Snowball earth

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Snowball Earth
Time scale in Ma
Tonium Cryogenium Ediacarium

Snowball Earth ( English Snowball Earth ) or Snowball Earth is a geoscientific hypothesis about several global glaciation in the late Precambrian ( Neoproterozoic ), whose last phase ended about 580 million years ago. During this ice age , glaciers advanced from the poles to near the equator , the sea was largely frozen over and almost the entire surface of the earth was covered by ice.

hypothesis

The hypothesis was put forward in 1992 by the US geologist Joseph L. Kirschvink and named Snowball Earth after other authors had already suspected the corresponding ice ages . Snowball Earth refers to the fact that the earth could have looked like a gigantic snowball from space because of the closed ice cover over the oceans and the continents . After a Neoproterozoic or Eocambrian Ice Age at the end of the Proterozoic or in the early Cambrian , or several variants of a Varanger Ice Age, were postulated, two now well characterizable and datable glaciations in the Cryogenian , the Sturtische and the Marino Ice Age , are possible snowballs - Earth episodes under discussion. There are still unclear indications of a possible third, earlier Ice Age in the Cryogenium. The Gaskiers Ice Age , later in the Ediacarium , is no longer considered a possible candidate today because of its much shorter duration. Too little data is available about much older earlier glaciation periods such as the Huronian Ice Age , so that they are hardly discussed in connection with the hypothesis.

The idea of ​​a total freezing of the earth is popular, but scientifically still controversial. The scientific discussion casts doubt on the reliability and interpretation of the geological findings for this hypothesis, as there are clear indications against total freezing at the time and the proposed physical mechanism that should have led to such freezing is questioned. On the other hand, the snowball hypothesis is sometimes referred to as a geoscientific paradox due to the seemingly contradicting geological findings .

Geological findings on the hypothesis

From the late Proterozoic there are glacial deposits such as Tillite , which are often only a few meters thick, but in some places up to 2000 m thick. These deposits have been detected on all continents with the exception of Antarctica . Direct dating of the sediments is not possible, but their formation time can be limited by overlying and underlying rocks. According to paleomagnetic reconstructions, at least some of the corresponding deposit sites were located near the equator during the entire late Proterozoic. In addition, they are closely associated with rocks that indicate formation under more tropical conditions, such as carbonate rocks , red sediments and evaporites . This finding leads to the assumption that the earth was covered by ice up to the equator during this time.

At least four glaciations in the late Proterozoic 750 to 580 million years ago can be detected in almost all parts of the world. A total freezing of the earth is assumed for at least two of these glaciations, the Sturtic Ice Age (715 to 680 million years ago) and the Marino Ice Age (660 to 635 million years ago). In addition, there are traces of much earlier icing. The so-called Huron Ice Age , which can be deduced from the rocks around the Huron Sea, took place around 2.3 to 2.2 billion years ago. The paleomagnetic findings from the Canadian rocks are controversial, for other glacial rocks of this age a deposit near the equator is being discussed.

The reason for the icing is said to have been the breakup of the then existing supercontinent Rodinia . Precipitation could again reach areas that were previously dry and desert-like due to the size of the supercontinent. As a result, the rocks on the surface were exposed to both physical and chemical weathering . The atmospheric carbon dioxide dissolved in the rainwater played an important role in the decomposition of certain minerals in these rocks (see →  Carbonic acid weathering ). The carbon dioxide was converted into hydrogen carbonate and washed with the rainwater running off into the rivers and finally into the sea, where it was precipitated as lime and deposited on the seabed. Because of the removal of the greenhouse gas from the atmosphere , temperatures dropped and triggered earth-wide glaciation.

Francis Macdonald and Robin Wordsworth from Harvard University in Cambridge (Massachusetts) suspect in an article published in 2017 that the Sturtic icing was triggered by the so-called Franklin flood basalts in what is now northern Canada. Since the lavas had to make their way through previously deposited sulphatic evaporite rocks , this volcanism released unusually large amounts of sulfur gases ( SO 2 , H 2 S ), which rose into the stratosphere , formed aerosols and reflected the sunlight.

The retreat of the ice is also attributed to volcanism. The increased emissions of carbon dioxide by volcanoes at the edges of continental plates led to an increase in temperature and thus to the melting of the ice.

