Late Ordovician glaciation

The Late Ordovician Glaciation is a period at the end of the Ordovician that started at the border between the Katian and Hirnantian 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.[1] Evidence of this glaciation can be seen in places such as Morocco, Libya, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, Tropical ocean temperatures were about 5 °C cooler than present day, this would have been a major factor that aided in the glaciation process.[2]

The Late Ordovician is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life.[3]

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.[2] Also, glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the southern hemisphere.

Evidence

Isotopic

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.[4]

Lithologic indicators

Possible causes

Decreases in CO2

One of the factors that hindered glaciation was atmospheric CO2 concentrations, which at the time were somewhere between 8 and 20 times pre-industrial levels.[7] During this time though, CO2 concentrations are thought to have dropped significantly, which could have led to further glaciation, but the methods for the removal of CO2 during that time are not well known.[5] It could have been possible for glaciation to initiate with high levels of CO2, but it would have depended highly on continental configuration.[7]

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 CO2 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 CO2 from the atmosphere to lower levels than pre-volcanic activity.[9]

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.[7]

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 CO2 conditions.[7]

Unfortunately due to the paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician,[10] 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.[7]

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 CO2 concentrations, and the Ashgillian is more likely to produce glacial ice at low CO2 concentrations.[7]

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 CO2, it would have a harder time maintaining glacial coverage.[11]

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.[7] The Orbit at this time is thought to have been in a cold summer orbit for the southern hemisphere.[7] This type of Orbital configuration is a change in the 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.[10] 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.[7]

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.[12]

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.[12]

CO2

As the Ice sheets began to increase the weathering of the basaltic and silicate rocks decreased, which caused CO2 levels to rise again, this in turned helped push deglaciation. This deglaciation cause the basaltic weathering to start back up which caused glaciation to occur again.[4]

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.[13]

References

  1. Delabroye, A.; Vecoli, M. (2010). "The end-Ordovician glaciation and the Hirnantian Stage: A global review and questions about the Late Ordovician event stratigraphy". Earth-Science Reviews: 269–282. doi:10.1016/j.earscirev.2009.10.010.
  2. 1 2 Finnegan, S. (2011). "The Magnitude and Duration of the Late Ordovician-Early Silurian Glaciation". Science: 903–906. doi:10.1126/science.1200803.
  3. Sheehan, Peter M (1 May 2001). "The Late Ordovician Mass Extinction". Annual Review of Earth and Planetary Sciences. 29 (1): 331–364. doi:10.1146/annurev.earth.29.1.331. Retrieved 25 November 2012.
  4. 1 2 Seth A Young, M. R. (2012). "Did Changes in atmospheric CO2 coincide with latest Ordovician glacial-interglacial cycles?". Palaeogeography, Palaeoclimatology, Palaeoecology: 376–388. doi:10.1016/j.palaeo.2010.02.033.
  5. 1 2 3 4 Brenchley, P.J.; J. D. (1994). "Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period". Geology: 295–298. doi:10.1130/0091-7613(1994)022<0295:baiefa>2.3.co;2.
  6. Heron, D. P.; Howard, J. (2010). "Evidence for Late Ordovician Glaciation of Al Kufrah Basin, Libya". Journal of African Earth Sciences: 354–364. doi:10.1016/j.jafrearsci.2010.04.001.
  7. 1 2 3 4 5 6 7 8 9 10 11 Herrmann, A. D.; Patzkowsky, M.E.; Pollard, D. (2004). "The impact of paleogeography, pCO2, poleward ocean heat transport, and sea level change on global cooling during the Late Ordovician.". Palaeogeography, Palaeoclimatology, Palaeoecology: 59–74. doi:10.1016/j.palaeo.2003.12.019.
  8. 1 2 Holland, S. M.; Patzkowsky, M. E. (2012). "Sequence Architecture of the Bighorn Dolomite, Wyoming, USA: Transition to the Late Ordovician Icehouse". Journal of Sedimentary Research: 599–615.
  9. Lefebvre, V.; Servais, T.; Francois, L.; Averbuch, O. (2010). "Did a Katian large igneous province trigger the Late Ordovician glaciation? A hypothesis tested with a carbon cycle model.". Palaeogeography, Palaeoclimatology, Palaeoecology: 310–319. doi:10.1016/j.palaeo.2010.04.010.
  10. 1 2 Poussart, P.F; Weaver, A.J.; Bames, C.R. (1999). "Late Ordovician glaciation under high atmospheric CO2; a coupled model analysis". Palaeoceanography. 14 (4): 542–558. Bibcode:1999PalOc..14..542P. doi:10.1029/1999pa900021.
  11. Scotese, C.R.; McKerrow, W.S. (1990). "Revised world maps and introduction. In: Scotese, C.R., McKerrow, W.S. (Eds.), Palaeozoic Palaeogeography and Biogeography". Geological Society of London Memoir. 12: 1–21. doi:10.1144/gsl.mem.1990.012.01.01.
  12. 1 2 Moreau, J. (2011). "The Late Ordovician deglaciation sequence of the SW". Basin Research: 449–477.
  13. Hammarlund, E. U. (2012). "A Sulfidic Driver for the End-Ordovician Mass Extinction". Earth and Planetary Science Letters: 128–139. Bibcode:2012E&PSL.331..128H. doi:10.1016/j.epsl.2012.02.024.
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