The Hirnantian is the final internationally recognized stage of the Ordovician Period of the PaleozoicEra. It was of short duration, lasting about 1.4 million years, from 445.2 to 443.8Ma (million years ago).[8] The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.
Most scientists believe that this climatic oscillation caused the major extinction event that took place during this time. In fact, the Hirnantian (also known as the End Ordovician and the Ordovician-Silurian) mass extinction event represents the second largest such event in geologic history. Approximately 85% of marine (sea-dwelling) species died. Only the End Permianmass extinction was larger. Unlike many smaller extinction events, however, the long-term consequences of the End Ordovician event were relatively small. Following the climatic oscillation, the climate returned to its previous state, and the species that survived soon (within two or three million years) evolved into species very similar to the ones that existed before.
Naming and history
The Hirnantian was named after Cwm Hirnant south of Bala, in northern Wales. Cwm Hirnant means the "valley of the long stream" in Welsh.
The stage was introduced in 1933 by B.B. Bancroft.[9] As proposed by Bancroft, the Hirnantian included the Hirnant Limestone and related sedimentary formations. These formations were located at the very top of Ordovician deposits, and were dominated by a fauna which included brachiopods, trilobites, and other "shelly" or hard-coated animals. In 1966, D. A. Bassett, Harry Blackmore Whittington, and A. Williams, writing in the Journal of the Geological Society of London, proposed a refinement to the Hirnantian Stage. This expanded the stage to include all of the Foel-y-Ddinas Mudstones, of which the Hirnant Limestone is a part. This expansion brought the Hirnantian Stage to its current scope.[10]
The International Commission on Stratigraphy (ICS) originally divided the Upper Ordovician into two stages. After considerable research, however, it was determined that no single faunal zone existed that could accommodate the upper stage of this division. Therefore, in 2003, the ICS voted to add an additional stage to its official international time scale. This was named the Hirnantian after Bancroft's stage. In 2006 the ICS ratified the Wangjiawan section as the official Global Boundary Stratotype Section and Point (GSSP) for the Hirnantian Stage.[10][11]
Secondary markers are Normalograptus ojsuensi 4cm below the boundary, and the Hirnantia Fauna 39cm above the boundary.[12]
Major events
As mentioned above, there was a major climatic oscillation during the Hirnantian, which is believed to have caused a major mass extinction event. When the Hirnantian Stage began, the Earth's climate was hot and sea-levels were substantially higher than today. The seas were filled with a diverse fauna. However, there is considerable evidence to indicate that many of these species were already in trouble when the Hirnantian began, and that overall biological diversity was already in sharp decline.
As the climate cooled and glaciers formed during the early part of the Hirnantian, sea level dropped. Estimates of the depth of this drop range from more than 50 meters (based on studies in Nevada and Utah) to more than 100 meters (from studies in Norway and the United Kingdom). More recent research indicates a worldwide reduction in sea level of approximately 80 meters. This drop dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time, causing the extinction of large numbers of species who depended on this shallow water environment. Those species that survived were greatly reduced in numbers. The overall result was a major reduction in the biodiversity of the world's oceans.
During this ice-house period (as scientists sometimes call periods of global cold climate and glaciation), surviving species began to adapt. Cold environment species replaced the warm environment species that had thrived during the previous hot period (hot-house to scientists).
Just as the species were adapting, however, the climate changed again. During the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level rose to the same or perhaps an even higher level than before the glaciation. The exposed shelves were flooded, resulting in additional extinctions among the fauna that had survived the first extinction event.
Dating
While there are no major radiometric dates for the Hirnantian itself, there are two such dates which bracket the Hirnantian. Both dates are from the Dob's Linn area in the United Kingdom.
The older one comes from the local Hartfell shale formations. Zircon found in an ash deposit at the site was dated to 445.7 mya, with a margin of error of plus or minus 2.4 million years.
