View source for Earth:Andean-Saharan glaciation

{{short description|Glaciation 450 million to 420 million years ago}}
{{Life timeline}}
The '''Andean-Saharan glaciation''', also known as the '''Early Paleozoic Ice Age''' ('''EPIA'''),<ref name ="EPIA">{{cite journal |last1=Pohl |first1=Alexandre |last2=Donnadieu |first2=Yannick |last3=Le Hir |first3=Guillaume |last4=Ladant |first4=Jean-Baptiste |last5=Dumas |first5=Christophe |last6=Alvarez-Solas |first6=Jorge |last7=Vandenbroucke |first7=Thijs R. A. |date=28 May 2016 |title=Glacial onset predated Late Ordovician climate cooling |journal=[[Earth:Paleoceanography and Paleoclimatology|Paleoceanography and Paleoclimatology]] |volume=31 |issue=6 |pages=800–821 |doi=10.1002/2016PA002928 |s2cid=133243759 |doi-access=free |hdl=1854/LU-8057556 |hdl-access=free }}</ref> the '''Early Paleozoic Icehouse''',<ref name="GlacioeustasyBlackShalesEarlyPalaeozoicIcehouse" /> the '''Late Ordovician glaciation''', the '''end-Ordovician glaciation''', or the '''Hirnantian glaciation''', occurred during the [[Earth:Paleozoic|Paleozoic]] from approximately 460 Ma to around 420 Ma, during the [[Earth:Late Ordovician|Late Ordovician]] and the [[Earth:Silurian|Silurian]] period. The major glaciation during this period was formerly thought only to consist of the Hirnantian glaciation itself but has now been recognized as a longer, more gradual event,<ref>{{cite journal |last1=Vandenbroucke |first1=Thijs R. A. |last2=Armstrong |first2=Howard A. |last3=Williams |first3=Mark |last4=Paris |first4=Florentin |last5=Sabbe |first5=Koen |last6=Zalasiewicz |first6=Jan A. |last7=Nõlvak |first7=Jaak |last8=Verniers |first8=Jacques |date=15 August 2010 |title=Epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest Late Ordovician seas: Implications for a cooling Late Ordovician climate |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018209005215 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=294 |issue=3–4 |pages=202–219 |doi=10.1016/j.palaeo.2009.11.026 |access-date=29 December 2022}}</ref><ref>{{cite journal |last1=Rosenau |first1=Nicholas A. |last2=Hermann |first2=Achim D. |last3=Leslie |first3=Stephen A. |date=15 January 2012 |title=Conodont apatite δ18O values from a platform margin setting, Oklahoma, USA: Implications for initiation of Late Ordovician icehouse conditions |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018211005839 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=315-316 |pages=172–180 |doi=10.1016/j.palaeo.2011.12.003 |access-date=29 December 2022}}</ref><ref name="Munnecke2010">{{Cite journal |last1=Munnecke |first1=Axel |last2=Calner |first2=Mikael |last3=Harper |first3=David A. T. |last4=Servais |first4=Thomas |date=15 October 2010 |title=Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=296 |issue=3–4 |pages=389–413 |doi=10.1016/j.palaeo.2010.08.001 |authorlink3=David Harper (palaeontologist)}}</ref> which began as early as the [[Earth:Darriwilian|Darriwilian]],<ref name="EPIA"/> and possibly even the [[Earth:Floian|Floian]].<ref name="MayaElrick">{{cite journal |last1=Elrick |first1=Maya |date=1 October 2022 |title=Orbital-scale climate changes detected in Lower and Middle Ordovician cyclic limestones using oxygen isotopes of conodont apatite |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=603 |page=111209 |doi=10.1016/j.palaeo.2022.111209 |doi-access=free }}</ref> [[Philosophy:Evidence|Evidence]] of this glaciation can be seen in places such as Arabia, [[Place:North Africa|North Africa]], {{wipe|South Africa}}, {{wipe|Brazil}}, Peru, Bolivia, {{wipe|Chile}}, {{wipe|Argentina}}, and Wyoming.<ref name="Díaz-MartínezGrahn2007">{{cite journal |last1=Díaz-Martínez |first1=Enrique |last2=Grahn |first2=Yngve |date=7 March 2007 |title=Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina): Palaeogeographic and geodynamic setting |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018206003397#! |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=245 |issue=1–2 |pages=62–81 |doi=10.1016/j.palaeo.2006.02.018 |access-date=17 October 2022}}</ref><ref>{{cite journal |last1=Hambrey |first1=M. J. |date=October 1985 |title=The late Ordovician—Early Silurian glacial period |url=https://www.sciencedirect.com/science/article/abs/pii/0031018285900896 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=51 |issue=1–4 |pages=273–289 |doi=10.1016/0031-0182(85)90089-6 |access-date=16 October 2022}}</ref><ref>{{cite journal |last1=Van Staden |first1=Anelda |last2=Zimmermann |first2=Udo |last3=Chemale, Jr. |first3=Farid |last4=Gutzmer |first4=Jens |last5=Germs |first5=G. J. B. |date=1 January 2010 |title=Correlation of Ordovician diamictites from Argentina and South Africa using detrital zircon dating |url=https://pubs.geoscienceworld.org/jgs/article-abstract/167/1/217/372785/Correlation-of-Ordovician-diamictites-from |journal=Journal of the Geological Society |volume=167 |issue=1 |pages=217–220 |doi=10.1144/0016-76492009-023 |s2cid=128392767 |access-date=14 October 2022}}</ref><ref name="Bighorn Dolomite, Wyoming" /> More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5&nbsp;°C cooler than present day; this would have been a major factor that aided in the glaciation process.<ref name="Magnitude and Duration">{{cite journal |last=Finnegan |first=S. |year=2011 |title=The Magnitude and Duration of the Late Ordovician-Early Silurian Glaciation |url=https://authors.library.caltech.edu/22403/2/Finnegan.SOM.pdf |journal=Science |volume=331 |issue=6019 |pages=903–906 |bibcode=2011Sci...331..903F |doi=10.1126/science.1200803 |pmid=21273448|s2cid=35089938 }}<!--|accessdate=25 November 2012--></ref>

The Late Ordovician glaciation is widely considered to be the leading cause of the [[Earth:Late Ordovician mass extinction|Late Ordovician mass extinction]],<ref name="GlacioeustasyBlackShalesEarlyPalaeozoicIcehouse">{{cite book |chapter = Were transgressive black shales a negative feedback modulating glacioeustasy in the Early Palaeozoic Icehouse? |display-editors = 3 |author1 = Page, A.|series = Special Publication of the Geological Society of London|author2 = Zalasiewicz, J.|publisher = The Micropaleontology Society special publications |author3 = Williams, M. |author4 = Popov, L. |editor1= Williams, Mark |editor2 = Haywood, A. M. |editor3 = Gregory, J. |editor4 = Schmidt, D.N. |title = Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies |chapter-url = https://books.google.com/books?id=j4eCj1J8k0IC&pg=PA123|year=2007|isbn=978-1-86239-240-3}}</ref><ref name="end-Ordovician glaciation">{{cite journal |last=Delabroye |first=A. |author2=Vecoli, M. |year=2010 |title=The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about the Late Ordovician event stratigraphy |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=98 |issue=3–4 |pages=269–282 |bibcode=2010ESRv...98..269D |doi=10.1016/j.earscirev.2009.10.010}}<!--|accessdate=25 November 2012--></ref> and it is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life.<ref name="Mass Extinction">{{cite journal |last=Sheehan |first=Peter M |date=1 May 2001 |title=The Late Ordovician Mass Extinction |journal=Annual Review of Earth and Planetary Sciences |volume=29 |issue=1 |pages=331–364 |bibcode=2001AREPS..29..331S |doi=10.1146/annurev.earth.29.1.331}}</ref> Estimates of peak [[Earth:Ice sheet|ice sheet]] [[Physics:Volume|volume]] range from 50 to 250 million cubic kilometres, and its duration from 35 million to less than 1 million years. At its height during the [[Earth:Hirnantian|Hirnantian]], the ice age is believed to have been significantly more extreme than the [[Earth:Last Glacial Maximum|Last Glacial Maximum]] occurring during the terminal [[Earth:Pleistocene|Pleistocene]].<ref name="Magnitude and Duration" /> Glaciation of the [[Earth:Northern Hemisphere|Northern Hemisphere]] was minimal because a large amount of the land was in the [[Earth:Southern Hemisphere|Southern Hemisphere]].

