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Hipotesis '''Bumi Bola Salju''' merupakan hipotesis yang memperkirakan bahwa permukaan [[Bumi]] pernah beku sepenuhnya sekitar 650 juta tahun yang lalu. Pendukung hipotesis ini menyatakan bahwa penjelasan yang ditawarkan hipotesis Bumi Bola Salju mampu menjawab pertanyaan mengenai keberadaan endapan sedimen yang sifatnya glasial di lintang purba, sementara penentang hipotesis ini menolak simpulan yang ditarik dari bukti tersebut dan mempertanyakan kemungkinan terjadinya peristiwa ini.<ref name=Kirschvink1992>{{cite book|author = Kirschvink, J.L.|year = 1992|chapter = Late Proterozoic low-latitude global glaciation: The snowball Earth|title = The Proterozoic Biosphere: A Multidisciplinary Study|pages = 51–2|publisher = Cambridge University Press|editor = Schopf, JW, and Klein, C.|url=http://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf|format=PDF}}</ref><ref name=nature_geo>{{cite journal| doi = 10.1038/ngeo355| title = Sedimentary challenge to Snowball Earth| year = 2008| author = Allen, Philip A.| journal = Nature Geoscience| volume = 1| pages = 817| last2 = Etienne| first2 = James L.| issue=12}}</ref> Ada beberapa pertanyaan yang belum dijawab, seperti apakah Bumi sepenuhnya terlapisi oleh salju, atau hanya sebagian dan ada bagian kecil yang tetap cair (atau cair musiman).
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== Sejarah ==
[[Douglas Mawson]] (1882–1958), seorang geolog asal [[Australia]] dan penjelajah [[Antartika]], meluangkan banyak waktu kariernya mempelajari [[stratigrafi]] [[Neoproterozoik]] Australia Selatan ketika ia mengidentifikasi sedimen glasial atau es yang tebal dan ektensif, sehingga pada akhir kariernya berspekulasi kemungkinan adanya glasiasi global.<ref name="Mawson">{{cite doi|10.1098/rsbm.1960.0011}}</ref>
Untuk mendepositkan batuan kaya zat besi semacam itu harus ada keadaan anoxia di lautan, sehingga besi yang larut (sebagai [[ferrous oxide]]) dapat terakumulasi sebelum berkontak dengan oksidan yang akan mengendapkannya sebagai [[ferric]] oxide. Supaya lautan menjadi anoxik maka harus terjadi pembatasan pertukaran gas dengan atmosfer yang mengandung oksigen. Pendukung hipotesis ini berargumen bahwa kemunculan kembali BIF pada batuan sedimentari adalah hasil dari terbatasnya kadar oksigen di dalam laut yang tertutup oleh es di laut,<ref name="Kirschvink"/> sementara para penentang berpendapat bahwa jarangnya deposit BIF mengindikasikan pembentukan di laut yang ada di daratan.
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Being isolated from the oceans, such lakes may have been stagnant and anoxic at depth, much like today's [[Black Sea]]; a sufficient input of iron could provide the necessary conditions for BIF formation.<ref name=Eyles2004 /> A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments.<ref name=Young1995/> BIFs are also strikingly absent during the Marinoan glaciation.{{Citation needed|date=March 2008}}
===Cap carbonate rocks===
[[Image:Grosser Aletschgletscher 3178.JPG|thumb|left|A present day glacier]]
[[Image:Mahameru-volcano.jpeg|thumb|right|Volcanoes may have had a role in replenishing {{co2}}, possibly ending the global ice age that was the snowball Earth during the [[Cryogenian]] Period.]] Around the top of [[Neoproterozoic]] glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary [[limestone]] or [[dolostone]] metres to tens of metres thick.<ref name="Kennedy">{{cite journal
| author=M.J. Kennedy
| title=Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial camp dolostones: deglaciation, d13C excursions and carbonate precipitation
| journal=Journal of Sedimentary Research
| year=1996
| volume=66
| issue=6
| pages=1050–64
| doi=10.2110/jsr.66.1050|bibcode = 1996JSedR..66.1050K }}</ref> These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry.<ref name="Spencer">{{cite journal
| author=Spencer, A.M.
| title=Late Pre-Cambrian glaciation in Scotland
| journal=Mem. Geol. Soc. Lond.
| year=1971
| volume=6}}</ref>
These cap carbonates have unusual chemical composition, as well as strange sedimentary structures that are often interpreted as large ripples.<ref name="HoffmanSchrag">{{cite journal
| author=P. F. Hoffman; D. P. Schrag
| title=The snowball Earth hypothesis: testing the limits of global change
| journal=Terra Nova
| year=2002
| volume=14
| pages=129–55
| url=http://users.uoa.gr/~pjioannou/nonlin/Snowball.pdf
| format=PDF 1.3 Mb
| doi = 10.1046/j.1365-3121.2002.00408.x
| issue=3}}</ref>
The formation of such sedimentary rocks could be caused by a large influx of positively charged [[ions]], as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The {{delta|13|C|link}} isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle — such a low value is usually/could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value, and counterbalance the effects of photosynthesis.
The precise mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant {{co2}} from the [[atmosphere]] to form [[carbonic acid]], which would fall as [[acid rain]]. This would weather exposed [[silicate]] and [[carbonate]] [[rock (geology)|rock]] (including readily attacked glacial debris), releasing large amounts of [[calcium]], which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an [[abiotic]] "[[cap carbonate]]" sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis.