The younger date is from the early Silurian Birkhill shale formations. Radiometric dating placed zircon samples found in another ash deposit at 438.7 mya, with a margin of error of plus or minus 2.1 million years.
With these dates as a base, scientists were able to use biostratigraphic correlation techniques to determine close approximations for the timing of various events during the Hirnantian.
Subdivisions
There are two Graptolite biozones in the Hirnantian and these are of approximately equal length. The base (beginning) of the Hirnantian Stage is defined by the first appearance in the geologic record of the graptolite Normalograptus extraordinarius, and thus, the Normalograptus extraordinarius biozone defines the early (or lower) part of the Hirnantian. The later (or upper) part is defined by the first appearance of the graptolite Normalograptus persulptus, and the Normalograptus persulptus biozone lasts from then until the end of the Hirnantian. This also marks the end of the Ordovician Period.
These biozones allow for additional refinement in dating events occurring within the Hirnantian Stage.
Paleogeography
During the Hirnantian, much of the world's land mass was gathered into a supercontinent called Gondwana, which occupied extreme southern latitudes and covered the south pole. This included South America, Africa, most of Australia, the bulk of India, and Antarctica. What is now west Africa was then located at the pole, while South America was close by, joined to Africa along the latter's west coast. Along Africa's east coast were Antarctica and India, while Australia lay just to the north of them, straddling the equator. To the north of Australia was New Guinea. It may have been the northernmost point of land in the world, located just above 30 degrees north latitude. To the north of it lay a vast, uninterrupted sea, known today as the Panthalassic ocean.
Not yet joined with what would become North America were Florida, southern Georgia, and the coastal areas of Mississippi, Alabama, and South Carolina wedged into a gap between Africa and South America, and located very near the South pole.
The rest of North America (called Laurentia by scientists) lay to the north and west of Gondwana, with relatively warmer climates. Rotated almost 45 degrees from their modern orientation, the eastern states of today's U.S. were located along the southeast coast of the continent, while the coastal areas of what are now the southeastern states faced south.
The Hirnantian now represents an internationally accepted stage with a carefully specified global beginning and ending. However, it started out in the nineteenth century as a regional stage in the United Kingdom, where it remains as such today. Since various regions of the world have their own local divisions of geologic time below the Period level, the following list provides correlations between those regional stages (or epochs in some cases) and the internationally recognized Hirnantian Stage.
Australasia – The Hirnantian is equal to approximately the top 20% of the regional Bolindian epoch.
Baltica – The Hirnantian is equivalent to the upper 50% of the regional Porkuni stage.
China – The Hirnantian is roughly equal to the upper 25% of the regional Wufeng stage.
North America – The Hirnantian is equivalent to all of the regional Gamachian stage.
United Kingdom – The Hirnantian is nearly equal to the regional Hirnantian stage, which starts slightly (maybe 100,000 years) earlier. The regional Hirnantian constitutes roughly the upper 20% of the Ashgill epoch.
Related Research Articles
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Ma) to the start of the Silurian Period 443.8 Mya.
The Late Ordovician mass extinction (LOME), sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 million years ago (Ma). It is often considered to be the second-largest known extinction event, in terms of the percentage of genera that became extinct. Extinction was global during this interval, eliminating 49–60% of marine genera and nearly 85% of marine species. Under most tabulations, only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. Despite its taxonomic severity, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.
The Tremadocian is the lowest stage of Ordovician. Together with the later Floian Stage it forms the Lower Ordovician Epoch. The Tremadocian lasted from 485.4 to 477.7 million years ago. The base of the Tremadocian is defined as the first appearance of the conodont species Iapetognathus fluctivagus at the Global Boundary Stratotype Section and Point (GSSP) section on Newfoundland.
The Piacenzian is in the international geologic time scale the upper stage or latest age of the Pliocene. It spans the time between 3.6 ± 0.005 Ma and 2.588 ± 0.005 Ma. The Piacenzian is after the Zanclean and is followed by the Gelasian.
