Silurian Period
Silurian Period, in geologic time, the third period of the Paleozoic Era. It began 443.8 million years ago and ended 419.2 million years ago, extending from the close of the Ordovician Period to the beginning of the Devonian Period.
During the Silurian, continental elevations were generally much lower than in the present day, and global sea level was much higher. Sea level rose dramatically as the extensive glaciers from the Late Ordovician ice age melted. This rising prompted changes in climatic conditions that allowed many faunal groups to recover from the extinctions of Late Ordovician times. Large expanses of several continents became flooded with shallow seas, and mound-type coral reefs were very common. Fishes were widespread. Vascular plants began to colonize coastal lowlands during the Silurian Period, whereas continental interiors remained essentially barren of life.
The name of this period is derived from work done by Scottish geologist Roderick I. Murchison, who in 1835 named a sequence of rocks in Wales and its borderland with England in honour of a native people called the Silures. The Silures, under the leadership of Caratacus, resisted Roman conquest for 30 years until 78 ce, when they were finally overcome.
The Silurian environment
Paleogeography
During most of the Silurian Period, the vast Panthalassic Ocean covered the northern polar regions, the supercontinent of Gondwana stretched over the southern polar region, and a ring of at least six continents spanned the Equator and middle latitudes. The approximate orientations and locations of Silurian continents can be reconstructed using a combination of paleomagnetic, paleoclimatic, and biogeographic data. The Earth’s magnetic field leaves its signature on volcanic rocks and certain sedimentary rocks rich in such iron-bearing minerals as magnetite. As rocks capable of being magnetized are cooled or otherwise lithified, their component crystals (grains) are lined up with the Earth’s magnetic field. Unless the rocks are remelted by the heat from the planet’s interior or reworked by erosion, they retain this signature regardless of whether they change position or not. The Earth’s zonal climate also has an effect on global patterns of sedimentation.
The most unusual features of the Silurian that distinguish it from the present-day physical environment relate to conditions of low continental elevations combined with a much higher global stand in sea level. Extensive continental regions were flooded by shallow seas ranging in water depth from a few to little more than 100 metres (330 feet). Where these seas occupied a tropical to subtropical climatic zone, coral mound reefs with associated carbonate sediments were very common. Strata forming in arid regions differ from those formed in inundated areas or other regions with high annual rainfall. The deposition of evaporites (salts) was periodically set in motion as a result of reduced ocean circulation occurring in geographically restricted places such as shallow embayments.
The strong faunal endemism (the restriction or limiting of species to specific continents or isolated regions) present during the Ordovician Period was replaced during the Silurian Period by a situation where some species were distributed globally. Seafloor topography was muted over large areas of flooded continental platforms, and faunas of shelly invertebrates in different regions were remarkably consistent with one another. This has allowed geologists and stratigraphers to correlate layers of Silurian rock found on different present-day continents.
The geographic summary that follows is based on a global reconstruction specific to the Wenlock Epoch which spans the middle of the Silurian Period.
Laurentia
Much of North America (including Greenland), northwestern Ireland, Scotland, and the Chukotskiy Peninsula of northeastern Russia belonged to the paleocontinent Laurentia (a name derived from Quebec’s portion of the Canadian Shield). With respect to the present-day Great Lakes and Hudson Bay, Laurentia was rotated clockwise during Wenlock time to fit fully between the latitudes 30° N and 30° S of the paleoequator. The present south shore of Hudson Bay was at the centre of Laurentia, with the Wenlock paleoequator crossing near Southampton Island. The microcontinent of Barentsia, which included Norway’s island of Svalbard, was likely appended to Laurentia off eastern Greenland. Island arcs and highland areas, such as Taconica (a landmass that would become part of eastern North America) and Pearya (a landmass that would become the northern part of Ellesmere Island), rimmed the flooded continent. During the Llandovery and Wenlock epochs, more than 65 percent of Laurentia was flooded by shallow seas.
Baltica
The narrow, north-south Iapetus Ocean still separated Laurentia from another paleocontinent, Baltica, during Wenlock time. The Uralian and Variscan-Hercynian sutures—regions where earlier orogenies (mountain-building events) had welded landmasses together—marked the eastern and southern margins of this paleocontinent. The northern tip of Scandinavia was situated just below the paleoequator during the Wenlock Epoch, but the islands of Novaya Zemlya extended well to the north. The most prominent features were the Caledonian highlands of Norway, although a lowland attributed to the Fennoscandian Shield (rocks of Precambrian origin underlying the present-day Baltic Sea, the Kola Peninsula of Russia, and the countries of Norway, Sweden, and Finland) existed in the vicinity of Finland. Another lowland within Baltica is related to the Sarmatian Shield, in the region between the Vistula and Volga rivers in Poland and adjacent Russia. The microcontinent of Avalonia—its name derived from the Avalon Peninsula of eastern Newfoundland—was appended to Baltica by the end of Ordovician time. It included what are now England, Wales, southeastern Ireland, the Belgian Ardennes, northern France, eastern Newfoundland, part of Nova Scotia, southern New Brunswick, and coastal New England.
