Editing Snowball Earth (section)

==Evidence==
The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes.<ref name=Harland1964>{{cite journal
| author = Harland, W.B.
| year = 1964
| title = Critical evidence for a great infra-Cambrian glaciation
| journal = International Journal of Earth Sciences
| volume = 54
| issue = 1
| pages = 45–61
| bibcode = 1964GeoRu..54...45H
| doi = 10.1007/BF01821169
| s2cid = 128676272
}}</ref> According to modelling, an [[ice–albedo feedback]] would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25°<ref name="Meert1994pm" /> to 30°<ref name=Budyko1969>{{cite journal
| author = Budyko, M.I.
| year = 1969
| title = The effect of solar radiation variations on the climate of the earth
| journal = Tellus
| volume = 21
| pages = 611–9
| doi = 10.1111/j.2153-3490.1969.tb00466.x
| issue = 5
| bibcode = 1969Tell...21..611B
| citeseerx = 10.1.1.696.824
}}</ref> of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover.

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 three 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.
#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 [[biostratigraphy|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 [[radiometry|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 approximate latitudes of landmasses even as recently as 200 Ma can be riddled with difficulties.<ref name=Briden1971>{{cite journal
| author = Briden, J.C. |author2=Smith, A.G. |author3=Sallomy, J.T.
| year = 1971
| title = The geomagnetic field in Permo-Triassic time
| journal = Geophys. J. R. Astron. Soc.
| volume = 23
| pages = 101–117
| doi = 10.1111/j.1365-246X.1971.tb01805.x
|bibcode=1971GeoJ...23..101B | doi-access = free
}}</ref>

===Palaeomagnetism===
The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.

When [[sedimentary rock]]s form, magnetic minerals within them tend to align with [[Earth's magnetic field]]. Through the precise measurement of this palaeomagnetism, it is possible to estimate the [[latitude]] (but not the [[longitude]]) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator,<ref name="Evans">{{cite journal
| author=D.A.D. Evans
| title=Stratigraphic, geochronological, and palaeomagnetic constraints upon the Neoproterozoic climatic paradox
| journal=American Journal of Science
| year=2000
| volume=300
| issue=5
| pages=347–433
| doi = 10.2475/ajs.300.5.347| bibcode=2000AmJS..300..347E
}}</ref> although the accuracy of this reconstruction is in question.<ref name=Eyles2004 /> This palaeomagnetic location of apparently glacial sediments (such as [[dropstone]]s) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation or the existence of localized, possibly land-locked, glacial regimes.<ref name=Young1995>{{cite journal
| author = Young, G.M.
| date = 1 February 1995
| title = Are Neoproterozoic glacial deposits preserved on the margins of Laurentia related to the fragmentation of two supercontinents?
| journal = Geology
| volume = 23
| issue = 2
| pages = 153–6
| doi = 10.1130/0091-7613(1995)023<0153:ANGDPO>2.3.CO;2
|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 journal| doi = 10.1016/0012-821X(94)90253-4| title = The Neoproterozoic (1000–540 Ma) glacial intervals: No more snowball earth?| year = 1994| last1 = Meert | first1 = J. G.| last2 = Van Der Voo | first2 = R.| journal = Earth and Planetary Science Letters| volume = 123| issue = 1–3| pages = 1–13| bibcode=1994E&PSL.123....1M | hdl = 2027.42/31585| hdl-access = free}}</ref>

Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of [[Internal structure of Earth|Earth's core]], it is possible that during the Proterozoic, the [[Earth's magnetic field|magnetic field]] did not approximate a simple [[dipole|dipolar]] distribution, with north and south [[Poles of astronomical bodies|magnetic poles]] 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. Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a "west pole" rather than the [[north magnetic pole]]. Alternatively, Earth's dipolar field could have been 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 magnetic pole would occur around the same time as the [[Gaskiers glaciation]].<ref>{{Cite journal| doi = 10.1016/j.epsl.2010.02.038| title = Incompatible Ediacaran paleomagnetic directions suggest an equatorial geomagnetic dipole hypothesis| year = 2010| last1 = Abrajevitch | first1 = A.| last2 = Van Der Voo | first2 = R.| journal = Earth and Planetary Science Letters| volume = 293| issue = 1–2| pages = 164–170| bibcode=2010E&PSL.293..164A}}</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 [[Metamorphism|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.
| author2 = Van Der Voo, R.
| author3 = 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
| doi = 10.1029/93JB01723
| bibcode=1994JGR....99.4625M
}}</ref> Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles.<ref name=Font2010pm>{{cite journal
| author = Font, E
| author2 = C.F. Ponte Neto
| author3 = 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
| doi = 10.1016/j.gr.2011.04.005
| bibcode = 2011GondR..20..782F
}}</ref><ref name=Rowan2010pm>{{cite journal
| author = Rowan, C. J.
| author2 = Tait, J.
| year = 2010
| title = Oman's low latitude "Snowball Earth" pole revisited: Late Cretaceous remagnetisation of Late Neoproterozoic carbonates in Northern Oman
| journal = American Geophysical Union, Fall Meeting
| volume = 2010
| pages = GP33C–0959
| bibcode = 2010AGUFMGP33C0959R
}}</ref>

There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date is well-constrained, and the signal is demonstrably original.<ref name=Sohl1999>{{cite journal
| author = Sohl, L.E. |author2=Christie-blick, N. |author3=Kent, D.V.
| year = 1999
| title = Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia; implications for the duration of low-latitude glaciation in Neoproterozoic time
| journal = Bulletin of the Geological Society of America
| volume = 111
| issue = 8
| pages = 1120–39
| doi = 10.1130/0016-7606(1999)111<1120:PPRIMC>2.3.CO;2
|bibcode = 1999GSAB..111.1120S | url = https://academiccommons.columbia.edu/doi/10.7916/D80P18JS/download
}}</ref>

===Low-latitude glacial deposits===
[[Image:PocatelloFm.JPG|thumb|[[Diamictite]] of the [[Neoproterozoic]] Pocatello Formation, a "snowball Earth"-type deposit]]
[[File:Elatina Fm diamictite.JPG|thumb|Elatina Fm [[diamictite]] below [[Ediacaran]] [[Global Boundary Stratotype Section and Point|GSSP]] site in the [[Flinders Ranges National Park|Flinders Ranges NP]], South Australia. [[Australian one dollar coin|A$1 coin]] for scale.]]
Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis, many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, many sedimentary features traditionally associated with glaciers can also be formed by other means.<ref name=Arnaud2002>{{cite journal
| author = Arnaud, E.
| author2 = Eyles, C. H.
| year = 2002
| title = Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway
| journal = Sedimentology
| volume = 49
| issue = 4
| pages = 765–88
| doi = 10.1046/j.1365-3091.2002.00466.x
| bibcode = 2002Sedim..49..765A
| s2cid = 128719279
}}</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 tillites,<ref name="Evans" /> which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the Sturtian glaciation is accumulating.<ref name='Macdonald2010'>{{cite journal |last1=Macdonald |first1=F. A. |last2=Schmitz |first2=M. D. |last3=Crowley |first3=J. L. |last4=Roots |first4=C. F. |last5=Jones |first5=D. S. |last6=Maloof |first6=A. C. |last7=Strauss |first7=J. V. |last8=Cohen |first8=P. A. |last9=Johnston |first9=D. T. |last10=Schrag |first10=D. P. |title=Calibrating the Cryogenian |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1241–1243 |doi=10.1126/science.1183325 |bibcode=2010Sci...327.1241M |pmid=20203045|s2cid=40959063 }}</ref><ref>{{cite journal |last1=Kerr |first1=R. A. |title=Snowball Earth Has Melted Back To a Profound Wintry Mix |journal=Science |date=4 March 2010 |volume=327 |issue=5970 |pages=1186 |doi=10.1126/science.327.5970.1186 |pmid=20203019 |bibcode=2010Sci...327.1186K |doi-access=free }}</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, S. K.
| author2 = Pickerill, R. K.
|year = 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
| bibcode = 1997PPP...131..175D
}}</ref>
* [[Varves]] (annual sediment layers in periglacial lakes), which can form at higher temperatures.<ref name=Thunell1995>{{cite journal
| author1 = Thunell, R. C.|author-link1=Robert Thunell
| author2 = Tappa, E.
| author3 = 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
|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, P. A.
| author2 = 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
| doi = 10.1017/S0016756800008657
| bibcode = 1996GeoM..133..137J
| s2cid = 129260708
}}</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.
| author2 = 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
| bibcode=2004ESRv...65....1E
}}</ref>