According to an additional assumption, multicellular organisms ( metazoa ) developed during and as a result of these ice ages , which after the end of the ice age in the Ediacarium (635 to 541 million years ago) spread explosively ( Ediacara fauna ).

History of the hypothesis

Brian Harland from Cambridge University published in various geological journals in 1964 the conclusion of his geomagnetic investigations that the globally distributed glacial sediments of the late Proterozoic were deposited near the equator. Model calculations by the Russian scientist Mikhail Budyko , which he carried out at the State Hydrological Institute of the University of St. Petersburg , revealed the possibility of a positive feedback effect , which, if the degree of icing is to a certain extent, causes it to continue until the entire earth is frozen .

The further development of the hypothesis goes back to the geologist Joseph Kirschvink, who in 1987 at California's Caltech worked. He examined with the help of his student Dawn Sumner, a sample from the Elatina formation of the Flinders Range ( Flinders Ranges ) in South Australia . The sample came from a rhythmically banded siltstone , which was exposed together with rocks traced back to glaciation processes such as diamictites interpreted as tillites and dropstones deposited by icebergs . The sample exhibited remanent magnetization , indicating a deposition location near the equator. Since Kirschvink could not believe that glaciation could reach low latitudes , further investigations - also by other scientists and with different methods - were carried out on this and other samples from the Flinders Range, which confirmed the first results.

Further work on the Proterozoic glaciations was published in the late 1980s. Kirschvink finally published his conclusions in 1992 and introduced the name Snowball Earth ( Snowball Earth on). He found support from, among others, Paul Hoffman from the Department of Earth and Planetary Sciences at Harvard University , who, together with other scientists, examined Proterozoic sediments in Namibia.

The distribution of Proterozoic sediments and the reliability of the available data was examined by D. Evans from the University of Western Australia and the results were published in 1997 and 2000. He comes to the conclusion that only very few reliable paleomagnetic data are available for these sediments, but that mainly positions close to the equator can be derived from the group of reliable data.

The topic has been taken up several times by popular science magazines. In 2003, the background to the hypothesis was presented by Gabrielle Walker in her book Snowball Earth . In this book, she focuses primarily on the person of the geologist Paul Hoffman. The presentation of the topic is not undisputed.

Arguments

The origin of some series of sediments from glacial processes has been discussed since the beginning of the 20th century, for example in the sparagmites of Norway and rocks from the lower reaches of the Yangtze in China. The results of the investigations by the Australian geologist and Antarctic researcher Sir Douglas Mawson in the Precambrian parts of the Flinders Range in South Australia , published in 1949, showed that deposits of a shallow sea must also have taken place here under ice age conditions. Similar sediments have also been found in southern Africa. However, it remained open in which latitudes the continents were located at that time, whether near the poles or near the equator.

The model calculations by the Russian climatologist Michail Budyko predict a positive feedback effect as soon as the ice caps of the poles expand beyond the thirtieth degree of latitude towards the equator: The reflection of the solar radiation by the ice masses ( albedo ) would be so large that one disproportionate cooling of the entire surface of the earth would occur. Only the lower layers of the oceans remained frozen due to the earth's own heat.

2.1 billion year old iron band ore from North America (State Museum for Mineralogy and Geology, Dresden)

In addition to the results of the paleomagnetic investigations, Joseph Kirschvink put forward the argument in his 1992 publication that iron-ore-rich sediments ( ribbon ores of the Rapitan type), which were dated to the end of the Neoproterozoic, must have formed due to the lack of oxygen in the icy oceans. Without dissolved oxygen, the iron escaping from the earth's mantle could dissolve in the water in the form of divalent ions (Fe 2+ ). Kirschvink suspected that when the ice masses thawed, the proportion of dissolved oxygen in the oceans could rise again because the water surface was connected to the atmosphere again. In the course of this, the iron was oxidized and large amounts of compounds of trivalent iron had precipitated and were deposited in the sediments.

