|
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).
== Sejarah ==
[[Douglas Mawson]] (1882–1958), seorang geolog asal [[Australia]] dan penjelajah [[AntarktikaAntartika]], 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>
Ide Mawson mengenai glasiasi global dianggap merupakan asumsi keliru bahwa posisi geografi Australia, dan benua lain di mana deposit glasial ketinggian rendah (''low-latitude glacial deposit'') ditemukan, terus konstan sepanjang waktu. Dengan munculnya hipotesis pergeseran kontinental, dan kemudian teori lempek tektonik, nampaknyatampaknya dapat dijelaskan bahwa sedimen glasiogenik itu terbentuk ketika benua berada pada ketinggian lebih tinggi.
Pada tahun 1964, ide glasiasi skala global ini muncul lagi ketika [[W. Brian Harland]] menerbitkan makalah tentang data [[palaeomagnetik]] menunjukkan bahwa pengungkitan (''till'') glasias di [[Svalbard]] dan [[Greenland]] didepositkan pada latitude tropis.<ref name="Harland">{{cite journal
| pages=45–61
| bibcode=1964GeoRu..54...45H
| doi=10.1007/BF01821169}}</ref> Dari data palaeomagnetik ini dan bukti sedimentologi bahwa sedimen glasial memutus kelanjutan lapisan batu-batuan yang biasanya dikaitkan dengan latitude tropis dan temperat, ia berargumen adanya suatu zaman es yang sangat ekstrimekstrem sehingga menghasilkan deposisi batu glasial lain pada daerah tropis.
Pada tahun 1960-an, [[Mikhail Budyko]], seorang ahli klimatologi asal Rusia, mengembangkan model iklim keseimbangan energi sederhana untuk meneliti efek tutupan es pada iklim global. Dengan model ini Budyko menemukan bahwa jika lapisan es menjalar cukup jauh dari daerah kutub, maka ada lingkaran reaksi balik di mana peningkatan reflektif ([[albedo]]) es akan membawa pendinginan lebih lanjut dan pembentukan semakin banyak es, sehingga seluruh bumi tertutup es dan mencapai keseimbangan dalam suatu ekuilibrium baru yang stabil dalam keadaan ditutup oleh es.<ref name="Budyko">{{cite journal
|year=1992
|publisher=Cambridge University Press
|chapter=Late Proterozoic low-latitude global glaciation: the Snowball Earth}}</ref> Kontribusi utama dalam karya ini adalah: (1) pengenalan adanya formasi besi berbalut (''[[:en:banded iron formation|banded iron formation]]'') yang konsisten dengan episode glasial semacam itu dan (2) perkenalan mekanisme untuk lepas dari bumi yang tertutup es, akumulasi {{co2}} dari keluarnya gas vulkanik yang menyebabkan efek ''ultra-greenhouse''.
Penemuan [[Franklyn Van Houten]] akan pola geologi konsisten di mana ketinggian danau naik dan turun sekarang dikenal sebagai "Van Houten cycle." Studinya mengenai deposit [[fosfor]] dan ''[[:en:banded iron formations|banded iron formations]]'' dalam sedimen membuatnya pendukung awal hipotesis "snowball Earth" dengan postulasi bahwa permukaan planet beku lebih dari 650 juta tahun lalu.<ref>[http://www.princeton.edu/main/news/archive/S28/44/69O99/index.xml?section=topstories Princeton University - Franklyn Van Houten, expert on sedimentary rocks, dies at 96]</ref>
Ketertarikan akan "bumi bola salju" meningkat pesat setelah [[Paul F. Hoffman]], profesor geologi pada [[Harvard University]], dan rekan-rekan pengarang menerapkan ide Kirschvink pada kelanjutan sedimen Neoproterozoic di [[Namibia]], menjabarkan hipotesis ini dengan memasukkan pengamatan seperti terbentuknya [[:en:cap carbonate|cap carbonate]], dan menerbitkan hasil mereka pada jurnal ''Science'' pada tahun 1998.<ref name="Hoffman">{{cite doi | 10.1126/science.281.5381.1342}}</ref>