== Timeline ==
===Pre-Hirnantian glaciations===
The earliest evidence for possible glaciation comes from Floian conodont apatite oxygen isotope fluctuations, which display a periodicity characteristic of Milankovitch cycles and have been interpreted as reflecting cyclic waxing and waning of polar ice caps.<ref name="MayaElrick" /> A speculated glaciation in the middle Darriwilian corresponds to the MDICE positive carbon isotope excursion.<ref name="ClimateChangesPreHirnantian" /> Sea level changes likely reflective of glacioeustasy are known from this geologic stage, around 467 Ma.<ref name="EPIA"/> However, there are no known Middle Ordovician glacial deposits that would provide direct geological evidence of glaciation.<ref>{{Cite journal |last=Cocks |first=L. Robin M. |last2=Torsvik |first2=Trond H. |date=December 2021 |title=Ordovician palaeogeography and climate change |url=https://www.sciencedirect.com/science/article/pii/S1342937X20302756?via%3Dihub |journal=Gondwana Research |language=en |volume=100 |pages=53–72 |doi=10.1016/j.gr.2020.09.008 |access-date=17 September 2023|doi-access=free |hdl=10852/83447 |hdl-access=free }}</ref><ref>{{Cite journal |last=M. Marcilly |first=Chloé |last2=Maffre |first2=Pierre |last3=Le Hir |first3=Guillaume |last4=Pohl |first4=Alexandre |last5=Fluteau |first5=Frédéric |last6=Goddéris |first6=Yves |last7=Donnadieu |first7=Yannick |last8=H. Heimdal |first8=Thea |last9=Torsvik |first9=Trond H. |date=15 September 2022 |title=Understanding the early Paleozoic carbon cycle balance and climate change from modelling |url=https://www.sciencedirect.com/science/article/pii/S0012821X22003533#br0070 |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |language=en |volume=594 |pages=117717 |doi=10.1016/j.epsl.2022.117717 |access-date=17 September 2023|hdl=10852/94890 |hdl-access=free }}</ref> Isotopic evidence from the Sandbian reveals three possible glaciations: an early Sandbian glaciation, a middle Sandbian glaciation, a late Sandbian glaciation.<ref name="ClimateChangesPreHirnantian" /> Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, there is evidence suggesting the presence of glaciation by the [[Earth:Sandbian|Sandbian]] stage (approximately 451–461 Ma).<ref name="Bighorn Dolomite, Wyoming" /> Graptolite distribution during the time interval delineated by the ''Nemacanthus gracilis'' graptolite biozone indicates a latitudinal extent of the subtropics and tropics similar to that of today, as evidenced by a steep faunal gradient that is uncharacteristic of greenhouse periods, suggesting that Earth was in a mild icehouse state by the start of the Sandbian, around 460 Ma.<ref>{{cite journal |last1=Vandenbroucke |first1=Thijs R. A. |last2=Armstrong |first2=Howard A. |last3=Williams |first3=Mark |last4=Zalasiewicz |first4=Jan A. |last5=Sabbe |first5=Koen |date=20 October 2009 |title=Ground-truthing Late Ordovician climate models using the paleobiogeography of graptolites |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008PA001720 |journal=[[Earth:Paleoceanography and Paleoclimatology|Paleoceanography and Paleoclimatology]] |volume=24 |issue=4 |pages=1–19 |doi=10.1029/2008PA001720 |hdl=1854/LU-5645677 |access-date=21 October 2022|hdl-access=free }}</ref> Many possible short glaciation occurred during the Katian: three very short glaciations during the early Katian, the Rakvere glaciation during the late early Katian, a middle Katian glaciation, the Early Ashgill glaciation of the early late Katian, and a latest Katian glaciation that was followed by a rapid warming event in the ''Paraorthograptus pacificus'' graptolite biozone immediately before the Hirnantian glaciation itself.<ref name="ClimateChangesPreHirnantian">{{cite journal |last1=Männik |first1=Peep |last2=Lehnert |first2=Oliver |last3=Nõlvak |first3=Jaak |last4=Joachimski |first4=Michael M. |date=1 May 2021 |title=Climate changes in the pre-Hirnantian Late Ordovician based on δ18Ophos studies from Estonia |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018221001322 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=569 |page=110347 |doi=10.1016/j.palaeo.2021.110347 |s2cid=233644917 |access-date=26 December 2022}}</ref> Evidence of major changes in bottom water formation, which usually indicates a sudden change in global climate, is known from the Katian.<ref>{{cite journal |last1=Young |first1=Seth A. |last2=Saltzman |first2=Matthew R. |last3=Bergström |first3=Stig M. |last4=Leslie |first4=Stephen A. |last5=Xu |first5=Chen |date=1 December 2008 |title=Paired δ13Ccarb and δ13Corg records of Upper Ordovician (Sandbian–Katian) carbonates in North America and China: Implications for paleoceanographic change |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018208005312 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=270 |issue=1–2 |pages=166–178 |doi=10.1016/j.palaeo.2008.09.006 |access-date=29 December 2022}}</ref> Shifts in isotopic ratios of carbon and neodymium that correspond to graptolite biostratigraphy lend further evidence in favour of the existence of glacioeustatic cycles during the Katian,<ref>{{cite journal |last1=Holmden |first1=C. |last2=Mitchell |first2=C. E. |last3=LaPorte |first3=D. F. |last4=Patterson |first4=W. P. |last5=Melchin |first5=M. J. |last6=Finney |first6=S. C. |date=15 September 2013 |title=Nd isotope records of late Ordovician sea-level change—Implications for glaciation frequency and global stratigraphic correlation |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018213002472 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=386 |pages=131–144 |doi=10.1016/j.palaeo.2013.05.014 |access-date=13 May 2023}}</ref> as do conodont apatite δ<sup>18</sup>O fluctuations from Kentucky and Quebec that likely reflect glacioeustatic sea level changes.<ref>{{Cite journal |last=Elrick |first=M. |last2=Reardon |first2=D. |last3=Labor |first3=W. |last4=Martin |first4=J. |last5=Desrochers |first5=A. |last6=Pope |first6=M. |date=1 July 2013 |title=Orbital-scale climate change and glacioeustasy during the early Late Ordovician (pre-Hirnantian) determined from δ18O values in marine apatite |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/41/7/775/131295/Orbital-scale-climate-change-and-glacioeustasy?redirectedFrom=fulltext |journal=Geology |language=en |volume=41 |issue=7 |pages=775–778 |doi=10.1130/G34363.1 |issn=1943-2682 |access-date=17 September 2023}}</ref> However, the existence of glacials during the Katian remains controversial.<ref>{{cite journal |last1=Quinton |first1=Page C. |last2=MacLeod |first2=Kenneth G. |date=15 June 2014 |title=Oxygen isotopes from conodont apatite of the midcontinent, US: Implications for Late Ordovician climate evolution |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018214001710 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=404 |pages=57–66 |doi=10.1016/j.palaeo.2014.03.036 |access-date=29 December 2022}}</ref><ref>{{cite journal |last1=Ainsaar |first1=Leho |last2=Meidla |first2=Tõnu |last3=Martma |first3=Tõnu |date=1 January 1999 |title=Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia |url=https://www.cambridge.org/core/journals/geological-magazine/article/abs/evidence-for-a-widespread-carbon-isotopic-event-associated-with-late-middle-ordovician-sedimentological-and-faunal-changes-in-estonia/ADD801E7002863FA94230E1C2180B179 |journal=Geological Magazine |volume=136 |issue=1 |pages=49-62 |doi=10.1017/S001675689900223X |access-date=9 August 2023}}</ref> Katian brachiopod and seawater δ<sup>18</sup>O values from Cincinnati Arch indicate ocean temperatures characteristic of a global greenhouse state.<ref>{{cite journal |last1=Barney |first1=Bryce B. |last2=Grossman |first2=Ethan L. |date=11 February 2022 |title=Reassessment of ocean paleotemperatures during the Late Ordovician |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/50/5/572/611747/Reassessment-of-ocean-paleotemperatures-during-the |journal=Geology |volume=50 |issue=5 |pages=572-576 |doi=10.1130/G49422.1 |access-date=25 July 2023}}</ref>

