Snowball Earth
Snowball Earth (English Snowball Earth ) is the name of hypothesis of glaciation of the entire globe during the Erdurzeit in the Neoproterozoic. According to this hypothesis ranged during global glaciations the glaciers of the poles to the equator and near the oceans were largely frozen.
The hypothesis was set up in 1992 by the American geologist Joseph L. Kirschvink and designated Snowball Earth after appropriate ice ages had already been suggested by other authors. Snowball Earth refers to the fact that the earth at that time might have looked like because of the closed sea ice cover over the oceans and the continents like a gigantic snowball from space. One of the possible ice ages is the Neoproterozoic or Eokambrische ice age at the output of Proterozoic or early Cambrian.
The idea of a total glaciation of the Earth is indeed popular, but scientifically times to be controversial. The scientific discussion draws the reliability and interpretation of the geological evidence for this hypothesis in doubt, since there is strong evidence against a former total glaciation and the proposed physical mechanism that could have resulted in such icing should lead, is doubted.
Geological findings and hypothesis
Handed down from the late Proterozoic glaciogenic deposits are as tillites, often only a few meters, in some places, however, up to 2000 m are powerful. These deposits were detected with the exception of Antarctica on all continents. A direct dating of the sediments is not possible, their formation time can, however, be limited by over - and under overlying rocks. According to paleomagnetic reconstructions at least a portion of the corresponding deposit locations were throughout the late Proterozoic near the equator. Moreover, they are closely associated with rocks that indicate an origin from a more tropical conditions, such as carbonate rocks, red beds, and evaporites. This finding leads to the assumption that the earth was covered in that time until near the equator of ice.
The reason for the icing to have been the breakup of the supercontinent Rodinia then existing. Precipitates could again reach the areas that were dry and desert-like because of the size of the supercontinent before. The carbon dioxide contained in the rainwater led by this idea to a chemical weathering, which finally had the bond of the carbon dioxide in the deposit products result. Due to the distance of the greenhouse gas from the atmosphere decreased temperatures, setting off a erdweite from glaciation.
Also the retreat of the ice is attributed to plate tectonic processes and the associated with them volcanism along plate boundaries. The increased emissions of carbon dioxide by volcanoes led to an increase in the temperature and hence the melting of the ice.
One additional assumption according to have during and as a result of these ice ages multicellular living organisms ( metazoans ) developed after the end of the Ice Age in the Ediacaran ( million years ago 630-542 ) explosively popular ( Ediacaran fauna).
At least four glaciations in the late Proterozoic before 750-580 million years can be detected in almost all parts of the world. A total glaciation of the earth is for two of these glaciations, the stubbornness Tables Ice Age ( before 715-680 million years ago) and the Marinoische Ice Age ( million years ago 660-635 ), suspected. In addition, traces can still clearly earlier glaciations. The so-called Huron ice age, which can be derived from rocks to Lake Huron, was held in front of about 2.3 to 2.2 billion years. The paleomagnetic evidence from the Canadian rocks are controversial for other glaciogenic rocks of this age a deposit near the equator is discussed.
History of the hypothesis
Brian Harland of Cambridge University published in 1964 in different geological specialist magazines, the conclusion of its geomagnetic research shows that the worldwide spread glacial sediments of the late Proterozoic were deposited near the equator. Model calculations of the Russian scientist Mikhail Budyko, he performed at the University of St. Petersburg at the State Hydrological Institute, revealed the possibility of a positive feedback effect, which leads to a certain amount of icing that further continues this until the whole earth is covered with ice.
The development of the hypothesis goes back to the geologist Joseph Kirschvink, who worked at Caltech, California, 1987. He examined with the help of his student Dawn Sumner, a sample from the Elatina formation of Flinders Range ( Flinders Ranges ) in South Australia. The sample was from a rhythmically banded siltstone, which was digested with recirculated to Vergletscherungsvorgänge rocks as interpreted as tillites Diamiktiten and deposited by icebergs Dropstones. The sample showed a residual magnetization which pointed to a deposit position in the vicinity of the equator. Since Kirschvink could not believe that a glaciation could extend to low latitudes, were further study - also by other scientists and with other methods - carried out in this and other samples from the Flinders Range, which confirmed the initial results.
The Proterozoic glaciations in the late 1980s further work has been published. Kirschvink finally published his conclusions in 1992 and introduced the term Snowball Earth ( Snowball Earth ). He found support among others, Paul Hoffman from the Department of Earth and Planetary Sciences at Harvard University, who examined with other scientists Proterozoic sediments in Namibia.