Siberia, Kazakhstania, and other continents
Separated from Baltica by the Pleionic Ocean (essentially the northwestern arm of the Paleotethys Sea), the paleocontinent of Siberia assumed an orientation rotated 180° from its present alignment (as recognized by the inverted position of Lake Baikal). A huge Siberian platform sea extended southward. Similarly, Kazakhstania was a neighbouring continent to the east in the same northern middle latitudes. North China (including Manchuria and Korea) and South China (the Yangtze platform) were two separate continents situated in a more equatorial position. In contrast to Siberia and Kazakhstania, most of North and South China were elevated above sea level during Wenlock time.
Gondwana
The vast supercontinent of Gondwana was centred over the South Pole. In addition to Australia, Antarctica, India, Arabia, Africa, and South America, Silurian Gondwana also included smaller pieces of Florida, southern Europe, and the Cimmerian terranes—namely, Turkey, Iran, Afghanistan, Tibet, and the Malay Peninsula—on its outer fringes. Either present-day Brazil or contiguous West Africa was stationed directly over the South Pole, buried by an ice cap likely comparable in size to the one capping Antarctica today. During Wenlock time, India, Tibet, the Malay Peninsula, and Australia projected into subtropical or tropical latitudes. The east-west ocean separating the southern European sector of Gondwana from northern Europe (Baltica) is called the Rheic Ocean and was essentially a southwestern extension of the Paleotethys Sea. The flooded margin of eastern Australia had a more-varied seafloor topography than the other shallow seas because of the extensive volcanism occurring there during Silurian time, but it shared many of the same faunal elements because of its tropical latitude.
Paleoclimate
Atmospheric circulation
Broad-scale Silurian climatic conditions can be inferred by determining the positions and orientations of the paleocontinents and assuming that atmospheric circulation functioned according to the same basic principles during Silurian times as it does today. The global paleoclimate was effectively driven by major contrast in the proportions of land and water between the Northern and Southern hemispheres. A zonally uniform climate would be expected in the Northern Hemisphere during the Silurian, because it was dominated by a North Polar ocean. Wind patterns must have included strong polar easterlies at high northern latitudes, prevailing westerlies at midlatitudes, and northeast trade winds in the tropics. In contrast, with Gondwana centred over the South Pole, climate in the Southern Hemisphere must have been dominated by the interaction of cellular air masses over land and water. The large continent would have experienced wide temperature variations due to summer heating and winter cooling.
Atmospheric circulation patterns interpreted for an early Silurian summer in the Northern Hemisphere indicate high pressure over the polar ocean with a zone of low pressure around 60° N latitude. Distinct high-pressure cells formed above subtropical oceans, much like the persistent Bermuda high-pressure centre over the present subtropical North Atlantic. Another zone of low pressure formed above the thermal equator, or the region of most intense solar warming. This somewhat migratory zone was the Silurian intertropical convergence zone (ITCZ), where the convergence of Northern and Southern Hemispheric trade winds caused the warm tropical air to rise, which in turn produced regular cloud cover and precipitation.
Mostly, the ITCZ remained near the Equator, but it may have migrated slightly to the north in response to strong summer heating on Laurentia, Baltica, and possibly Kazakhstania. This tendency would have been strongest along the eastern margins of tropical continents, where anticyclonic circulation around subtropical highs pulled warm, moisture-laden air northwestward from equatorial oceans. Subtropical high pressure probably spread onto Gondwana, particularly the Australian and Antarctic sectors. A pressure ridge may have merged with these subtropical highs to form a massive cold cell penetrating to higher latitudes over the continental interior of Gondwana. Low-pressure systems over Gondwana’s midlatitude shelf were not unlike the Aleutian and Icelandic lows of today.
Pressure systems should have moved somewhat southward during the Northern Hemispheric winter—particularly the ITCZ, which generally follows the region of maximum heating to the Southern Hemisphere during this season. A low-pressure system between Laurentia and Baltica is consistent with the erosion of thick clastic rocks (sedimentary rock composed of fragments of older rocks) derived from the Taconic and Caledonian highlands. The most significant seasonal variation surely occurred in the eastern Australian and Antarctic sectors of Gondwana, where summer heating abolished winter high-pressure cells and pulled the ITCZ more poleward. This is comparable to today’s monsoons, which pull the ITCZ in the opposite direction over the subcontinent of India during the Northern Hemispheric summer. Subtropical highs intensified over the ocean waters of the Southern Hemisphere and probably insulated the arid climate of Western Australia.
Oceanic anoxia
The broadly flooded shelf of Gondwana had a circumference of at least 28,000 km (17,400 miles), most of which sat in the belt of persistent westerly winds. This had the effect of producing intense surface upwelling of oceanic waters along the margins of the supercontinent and significantly boosting biological productivity. After the organisms living in these waters died, their remains sank into a bottom layer of water characterized by low levels of dissolved oxygen. This dearth of oxygen (anoxia) significantly decelerated the decomposition process, thereby providing the right conditions for the development of black shales. Today, black shale deposits are widespread around the perimeter areas of the former supercontinent, as evidenced by the rich source rocks yielding concentrations of Silurian oil in present-day Algeria and Saudi Arabia.
