Editing Snowball Earth (section)

===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>