However, there are some problems with the designation of a glacial origin to cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within — starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin.<ref name=Eyles2004 /> An alternative mechanism, which may have produced the [[Doushantuo]] cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low — as low as −48 ‰ — {{delta|13|C|}} values — as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments.<ref>{{cite journal
| title=Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China
| url = http://geology.geoscienceworld.org/cgi/reprint/36/5/347.pdf
|format=PDF| doi = 10.1130/G24513A.1
| year=2008
| author=Wang, Jiasheng
| journal=Geology
| volume=36
| page=347
| last2=Jiang
| first2=Ganqing
| last3=Xiao
| first3=Shuhai
| last4=Li
| first4=Qing
| last5=Wei
| first5=Qing
| issue=5
}}</ref>
===Changing acidity===
Isotopes of the element [[boron]] suggest that the [[pH]] of the oceans dropped dramatically before and after the [[Marinoan]] glaciation.<ref name=Kasemann2005>δ<sup>11</sup>B, in {{cite journal
| author = Kasemann, S.A.
| coauthors = Hawkesworth, C.J., Prave, A.R., Fallick, A.E., Pearson, P.N.
| year = 2005
| title = Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change
| journal = [[Earth and Planetary Science Letters]]
| volume = 231
| issue = 1–2
| pages = 73–86
| doi = 10.1016/j.epsl.2004.12.006
| url = http://eprints.gla.ac.uk/2044/
| accessdate =4 May 2007
| bibcode=2005E&PSL.231...73K
}}</ref>
This may indicate a build up of [[carbon dioxide]] in the atmosphere, some of which would dissolve into the oceans to form [[carbonic acid]]. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation.
===Space dust===
The Earth's surface is very depleted in the element [[iridium]], which primarily resides in the [[Inner core|Earth's core]]. The only significant source of the element at the surface is [[cosmogenic|cosmic particles]] that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An [[iridium anomaly]] has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years,<ref name=Bodiselitsch>{{cite journal
| author = Bodiselitsch, Bernd.
| coauthors = Koeberl, C., Master, S., Reimold, W.U.
| date = 8 April 2005
| title =Estimating Duration and Intensity of Neoproterozoic Snowball Glaciations from Ir Anomalies
| journal = Science
| volume = 308
| issue = 5719
| pages = 239–42
| doi = 10.1126/science.1104657
| url = http://www.sciencemag.org/cgi/content/abstract/308/5719/239
| accessdate =4 May 2007
| pmid =15821088
|bibcode = 2005Sci...308..239B }}</ref> but this does not necessarily imply a ''global'' extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large [[meteorite]].<ref name=Grey2003>{{cite journal
| author = Grey, K.
| coauthors = Walter, M.R.; Calver, C.R.
| date = 1 May 2003
| title = Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?
| journal = Geology
| volume = 31
| issue = 5
| pages = 459–62
| doi = 10.1130/0091-7613(2003)031<0459:NBDSEO>2.0.CO;2
| url = http://geology.geoscienceworld.org/cgi/content/abstract/31/5/459
| accessdate =29 May 2007
| bibcode=2003Geo....31..459G
}}</ref>
===Cyclic climate fluctuations===
Using the ratio of mobile [[cation]]s to those that remain in soils during [[chemical weathering]] (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods.<ref name="Rieu">{{cite journal
| author=R. Rieu; P.A. Allen; M. Plotze; T. Pettke
| title=Climatic cycles during a Neoproterozoic "snowball" glacial epoch
| journal=Geology
| year=2007
| volume=35
| issue=4
| pages=299–302
| url=http://geology.geoscienceworld.org/cgi/reprint/35/4/299.pdf
| format=PDF
| doi = 10.1130/G23400A.1|bibcode = 2007Geo....35..299R }}</ref> This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to [[Timeline of glaciation#Pleistocene glacial cycles|Pleistocene]] [[ice age]] cycles than to a completely frozen Earth.
What's more, glacial sediments of the [[Port Askaig|Portaskaig]] [[Portaskaig formation|formation]] in Scotland clearly show interbedded cycles of glacial and shallow marine sediments.<ref name=Young1999>{{cite journal
| author = Young, G.M.
| year = 1999
| title = Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland
| journal = Journal of the Geological Society
| volume = 156
| issue = 6
| pages = 1097–1111
| doi = 10.1144/gsjgs.156.6.1097
}}</ref> The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Portaskaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth.
==Mechanisms==
[[Image:SnowballSimulations.jpg|350px|thumb|right|One computer simulation of conditions during a snowball Earth period<ref name=NatureAttrib>Reprinted by permission from Macmillan Publishers Ltd: Nature 405:425-429, copyright 2000. See Hyde ''et al'' (2000).</ref>]]
The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in the Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase the Earth's [[albedo]], which would result in [[positive feedback]] for cooling. If enough snow and ice accumulates, runaway cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where [[solar radiation]] is most direct.
Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a [[supervolcano]], a reduction in the atmospheric concentration of [[greenhouse gas]]es such as [[methane]] and/or [[carbon dioxide]], changes in [[solar variation|solar energy output]], or perturbations of the [[Earth's orbit]]. Regardless of the trigger, initial cooling results in an increase in the area of the Earth's surface covered by ice and snow, and the additional ice and snow reflects more solar energy back to space, further cooling the Earth and further increasing the area of the Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen [[equator]] as cold as modern-day [[Antarctica]].
[[Global warming]] associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of the Earth's surface would require as little as 1,000 years.
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