===Open-water deposits===
It appears that some deposits formed during the snowball period could only have formed in the presence of an active [[Water cycle|hydrological cycle]]. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed 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.
|author2=Prave, A.R. |author3=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
|bibcode = 2002Geo....30...35C
}}</ref> It is considered{{by whom|date=June 2015}} possible that [[ice stream]]s such as seen in [[Antarctica]] today could have caused 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 [[Photosynthesis|photosynthetic]] activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent "[[Oasis|oases]]" of [[meltwater]] on a completely frozen Earth,<ref name=Halverson2004>{{cite journal
| author = Halverson, G.P.
|author2=Maloof, A.C. |author3=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
| bibcode = 2004BasR...16..297H
|citeseerx=10.1.1.368.2815 |s2cid=53588955 }}</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.
| editor= Jenkins, G.S.
| editor2= McMenamin, M.A.S.
| editor3= McKey, C.P.
| editor4= 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>

===Carbon isotope ratios===
There are two stable [[isotope]]s of carbon in sea water: [[carbon-12]] (<sup>12</sup>C) and the rare [[carbon-13]] (<sup>13</sup>C), which makes up about 1.109 percent of carbon atoms. [[Biochemistry|Biochemical]] processes, of which [[photosynthesis]] is one, tend to preferentially incorporate the lighter <sup>12</sup>C isotope. Thus ocean-dwelling photosynthesizers, both [[protist]]s and [[algae]], tend to be very slightly depleted in <sup>13</sup>C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower <sup>13</sup>C/<sup>12</sup>C ratio within organic remains and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in <sup>13</sup>C.

[[Carbonate–silicate cycle|Silicate weathering]], an inorganic process by which carbon dioxide is drawn out of the atmosphere and deposited in rock, also fractionates carbon. The emplacement of several [[large igneous province]]s shortly before the Cryogenian and the subsequent chemical [[weathering]] of the enormous continental [[flood basalt]]s created by them, aided by the breakup of Rodinia that exposed many of these flood basalts to warmer, moister conditions closer to the coast and accelerated chemical weathering, is also believed to have caused a major positive shift in carbon isotopic ratios and contributed to the beginning of the Sturtian glaciation.<ref name="FloodBasaltWeathering">{{cite journal |last1=Cox |first1=Grant M. |last2=Halverson |first2=Galen P. |last3=Stevenson |first3=Ross K. |last4=Vokaty |first4=Michelle |last5=Poirier |first5=André |last6=Kunzmann |first6=Marcus |last7=Li |first7=Zheng-Xiang |last8=Denyszyn |first8=Steven W. |last9=Strauss |first9=Justin V. |last10=Macdonald |first10=Francis A. |date=15 July 2016 |title=Continental flood basalt weathering as a trigger for Neoproterozoic Snowball Earth |journal=[[Earth and Planetary Science Letters]] |volume=446 |issue= |pages=89–99 |doi=10.1016/j.epsl.2016.04.016 |bibcode=2016E&PSL.446...89C |doi-access=free }}</ref>

