Magnetite, known to the ancients as lodestone, considerably. Huronian time was a glacial age, is the best-known example. When these with temperatures as cold as those today, if not minerals form or are deposited in sediments, colder. their own magnetic fields align with that of the Earth. If they formed at the equator, their Taking Ancient fields will be horizontal; at the poles, they will be vertical. Using sensitive magnetometers, One major open question about the paleomagnetists can detect the magnetic fields Huronian glaciations is how close to the preserved by these minerals in rocks and equator that occurred. In the recent ice ages, determine the at which a rock formed. glaciers in North America extended to around Unfortunately, magnetic orientations 40ºN latitude, somewhat farther than Chicago. are not forever. If a rock is heated to too high a As Eiichi Tajika discussed earlier, climate temperature, or subjected to too high a modelers such as Ken Calderia of Lawrence pressure, its magnetic field will be reset and Livermore National Laboratory and Jim will be oriented with the field it experienced at Kasting of Pennsylvania State University have the time of alteration. When the of a shown that, if glaciers penetrated below about site is favorable, paleomagnetists can conduct 30° in latitude, they would trigger an “ice- tests to determine whether alteration has runaway”. Because ice reflects most of occurred. the Sun’s light, extending the glaciers would One such test, the fold test, can be cool the planet further until all the continents, applied to sedimentary rocks that have been and perhaps all the oceans as well, became folded. Folding can occur either before encased in ice, in what is called a “Snowball sediments harden into rock or later, when they Earth” event. are subjected to pressure from other rocks. In In order to determine whether the a folded rock that preserves its original Huronian diamictites record Snowball Earth magnetic orientation, the measured events or more ordinary ice ages, scientists orientations will vary across the fold. When need a way of determining the latitude at the rock is mathematically unfolded, the which they formed. If the diamictites were directions will converge. On the other hand, if shown to have formed at low latitudes, they the magnetic orientations were reset after would suggest a Snowball Earth event had folding, the measured orientations will not occurred. , the study of

2 vary across the fold and will diverge when the As a graduate student at Caltech, David rock is unfolded. Evans, now a professor at Yale, studied the Unfortunately, many old rocks have Ongeluk lava. He found that its magnetic been beaten up rather badly by geological orientation indicated that it formed only 11º of processes. They sometimes get cooked and latitude from the equator. Moreover, he altered to the point where their original conducted a breccia test, a test analogous to the magnetic minerals loose all memory of their fold test based on the measurement of the original magnetism. This seems to be the case directions preserved in different clasts of a for the Huronian rocks in Canada. On a field rock, and proved that the low latitude was, trip to Canada in 2002 with colleagues from indeed, original. The Huronian glaciations the University of Tokyo, we found a well- may not have been Snowballs, but the preserved fold in an outcrop of hematite-rich magnetic data indicates the Makganyene sediments that formed just after one of the glaciation was. It is also followed by some of major glaciations. Magnetic patterns from the the most unusual sediments on this planet. fold demonstrated conclusively that none of the original magnetic directions had been Bacteria Throwing Snowballs preserved. There is therefore no evidence that the glaciations preserved in Canada were low- What caused the Earth to plunge into latitude, Snowball Earth events;, they could its coldest and longest ? The answer to instead be mid-latitude glaciations like those that question depends upon whether the that have occurred in the last ten million years. Makganyene Snowball was an isolated event, Several geologists have also noted that these or whether the Huronian glaciations that rocks do not have the pattern of post-snowball preceded it were also Snowballs David that would form from the large Evans believes that the Huronian glaciations – concentration of atmospheric CO2 built up indeed, every glaciation of the during the glaciation.The precise age of the epoch, which runs from the formation of the Huronian glaciations is not known, but a Earth to the time of the rise of animal life 540 minimum age is. An igneous intrusion, the million years ago – was a Snowball glaciation. Nipissing diabase, penetrates the Huronian He suggests that this change may be the result rocks and is therefore younger than them. By of long-term changes. Perhaps, by 540 million measuring the concentrations of different years ago, the Sun was warm enough to isotopes of lead produced by the decay of prevent Snowballs. Perhaps, as Paul Hoffman uranium, several groups of researchers have of Harvard University suggests, animals, by dated the Nipissing diabase to 2.22-2.21 billion disturbing soil and sediments, decreased the years ago. Because we know this date, we can effectiveness with which photosynthesizers look elsewhere on the planet for rocks of the could remove atmospheric carbon dioxide and same age. bury it as organic matter. The least altered rocks from this time In contrast to Evans, we think that the are located in southern Africa. Remarkably, the Makganyene glaciation may have been the Ongeluk basalt, a lava flow in the Transvaal only Snowball in the Paleoproterozoic. Supergroup there, bears the same 2.22 billion Diamictites formed during Snowball events are year age as the Nipissing diabase. The Ongeluk generally overlain with distinctive and unusual lava occurs mixed with a diamictite formation, sediments – cap and banded iron the Makganyene diamictite. The diamictite formations. These are present after the includes multiply-striated clasts, the fingerprint Makganyene glaciation, but not after any of the indicating that it was produced by a glaciation. Huronian glaciations. Thus, in the absence of