===Hirnantian glaciation===
[[File:Carbon_13_and_time_scale_during_the_Ordovician.png|alt=Ordovician Carbon 13 time scale|thumb|In this graph the time period that represents the Late Ordovician is at the very top. There is a sharp shift in carbon 13, as well as a sharp decline in sea surface temperatures.<ref name="CO2 exit glaciation">{{cite journal |last=Seth A Young |first=M. R. |year=2012 |title=Did Changes in atmospheric CO2 coincide with latest Ordovician glacial-interglacial cycles? |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=296 |issue=3–4 |pages=376–388 |doi=10.1016/j.palaeo.2010.02.033}}</ref>]]
At the Katian-Hirnantian boundary, a sudden cooling event caused a rapid expansion of glaciers, resulting in one of the most severe glaciations of the Phanerozoic, an extreme cooling event generally believed to be coincident with the first pulse of the Late Ordovician mass extinction.<ref>{{cite journal |last1=Saupe |first1=Erin E. |last2=Qiao |first2=Huijie |last3=Donnadieu |first3=Yannick |last4=Farnsworth |first4=Alexander |last5=Kennedy-Asser |first5=Alan T. |last6=Ladant |first6=Jean-Baptiste |last7=Lunt |first7=Daniel J. |last8=Pohl |first8=Alexandre |last9=Valdes |first9=Paul |last10=Finnegan |first10=Seth |date=16 December 2019 |title=Extinction intensity during Ordovician and Cenozoic glaciations explained by cooling and palaeogeography |url=https://www.nature.com/articles/s41561-019-0504-6#citeas |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=13 |issue=1 |pages=65–70 |doi=10.1038/s41561-019-0504-6 |s2cid=209381464 |access-date=22 October 2022|hdl=1983/c88c3d46-e95d-43e6-aeaf-685580089635 |hdl-access=free }}</ref> An δ<sup>18</sup>O shift occurs at the start of the Hirnantian; the magnitude of this shift (+2-4‰) was extraordinary.<ref>{{cite journal |last1=Wang |first1=K. |last2=Chatterton |first2=B. D. E. |last3=Wang |first3=K. |date=August 1997 |title=An organic carbon isotope record of Late Ordovician to Early Silurian marine sedimentary rocks, Yangtze Sea, South China: Implications for CO2 changes during the Hirnantian glaciation |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018297000461 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=132 |issue=1-4 |pages=147-158 |doi=10.1016/S0031-0182(97)00046-1 |access-date=23 July 2023}}</ref> Its direction implies glacial cooling and possibly increases in ice-volume. The observed shifts in the δ<sup>18</sup>O isotopic indicator would require a sea-level fall of 100 meters and a drop of 10&nbsp;°C in tropical ocean temperatures to have occurred during this glacial episode.<ref name="Bathymetric and Isotopic evidence" /> Sedimentological data shows that Late Ordovician ice sheets glacierized the Al Kufrah Basin. Ice sheets also probably formed continuous ice cover over North Africa and the Arabian Peninsula. In all areas of North Africa where [[Earth:Llandovery epoch|Early Silurian]] shale occurs, Late Ordovician glaciogenic deposits occur beneath, likely due to the [[Earth:Anoxic waters|anoxia]] promoted in these basins.<ref name="Evidence Al Kufrah">{{cite journal |last=Heron |first=D. P. |author2=Howard, J. |year=2010 |title=Evidence for Late Ordovician Glaciation of Al Kufrah Basin, Libya |journal=Journal of African Earth Sciences |volume=58 |issue=2 |pages=354–364 |bibcode=2010JAfES..58..354L |doi=10.1016/j.jafrearsci.2010.04.001}}<!--|accessdate=25 November 2012--></ref>

At the end of the Hirnantian, an abrupt retreat of glaciers concurrent with the second pulse of the Late Ordovician mass extinction occurred,<ref>{{cite journal |last1=Melchin |first1=Michael J. |last2=Mitchell |first2=Charles E. |last3=Holmden |first3=Chris |last4=Štorch |first4=Petr |date=1 November 2013 |title=Environmental changes in the Late Ordovician–early Silurian: Review and new insights from black shales and nitrogen isotopes |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/125/11-12/1635/125852/Environmental-changes-in-the-Late-Ordovician-early |journal=Geological Society of America Bulletin |volume=125 |issue=11-12 |pages=1635-1670 |doi=10.1130/B30812.1 |access-date=22 July 2023}}</ref> after which Earth receded back into a much warmer climate during the Rhuddanian.<ref>{{cite journal |last1=Cai |first1=Quansheng |last2=Hu |first2=Mingyi |last3=Kane |first3=Oumar Ibrahima |last4=Li |first4=Mingtao |last5=Zhang |first5=Baomin |last6=Hu |first6=Zhonggui |last7=Deng |first7=Qingjie |last8=Xing |first8=Niu |date=February 2022 |title=Cyclic variations in paleoenvironment and organic matter accumulation of the Upper Ordovician–Lower Silurian black shale in the Middle Yangtze Region, South China: Implications for tectonic setting, paleoclimate, and sea-level change |url=https://www.sciencedirect.com/science/article/abs/pii/S0264817221005808 |journal=Marine and Petroleum Geology |volume=136 |doi=10.1016/j.marpetgeo.2021.105477 |access-date=22 July 2023}}</ref> Late Hirnantian warming was marked by a similarly meteoric shift in δ<sup>18</sup>O towards more negative values.<ref>{{cite journal |last1=Brenchley |first1=P. J. |last2=Carden |first2=G. A. |last3=Hints |first3=L. |last4=Kaljo |first4=D. |last5=Marshall |first5=J. D. |last6=Martma |first6=T. |last7=Meidla |first7=T. |last8=Nõlvak |first8=J. |date=1 January 2003 |title=High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/115/1/89/183914/High-resolution-stable-isotope-stratigraphy-of?redirectedFrom=fulltext |journal=Geological Society of America Bulletin |volume=115 |issue=1 |pages=89-104 |doi=10.1130/0016-7606(2003)115<0089:HRSISO>2.0.CO;2 |access-date=23 July 2023}}</ref> δ<sup>13</sup>C values likewise fall sharply at the beginning of the [[Earth:Silurian|Silurian]].<ref name="Bathymetric and Isotopic evidence">{{cite journal |last=Brenchley |first=P.J. |author2=J. D. |year=1994 |title=Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period |journal=Geology |volume=22 |issue=4 |pages=295–298 |bibcode=1994Geo....22..295B |doi=10.1130/0091-7613(1994)022<0295:baiefa>2.3.co;2}}<!--|accessdate=25 November 2012--></ref>