During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of <sup>13</sup>C to <sup>12</sup>C.<ref name=Rothman2003>{{cite journal |author1=D.H. Rothman |author2=J.M. Hayes |author3=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 |doi-access=free }}</ref> Close analysis of the timing of <sup>13</sup>C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic.<ref name=Kaufman1997>{{cite journal
| author = Kaufman, Alan J.
|author2=Knoll, Andrew H. |author3=Narbonne, Guy M. 
| date = 24 June 1997
| title =Isotopes, ice ages, and terminal Proterozoic earth history
| journal = Proc. Natl. Acad. Sci. U.S.A.
| volume = 94
| issue = 13
| pages = 6600–5
| doi = 10.1073/pnas.94.13.6600
| pmid =11038552
| pmc = 21204
|bibcode = 1997PNAS...94.6600K |doi-access=free }}</ref>

===Banded iron formations===
[[Image:Black-band ironstone (aka).jpg|thumb|2.1 billion-year-old rock with black-band ironstone]]
Banded iron formations (BIF) are sedimentary rocks of layered [[iron oxide]] and iron-poor [[chert]]. In the presence of oxygen, [[iron]] naturally [[Rust|rusts]] and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of Earth's atmosphere during the [[Palaeoproterozoic]] era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and [[Precipitation (chemistry)|precipitated]] out as iron oxide.

The bands were produced at the [[Tipping points in the climate system|tipping point]] between an [[Anoxic waters|anoxic]] and an oxygenated ocean. Since today's atmosphere is [[oxygen]]-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with [[Cryogenian]] glacial deposits.

For such iron-rich rocks to be deposited there would have to be [[Anoxic event|anoxia]] in the ocean, so that much dissolved iron (as [[Iron(II) oxide|ferrous oxide]]) could accumulate before it met an [[Oxidizing agent|oxidant]] that would precipitate it as [[Iron(III) oxide|ferric oxide]]. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice.<ref name="Kirschvink">{{cite book
| last=Kirschvink
| first=Joseph
| editor=J. W. Schopf
| editor2=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> Near the end of a glaciation period, a reestablishment of gas exchange between the ocean and atmosphere oxidised seawater rich in ferrous iron would occur.<ref>{{cite journal |last1=Wu |first1=Chang-Zhi |last2=Zhao |first2=Fei-Fan |last3=Yang |first3=Tao |last4=Lei |first4=Ru-Xiong |last5=Ye |first5=Hui |last6=Gao |first6=Bing-Fei |last7=Li |first7=Feiqiang |date=15 July 2022 |title=Genesis of the Fulu Cryogenian iron formation in South China: Synglacial or interglacial? |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926822001334 |journal=[[Precambrian Research]] |volume=376 |page=106689 |doi=10.1016/j.precamres.2022.106689 |bibcode=2022PreR..37606689W |s2cid=248622156 |access-date=20 May 2023}}</ref> A positive shift in δ<sup>56</sup>Fe<sub>IRMM-014</sub> from the lower to upper layers of Cryogenian BIFs may reflect an increase in ocean acidification, as the upper layers were deposited as more and more oceanic ice cover melted away and more carbon dioxide was dissolved by the ocean.<ref>{{cite journal |last1=Zhu |first1=Xiang-Kun |last2=Sun |first2=Jian |last3=Li |first3=Zhi-Hong |date=1 September 2019 |title=Iron isotopic variations of the Cryogenian banded iron formations: A new model |url=https://www.sciencedirect.com/science/article/abs/pii/S0301926818303243 |journal=[[Precambrian Research]] |volume=331 |page=105359 |doi=10.1016/j.precamres.2019.105359 |bibcode=2019PreR..33105359Z |s2cid=189975438 |access-date=21 May 2023}}</ref> 

