The Hirnantian glaciation, also known as the Andean-Saharan glaciation, Early Paleozoic Ice Age (EPIA), the Early Paleozoic Icehouse, the Late Ordovician glaciation, or the end-Ordovician glaciation, occurred during the Paleozoic from around 460 to 420 million years ago. 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, which began as early as the Darriwilian, and possibly even the Floian. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, 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. Cold temperatures are widely considered to be the leading cause of the Late Ordovician mass extinction, and it is the only glacial episode that appears to have coincided with a major mass extinction of nearly 61% of marine life. Estimates of peak ice sheet 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 Hirnantian, the ice age is believed to have been significantly more extreme than the Last Glacial Maximum occurring during the terminal Pleistocene. Glaciation of the Northern Hemisphere was minimal because a large amount of the land was in the 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. A speculated glaciation in the middle Darriwilian corresponds to the MDICE positive carbon isotope excursion. Sea level changes likely reflective of glacioeustasy are known from this geologic stage, around 467 million years ago. However, there are no known Middle Ordovician glacial deposits that would provide direct geological evidence of glaciation. Isotopic evidence from the Sandbian reveals three possible glaciations: an early Sandbian glaciation, a middle Sandbian glaciation, a late Sandbian glaciation. Although biostratigraphy dating the glacial deposits in Gondwana has been problematic, there is evidence suggesting the presence of glaciation by the Sandbian stage. 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 million years ago. 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. Evidence of major changes in bottom water formation, which usually indicates a sudden change in global climate, is known from the Katian. 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, as do conodont apatite δ18O fluctuations from Kentucky and Quebec that likely reflect glacioeustatic sea level changes. However, the existence of glacials during the Katian remains controversial. Katian brachiopod and seawater δ18O values from Cincinnati Arch indicate ocean temperatures characteristic of a global greenhouse state.
Hirnantian glaciation
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. An δ18O shift occurs at the start of the Hirnantian; the magnitude of this shift (+2-4‰) was extraordinary. Its direction implies glacial cooling and possibly increases in ice-volume. 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. 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 Early Silurian shale occurs, Late Ordovician glaciogenic deposits occur beneath, likely due to the anoxia promoted in these basins. At the end of the Hirnantian, an abrupt retreat of glaciers concurrent with the second pulse of the Late Ordovician mass extinction occurred, after which Earth receded back into a much warmer climate during the Rhuddanian. Late Hirnantian warming was marked by a similarly meteoric shift in δ18O towards more negative values. δ13C values likewise fall sharply at the beginning of the Silurian.
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