===Silurian glaciations===
Following the relatively warm Rhuddanian, glacial events occurred during the early and latest Aeronian.<ref>{{cite journal |last1=Azmy |first1=Karem |last2=Veizer |first2=Ján |last3=Bassett |first3=Michael G. |last4=Copper |first4=Paul |date=1 November 1998 |title=Oxygen and carbon isotopic composition of Silurian brachiopods: Implications for coeval seawater and glaciations |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article/110/11/1499/183331/Oxygen-and-carbon-isotopic-composition-of-Silurian |journal=Geological Society of America Bulletin |volume=110 |issue=11 |pages=1499–1512 |doi=10.1130/0016-7606(1998)110<1499:OACICO>2.3.CO;2 |access-date=26 December 2022}}</ref> A further glaciation occurred from the late Telychian to middle Sheinwoodian.<ref>{{cite journal |last1=Lehnert |first1=Oliver |last2=Männik |first2=Peep |last3=Joachimski |first3=Michael M. |last4=Calner |first4=Mikael |last5=Frýda |first5=Jiři |date=15 October 2010 |title=Palaeoclimate perturbations before the Sheinwoodian glaciation: A trigger for extinctions during the 'Ireviken Event' |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018210000106 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=296 |issue=3–4 |pages=320–331 |doi=10.1016/j.palaeo.2010.01.009 |access-date=26 December 2022|doi-access=free }}</ref><ref>{{cite journal |last1=Loydell |first1=David K. |date=2 July 2007 |title=Early Silurian positive δ13C excursions and their relationship to glaciations, sea-level changes and extinction events |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/gj.1090 |journal=Geological Journal |volume=42 |issue=5 |pages=531-546 |doi=10.1002/gj.1090 |access-date=25 July 2023|doi-access=free }}</ref> From the early to late Homerian, Earth was in yet another glacial phase.<ref name="TrotterEtAl2016PPP">{{cite journal |last1=Trotter |first1=Julie A. |last2=Williams |first2=Ian S. |last3=Barnes |first3=Christopher R. |last4=Männik |first4=Peep |last5=Simpson |first5=Andrew |title=New conodont δ18O records of Silurian climate change: Implications for environmental and biological events |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |date=February 2016 |volume=443 |pages=34–48 |doi=10.1016/j.palaeo.2015.11.011 |bibcode=2016PPP...443...34T }}</ref> The last major glaciation of the EPIA occurred during the Ludfordian and was associated with the Lau event.<ref name ="LudfordianGlaciation">{{cite journal |last1=Frýda |first1=Jiří |last2=Lehnert |first2=Oliver |last3=Joachimski |first3=Michael M. |last4=Männik |first4=Peep |last5=Kubajko |first5=Michal |last6=Mergl |first6=Michal |last7=Farkaš |first7=Juraj |last8=Frýdová |first8=Barbora |date=September 2021 |title=The Mid-Ludfordian (late Silurian) Glaciation: A link with global changes in ocean chemistry and ecosystem overturns |url=https://www.sciencedirect.com/science/article/abs/pii/S0012825221001537 |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=220 |page=103652 |doi=10.1016/j.earscirev.2021.103652 |access-date=26 December 2022}}</ref>

During this period, glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes, and the centre of glaciation is known to have migrated from the Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma). According to Eyles and Young, "A major glacial episode at c. 440 [[Astronomy:Myr|Ma]], is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the [[Earth:Llandovery epoch|Early Silurian]] the centre of glaciation moved from northern Africa to southwestern South America."<ref name="Eyles">{{cite book |last1=Eyles |first1=Nicholas |last2=Young |first2=Grant |editor1-last=Deynoux |editor1-first=M. |editor2-last=Miller |editor2-first=J.M.G. |editor3-last=Domack |editor3-first=E.W. |editor4-last=Eyles |editor4-first=N. |editor5-last=Fairchild |editor5-first=I.J. |editor6-last=Young |editor6-first=G.M.|title=Geodynamic controls on glaciation in Earth history, in Earth's Glacial Record |date=1994 |publisher=Cambridge University Press |location=Cambridge |isbn=0521548039 |pages=[https://archive.org/details/earthsglacialrec0000unse/page/10 10–18] |url=https://archive.org/details/earthsglacialrec0000unse/page/10}}</ref> Continental glaciers developed in Africa and eastern Brazil, while alpine glaciers formed in the Andes.<ref>{{Cite web | last = Aber | first = James S. | title = ES 331/767 Lab III | publisher = Emporia State University | year = 2008 | url = http://academic.emporia.edu/aberjame/ice/labs/lab03.htm | access-date = 7 November 2015 | archive-date = 10 July 2016 | archive-url = https://web.archive.org/web/20160710071747/http://academic.emporia.edu/aberjame/ice/labs/lab03.htm | url-status = dead }}</ref> In western South America (Peru, Bolivia and northern Argentina) were found glacio-marine diamictites interbedded with turbidites, shales, mud flows and debris flows, dated as early Silurian (Llandonvery), with a southward extension into northern Argentina and western Paraguay, and with a probably northern extension into Peru, Ecuador and Colombia.<ref name="Díaz-MartínezGrahn2007" />

A major ice age, the Andean-Saharan was preceded by the [[Earth:Cryogenian|Cryogenian]] ice ages (720–630 Ma, the [[Earth:Sturtian glaciation|Sturtian]] and [[Earth:Marinoan glaciation|Marinoan glaciation]]s), often referred to as [[Earth:Snowball Earth|Snowball Earth]], and followed by the [[Earth:Karoo Ice Age|Karoo Ice Age]] (350–260 Ma).<ref>{{Citation | last = Högele | first = M. A. | title = Metastability of the Chafee-Infante equation with small heavy-tailed Lévy Noise | year = 2011 | publisher = Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät II | url = http://edoc.hu-berlin.de/dissertationen/hoegele-michael-anton-2010-12-02/PDF/hoegele.pdf | access-date = 7 November 2015 | archive-date = 2017-03-15 | archive-url = https://web.archive.org/web/20170315174321/http://edoc.hu-berlin.de/dissertationen/hoegele-michael-anton-2010-12-02/PDF/hoegele.pdf | url-status = dead }}</ref>

== Evidence ==
=== Lithologic ===
* The sequence of the stratigraphic architecture of the Bighorn Dolomite (which represents end of the Ordovician period), is consistent with the gradual buildup of glacial ice.  The sequences of the Bighorn Dolomite display systematic changes in their component cycles, and the changes in these cycles are interpreted as being a change from a greenhouse climate to a transitional ice house climate.<ref name="Bighorn Dolomite, Wyoming">{{cite journal |last=Holland |first=S. M. |author2=Patzkowsky, M. E. |year=2012 |title=Sequence Architecture of the Bighorn Dolomite, Wyoming, USA: Transition to the Late Ordovician Icehouse |journal=Journal of Sedimentary Research |volume=82 |issue=8 |pages=599–615 |bibcode=2012JSedR..82..599H |doi=10.2110/jsr.2012.52}}</ref>