Saat ini, aspek-aspek hipotesis ini masih diperdebatkan, terutama di dalam lingkungan International Geoscience Programme (IGCP) Project 512: Neoproterozoic Ice Ages.<ref>Detailed information on International Geoscience Programme (IGCP) Project 512: Neoproterozoic Ice Ages can be found at http://www.igcp512.com/ {{Webarchive|url=https://web.archive.org/web/20070422082321/http://www.igcp512.com/ |date=2007-04-22 }}</ref>
Pada bulan Maret 2010, jurnal ''[[:en:Science (journal)|Science]]'' menerbitkan artikel berjudul "Calibrating the [[:en:Cryogenian|Cryogenian]]" yang menyimpulkan bahwa "Maka es terdampar di bawah permukaan laut pada paleolatitude sangat rendah, yang meyiratkan bahwa glasiasi Sturtian berjangkauan global".<ref>[http://www.sciencemag.org/cgi/content/abstract/327/5970/1241 Calibrating the [[Cryogenian]], Abstract only:] "Ice ... implies that the Sturtian glaciation was global in extent". 5 March 2010.</ref> Suatu penjelasan populer kesimpulan ini diterbitkan dalam [[Science Daily]].<ref>[http://www.sciencedaily.com/releases/2010/03/100304142228.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:%20sciencedaily%20%28ScienceDaily:%20Latest%20Science%20News%29&utm_content=Google%20Feedfetcher Snowball Earth: New Evidence Hints at Global Glaciation 716.5 Million Years Ago] Geologists have found evidence that sea ice extended to the equator 716.5 million years ago. 5 March 2010.</ref>
== Bukti ==
| pages = 45–61
| url = http://www.springerlink.com/index/KW2790433113J4LX.pdf
| format = PDF
| accessdate = 11 March 2008
| bibcode = 1964GeoRu..54...45H
| doi = 10.1007/BF01821169
}}{{Pranala mati|date=Maret 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Model yang ada menunjukkan bahwa suatu kali glasier itu menyebar sampai 30° jaraknya dari [[kathulistiwa]], suatu [[:en:ice-albedoumpan feedback|icebalik es-albedo feedback]] akan menghasilkan es yang dengan cepat merambah ke arah kathulistiwa <ref name=Budyko1969>{{cite journal
| author = Budyko, M.I.
| year = 1969
| doi = 10.1111/j.2153-3490.1969.tb00466.x
| issue = 5
}}</ref> (model yang berikutnya menunjukkan bahwa es itu bahkan mendekat sampai 25° atau kurang dari kathulistiwa tanpa memulai glasiasi total<ref name=Meert1994pm/>). Jadi, kehadiran deposit glasial dekat dengan wilayah tropis nampaknyatampaknya menunjuk kepada penutupan es global.
<!--
Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove two things:
# that a bed contains sedimentary structures that could have been created only by glacial activity;
# that the bed lay within the tropics when it was deposited.
=== Palaeomagnetisme ===
During a period of global glaciation, it must also be demonstrated that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.
This last point is very difficult to prove. Before the [[Ediacaran]], the biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at the same time. The best that can be done is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so.<ref name=Eyles2004/>
The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the latitude of landmasses even as little as {{Ma|200}} can be riddled with difficulties.<ref name=Briden1971>{{cite journal
| author = Briden, J.C.
| coauthors = Smith, A.G.; Sallomy, J.T.
| year = 1971
| title = The geomagnetic field in Permo-Triassic time
| journal = Geophys. JR astr. Soc.