The Andean-Saharan glaciation, also known as the Early Paleozoic Ice Age (EPIA), the Early Paleozoic Icehouse, the Late Ordovician glaciation, the end-Ordovician glaciation, or the Hirnantian glaciation, occurred during the Paleozoic from approximately 460 Ma to around 420 Ma, during the Late Ordovician and the 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, 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.
In the geologic timescale, the Capitanian is an age or stage of the Permian. It is also the uppermost or latest of three subdivisions of the Guadalupian Epoch or Series. The Capitanian lasted between 264.28 and 259.51 million years ago. It was preceded by the Wordian and followed by the Wuchiapingian.
The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene Epoch or Series. It spans the time between 37.71 and33.9 Ma. The Priabonian is preceded by the Bartonian and is followed by the Rupelian, the lowest stage of the Oligocene.
The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene Epoch/Series. It spans the time between 33.9 and27.82 Ma. It is preceded by the Priabonian Stage and is followed by the Chattian Stage.
The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/Series. It spans the time between 27.82 and23.03 Ma. The Chattian is preceded by the Rupelian and is followed by the Aquitanian.
The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding age. Both age and stage bear the same name.
The Pliensbachian is an age of the geologic timescale and stage in the stratigraphic column. It is part of the Early or Lower Jurassic Epoch or Series and spans the time between 192.9 ±0.3 Ma and 184.2 ±0.3 Ma. The Pliensbachian is preceded by the Sinemurian and followed by the Toarcian.
In the geologic timescale, the Wuchiapingian or Wujiapingian is an age or stage of the Permian. It is also the lower or earlier of two subdivisions of the Lopingian Epoch or Series. The Wuchiapingian spans the time between 259.51 and 254.14 million years ago (Ma). It was preceded by the Capitanian and followed by the Changhsingian.
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 242 Ma and ~237 Ma. The Ladinian was preceded by the Anisian and succeeded by the Carnian.
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around 467.3 million years ago. It lasted for about 8.9 million years until the beginning of the Sandbian around 458.4 million years ago. This stage of the Ordovician was marked by the beginning of the Andean-Saharan glaciation.
In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.
The Katian is the second stage of the Upper Ordovician. It is preceded by the Sandbian and succeeded by the Hirnantian Stage. The Katian began 453 million years ago and lasted for about 7.8 million years until the beginning of the Hirnantian 445.2 million years ago. During the Katian the climate cooled which started the Late Ordovician glaciation.
The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species Nemagraptus gracilis around 458.4 million years ago. The Sandbian lasted for about 5.4 million years until the beginning of the Katian around 453 million years ago.
The Dapingian is the third stage of the Ordovician period and the first stage of the Middle Ordovician series. It is preceded by the Floian and succeeded by the Darriwilian. The top of the Floian is defined as the first appearance of the conodont species Baltoniodus triangularis which happened about 470 million years ago. The Dapingian lasted for about 2.7 million years until about 467.3 million years ago.
Wangjiawan is a village in the municipality of Fenxiang Town and part of the Yiling District, which itself is a part of the Yichang of Hubei province.
↑ Bancroft, B. B. (1933). Correlation tables of the stages Costonian-Onnian in England and Wales. Blakeney, Gloucestershire: Privately printed. pp.1–4.
Salvador, Amos, ed., 1994. International Stratigraphic Guide (The International Union of Geological Sciences and The American Geological Society).
Sheehan, Peter, "The Late Ordovician Mass Extinction" (Annual Review of Earth and Planetary Sciences, 2001, pp.331–364).
Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).
Further reading
Brenchley, P. J., 1984. "Late Ordovician extinction and their relationship to the Gondwana glaciation", in Brenchley, P. J., ed., Fossils and Climate. pp.291–316, (John Wiley & Sons).
Hallam, Anthony and Paul B. Wignall, 1997. Mass extinctions and their aftermath (Oxford University Press).
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