== Possible causes ==

=== CO<sub>2</sub> depletion ===
One of the factors that hindered glaciation during the early Paleozoic was atmospheric CO<sub>2</sub> concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels.<ref name="impacts" /> However, [[Astronomy:Solar irradiance|solar irradiance]] was significantly lower during the Late Ordovician; 450 million years ago, solar irradiance of Earth was about 1312.00 W{{nnbsp}}m<sup>−2</sup> compared to 1360.89 W{{nnbsp}}m<sup>−2</sup> in the present day.<ref>{{cite journal |last1=Li |first1=Xiang |last2=Hu |first2=Yongyun |last3=Guo |first3=Jiaqi |last4=Lan |first4=Jiawenjing |last5=Lin |first5=Qifan |last6=Bao |first6=Xiujuan |last7=Yuan |first7=Shuai |last8=Wei |first8=Mengyu |last9=Li |first9=Zhibo |last10=Man |first10=Kai |last11=Yin |first11=Zihan |last12=Han |first12=Jing |last13=Zhang |first13=Jian |last14=Zhu |first14=Chenguang |last15=Zhao |first15=Zhouqiao |last16=Liu |first16=Yonggang |last17=Yang |first17=Jun |last18=Nie |first18=Ji |date=28 June 2022 |title=A high-resolution climate simulation dataset for the past 540 million years |journal=Scientific Data |volume=9 |issue=371 |page=371 |doi=10.1038/s41597-022-01490-4 |pmid=35764652 |pmc=9240078 }}</ref> Furthermore, CO<sub>2</sub> concentrations are thought to have dropped significantly in the Hirnantian, which could have induced widespread glaciation during an overall cooling trend.<ref>{{cite journal |last1=Vandenbroucke |first1=Thijs R. A. |last2=Armstrong |first2=Howard A. |last3=Williams |first3=Mark |last4=Paris |first4=Florentin |last5=Zalasiewicz |first5=Jan A. |last6=Sabbe |first6=Koen |last7=Nõlvak |first7=Jaak |last8=Challands |first8=Thomas J. |last9=Verniers |first9=Jacques |last10=Servais |first10=Thomas |date=9 August 2010 |title=Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=34 |pages=14983–14986 |doi=10.1073/pnas.1003220107 |pmid=20696937 |doi-access=free |pmc=2930542 }}</ref> Methods for the removal of CO<sub>2</sub> during this time were not well known,<ref name="Bathymetric and Isotopic evidence" /> and are still hotly debated, with the radiation of terrestrial plants,<ref name="Lenton2012">{{cite journal |last1=Lenton |first1=Timothy M. |last2=Crouch |first2=Michael |last3=Johnson |first3=Martin |last4=Pires |first4=Nuno |last5=Dolan |first5=Liam |date=1 February 2012 |title=First plants cooled the Ordovician |url=https://www.nature.com/articles/ngeo1390 |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |language=en |volume=5 |issue=2 |pages=86–89 |doi=10.1038/ngeo1390 |issn=1752-0908 |access-date=18 October 2022}}</ref> enhanced oceanic organic carbon burial,<ref>{{cite journal |last1=Sproson |first1=Adam D. |last2=Von Strandmann |first2=Philip A. E. Pogge |last3=Selby |first3=David |last4=Jarochowska |first4=Emilia |last5=Frýda |first5=Jiří  |last6=Hladil |first6=Jindřich |last7=Loydell |first7=David K. |last8=Slavík |first8=Ladislav |last9=Calner |first9=Mikael |last10=Maier |first10=Georg |last11=Munnecke |first11=Axel |last12=Lenton |first12=Timothy M. |date=1 January 2022 |title=Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X21005161#! |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=577 |page=117260 |doi=10.1016/j.epsl.2021.117260 |s2cid=243795224 |access-date=18 October 2022}}</ref><ref>{{cite journal |last1=Lv |first1=Y. |last2=Liu |first2=S.-A. |last3=Wu |first3=H. |last4=Sun |first4=Z. |last5=Li |first5=C. |last6=Fan |first6=J. X. |date=25 March 2022 |title=Enhanced organic carbon burial intensified the end-Ordovician glaciation |url=https://www.geochemicalperspectivesletters.org/article2210/ |journal=[[Chemistry:Geochemical Perspectives Letters|Geochemical Perspectives Letters]] |volume=21 |pages=13–17 |doi=10.7185/geochemlet.2210 |s2cid=247721878 |access-date=14 May 2023|doi-access=free }}</ref> and a reduction in volcanic outgassing of carbon dioxide having been proposed.<ref>{{cite journal |last1=Young |first1=Seth A. |last2=Saltzman |first2=Matthew R. |last3=Foland |first3=Kenneth A. |last4=Linder |first4=Jeff S. |last5=Kump |first5=Lee R. |date=1 October 2009 |title=A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate? |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/37/10/951/103964/A-major-drop-in-seawater-87Sr-86Sr-during-the |journal=Geology |volume=37 |issue=10 |pages=951–954 |doi=10.1130/G30152A.1 |access-date=19 October 2022}}</ref> It could have been possible for glaciation to initiate with high levels of CO<sub>2</sub>, but it would have depended highly on continental configuration.<ref name="impacts" />

====Silicate weathering====
Long-term [[Chemistry:Carbonate–silicate cycle|silicate weathering]] is a major mechanism through which CO<sub>2</sub> is removed from the atmosphere, converting it into [[Chemistry:Bicarbonate|bicarbonate]] which is stored in marine sediments. This has often been linked to the Taconic Orogeny, a mountain-building event on the east coast of Laurentia (present-day North America).<ref name=":4">{{Cite journal |last1=Harper |first1=D. A. T. |last2=Hammarlund |first2=E. U. |last3=Rasmussen |first3=C. M. Ø. |date=May 2014 |title=End Ordovician extinctions: A coincidence of causes |journal=Gondwana Research |volume=25 |issue=4 |pages=1294–1307 |bibcode=2014GondR..25.1294H |doi=10.1016/j.gr.2012.12.021}}</ref> Another hypothesis is that a hypothetical large igneous province in the Katian led to basaltic flooding caused by high continental volcanic activity during that period. In the short term, this would have released a large amount of CO<sub>2</sub> into the atmosphere, which may explain a warming pulse in the Katian. However, in the long term flood basalts would have left behind plains of basaltic rock, replacing exposures of granitic rock. Basaltic rocks weather substantially faster than granitic rocks, which would quickly remove CO<sub>2</sub> from the atmosphere at a much faster rate than before the volcanic activity.<ref name="Katian Large Igneous">{{cite journal |last1=Lefebvre |first1=Vincent |last2=Servais |first2=Thomas |last3=François |first3=Louis |last4=Averbuch |first4=Olivier |date=15 October 2010 |title=Did a Katian large igneous province trigger the Late Ordovician glaciation? A hypothesis tested with a carbon cycle model |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018210002191 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=296 |issue=3-4 |pages=310–319 |doi=10.1016/j.palaeo.2010.04.010 |access-date=23 July 2023}}</ref> CO<sub>2</sub> levels could also have decreased due to accelerated silicate weathering caused by the expansion of terrestrial non-vascular plants. Vascular plants only appeared 15 Ma after the glaciation.<ref>{{cite news |last1=Ghosh |first1=Pallab |date=2 February 2012 |title=Humble moss 'brought on ice ages' |work=BBC News |url=https://www.bbc.com/news/science-environment-16814669 |accessdate=27 March 2020}}</ref><ref name="Lenton2012" />

