Late Ordovician glaciation: Difference between revisions

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+#REDIRECT [[Hirnantian glaciation]]
-The '''Late Ordovician Glaciation''' is the first part of the [[Andean-Saharan glaciation]]. It was centered on the Sahara region in late [[Ordovician]], about  440–460 Ma (million years ago).  The major glaciation during this period is widely considered to be the leading cause of the [[Ordovician-Silurian extinction event]].<ref name="end-Ordovician glaciation">{{cite journal|last=Delabroye|first=A.|author2=Vecoli, M. |title=The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about the Late Ordovician event stratigraphy|journal=Earth-Science Reviews|year=2010|pages=269–282|doi=10.1016/j.earscirev.2009.10.010|volume=98}}<!--|accessdate=25 November 2012--></ref>  Evidence of this glaciation can be seen in places such as [[Morocco]], [[South Africa]], [[Libya]], and [[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.|title=The Magnitude and Duration of the Late Ordovician-Early Silurian Glaciation|journal=Science|year=2011|pages=903–906|doi=10.1126/science.1200803|volume=331|pmid=21273448}}<!--|accessdate=25 November 2012--></ref>
-The Late Ordovician 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|title=The Late Ordovician Mass Extinction|journal=Annual Review of Earth and Planetary Sciences|date=1 May 2001|volume=29|issue=1|pages=331–364|doi=10.1146/annurev.earth.29.1.331|url=http://www.annualreviews.org/doi/abs/10.1146/annurev.earth.29.1.331?journalCode=earth&|accessdate=25 November 2012}}</ref>
-Estimates of peak ice sheet volume range from 50 to 250 million cubic kilometers, and its duration from 35 million to less than 1 million years.   There were also two peaks of glaciation.<ref name="Magnitude and Duration" />   Also, glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the southern hemisphere.
-==Evidence==
-===Isotopic===
-[[File:Carbon 13 and time scale during the Ordovician.png|thumb|upright=1.3|alt=Ordovician Carbon 13 time scale|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" />]]
-*Isotopic evidence points to a global [[Hirnantian]] positive shift in marine carbonate <sup>18</sup>O, and at nearly the same time a shift in <sup>13</sup>C in organic and inorganic carbon.  This evidence is further aided by the observation that both <sup>18</sup>O and <sup>13</sup>C fall sharply at the beginning of the [[Silurian]].<ref name="Bathymetric and Isotopic evidence" />
-*The direction of the <sup>18</sup>O shift can imply glacial-cooling and possibly increases in ice-volume, and the magnitude of this shift (+4‰) was extraordinary.  The direction and magnitude of the <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.<ref name="Bathymetric and Isotopic evidence" />
-*The shift in <sup>13</sup>C implies a change in the [[carbon cycle]] leading to more burial of carbon, or at the very least production of more carbon with the removal of <sup>12</sup>C in surface waters.  This decrease points toward a decrease in the atmospheric CO<sub>2</sub> levels which would have an inverse greenhouse effect, which would allow glaciation to occur more readily.<ref name="Bathymetric and Isotopic evidence" />
-=== Lithologic indicators ===
-*Sedimentological data shows that Late Ordovician ice sheets glacierized the Al Kufrah Basin.  Ice sheets also probably formed continuous ice cover over North African and the Arabian Peninsula.  In all areas of North African where [[Llandovery epoch|Early Silurian]] shale occurs, Late Ordovician glaciogenic deposits occur beneath, likely due to the [[Anoxic waters|anoxia]] promoted in these basins.<ref name="Evidence Al Kufrah">{{cite journal|last=Heron|first=D. P.|author2=Howard, J. |title=Evidence for Late Ordovician Glaciation of Al Kufrah Basin, Libya|journal=Journal of African Earth Sciences|year=2010|pages=354–364|doi=10.1016/j.jafrearsci.2010.04.001|volume=58}}<!--|accessdate=25 November 2012--></ref>
-*From what we know about [[tectonic]] movement, the time span required to allow the southward movement of [[Gondwana]] toward the South Pole would have been too long to trigger this glaciation.<ref name=impacts />   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 />
-*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" />
-*Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, some evidence suggested an onset of glaciation as early as the [[Sandbian]] Stage (approximately 451–461 Ma).<ref name="Bighorn Dolomite, Wyoming">{{cite journal|last=Holland|first=S. M.|author2=Patzkowsky, M. E. |title=Sequence Architecture of the Bighorn Dolomite, Wyoming, USA: Transition to the Late Ordovician Icehouse|journal=Journal of Sedimentary Research|year=2012|pages=599–615}}</ref>
-== Possible causes ==
-=== Decreases in CO<sub>2</sub> ===