In 1998 Paul Hoffman, Daniel Schrag and other authors published the results of their investigations into carbonate rocks by the Otavi Group in northern Namibia, which are in sharp contact with the ice age sediments ( cap carbonates , 'cover carbonates'). The very pronounced negative δ 13 C anomaly (up to −6 ‰) found in it, i.e. a significantly higher proportion of the carbon isotope 12 C in relation to the carbon isotope 13 C, was a further indication of a far-reaching icy earth with only low biological activity interpreted. Because organisms prefer to take up the lighter carbon isotope 12 C, this is depleted in geological periods with normal biomass production in the carbon cycle and deposited in the form of organic-rich sediments, which leads to a relative increase of 13 C in simultaneously formed inorganic carbon deposits (carbonates). Since the cover carbonates of the Otavi Group have a relatively high amount of 12 C, Hoffman and his co-authors conclude from this that there is a global collapse in biomass production, which they in turn attribute to the previous extensive icing of the earth. According to their model, the end of the glaciation was brought about by volcanic outgassing, which caused the carbon dioxide content of the earth's atmosphere to rise to 350 times its current level and resulted in a rapid, significant warming of the earth and the abrupt melting of the ice. The entry of atmospheric carbon dioxide into the now ice-free oceans finally led to the formation of cover carbonates through abiogenic precipitation of calcium carbonate in warm shelf seas.

Counter arguments

The criticism of the hypothesis of the snowball earth is mainly based on the fact that very far-reaching conclusions are drawn from few and inadequate data. Paleomagnetic reconstructions of rocks from the Proterozoic Era are fraught with considerable uncertainty. At the time the hypothesis was formulated, the position and extent of the continents at that time were not reliably reconstructed or are possible in different ways. Although many indications speak for one or more ice ages and also for extensive continental ice sheets in these periods, a largely icy earth as in the scenario of the "snowball earth" cannot be derived from this.

In addition, sedimentary structures have survived from the period in question, suggesting open oceans (during an ice age). Furthermore, there are indications in the sediment tradition that a temperate climate prevailed in the ice edge layers. The high thickness of the neoproterozoic glacial deposits also suggests that no catastrophic rapid warming of the earth and thus no rapid melting of the glaciers could have taken place, but that the climate has gradually changed. In addition, the δ 13 C data of the cover carbonates showed that the δ 13 C value did not decrease but increased from its base to the hanging wall (i.e. with decreasing age of the rock). On the other hand, it is to be expected that the δ 13 C minimum value was reached during the high phase of the glaciation and not only during the sedimentation of the carbonate rocks on the ice-free shelf again. The persistently low or still falling δ 13 C value during the carbonate sedimentation would, according to Hoffman's interpretation as a result of the abiogenic precipitation of atmospheric carbon dioxide, mean that the post-glacial ocean was in fact inanimate for an unrealistically long period of time.

A complete icing lasting over millions of years would have made oxygen-producing life forms almost impossible on the basis of photosynthesis . An oxidizing atmosphere with inferences about the corresponding life forms has been documented since the Archean, i.e. at least 2.4 billion years ago, flowing waters and widespread life forms for over 3.5 billion years. Other metabolic mechanisms are known and still exist today, for example in communities of black smokers in the deep sea , in lightless rooms with chemoautotrophic archaea and sulfur bacteria and in hot hydrothermal springs. Finds of hydrothermal ores in around 535 million year old sedimentary layers of the Yangtze River platform in southeast China are also cited as an indication of such processes in the early Cambrian. A complete conversion of the metabolic processes and subsequent reinvention is considered rather unlikely.

literature

  • Gabrielle Walker: Snowball Earth: The Story of a Maverick Scientist and His Theory of the Global Catastrophe That Spawned Life as We Know It . Three Rivers Press 2004, ISBN 1-4000-5125-8 .
  • Gabrielle Walker: Snowball Earth . Bvt Berliner Taschenbuch Verlag, Berlin 2005, ISBN 3-8333-0138-4 .
  • Peter Ward, Joe Kirschvink: A New Story of Life. How catastrophes determined the course of evolution . Pantheon, Verlagsgruppe Random House GmbH 2018, ISBN 978-3-570-55307-7 .