| volume = 23
| pages = 101–117
| doi = 10.1111/j.1365-246X.1971.tb01805.x
}}</ref>
-->
===Palaeomagnetisme ===
Mengingat plat tektonik bergerak setiap saat, penentuan posisinya pada suatu waktu dalam sejarah tidaklah mudah. Selain pertimbangan bagaimana daratan dapat cocok merapat, latitude deposit batu-batuan dapat dibatasi oleh palaeomagnetisme. Ketika batuan sedimentari terbentuk, mineral bermagnet di dalamnya cenderung menyesuaikan diri dengan medan magnet bumi. Melalui pengukuran cermat [[palaeomagnetisme]], dimungkinkan untuk memperkirakan [[latitude]] (tapi [[longitude]] tidak) di mana matriks batuan itu didepositkan. Pengukuran paleomagnet mengindikasikan bahwa sejumlah sedimen glasial pada catatan batuan era [[Neoproterozoic]] didepositkan di dalam jangkauan 10 derajat dari kathulistiwa,<ref name="Evans">{{cite journal
| author=D.A.D. Evans
| pages=347–433
| doi = 10.2475/ajs.300.5.347}}</ref> meskipun keakurasian rekonstruksi ini masih dipertanyakan.<ref name=Eyles2004 />
Lokasi palaeomagnet sedimen glasial ini (misalnya [[:en:dropstone|dropstone]]) menunjukkan bahwa glasier menyebar sampai permukaan laut pada latitude tropis. Tidak jelas apakah dapat disiratkan adanya glasiasi global, atau keberadaan wilayah glasial yang terlokalisasi atau terkurung daratan.<ref name=Young1995>{{cite journal
| author = Young, G.M.
| date = 1 February 1995
| url = http://geology.geoscienceworld.org/cgi/content/abstract/23/2/153
| accessdate =27 April 2007
|bibcode = 1995Geo....23..153Y }}</ref> <!--Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.<ref name=Meert1994nmse>{{cite doi
| 10.1016/0012-821X(94)90253-4}}</ref>
Skeptics suggest that the palaeomagnetic data could be corrupted if the Earth's magnetic field was substantially different from today's. Depending on the rate of cooling of the Earth's core, it is possible that during the Proterozoic, its [[magnetic field]] did not approximate a [[dipole|dipolar]] distribution, with a North and South pole roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Paleomagnetic data would then have to be re-interpreted as particles could align pointing to a 'West Pole' rather than the North Pole. Alternatively, the Earth's dipolar field could have oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north pole would occur around the same time as the Gaskiers glaciation.<ref>{{cite doi
| 10.1016/j.epsl.2010.02.038}}</ref>
Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building [[orogeny]] releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations.<ref
name=Meert1994pm>{{cite journal
| author = Meert, J.G.
| coauthors = Van Der Voo, R.; Payne, T.W.
| year = 1994
| title = Paleomagnetism of the Catoctin volcanic province: A new Vendian-Cambrian apparent polar wander path for North America
| journal = Journal of Geophysical Research
| volume = 99
| issue = B3
| pages = 4625–41
| url = http://www.agu.org/pubs/crossref/1994.../93JB01723.shtml
| accessdate =11 March 2008
| doi = 10.1029/93JB01723
| bibcode=1994JGR....99.4625M
}}</ref> Moreover, further evidence is accumulating that large-scale remagnetization events have taken place, that may require revision of the position of the paleomagnetic poles.<ref
name=Font2010pm>{{cite journal
| author = Font, E
| coauthors = C.F. Ponte Neto, M. Ernesto
| year = 2011
| title = Paleomagnetism and rock magnetism of the Neoproterozoic Itajaí Basin of the Rio de la Plata craton (Brazil): Cambrian to Cretaceous widespread remagnetizations of South America
| journal = Gondwana Research
| volume = 20
| issue = 4
| pages = 782–797
| url = http://www.sciencedirect.com/science/article/pii/S1342937X11001250
| accessdate =6 May 2011
| doi = 10.1016/j.gr.2011.04.005
}}</ref><ref name=Rowan2010pm>{{cite journal
| author = Rowan, C. J.
| coauthors = Tait, J.
| year = 2010
| title = Oman's low latitude "Snowball Earth" pole revisited: Late Cretaceous remagnetisation of Late Neoproterozoic carbonates in Northern Oman| journal = The Smithsonian/NASA Astrophysics Data System
| volume = American Geophysical Union, Fall Meeting 2010
| issue = abstract #GP33C–0959
| bibcode = 2010AGUFMGP33C0959R
| last2 = Tait
| pages = 0959
}}</ref>
-->
Ada satu deposit, Elatina di Australia, yang jelas didepositkan pada latitude rendah; tarikhnya sungguh terbataas, dan signalnya benar-benar asli.<ref name=Sohl1999>{{cite journal
| author = Sohl, L.E.