Opponents of the hypothesis suggest that the rarity of the BIF deposits may indicate that they formed in inland seas. Being isolated from the oceans, such lakes could 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/> such interbedding has been suggested to be an artefact of [[Milankovitch cycles]], which would have periodically warmed the seas enough to allow gas exchange between the atmosphere and ocean and precipitate BIFs.<ref>{{cite journal |last1=Mitchell |first1=Ross N. |last2=Gernon |first2=Thomas M. |last3=Cox |first3=Grant M. |last4=Nordsvan |first4=Adam R. |last5=Kirscher |first5=Uwe |last6=Xuan |first6=Chuang |last7=Liu |first7=Yebo |last8=Liu |first8=Xu |last9=He |first9=Xiaofang |date=7 July 2021 |title=Orbital forcing of ice sheets during snowball Earth |journal=[[Nature Communications]] |volume=12 |issue=1 |page=4187 |doi=10.1038/s41467-021-24439-4 |pmid=34234152 |pmc=8263735 |bibcode=2021NatCo..12.4187M |hdl=20.500.11937/90462 |hdl-access=free }}</ref>

===Cap carbonate rocks===
[[Image:Grosser Aletschgletscher 3178.JPG|thumb|left|A present-day glacier]]

Around the top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary [[limestone]] or [[Dolomite (rock)|dolomite]] 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 cap dolomite: 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| issue=1
| page=5
| doi=10.1144/GSL.MEM.1971.006.01.02
| bibcode=1971GSLMm...6....5.
}}</ref>
[[File:Redoubt pre-eruption 2009.jpg|thumb|right|Volcanoes may have had a role in replenishing {{co2}}, possibly ending the global ice age of the [[Cryogenian]] Period.]]

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
|author1=P. F. Hoffman |author2=D. P. Schrag | title=The snowball Earth hypothesis: testing the limits of global change
| journal=Terra Nova
| year=2002
| volume=14
| pages=129–55
| doi = 10.1046/j.1365-3121.2002.00408.x
| issue=3| bibcode=2002TeNov..14..129H| doi-access=free
}}</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&nbsp;‰, consistent with the value of the mantle—such a low value 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 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 (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. 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. 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 Formation|Doushantuo]] cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48&nbsp;‰—{{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
| doi = 10.1130/G24513A.1
| year=2008
| author=Wang, Jiasheng
| journal=Geology
| volume=36
| pages=347–350
| last2=Jiang
| first2=Ganqing
| last3=Xiao
| first3=Shuhai
| last4=Li
| first4=Qing
| last5=Wei
| first5=Qing
| issue=5
| bibcode=2008Geo....36..347W
}}</ref>

===Changing acidity===
Isotopes of [[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.
|author2=Hawkesworth, C.J. |author3=Prave, A.R. |author4=Fallick, A.E. |author5=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
| bibcode=2005E&PSL.231...73K
}}</ref>
This may indicate a buildup 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===
Earth's surface is very depleted in [[iridium]], which primarily resides in Earth's core. The only significant source of the element at the surface is 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.
|author2=Koeberl, C. |author3=Master, S. |author4=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
| pmid =15821088
|bibcode = 2005Sci...308..239B |s2cid=12231751 }}</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. |author2=Walter, M.R. |author3=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
| 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
|author1=R. Rieu |author2=P.A. Allen |author3=M. Plötze |author4=T. Pettke | title=Climatic cycles during a Neoproterozoic "snowball" glacial epoch
| journal=Geology
| year=2007
| volume=35
| issue=4
| pages=299–302
| 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#Nomenclature of Quaternary glacial cycles|Pleistocene]] [[ice age]] cycles than to a completely frozen Earth.

In addition, glacial sediments of the [[Port Askaig Tillite 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
| bibcode = 1999JGSoc.156.1097Y
| s2cid = 128600222
}}</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 Port Askaig group is 8&nbsp;km stratigraphically above the beds of interest. Its dating to 600&nbsp;Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth.