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Cambrian

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For other uses, see Cambrian (disambiguation). Not to be confused with Cambria or Cumbria.
"Cambrian fauna" redirects here. For the first evolutionary fauna, see Evolutionary fauna § Cambrian fauna.

Cambrian
538.8 ± 0.2 – 485.4 ± 1.9 Ma
A map of Earth as it appeared 510 million years ago during the Cambrian Period, Series 2 epoch
Chronology
Etymology
Name formalityFormal
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitPeriod
Stratigraphic unitSystem
First proposed byAdam Sedgwick, 1835
Time span formalityFormal
Lower boundary definitionAppearance of the Ichnofossil Treptichnus pedum
Lower boundary GSSPFortune Head section, Newfoundland, Canada
47°04′34″N 55°49′52″W / 47.0762°N 55.8310°W / 47.0762; -55.8310
Lower GSSP ratified1992[2]
Upper boundary definitionFAD of the Conodont Iapetognathus fluctivagus.
Upper boundary GSSPGreenpoint section, Green Point, Newfoundland, Canada
49°40′58″N 57°57′55″W / 49.6829°N 57.9653°W / 49.6829; -57.9653
Upper GSSP ratified2000[3]
Atmospheric and climatic data
Sea level above present dayRising steadily from 4 m to 90 m[4]
Part of a series on
The Cambrian explosion
Fossil localities
Key organisms
Ediacaran biota
Burgess-type
Small shelly fauna
Evolutionary concepts
Trends
Themes

The Cambrian ( /ˈkæmbri.ən, ˈkeɪm-/ KAM-bree-ən, KAYM-) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon.[5] The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovician Period 485.4 Ma.[6]

Most of the continents lay in the southern hemisphere surrounded by the vast Panthalassa Ocean.[7] The assembly of Gondwana during the Ediacaran and early Cambrian led to the development of new convergent plate boundaries and continental-margin arc magmatism along its margins that helped drive up global temperatures.[8] Laurentia lay across the equator, separated from Gondwana by the opening Iapetus Ocean.[7]

The Cambrian was a time of greenhouse climate conditions, with high levels of atmospheric carbon dioxide and low levels of oxygen in the atmosphere and seas. Upwellings of anoxic deep ocean waters into shallow marine environments led to extinction events, whilst periods of raised oxygenation led to increased biodiversity.[9]

The Cambrian marked a profound change in life on Earth; prior to the Period, the majority of living organisms were small, unicellular and poorly preserved. Complex, multicellular organisms gradually became more common during the Ediacaran, but it was not until the Cambrian that the rapid diversification of lifeforms, known as the Cambrian explosion, produced the first representatives of most modern animal phyla.[10] The Period is also unique in its unusually high proportion of lagerstätte deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells.[11]

By the end of the Cambrian, myriapods,[12][13] arachnids,[14] and hexapods[15] started adapting to the land, along with the first plants.[16][17]

Etymology and history

[edit]

The term Cambrian is derived from the Latin version of Cymru, the Welsh name for Wales, where rocks of this age were first studied. It was named by Adam Sedgwick in 1835, who divided it into three groups; the Lower, Middle, and Upper.[18] He defined the boundary between the Cambrian and the overlying Silurian, together with Roderick Murchison, in their joint paper "On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales". This early agreement did not last.[19]

Due to the scarcity of fossils, Sedgwick used rock types to identify Cambrian strata. He was also slow in publishing further work. The clear fossil record of the Silurian, however, allowed Murchison to correlate rocks of a similar age across Europe and Russia, and on these he published extensively. As increasing numbers of fossils were identified in older rocks, he extended the base of the Silurian downwards into the Sedgwick's "Upper Cambrian", claiming all fossilised strata for "his" Silurian series. Matters were complicated further when, in 1852, fieldwork carried out by Sedgwick and others revealed an unconformity within the Silurian, with a clear difference in fauna between the two.[20][19] This allowed Sedgwick to now claim a large section of the Silurian for "his" Cambrian and gave the Cambrian an identifiable fossil record. The dispute between the two geologists and their supporters, over the boundary between the Cambrian and Silurian, would extend beyond the life times of both Sedgwick and Murchison. It was not resolved until 1879, when Charles Lapworth proposed the disputed strata belong to its own system, which he named the Ordovician.[19]

The term Cambrian for the oldest period of the Paleozoic was officially agreed in 1960, at the 21st International Geological Congress. It only includes Sedgwick's "Lower Cambrian series", but its base has been extended into much older rocks.[18]

Geology

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Stratigraphy

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Systems, series and stages can be defined globally or regionally. For global stratigraphic correlation, the ICS ratify rock units based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary of the unit. Currently the boundaries of the Cambrian System, three series and six stages are defined by global stratotype sections and points.[6]

Ediacaran-Cambrian boundary

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The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites. The recognition of small shelly fossils before the first trilobites, and Ediacara biota substantially earlier, has led to calls for a more precisely defined base to the Cambrian Period.[21]

Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian Period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum.[21] Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, and it is the T. pedum ichnofossil assemblage that is now formally used to correlate the base of the Cambrian.[21][22]

This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. An early date of 570 Ma quickly gained favour,[21] though the methods used to obtain this number are now considered to be unsuitable and inaccurate. A more precise analysis using modern radiometric dating yields a date of 538.8 ± 0.2 Ma.[6] The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, and to the disappearance of distinctive Ediacaran fossils (Namacalathus, Cloudina). Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata – which would mean that dates from other sections, ranging from 544 to 542 Ma, are more suitable.[21]

Approximate correlation of global and regional stages in Cambrian stratigraphy[18]
International Series International Stage Chinese Australian Russian-Kazakhian North American European
C
a
m
b
r
i
a
n 		Furongian "Stage 10" Niuchehean Datsonian Batyrbaian Skullrockian / Ibexian (part) Merionethian
Payntonian Sunwaptan / Trempealeauan
Jiangshanian Jiangshanian Iverian Aksaian
Sakian
Paibian Paibian Idamean Steptoean / Franconian
Miaolingian Guzhangian Guzhangian Mindyallan Ayusokkanian Marjuman / Dresbachian
Boomerangian Mayan Acadian / St. David's
Drumian Wangcunian Undillian
Florian
Wuliuan Wuliuan Templetonian Amgan / Amgaian Topazan
Ordian Delmaran
Cambrian Series 2 "Stage 4" Duyunian Branchian / Comley (part)
Toyonian
Dyeran
Botomian
"Stage 3" Nangaoan
Atdabanian Montezuman
Placentian / Comley (part)
Terreneuvian "Stage 2" Meishucunian Tommotian* Begadean
Jinningian Nemakit-Daldynian*
Fortunian
Ediacaran Sinian Adelaidean Sakharan / Vendian Hadrynian

*Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.[23][24][25]

Photograph of the layered rocks that make up the headland at Fortune Head GSSP
Ediacaran-Cambrian boundary section at Fortune Head, Newfoundland, GSSP

Terreneuvian

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The Terreneuvian is the lowermost series/epoch of the Cambrian, lasting from 538.8 ± 0.2 Ma to c. 521 Ma. It is divided into two stages: the Fortunian stage, 538.8 ± 0.2 Ma to c. 529 Ma; and the unnamed Stage 2, c. 529 Ma to c. 521 Ma.[6] The name Terreneuvian was ratified by the International Union of Geological Sciences (IUGS) in 2007, replacing the previous "Cambrian Series 1". The GSSP defining its base is at Fortune Head on the Burin Peninsula, eastern Newfoundland, Canada (see Ediacaran - Cambrian boundary above). The Terreneuvian is the only series in the Cambrian to contain no trilobite fossils. Its lower part is characterised by complex, sediment-penetrating Phanerozoic-type trace fossils, and its upper part by small shelly fossils.[18]

Cambrian Series 2

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The second series/epoch of the Cambrian is currently unnamed and known as Cambrian Series 2. It lasted from c. 521 Ma to c. 509 Ma. Its two stages are also unnamed and known as Cambrian Stage 3, c. 521 Ma to c. 514 Ma, and Cambrian Stage 4, c. 514 Ma to c. 509 Ma.[6] The base of Series 2 does not yet have a GSSP, but it is expected to be defined in strata marking the first appearance of trilobites in Gondwana. There was a rapid diversification of metazoans during this epoch, but their restricted geographic distribution, particularly of the trilobites and archaeocyaths, have made global correlations difficult, hence ongoing efforts to establish a GSSP.[18]

Miaolingian

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Diorama of the Burgess Shale Biota

The Miaolingian is the third series/epoch of the Cambrian, lasting from c. 509 Ma to c. 497 Ma, and roughly identical to the middle Cambrian in older literature [1]. It is divided into three stages: the Wuliuan c. 509 Ma to 504.5 Ma; the Drumian c. 504.5 Ma to c. 500.5 Ma; and the Guzhangian c. 500.5 Ma to c. 497 Ma.[6] The name replaces Cambrian Series 3 and was ratified by the IUGS in 2018.[26] It is named after the Miaoling Mountains in southeastern Guizhou Province, South China, where the GSSP marking its base is found. This is defined by the first appearance of the oryctocephalid trilobite Oryctocephalus indicus. Secondary markers for the base of the Miaolingian include the appearance of many acritarchs forms, a global marine transgression, and the disappearance of the polymerid trilobites, Bathynotus or Ovatoryctocara. Unlike the Terreneuvian and Series 2, all the stages of the Miaolingian are defined by GSSPs.[26]

The olenellids, eodiscids, and most redlichiids trilobites went extinct at the boundary between Series 2 and the Miaolingian. This is considered the oldest mass extinction of trilobites.[18]

Furongian

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The Furongian, c. 497 Ma to 485.4 ± 1.9 Ma, is the fourth and uppermost series/epoch of the Cambrian. The name was ratified by the IUGS in 2003 and replaces Cambrian Series 4 and the traditional "Upper Cambrian". The GSSP for the base of the Furongian is in the Wuling Mountains, in northwestern Hunan Province, China. It coincides with the first appearance of the agnostoid trilobite Glyptagnostus reticulatus, and is near the beginning of a large positive δ13C isotopic excursion.[18]