The distribution of Proterozoic sediments and the reliability of the available data was examined by D. Evans of the University of Western Australia and published the results in 1997 and 2000. He comes to the conclusion that only exist very few reliable paleomagnetic data for these sediments, however, that leave from the group of reliable data derived mainly equatorial positions.
The topic has been repeatedly taken up by popular science magazines. 2003, the background of the hypothesis by Gabrielle Walker has shown in her book Snowball Earth. It focuses in this book, especially to the person of the geologist Paul Hoffman. The presentation of the subject is not without controversy.
Arguments
The origin of some sedimentary series of glacial processes has been discussed since the early 20th century, as in the Sparagmiten Norway and rocks from the lower reaches of the Yangtze River in China. The 1949 published results of studies of Australian geologist and Antarctic explorer Sir Douglas Mawson in the Precambrian units of the Flinders Ranges in South Australia (South Australia ) showed that this had to have taken place also under glacial conditions deposits of a flat sea. Similar sediments were also found in southern Africa. It remained open, however, on which latitude the continents were at that time located whether. Near the poles or near the equator
The model calculations of the Russian climatologist Mikhail Budyko say a positive feedback effect before, once to expand the polar ice caps on the thirtieth degree of latitude towards the equator because: The reflection of the sunlight through the ice ( albedo) would by the incident angle at these latitudes so large that a above-average cooling of the earth's total surface one devices. Only the lower layers of the oceans remained then by the sensible heat of the earth unvereist.
Joseph Kirschvink brought in 1992 in his publication alongside the results of paleomagnetic studies, the argument before that iron ore -rich sediments ( Bändererze of Rapitan - type ), which were dated to the end of the Neoproterozoic, had to be created due to lack of oxygen to the icy oceans. Emerging from the mantle iron itself could solve in the form of divalent ions ( Fe2 ) in the water without dissolved oxygen. Kirschvink suspected that the defrosting of the ice masses of the amount of dissolved oxygen could rise again in the oceans, there again was a connection to the water surface to the atmosphere. As part of the iron was oxidized and large amounts of trivalent iron compounds are unusual and have been deposited in the sediments.
Published in 1998, Paul Hoffman, Daniel Schrag and other authors, the results of their investigations of carbonate rocks of the Otavi Group in northern Namibia, the store with a sharp boundary on the sediments of the glaciations. The fact determined relative lack of carbon isotope 13C compared to carbon isotope 12C in Neoproterozoic sediments was interpreted as a further indication of a frozen earth with only weak biological activity. This was due to the preferential uptake of carbon isotope 12C in biological activity. Biomass thus is no longer present from the 12C - isotope, in which a strong production of biomass results in a relative increase of 13 C in carbon deposits inorganic ( carbonates ). Since the Neoproterozoic sediments deficient in 13C, close Hoffman and his co-authors from biomass production to a minimum during this time, which is attributed to the glaciation of the earth. The end of the glaciation was caused by their model by volcanic outgassing, which allowed to increase to 350 fold of today the content of carbon dioxide and suffered an extreme warming of the Earth and the abrupt melting of the ice by itself.
Counterarguments
Criticism of the hypothesis of the Snowball Earth is based primarily on that very far-reaching conclusions from the few and insufficient data are drawn. Paleomagnetic reconstructions of Proterozoic rocks are subject to considerable uncertainty. Location and extent of the continents at that time were not reconstructed at the time of formulation of the hypothesis reliably or are different possible. Although many references speak well for one or more ice ages, a largely ice-covered earth, as in the scenario of the " Snowball Earth " However, this does not imply.
Widespread icing can also be interpreted as tectonically so by changes in circulation in the ocean and in the atmosphere. In addition, sedimentary structures are preserved during the time in question, suggesting open oceans ( during an ice age ). Abrupt melting within less than 100,000 years, as postulated by Hoffman and his co-authors would have led to significant material flows that are not proven. The unusual isotopic signature of carbonate rocks indicates longer periods during re- increase in biomass production.
A lasting over millions of years complete glaciation would have made almost impossible on the basis of the photosynthetic oxygen- producing life forms. An oxidizing atmosphere with conclusion on appropriate forms of life but since the Archean, so thick with at least 2.4 billion years ago, rivers and widespread forms of life for over 3.5 billion years. Other metabolic mechanisms are known and are still existent today, as in communities of black smokers of the deep sea, in lightless rooms at chemoautotrophic archaea and sulfur bacteria and in hot hydrothermal vents. Also finds of hydrothermal ore in about 535 Ma are given sediment layers of the Yangtze platform in southeast China as an indication of such processes in the early Cambrian. An utter conversion of metabolic processes and subsequent re- invention is considered unlikely.