Hirnantian glaciation
[7][8][9][10] 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.
At its height during the Hirnantian, the ice age is believed to have been significantly more extreme than the Last Glacial Maximum occurring during the terminal Pleistocene.
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.
[14] Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, there is evidence suggesting the presence of glaciation by the Sandbian stage (approximately 451–461 Ma).
[10] 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.
[21][22] Katian brachiopod and seawater δ18O values from Cincinnati Arch indicate ocean temperatures characteristic of a global greenhouse state.
The observed shifts in the δ18O isotopic indicator would require a sea-level fall of 100 meters and a drop of 10 °C in tropical ocean temperatures to have occurred during this glacial episode.
[28] At the end of the Hirnantian, an abrupt retreat of glaciers concurrent with the second pulse of the Late Ordovician mass extinction occurred,[29] after which Earth receded back into a much warmer climate during the Rhuddanian.
According to Eyles and Young, "A major glacial episode at c. 440 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 Early Silurian the centre of glaciation moved from northern Africa to southwestern South America.
[47] 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.
[53][54] This enhanced organic carbon burial resulted in a decrease in the atmospheric CO2 levels and an inverse greenhouse effect, allowing glaciation to occur more readily.
[56] The breakup of the L-chondrite parent body caused a rain of extraterrestrial material onto the Earth called the 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.
There is good evidence for elevated volcanic activity through the Hirnantian, based on anomalously high concentrations of mercury (Hg) in many areas.
Ash beds are common in the Late Ordovician, and Hirnantian pyrite records sulphur isotope anomalies consistent with stratospheric eruptions.
[40] Alternatively, 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.
[40] Due to the paleogeographic configuration of the continents, global ocean heat transport is thought to have been stronger in the Late Ordovician.
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.
The ice sheet initially stabilized once it reached as far north as Ghat, Libya and developed a large proglacial fan-delta system.
This hypothesis is supported by glacial deposits and large land formations found in Ghat, Libya which is part of the Murzuq Basin.
An exception to this declining trend of chitinozoan diversity was exhibited in Laurentia due to its low latitude position and warmer climate.
[69] 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 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.