==== Organic carbon burial ====
Isotopic evidence points to a global [[Earth:Hirnantian|Hirnantian]] positive shift in δ<sup>13</sup>C at nearly the same time as the positive shift in marine carbonate δ<sup>18</sup>O.<ref name="Wang1997">{{cite journal |last1=Wang |first1=K. |last2=Chatterton |first2=B. D. E. |last3=Wang |first3=Y. |date=August 1997 |title=An organic carbon isotope record of Late Ordovician to Early Silurian marine sedimentary rocks, Yangtze Sea, South China: Implications for CO2 changes during the Hirnantian glaciation |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018297000461 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=132 |issue=1–4 |pages=147–158 |doi=10.1016/S0031-0182(97)00046-1 |access-date=19 October 2022}}</ref> This shift is known as the Hirnantian Isotopic Carbon Excursion (HICE).<ref name="Men2022">{{cite journal |last1=Men |first1=Xin |last2=Mou |first2=Chuanlong |last3=Ge |first3=Xiangying |date=1 August 2022 |title=Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition |journal=Scientific Reports |volume=12 |issue=1 |page=13186 |doi=10.1038/s41598-022-17105-2 |pmid=35915216 |pmc=9343391 }}</ref> The positive shift in δ<sup>13</sup>C implies a change in the [[Earth:Carbon cycle|carbon cycle]] leading to more burial of organic carbon,<ref name="Men2022" /><ref>{{cite journal |last1=Jones |first1=David S. |last2=Creel |first2=Roger C. |last3=Rios |first3=Bernardo A. |date=15 September 2016 |title=Carbon isotope stratigraphy and correlation of depositional sequences in the Upper Ordovician Ely Springs Dolostone, eastern Great Basin, USA |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018216000444 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=458 |pages=85-101 |doi=10.1016/j.palaeo.2016.01.036 |access-date=23 July 2023}}</ref> though some researchers hold a conflicting interpretation of this δ<sup>13</sup>C change as being caused by increased weathering of carbonate platforms exposed by sea level fall.<ref>{{cite journal |last1=Melchin |first1=Michael J. |last2=Holmden |first2=Chris |date=18 May 2006 |title=Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018205005821 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=234 |issue=2-4 |pages=186-200 |doi=10.1016/j.palaeo.2005.10.009 |access-date=23 July 2023}}</ref><ref>{{cite journal |last1=Gorjan |first1=Paul |last2=Kaiho |first2=Kunio |last3=Fike |first3=David A. |last4=Xu |first4=Chen |date=15 June 2012 |title=Carbon- and sulfur-isotope geochemistry of the Hirnantian (Late Ordovician) Wangjiawan (Riverside) section, South China: Global correlation and environmental event interpretation |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018212001617 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=337-338 |pages=14-22 |doi=10.1016/j.palaeo.2012.03.021 |access-date=23 July 2023}}</ref> This enhanced organic carbon burial resulted in a decrease in the atmospheric CO<sub>2</sub> levels and an inverse greenhouse effect, allowing glaciation to occur more readily.<ref name="Bathymetric and Isotopic evidence" />

==== Gamma-ray burst ====
A gamma-ray burst (GRB) has been suggested by some researchers as the cause of the abrupt glaciation at the beginning of the Hirnantian.<ref name="GammaRayBurstLOME">{{cite journal |last1=Melott |first1=Adrian L. |last2=Lieberman |first2=B. S. |last3=Laird |first3=Claude M. |last4=Martin |first4=L. D. |last5=Medvedev |first5=M. V. |last6=Thomas |first6=Brian C. |last7=Cannizzo |first7=John K. |last8=Gehrels |first8=Neil |last9=Jackman |first9=Charles H. |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/did-a-gammaray-burst-initiate-the-late-ordovician-mass-extinction/F37A58C811EB82496CEF6CF989159807 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |journal=International Journal of Astrobiology |date=5 August 2004 |volume=3 | issue=2 | pages=55–61 |arxiv=astro-ph/0309415 |doi=10.1017/S1473550404001910 |bibcode=2004IJAsB...3...55M |hdl=1808/9204 |s2cid=13124815 |access-date=26 December 2022}}</ref> The effects of a ten second GRB occurring within two [[Astronomy:Parsec|kiloparsecs]] of Earth would have delivered it a fluence of 100 kilojoules per square metre. This would have resulted in large amounts of [[Chemistry:Nitric acid|nitric acid]] raining down on Earth's surface in the aftermath of the gamma-ray burst, causing blooms of nitrate-limited photosynthesisers that would have sequestered large amounts of carbon dioxide from the atmosphere. Additionally, the GRB would have initiated a major depletion of [[Chemistry:Ozone|ozone]], another potent greenhouse gas, through its reaction with [[Chemistry:Nitric oxide|nitric oxide]] produced as a result of the GRB's dissociation of diatomic nitrogen and subsequent reaction of nitrogen atoms with oxygen.<ref name="TerrestrialOzoneDepletion">{{cite journal |last1=Thomas |first1=Brian C. |last2=Jackman |first2=Charles H. |last3=Melott |first3=Adrian L. |last4=Laird |first4=Claude M. |last5=Stolarski |first5=Richard S. |last6=Gehrels |first6=Neil |last7=Cannizzo |first7=John K. |last8=Hogan |first8=Daniel P. |date=28 February 2005 |title=Terrestrial Ozone Depletion due to a Milky Way Gamma-Ray Burst |url=https://iopscience.iop.org/article/10.1086/429799/meta |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=622 |issue=2 |pages=L153–L156 |doi=10.1086/429799 |arxiv=astro-ph/0411284 |bibcode=2005ApJ...622L.153T |hdl=2060/20050179464 |s2cid=11199820 |access-date=26 December 2022}}</ref>

=== Asteroid impact ===
==== Ordovician meteor event ====
The breakup of the L-chondrite parent body caused a rain of extraterrestrial material onto the Earth called the [[Earth:Ordovician meteor event|Ordovician meteor event]]. This event increased stratospheric dust by 3 or 4 orders of magnitude and may have triggered the ice age by reflecting sunlight back into space.<ref>{{cite journal |last1=Schmitz |first1=Birger |last2=Farley |first2=Kenneth A. |last3=Goderis |first3=Steven |last4=Heck |first4=Philipp R. |last5=Bergström |first5=Stig M. |last6=Boschi |first6=Samuele |last7=Claeys |first7=Philippe |last8=Debaille |first8=Vinciane |last9=Dronov |first9=Andrei |last10=Van Ginneken |first10=Matthias |last11=Harper |first11=David A.T. |last12=Iqbal |first12=Faisal |last13=Friberg |first13=Johan |last14=Liao |first14=Shiyong |last15=Martin |first15=Ellinor |date=18 September 2019 |title=An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body |journal=Science Advances |volume=5 |issue=9 |pages=eaax4184 |doi=10.1126/sciadv.aax4184 |pmc=6750910 |pmid=31555741 |last16=Meier |first16=Matthias M. M. |last17=Peucker-Ehrenbrink |first17=Bernhard |last18=Soens |first18=Bastien |last19=Wieler |first19=Rainer |last20=Terfelt |first20=Fredrik}}</ref>

==== Deniliquin impact structure ====
A 2023 paper has proposed that the Hirnantian glaciation could have come about due to an [[Astronomy:Impact winter|impact winter]] generated by the impact that formed the Deniliquin multiple-ring feature in what is now southeastern Australia, although this hypothesis currently remains untested.<ref name="AsteroidImpactOrigin">{{cite journal |last1=Glikson |first1=Andrew Yoram |date=June 2023 |title=An asteroid impact origin of the Hirnantian (end-Ordovician) glaciation and mass extinction |url=https://www.sciencedirect.com/science/article/abs/pii/S1342937X23000655 |journal=Gondwana Research |volume=118 |pages=153-159 |doi=10.1016/j.gr.2023.02.019 |access-date=12 August 2023}}</ref>

