The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/atmospheric-carbon-dioxide-concentration-across-mid-pleistocene-transition
[2] http://admin.indiaenvironmentportal.org.in/category/author/david-archer-et-al
[3] http://admin.indiaenvironmentportal.org.in/category/author/brbel-hnisch
[4] http://admin.indiaenvironmentportal.org.in/category/author/n-gary-hemming
[5] http://admin.indiaenvironmentportal.org.in/category/journal/science
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/carbon-dioxide
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/atmosphere
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-change
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate-science
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/climate