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1 the Earth's Worst Climate Disaster by Robert Kopp and Joseph The Earth’s Worst Climate 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 climate change. 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 atmosphere was essentially free of oxygen. pouring off of North America shift the Methane concentrations could build up to circulation of the North Atlantic Ocean and levels a thousand times higher than at present. plunge Europe into an Ice Age? Yet, as High levels of methane, combined with high worrisome as these prospects are, the Earth has levels of carbon dioxide, kept the planet toasty. faced far greater climatic catastrophes in the Life was only single-celled. Because past. The greatest among these was the most single-celled organisms make poor Paleoproterozoic 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 cyanobacteria Canadian Shield of North America, the and eukaryotes. Pilsbara Craton of Australia, the Kalahari Cyanobacteria, once known as blue- Craton of southern Africa – had just begun to green algae, are bacteria that act like plants -- form. they use energy from light to transform water Though the Sun 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 isotopes of ancient Today, they are the main producers of certain chert 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 rock 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 glaciers 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 greenhouse gas, 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 iron circulating in the Earth’s core produces the collapse – pebbles, cobbles, and boulders, the planet’s magnetic field. 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 sedimentary rock called a diamictite. Glaciers from solar radiation and allows people to are not the only agents that can form navigate using a simple magnetic compass. At diamictites, but glacial transport leaves the magnetic equator, 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 glacier 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 Latitudes 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 latitude 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 geology 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 albedo 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. Paleomagnetism, 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 ice age? 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 limestones that would form from the large Evans believes that the Huronian glaciations – concentration of atmospheric CO2 built up indeed, every glaciation of the Precambrian 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.
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