=== Volcanic aerosols ===
Although volcanic activity often leads to warming through the release of greenhouse gasses, it may also lead to cooling via the production of [[Chemistry:Aerosol|aerosol]]s, light-blocking particles. There is good evidence for elevated volcanic activity through the Hirnantian, based on anomalously high concentrations of mercury (Hg) in many areas. Sulphur dioxide (SO<sub>2</sub>) and other sulphurous volcanic gasses are converted into [[Chemistry:Sulfate aerosol|sulphate aerosols]] in the [[Earth:Stratosphere|stratosphere]], and short, periodic large igneous province eruptions may be able to account for cooling in this way.<ref>{{Cite journal |last1=Jones |first1=David S. |last2=Martini |first2=Anna M. |last3=Fike |first3=David A. |last4=Kaiho |first4=Kunio |date=2017-07-01 |title=A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia |journal=Geology |volume=45 |issue=7 |pages=631–634 |doi=10.1130/G38940.1 |issn=0091-7613|doi-access=free }}</ref> Although there is no direct evidence for a large igneous province during the Hirnantian, volcanism could still be a major factor. Explosive volcanic eruptions, which regularly send debris and volatiles into the stratosphere, would be even more effective at producing sulfate aerosols. Ash beds are common in the Late Ordovician, and Hirnantian pyrite records sulphur isotope anomalies consistent with stratospheric eruptions.<ref>{{Cite journal |last1=Hu |first1=Dongping |last2=Li |first2=Menghan |last3=Zhang |first3=Xiaolin |last4=Turchyn |first4=Alexandra V. |last5=Gong |first5=Yizhe |last6=Shen |first6=Yanan |date=2020-05-08 |title=Large mass-independent sulphur isotope anomalies link stratospheric volcanism to the Late Ordovician mass extinction |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=2297 |doi=10.1038/s41467-020-16228-2 |pmid=32385286 |pmc=7210970 |s2cid=218540475 |issn=2041-1723|doi-access=free }}</ref> The enormous megaeruption that formed the [[Chemistry:Deicke and Millbrig bentonite layers|Deicke bentonite layer]] in particular has been linked to global cooling due to it coinciding with a major positive oxygen isotope excursion and the high sulphur concentration observed in its bentonite layer.<ref>{{cite journal |last1=Buggisch |first1=Werner |last2=Joachimski |first2=Michael M. |last3=Lehnert |first3=Oliver |last4=Bergström |first4=Stig M. |last5=Repetski |first5=John A. |last6=Webers |first6=Gerald F. |date=1 April 2010 |title=Did intense volcanism trigger the first Late Ordovician icehouse? |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/38/4/327/130212/Did-intense-volcanism-trigger-the-first-Late?redirectedFrom=fulltext |journal=Geology |volume=38 |issue=4 |pages=327–330 |doi=10.1130/G30577.1 |access-date=19 October 2022}}</ref>

=== Sea level change ===
One of the possible causes for the temperature drop during this period is a drop in sea level. Sea level must drop prior to the initiation of extensive ice sheets in order for it to be a possible trigger. A drop in sea level allows more land to become available for ice sheet growth.  There is wide debate on the timing of sea level change, but there is some evidence that a sea level drop started before the Ashgillian, which would have made it a contributing factor to glaciation.<ref name="impacts" />

=== Palaeogeography ===
The possible setup of the paleogeography during the period from 460 Ma to 440 Ma falls in a range between the Caradocian and the Ashgillian. The choice of setup is important, because the Caradocian setup is more likely to produce glacial ice at high CO<sub>2</sub> concentrations, and the Ashgillian is more likely to produce glacial ice at low CO<sub>2</sub> concentrations.<ref name="impacts" />

The height of the land mass above sea level also plays an important role, especially after ice sheets have been established.  A higher elevation allows ice sheets to remain with more stability, but a lower elevation allows ice sheets to develop more readily. The Caradocian is considered to have a lower surface elevation, and though it would be better for initiation during high CO<sub>2</sub>, it would have a harder time maintaining glacial coverage.<ref name="revised world maps">{{cite journal |last=Scotese |first=C.R. |author2=McKerrow, W.S. |year=1990 |title=Revised world maps and introduction. In: Scotese, C.R., McKerrow, W.S. (Eds.), Palaeozoic Palaeogeography and Biogeography |journal=Geological Society of London Memoir |volume=12 |pages=1–21 |doi=10.1144/gsl.mem.1990.012.01.01 |doi-access=}}</ref>

From what we know about tectonic movement, the time span required to allow the southward movement of [[Earth:Gondwana|Gondwana]] toward the South Pole would have been too long to trigger this glaciation. Tectonic movement tends to take several million years, but the scale of the glaciation seems to have occurred in less than 1 million years, but the exact time frame of glaciation ranges from less than 1 million years to 35 million years, so it could still be possible for tectonic movement to have triggered this glacial period.<ref name="impacts">{{cite journal |last1=Herrmann |first1=Achim D. |last2=Patzkowsky |first2=Mark E. |last3=Pollard |first3=David |date=13 April 2004 |title=The impact of paleogeography, pCO2, poleward ocean heat transport, and sea level change on global cooling during the Late Ordovician |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=206 |issue=1–2 |pages=59–74 |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018204000082 |bibcode=2004PPP...206...59H |doi=10.1016/j.palaeo.2003.12.019 |access-date=9 August 2023}}</ref> Alternatively, [[Earth:True polar wander|true polar wander]] (TPW) and not conventional plate motion may have been responsible for the initiation of the Hirnantian glaciation. Palaeomagnetic data from between 450 and 440 Ma indicates a TPW of around ~50˚ occurring at a maximum speed of ~55 cm per year, which better explains the rapid motion of the continents than conventional plate tectonics.<ref>{{cite journal |last1=Jing |first1=Xianqing |last2=Yang |first2=Zhenyu |last3=Mitchell |first3=Ross N. |last4=Tong |first4=Yabo |last5=Zhu |first5=Min |last6=Wan |first6=Bo |date=26 December 2022 |title=Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction |url=https://www.nature.com/articles/s41467-022-35609-3 |journal=Nature Communications |volume=13 |issue=1 |page=7941 |doi=10.1038/s41467-022-35609-3 |pmid=36572674 |pmc=9792554 |bibcode=2022NatCo..13.7941J |doi-access=free |access-date=22 July 2023}}</ref>

==== Poleward ocean heat transport ====
Ocean heat transport is a major driver in the warming of the poles, taking warm water from the equator and distributing it to higher latitudes. A weakening of this heat transport may have allowed the poles to cool enough to form ice under high CO<sub>2</sub> conditions.<ref name="impacts" /> Due to the paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician.<ref name="glaciation under high CO2">{{cite journal |last=Poussart |first=P.F |author2=Weaver, A.J. |author3=Bames, C.R. |year=1999 |title=Late Ordovician glaciation under high atmospheric CO2; a coupled model analysis |journal=[[Earth:Paleoceanography and Paleoclimatology|Paleoceanography and Paleoclimatology]] |volume=14 |issue=4 |pages=542–558 |bibcode=1999PalOc..14..542P |doi=10.1029/1999pa900021|doi-access=free }}</ref> However, research shows that in order for glaciation to occur, poleward heat transport had to be lower, which creates a discrepancy in what is known.<ref name="impacts" />

=== Orbital parameters ===
Orbital parameters may have acted in conjunction with some of the above parameters to help start glaciation. The variation of the earth's precession, and eccentricity, could have set the off the tipping point for initiation of glaciation.<ref name="impacts" />  The Orbit at this time is thought to have been in a cold summer orbit for the Southern Hemisphere.<ref name="impacts" /> This type of orbital configuration is a change in the [[Astronomy:Apsidal precession|orbital precession]] such that during the summer when the hemisphere is tilted toward the sun (in this case the earth) the earth is furthest away from the sun, and [[Astronomy:Orbital eccentricity|orbital eccentricity]] such that the orbit of the earth is more elongated which would enhance the effect of precession.