3 paleolatitude data and the rocks indicative of a able to destroy the greenhouse in less than Snowball, we think the Huronian glaciations 100,000 years, although it might have taken were mid-latitude glaciations like the millions of years under a more conservative set glaciations of more recent times and that the of assumptions. In either case, the results Makganyene Snowball was a symptom of a suggest that the cyanobacteria evolved later major transformation brought about by life. than Brocks and Summons’ organic molecules Cyanobacteria, the blue-green microbes suggest they did, and that, when they did that transform water into oxygen in the process evolve, the global consequences were severe. of turning carbon dioxide into organic matter, Without the methane greenhouse, average are not the only bacteria that can perform global temperatures would have plummeted to . Other, more ancient bacteria, –50ºC. Ice would have sheathed the oceans called purple and green bacteria because of and the continents. their distinctive colors, can also transform Yet life managed to survive this carbon dioxide into organic matter, but they disaster. Perhaps, in certain spots, the ocean’s require certain – typically, iron or sulfide ice cover was thin enough that sunlight could – to do so. Of all microbes, cyanobacteria still penetrate beneath. Photosynthetic alone can photosynthesize using only water. microbes might then continue to eke out a The more ancient bacteria could minimal existence, as could the microbes that therefore thrive only in regions rich in iron or rely on photosynthesizers to produce organic sulfide. When cyanobacteria finally evolved, carbon. At deep-sea vents, heat-loving photosynthesis was unleashed upon the low- bacteria would have continued on with their iron and low-sulfide portions of the world, existence, unaffected by the cold happenings limited only by the availability of nutrients like kilometers above them. phosphate. Their greater range should have Too, we do not know how well life did allowed the cyanobacteria to come to dominate survive the Snowball. Since most microbes life on Earth quickly and start releasing large leave at best crude fossils, scientists do not amounts of oxygen. know what microbes were around 2.2 billion Today, oxygen is both a dangerous years ago. By comparing the genomes of toxin and an element essential for complex life. extent organisms, molecular biologists can The damaging effects of oxygen are why anti- reconstruct ancestral trees and estimate when oxidants have become a dietary fad. But the organisms evolved. But the method is limited: Earth’s atmosphere has contained significant were our only record of the the amounts of oxygen for about half the planet’s genomes of , we would know nothing of life, and we have all evolved mechanisms for the gigantic herbivore Apatosaurus or the coping with the damage oxygen causes. When fierce Tyrannosaurus. Likewise, the cyanobacteria first appeared, no organisms Paleoproterozoic Snowball may have driven could deal with oxygen poisoning. many lineages of organisms extinct without Cyanobacteria, of course, had to develop a leaving us any traces. mechanism for surviving oxygen before they could thrive; once they did so, their ability to Aftermath poison other organisms would have strengthened their dominant position. For a long time, climate researchers The oxygen would have destabilized dismissed Snowball Earth scenarios as a flaw the methane greenhouse that kept the planet in their models. Once the Earth entered a warm. Based on simple models we have Snowball, they did not see how it could get constructed, cyanobacteria might have been out. In 1992, however, one of us (Joe

4 Kirschvink) realized the answer. Throughout with oxygen to form the world’s largest the Snowball period, volcanoes remained manganese deposit, the Kalahari Manganese active. Volcanic gases, including carbon Field. In parts, the Kalahari Manganese Field, dioxide, built up in the atmosphere. Under which dominates the world’s manganese normal conditions, carbon dioxide dissolves in production, holds high-grade deposits of the ocean and reacts with dissolved ions like manganese ore nearly 80 meters thick. to form rocks. Carbon The hot conditions of the post-glacial dioxide cannot, however, dissolve in an ice- world fostered gigantic storms and rapid covered ocean; it can only accumulate in the of the continents. Ions like calcium atmosphere. Eventually, enough carbon were carried from rocks dissolved on to the dioxide would build up to heat the planet and continents into the oceans. These ions reacted end the Snowball. with dissolved carbon dioxide, the For tens of millions of years after the concentration of which was driven to extreme onset of the Paleoproterozoic Snowball Earth, levels by the thick atmosphere. The result was the Earth remained a frozen planet. After a the rapid formation of carbonate rocks, first long time – perhaps thirty million years, recognized as being linked to the Snowball perhaps seventy million years, a span longer aftermath by Paul Hoffman, called ‘cap than the time period that separates us from the carbonates’ because of their position above the dinosaurs – carbon dioxide became the most Snowball diamictite. Carbonate formation abundant gas in the atmosphere. As the bright decreased the amount of carbon dioxide in the ice melted, the darker oceans and continents atmosphere and brought milder global were exposed and sped the planetary warming. temperatures. By the time the last ice vanished from the Life recovered from the Snowball Earth, average global temperatures may have catastrophe, but the Earth was permanently exceeded 50ºC. changed. The onset of the Snowball marked During the long glacial interval, carbon the beginning of the reign of cyanobacteria and dioxide was not the only substance the age of oxygen. Many of the microbes that accumulating. Far beneath the oceans’ icy dominated the pre-Snowball world survived, tops, deep-sea vents released iron, manganese, but their descendants were driven off the and various trace metals and nutrients. Most Earth’s surface, into low-oxygen environments forms of life need such trace metals to thrive. in the soil, the sediments, and low-oxygen Since life, barely surviving during the water bodies like the . Though Snowball, was not able to use all the metals disastrous, the rise of oxygen and the being released, they built up and fertilized the Paleoproterozoic Snowball allowed the oceans. development of more complex life forms, The melting of the ice fostered a global including – though they would not appear for bloom. Cyanobacteria once again released more than a billion years – plants and animals. oxygen into the environment, but since the We owe our existence to the bacteria that may Earth was no longer dependent on a methane have caused the world’s worst climate disaster. greenhouse, it was safe from a second Snowball. The oxygen did, however, react For further reading with the iron and manganese dissolved into the oceans. The iron rusted and precipitated out in THE SNOWBALL EARTH. P. F. Hoffman finely layered deposits called banded iron and D. P. Schrag in Scientific American, v. formations, which extended over hundreds of 282, no. 1 (January), p.p. 68-75. square kilometers. The manganese reacted

5 PALEOPROTEROZOIC SNOWBALL EARTH: EXTREME CLIMATIC AND GEOCHEMICAL GLOBAL CHANGE AND ITS BIOLOGICAL CONSEQUENCES. J. L. Kirschvink, E. J. Gaidos, L. E. Bertani, N. J. Beukes, J. Gutzmer, L. N. Maepa, and R. E. Steinberger in Proceedings of the National Academy of the Sciences, vol. 97, no. 4, p.p. 1400-1405.

SNOWBALL EARTH. G. Walker; Crown Press, New York, 288 pages, 2003.

6 Figures

Figure 1: Striations formed by a glacier in a cobble from the Makganyene diamictite, South Africa.

Figure 2: The folded rock from Canada that demonstrated that we do not know the latitude at which the Huronian glaciations occurred.

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Figure 3: Timeline of events (Huronian and Makganyene)

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