=== Deposit glasial latitude rendah ===
[[ImageBerkas:PocatelloFm.JPG|thumbjmpl|[[Diamictite]] dari [[Neoproterozoic]] Pocatello Formation, suatu deposit berjenis 'snowball Earth']]
[[FileBerkas:Elatina Fm diamictite.JPG|thumbjmpl|Elatina Fm [[diamictite]] di bawah situs [[Ediacaran]] [[Global Boundary Stratotype Section and Point|GSSP]] pada [[Flinders Ranges National Park|Flinders Ranges NP]], South Australia. Koin A$1 untuk skala.]]
Batuan sedimentari yang didepositkan oleh glasier mempunyai ciri khas sehingga dapat diidentifikasi. Jauh sebelum munculnya hipotesis ''snowball Earth'' banyak sedimen [[Neoproterozoic]] telah ditafsirkan mempunyai suatu asal mula glasial, termasuk beberapa yang berada pada latitude tropis pada waktu deposisinya. Namun, perlu diingat bahwa banyak ciri sedimentari yang secara tradisional dikaitkan dengan glasier dapat pula dibentuk dengan cara lain.<ref name=Arnaud2002>{{cite journal
| author = Arnaud, E.
| doi = 10.1046/j.1365-3091.2002.00466.x
| url =
}}</ref> <!--Thus the glacial origin of many of the key occurrences for snowball Earth has been contested.<ref name="Eyles2004" />
As of 2007, there was only one "very reliable" – still challenged<ref name="Eyles2004" /> – datum point identifying tropical [[tillite]]s,<ref name="Evans" /> which makes statements of equatorial ice cover somewhat presumptuous. However evidence of sea-level glaciation in the tropics during the [[Sturtian]] is accumulating.<ref name='Macdonald2010'>{{cite doi
| 10.1126/science.1183325
| laysummary=10.1126/science.327.5970.1186 }}</ref>
Evidence of possible glacial origin of sediment includes:
* [[Dropstones]] (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena.<ref name=Donovan1997>{{cite journal
| author = Donovan, SK
| coauthors = Pickerill, RK
| date = 27 April 2007 1997
| title = Dropstones: their origin and significance: a comment
| journal = Palaeogeography, Palaeoclimatology, Palaeoecology
| volume = 131
| issue = 1
| pages = 175–8
| doi = 10.1016/S0031-0182(96)00150-2
}}</ref>
* [[Varves]] (annual sediment layers in periglacial lakes), which can form at higher temperatures.<ref name=Thunell1995>{{cite journal
| author = Thunell, R.C.
| coauthors = Tappa, E., Anderson, D.M.
| date = 1 December 1995
| title = Sediment fluxes and varve formation in Santa Barbara Basin, offshore California
| journal = Geology
| volume = 23
| issue = 12
| pages = 1083–6
| doi = 10.1130/0091-7613(1995)023<1083:SFAVFI>2.3.CO;2
| url = http://geology.geoscienceworld.org/cgi/content/abstract/23/12/1083
| accessdate =27 April 2007
|bibcode = 1995Geo....23.1083T }}</ref>
* [[Glacial striation]]s (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by [[mudflow]]s or tectonic movements.<ref name=Jensen1996>{{cite journal
| author = Jensen, PA
| coauthors = Wulff-pedersen, E.
| date = 1 March 1996
| title = Glacial or non-glacial origin for the Bigganjargga tillite, Finnmark, Northern Norway
| journal = Geological Magazine
| volume = 133
| issue = 2
| pages = 137–45
| url = http://geolmag.geoscienceworld.org/cgi/content/abstract/133/2/137
| accessdate =27 April 2007
| doi = 10.1017/S0016756800008657
}}</ref>
* [[Diamictite]]s (poorly sorted conglomerates). Originally described as glacial [[till]], most were in fact formed by [[debris flow]]s.<ref name=Eyles2004>{{cite journal
| author = Eyles, N.
| coauthors = Januszczak, N.
