Editing Snowball Earth (section)

===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>