-One of the factors that hindered glaciation was atmospheric CO<sub>2</sub> concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels.<ref name=impacts>{{cite journal|last=Herrmann|first=A. D.|author2=Patzkowsky, M.E. |author3=Pollard, D. |title=The impact of paleogeography, pCO2, poleward ocean heat transport, and sea level change on global cooling during the Late Ordovician.|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|year=2004|pages=59–74|doi=10.1016/j.palaeo.2003.12.019|volume=206}}</ref>   During this time though, CO<sub>2</sub> concentrations are thought to have dropped significantly, which could have led to further glaciation, but the methods for the removal of CO<sub>2</sub> during that time are not well known.<ref name="Bathymetric and Isotopic evidence">{{cite journal|last=Brenchley|first=P.J.|author2=J. D. |title=Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period|journal=Geology|year=1994|pages=295–298|doi=10.1130/0091-7613(1994)022<0295:baiefa>2.3.co;2|volume=22}}<!--|accessdate=25 November 2012--></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 />
-One theory is that the Katian large igneous province had basaltic flooding caused by high continental volcanic activity during that period. This would have released a large amount of CO<sub>2</sub> into the atmosphere but would have left behind basaltic plains replacing the granitic rock.  Basaltic rocks weather substantially faster than granitic rocks, which would quickly remove CO<sub>2</sub> from the atmosphere to lower levels than pre-volcanic activity.<ref name="Katian Large Igneous">{{cite journal|last=Lefebvre|first=V. |author2=Servais, T. |author3=Francois, L. |author4=Averbuch, O.|title=Did a Katian large igneous province trigger the Late Ordovician glaciation? A hypothesis tested with a carbon cycle model.|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|year=2010|pages=310–319|doi=10.1016/j.palaeo.2010.04.010 |volume=296}}<!--|accessdate=25 November 2012--></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 />
-===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 />
-Unfortunately 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. |title=Late Ordovician glaciation under high atmospheric CO2; a coupled model analysis|journal=Palaeoceanography|year=1999|volume=14|issue=4|pages=542–558|doi=10.1029/1999pa900021|bibcode=1999PalOc..14..542P}}</ref> but 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 />
-===Paleogeography===
-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. |title=Revised world maps and introduction. In: Scotese, C.R., McKerrow, W.S. (Eds.), Palaeozoic Palaeogeography and Biogeography|journal=Geological Society of London Memoir|year=1990|volume=12|pages=1–21|doi=10.1144/gsl.mem.1990.012.01.01}}</ref>
-===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 [[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 [[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 ==
-=== Causes ===
-The cause for the end of the Late Ordovician Glaciation is a matter of intense research, but evidence shows that it may have occurred abruptly, as Silurian strata marks a significant change from the glacial deposits left during the Late Ordovician.  Most evidence points to an abrupt change rather than a gradual change.<ref name="deglaciation sequence" />
-==== Ice collapse ====
-One of the possible causes for the end of this glacial event is 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">{{cite journal|last=Moreau|first=J.|title=The Late Ordovician deglaciation sequence of the SW|journal=Basin Research|year=2011|pages=449–477}}</ref>
-====CO<sub>2</sub>====
-As the Ice sheets began to increase the weathering of [[silicate]] rocks and basaltic important to carbon sequestration (the silicates through the [[Carbonate–silicate cycle]], the [[basalt]] through forming [[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">{{cite journal|last=Seth A Young|first=M. R.|title=Did Changes in atmospheric CO2 coincide with latest Ordovician glacial-interglacial cycles?|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|year=2012|pages=376–388|doi=10.1016/j.palaeo.2010.02.033|volume=296}}</ref>
-==Significance==
-The Late Ordovician Glaciation coincided with the second largest of the 5 major [[extinction events]], known as the [[Ordovician–Silurian extinction event]].  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 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.|title=A Sulfidic Driver for the End-Ordovician Mass Extinction|journal=Earth and Planetary Science Letters|year=2012|pages=128–139|doi=10.1016/j.epsl.2012.02.024|bibcode=2012E&PSL.331..128H|volume=331–332}}<!--|accessdate=25 November 2012--></ref>
-== References ==
-{{Reflist}}
-[[Category:Ordovician events]]
-[[Category:Late Ordovician]]
-[[Category:Late Ordovician extinctions|*]]
-[[Category:Ice ages]]
-[[Category:Climate change science]]