| year = 2004
| title = 'Zipper-rift': A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma
| journal = Earth-Science Reviews
| volume = 65
| issue = 1–2
| pages = 1–73
| doi = 10.1016/S0012-8252(03)00080-1
| url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V62-4B723V6-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=757070dcaa8d8b501170dfde579d792f
|format=PDF| accessdate =4 May 2007
| bibcode=2004ESRv...65....1E
}}</ref>
===Open-water deposits===
It appears that some deposits formed during the snowball period could only have been formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers were melting and re-forming repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition.<ref name=Condon2002>{{cite journal
| author = Condon, D.J.
| coauthors = Prave, A.R., Benn, D.I.
| date = 1 January 2002
| title = Neoproterozoic glacial-rainout intervals: Observations and implications
| journal = Geology
| volume = 30
| issue = 1
| pages = 35–38
| doi = 10.1130/0091-7613(2002)030<0035:NGRIOA>2.0.CO;2
| url = http://geology.geoscienceworld.org/cgi/content/abstract/30/1/35
| accessdate =4 May 2007
|bibcode = 2002Geo....30...35C
}}</ref> It is considered possible that [[ice stream]]s such as seen in [[Antarctica]] today could be responsible for these sequences.
Further, sedimentary features that could only form in open water, for example [[wave-formed ripples]], far-traveled [[ice-rafted debris]] and indicators of photosynthetic activity, can be found throughout sediments dating from the snowball Earth periods. While these may represent 'oases' of [[meltwater]] on a completely frozen Earth,<ref name=Halverson2004>{{cite journal
| author = Halverson, G.P.
| coauthors = Maloof, A.C., Hoffman, P.F.
| year = 2004
| title = The Marinoan glaciation (Neoproterozoic) in northeast Svalbard
| journal = Basin Research
| volume = 16
| issue = 3
| pages = 297–324
| doi = 10.1111/j.1365-2117.2004.00234.x
| url = http://geoweb.princeton.edu/people/maloof/downloads/marinoan.pdf
| accessdate =5 May 2007
}}</ref> computer modelling suggests that large areas of the ocean must have remained ice free arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models.<ref name="Peltier">{{cite book
| last= Peltier
| first=W.R.
| authorlink=
| editor=Jenkins, G.S., McMenamin, M.A.S., McKey, C.P., & Sohl, L.
| title=The Extreme Proterozoic: Geology, Geochemistry, and Climate
| year=2004 |publisher=American Geophysical union |pages=107–124
| chapter=Climate dynamics in deep time: modeling the "snowball bifurcation" and assessing the plausibility of its occurrence}}</ref>
-->
=== Rasio isotop karbon ===
Ada dua isotop karbon stabil di air laut: [[karbon-12]] (<sup>12</sup>C) dan [[karbon-13]] (<sup>13</sup>C) yang jarang ada, keseluruhan membentuk 1.109 persen atom karbon. Proses biokimia, di antaranya [[fotosintesis]], cenderung memilih melibatkan isotop <sup>12</sup>C yang lebih ringan. Jadi pelaku fotosintesis di lautan, baik [[protista]] dan [[algae]], cenderung kekurangan <sup>13</sup>C, relatif terhadap yang banyak ditemukan di sumber vulkanik primer untuk karbon di bumi. Maka, suatu lautan dengan kehidupan fotosintesis akan mengandung rasio <sup>13</sup>C/<sup>12</sup>C yang lebih kecil dalam bekas-bekas organik, terutama dibandingkan dengan air laut. Komponen organik sedimen membatu (''lithified sediment'') akan selamanya sedikit, tapitetapi terukur, kekurangan <sup>13</sup>C.
Selama peristiwa bumi bola salju, ditemukan ekskursi cepat dan sangat negatif pada rasio <sup>13</sup>C to <sup>12</sup>C.<ref name=Rothman2003>{{cite journal | author=D.H. Rothman; J.M. Hayes; R.E. Summons | title=Dynamics of the Neoproterozoic carbon cycle | journal=Proc. Natl. Acad. Sci. U.S.A. | year=2003 | volume=100 | issue=14 | pages=124–9 | doi = 10.1073/pnas.0832439100 | pmid=12824461 | pmc=166193|bibcode = 2003PNAS..100.8124R }}</ref> Ini konsisten dengan kebekuan dalam yang membunuh semua atau hampir semua kehidupan fotosintesis – meskipun mekanisme lain, misalnya [[:en:Clathrate compound|pelepasan ''clathrate'']], dapat pula menyebabkan gangguan semacam itu. Analisis cermat mengenai waktu lonjakan <sup>13</sup>C pada deposit di seluruh dunia menunjukkan adanya empat, mungkin lima, peristiwa glasial pada akhir zaman Neoproterozoic.<ref name=Kaufman1997>{{cite journal
| author = Kaufman, Alan J.
