Editing Geologic temperature record (section)

=== Late Proterozoic ice ages ===

The [[Neoproterozoic]] era ({{Ma|Neoproterozoic|Phanerozoic}}), provides evidence of at least two and possibly more major glaciations. The more recent of these ice ages, encompassing the Marinoan & Varangian glacial maxima (about {{Ma|560|650}}), has been proposed as a [[snowball Earth]] event with continuous sea ice reaching nearly to the equator. This is significantly more severe than the ice ages during the Phanerozoic. Because this ice age terminated only slightly before the rapid diversification of life during the [[Cambrian explosion]], it has been proposed that this ice age (or at least its end) created conditions favorable to evolution. The earlier Sturtian glacial maxima (~730 million years) may also have been a snowball Earth event though this is unproven.

The changes that lead to the initiation of snowball Earth events are not well known, but it has been argued that they necessarily led to their own end. The widespread sea ice prevents the deposition of fresh carbonates in [[ocean sediment]]. Since such carbonates are part of the natural process for recycling carbon dioxide, short-circuiting this process allows carbon dioxide to accumulate in the atmosphere. This increases the [[greenhouse effect]] and eventually leads to higher temperatures and the retreat of sea ice.<ref name="Eyles2004">{{cite journal |last=Eyles |first=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>