Web links

Individual evidence

  1. ↑ Ute Kehse: No snowball earth. Image of Science Online, December 1, 2008, article based on a publication by Philip Allen, James Etienne: Sedimentary challenge to Snowball Earth. In: Nature Geoscience. 1, 2008, pp. 817-825. Published online November 30, 2008.
  2. ^ David AD Evans: Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. In: American Journal of Science. Volume 300, No. 5, ISSN  0002-9599 , pp. 347-433, doi: 10.2475 / ajs.300.5.347 .
  3. ↑ a b Roland Walter: Geological history: The emergence of the continents and oceans. 5th edition. Walter de Gruyter Verlag, 2003, ISBN 3-11-017697-1 , p. 61.
  4. ^ A b D. A. Evans et al .: Low-latitude glaciation in the Palaeoproterozoic era. In: Nature. 386, 1997, pp. 262-266. (Abstract)
  5. ^ GA Shields-Zhou, AC Hill, BA Macgabhann: The Cryogenian Period. In: Felix Gradstein, James Ogg, Mark Schmitz, Gabi Ogg (eds.): The Geologic Time Scale 2012. Elsevier, 2012, ISBN 978-0-444-59425-9 , Chapter 17. doi: 10.1016 / B978-0- 444-59425-9.00017-2
  6. ↑ Phillip W. Schmidt, George E. Williams: Paleomagnetism of the Lorrain Formation, Quebec, and Implications for The Latitude of Huronian Glaciation. In: Geophysical Research Abstracts. Vol. 5, 08262, 2003. (PDF, 23 kB)
  7. ↑ a b c Joachim Schüring: Snowball Earth. ( Memento from February 12, 2013 in the web archive archive.today ) Spektrumdirekt, August 13, 2001.
  8. ^ FA Macdonald, R. Wordsworth: Initiation of Snowball Earth with volcanic sulfur aerosol emissions. In: Geophysical Research Letters. 44, No. 4, 2017, pp. 1938–1946, doi: 10.1002 / 2016GL072335 .
  9. ↑ See Lars Fischer: Geoengineering gone bad. Did sulfur drops turn the earth into an ice ball? Spektrum.de, May 10, 2017, accessed on May 11, 2017.
  10. ^ WB Harland: Critical evidence for a great infra-Cambrian glaciation. In: Geologische Rundschau. 54, 1964, pp. 45-61.
  11. ^ Snowball Earth - Introduction , Snowballearth.com . Retrieved February 4, 2008.
  12. ↑ a b Bibliography of general papers on the snowball Earth hypothesis , Snowballearth.com
  13. ^ A b J. L. Kirschvink: Late Proterozoic low-latitude glaciation: the snowball Earth. In: JW Schopf, C. Klein (Ed.): The Proterozoic Biosphere. Cambridge University Press, Cambridge 1992, pp. 51-52.
  14. ^ A b Paul F. Hoffman et al: A Neoproterozoic Snowball Earth. In: Science. Volume 281, No. 5381, August 28, 1998, pp. 1342-1346.
  15. ^ A b D. AD Evans: Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. In: Tectonics. 16 (1), 1997, pp. 161-171. (Abstract, PDF, 60 kB)
  16. ^ DAD Evans: Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. In: American Journal of Science. 300, 2000, pp. 347-433. (Abstract)
  17. ^ WB Harland, MJS Rudwick : The great infra-Cambrian ice age. In: Scientific American. August 1964, pp. 42-49.
  18. ↑ O. Holte Dahl: A tillite-like conglomerate in the "Eocambrian" sparagmite of southern Norway. In: American Journal of Science. 4, 1922, pp. 165-173.
  19. ↑ YY Lee: The Sinian glaciation in the lower Yangtze valley. In: Bulletin of the Geological Society of China. 15, 1936, pp. 131-000.
  20. ↑ MI Budyko: The Effect of Solar Radiation Variations on the Climate of the Earth. In: Tellus. Volume 21, 1969, pp. 611-619.
  21. ^ Thayer Watkins: Mikhail I. Budyko's Ice-Albedo Feedback Model. Department of Economics, San José State University.
  22. ^ CR Scotese: More Information About the Late Precambrian. PALEOMAP Project
  23. ^ Philip Allen, James Etienne: Sedimentary challenge to Snowball Earth. In: Nature Geoscience. 1, 2008, pp. 817-825. Published online November 30, 2008.
  24. ^ A b c Nicholas Christie-Blick et al: Considering a Neoproterozoic Snowball Earth. In: Science. Volume 284, No. 5417, May 14, 1999, p. 1087.
  25. ^ B. Windley: The Evolving Continents. Wiley Press, New York 1984.
  26. ↑ J. Schopf: Earth's Earliest Biosphere: Its Origin and Evolution. Princeton University Press, Princeton, NJ 1983.