| coauthors = Knoll, Andrew H., Narbonne, Guy M.
|bibcode = 1997PNAS...94.6600K }}</ref>
=== Formasi besi berbalut ===
[[ImageBerkas:Black-band ironstone (aka).jpg|thumbjmpl|2.1 billion year old rock with black-band ironstone]]
Formasi besi berbalut (''[[:en:Banded iron formations|Banded iron formations]]''; BIF) adalah batuan sedimentari yang terdiri dari lembaran [[besi oksida]] dan [[chert]] yang kekurangan zat besi. Dengan adanya oksigen, [[besi]] secara alamiah mengalami perkaratan dan menjadi tidak larut dalam air. Formasi besi berbalut umumnya sangat tua dan deposisi yang sering dikaitkan dengan oksidasi atmosfer bumi pada era [[Paleoproterozoic]], ketika besi yang larut di dalam lautan berkontak dengan oksigen yang dihasilkan dari fotosintesis dan mengendap sebagai besi oksida.
Balutan-balutan ini dihasilkan pada [[:en:Tipping point (climatology)|tipping point]] antara [[:en:Hypoxia (environmental)|anoxic]] dan lautan yang beroksigen. Karena atmosfer saat ini kaya akan oksigen (hampir 21 persen volume) dan berkontak dengan lautan, tidak mungkin untuk mengakumulasi cukup besi oksida untuk dideposisi dalam formasi berbalut. Satu-satunya pembentukan ekstensif besi yang didepositkan setelah era Paleoproterozoic (setelah 1,8 miliar tahun lalu) dikatikan dengan deposit glasial [[:en:Cryogenian|Cryogenian]].
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">{{cite book
|last=Kirschvink
|first=Joseph
|editor=J. W. Schopf; C. Klein
|title=The Proterozoic Biosphere: A Multidisciplinary Study
|year=1992
|publisher=Cambridge University Press
|chapter=Late Proterozoic low-latitude global glaciation: the Snowball Earth}}</ref> sementara para penentang berpendapat bahwa jarangnya deposit BIF mengindikasikan pembentukan di laut yang ada di daratan.
<!--
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]].
Balutan-balutan ini dihasilkan pada [[Tipping point (climatology)|tipping point]] antara [[Hypoxia (environmental)|anoxic]] dan lautan yang beroksigen. Karena atmosfer saat ini kaya akan oksigen (hampir 21 persen volume) dan berkontak dengan lautan, tidak mungkin untuk mengakumulasi cukup besi oksida untuk dideposisi dalam formasi berbalut. Satu-satunya pembentukan ekstensif besi yang didepositkan setelah era Paleoproterozoic (setelah 1,8 miliar tahun lalu) dikatikan dengan deposit glasial [[Cryogenian|Cryogenian]].
[[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.
-->
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.
== Lihat pula ==
==Linimasa==
* [[Geologi air bah]]
{|
| '''[[Paleoproterozoikum]]'''
|
|-
| [[Glasiasi Huron]]
| 2,400 – 2,100 jtl
|-
| '''[[Neoproterozoikum]]'''
|
|-
|[[Glasiasi Kaigas]]
| 825 – 730 jtl
|-
|[[Glasiasi Sturtian]]
|720 – 635 jtl
|-
|[[Glasiasi Marinoan]]
|650 – 635 jtl
|}
== Referensi ==
{{reflist}}
{{geologi-stub}}
[[Kategori:Glasiologi]]
[[Kategori:Zaman es]]
[[Kategori:Peristiwa kepunahan]]
[[Kategori:HipotesisHipotesa meteorologis]]
[[Kategori:Kitab KejadianKriogenium]]
[[Kategori:Paleoklimatologi]]
[[Kategori:Peristiwa proterozoikum]]
{{Geologi-stub}}
| 