Coupled models have shown that in order to maintain ice at the pole in the Southern Hemisphere, the earth would have to be in a cold summer configuration.<ref name="glaciation under high CO2" /> The glaciation was most likely to start during a cold summer period because this configuration enhances the chance of snow and ice surviving throughout the summer.<ref name="impacts" />

== End of the event ==
The cause for the end of the Late Ordovician Glaciation is a matter of intense research, but evidence shows that the deglaciation in the terminal Hirnantian may have occurred abruptly, as Silurian strata marks a significant change from the glacial deposits left during the Late Ordovician.<ref name="deglaciation sequence">{{cite journal |last=Moreau |first=J. |year=2011 |title=The Late Ordovician deglaciation sequence of the SW |journal=Basin Research |volume=23 |pages=449–477 |doi=10.1111/j.1365-2117.2010.00499.x|s2cid=129897765 }}</ref> Though the Hirnantian glaciation ended rapidly, milder glaciations continued to occur throughout the subsequent Silurian period,<ref name="TrotterEtAl2016PPP" /> with the last glacial phase occurring in the Late Silurian.<ref name ="LudfordianGlaciation" />

=== Ice collapse ===
One of the possible causes for the end of the Hirnantian glaciation is that during the glacial maximum, the ice reached out too far and began collapsing on itself. The ice sheet initially stabilized once it reached as far north as  Ghat, Libya and developed a large proglacial fan-delta system. A glaciotectonic fold and thrust belt began to form from repeated small-scale fluctuations in the ice. The glaciotectonic fold and thrust belt eventually led to ice sheet collapse and retreat of the ice to south of Ghat. Once stabilized south of Ghat, the ice sheet began advancing north again. This cycle slowly shrank more south each time which lead to further retreat and further collapse of glacial conditions. This recursion allowed the melting of the ice sheet, and rising sea level. This hypothesis is supported by glacial deposits and large land formations found in Ghat, Libya which is part of the Murzuq Basin.<ref name="deglaciation sequence" />

=== CO<sub>2</sub> ===
As the Ice sheets began to increase the weathering of [[Chemistry:Silicate|silicate]] rocks and basaltic important to [[Earth:Carbon sequestration|carbon sequestration]] (the silicates through the [[Chemistry:Carbonate–silicate cycle|Carbonate–silicate cycle]], the [[Earth:Basalt|basalt]] through forming [[Chemistry:Calcium carbonate|calcium carbonate]]) decreased, which caused {{CO2}} levels to rise again, this in turned helped push deglaciation. This deglaciation cause the transformation of silicates exposed to the air (thus given the opportunity to bind to its {{CO2}}) and weathering of basaltic rock to start back up which caused glaciation to occur again.<ref name="CO2 exit glaciation" />

== Significance ==
Even before the mass extinction at the end of the Ordovician, which resulted in a significant drop in chitinozoan diversity and abundance,<ref>{{cite journal |last1=Paris |first1=F. |last2=Bourahrouh |first2=A. |last3=Hérissé |first3=A. L. |date=December 2000 |title=The effects of the final stages of the Late Ordovician glaciation on marine palynomorphs (chitinozoans, acritarchs, leiospheres) in well Nl-2 (NE Algerian Sahara) |url=https://www.sciencedirect.com/science/article/abs/pii/S0034666700000543 |journal=Review of Palaeobotany and Palynology |volume=113 |issue=1–3 |pages=87–104 |doi=10.1016/S0034-6667(00)00054-3 |pmid=11164214 |access-date=10 January 2023}}</ref> the biodiversity of [[Biology:Chitinozoan|chitinozoan]]s was adversely impacted by the onset of the Andean-Saharan glaciation. Following a peak in diversity in the late Darriwilian, chitinozoans declined in diversity as the Late Ordovician progressed. An exception to this declining trend of chitinozoan diversity was exhibited in Laurentia due to its low latitude position and warmer climate.<ref>{{cite journal |last1=Achab |first1=Aïcha |last2=Paris |first2=Florentin |date=7 March 2007 |title=The Ordovician chitinozoan biodiversification and its leading factors |url=https://www.sciencedirect.com/science/article/abs/pii/S003101820600335X |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=245 |issue=1–2 |pages=5–19 |doi=10.1016/j.palaeo.2006.02.030 |access-date=16 October 2022}}</ref>

The Late Ordovician Glaciation coincided with the second largest of the five major extinction events, known as the [[Earth:Late Ordovician mass extinction|Late Ordovician mass extinction]]. This period is the only known glaciation to occur alongside of a mass extinction event. The extinction event consisted of two discrete pulses. The first pulse of extinctions is thought to have taken place because of the rapid cooling, and increased oxygenation of the water column. This first pulse was the larger of the two and caused the extinction of most of the marine animal species that existed in the shallow and deep oceans. The second phase of extinction was associated with strong [[Earth:Sea level rise|sea level rise]], and due to the atmospheric conditions, namely oxygen levels being at or below 50% of present-day levels, high levels of anoxic waters would have been common. This anoxia would have killed off many of the survivors of the first extinction pulse. In all the extinction event of the Late Ordovician saw a loss of 85% of marine animal species and 26% of animal families.<ref name="Sulfidic driver">{{cite journal |last=Hammarlund |first=E. U. |year=2012 |title=A Sulfidic Driver for the End-Ordovician Mass Extinction |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=331–332 |pages=128–139 |bibcode=2012E&PSL.331..128H |doi=10.1016/j.epsl.2012.02.024}}<!--|accessdate=25 November 2012--></ref>

The deglaciation at the end of the Homerian glacial interval was coeval with the first major radiation of trilete spore-producing plants, harbingering the dawn of the [[Biology:Silurian-Devonian Terrestrial Revolution|Silurian-Devonian Terrestrial Revolution]]. The later middle Ludfordian glaciation caused a sea level drop that created vast areas of new terrestrial habitats that were promptly colonised by land plants, further facilitating their diversification.<ref name="SilurianPlantDynamics">{{cite journal |last1=Pšenička |first1=Josef |last2=Bek |first2=Jiří |last3=Frýda |first3=Jiří |last4=Žárský |first4=Viktor |last5=Uhlířová |first5=Monika |last6=Štorch |first6=Petr |date=31 August 2022 |title=Dynamics of Silurian Plants as Response to Climate Changes |journal=Life |volume=11 |issue=9 |page=906 |doi=10.3390/life11090906 |pmid=34575055 |pmc=8470493 |doi-access=free }}</ref> The warming during the [[Earth:Pridoli Epoch|Pridoli]] that marked the end of the Andean-Saharan glaciation saw further floral expansion.<ref>{{cite journal |last1=Bek |first1=Jiří |last2=Štorch |first2=Petr |last3=Tonarová |first3=Petra |last4=Libertín |first4=Milan |year=2022 |title=Early Silurian (mid-Sheinwoodian) palynomorphs from the Loděnice-Špičatý vrch, Prague Basin, Czech Republic. |url=https://web.p.ebscohost.com/abstract?direct=true&profile=ehost&scope=site&authtype=crawler&jrnl=12141119&AN=163982020&h=J4hndCjVAEf8HgMrCxVarnB4jXNouJXI2MLEt1KUW9fo%2bXOJzJ9XohMEXUpR8bihoJuQJOAQHCSTgI%2bEs3PATQ%3d%3d&crl=c&resultNs=AdminWebAuth&resultLocal=ErrCrlNotAuth&crlhashurl=login.aspx%3fdirect%3dtrue%26profile%3dehost%26scope%3dsite%26authtype%3dcrawler%26jrnl%3d12141119%26AN%3d163982020 |journal=Bulletin of Geosciences |volume=97 |issue=3 |pages=385-396 |access-date=14 August 2023}}</ref>

== See also ==
* [[Earth:Timeline of glaciation|Timeline of glaciation]]
* [[Earth:Ordovician–Silurian extinction events|Ordovician–Silurian extinction events]]
* [[Earth:Late Paleozoic icehouse|Late Paleozoic icehouse]]

== References ==
{{Reflist}}

{{Ice ages}}

[[Category:Glaciology]]
[[Category:Ordovician events]]
[[Category:Late Ordovician extinctions|*]]

{{Sourceattribution|Andean-Saharan glaciation}}