The Furongian is divided into three stages: the Paibian, c. 497 Ma to c. 494 Ma, and the Jiangshanian c. 494 Ma to c. 489.5 Ma, which have defined GSSPs; and the unnamed Cambrian Stage 10, c. 489.5 Ma to 485.4 ± 1.9 Ma.[6]

Cambrian–Ordovician boundary

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The GSSP for the Cambrian–Ordovician boundary is at Green Point, western Newfoundland, Canada, and is dated at 485.4 Ma. It is defined by the appearance of the conodont Iapetognathus fluctivagus. Where these conodonts are not found the appearance of planktonic graptolites or the trilobite Jujuyaspis borealis can be used. The boundary also corresponds with the peak of the largest positive variation in the δ13C curve during the boundary time interval and with a global marine transgression.[27]

Impact structures

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Major meteorite impact structures include: the early Cambrian (c. 535 Ma) Neugrund crater in the Gulf of Finland, Estonia, a complex meteorite crater about 20 km in diameter, with two inner ridges of about 7 km and 6 km diameter, and an outer ridge of 8 km that formed as the result of an impact of an asteroid 1 km in diameter;[28] the 5 km diameter Gardnos crater (500±10 Ma) in Buskerud, Norway, where post-impact sediments indicate the impact occurred in a shallow marine environment with rock avalanches and debris flows occurring as the crater rim was breached not long after impact;[29] the 24 km diameter Presqu'ile crater (500 Ma or younger) Quebec, Canada; the 19 km diameter Glikson crater (c. 508 Ma) in Western Australia; the 5 km diameter Mizarai crater (500±10 Ma) in Lithuania; and the 3.2 km diameter Newporte structure (c. 500 Ma or slightly younger) in North Dakota, U.S.A.[30]

Paleogeography

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Reconstructing the position of the continents during the Cambrian is based on palaeomagnetic, palaeobiogeographic, tectonic, geological and palaeoclimatic data. However, these have different levels of uncertainty and can produce contradictory locations for the major continents.[31] This, together with the ongoing debate around the existence of the Neoproterozoic supercontinent of Pannotia, means that while most models agree the continents lay in the southern hemisphere, with the vast Panthalassa Ocean covering most of northern hemisphere, the exact distribution and timing of the movements of the Cambrian continents varies between models.[31]

Paleogeographic map showing Gondwana close to the south pole, Laurentia at the equator and Baltica in between.
Approximate positions of Gondwana, Laurentia and Baltica in the middle Cambrian (c. 500 Ma). AT: Armorican terrane, CA: Carolinia, CU: Cuyania, EA: East Avalonia, FA: Famatina arc, GA: Ganderia, IB: Iberia, MX: Mixteca–Oaxaca block, WA: West Avalonia. Plate boundaries: red - subduction; white - ridges; yellow - transform.[32][7]

Most models show Gondwana stretching from the south polar region to north of the equator.[7] Early in the Cambrian, the south pole corresponded with the western South American sector and as Gondwana rotated anti-clockwise, by the middle of the Cambrian, the south pole lay in the northwest African region.[31]

Laurentia lay across the equator, separated from Gondwana by the Iapetus Ocean.[7] Proponents of Pannotia have Laurentia and Baltica close to the Amazonia region of Gondwana with a narrow Iapetus Ocean that only began to open once Gondwana was fully assembled c. 520 Ma.[33] Those not in favour of the existence of Pannotia show the Iapetus opening during the Late Neoproterozoic, with up to c. 6,500 km (c. 4038 miles) between Laurentia and West Gondwana at the beginning of the Cambrian.[7]

Of the smaller continents, Baltica lay between Laurentia and Gondwana, the Ran Ocean (an arm of the Iapetus) opening between it and Gondwana. Siberia lay close to the western margin of Gondwana and to the north of Baltica.[34][7] Annamia and South China formed a single continent situated off north central Gondwana. The location of North China is unclear. It may have lain along the northeast Indian sector of Gondwana or already have been a separate continent.[7]

Laurentia

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During the Cambrian, Laurentia lay across or close to the equator.  It drifted south and rotated c. 20° anticlockwise during the middle Cambrian, before drifting north again in the late Cambrian.[7]

After the Late Neoproterozoic (or mid-Cambrian) rifting of Laurentia from Gondwana and the subsequent opening of the Iapetus Ocean, Laurentia was largely surrounded by passive margins with much of the continent covered by shallow seas.[7]

As Laurentia separated from Gondwana, a sliver of continental terrane rifted from Laurentia with the narrow Taconic seaway opening between them. The remains of this terrane are now found in southern Scotland, Ireland, and Newfoundland. Intra-oceanic subduction either to the southeast of this terrane in the Iapetus, or to its northwest in the Taconic seaway, resulted in the formation of an island arc. This accreted to the terrane in the late Cambrian, triggering southeast-dipping subduction beneath the terrane itself and consequent closure of the marginal seaway. The terrane collided with Laurentia in the Early Ordovician.[35]

Towards the end of the early Cambrian, rifting along Laurentia's southeastern margin led to the separation of Cuyania (now part of Argentina) from the Ouachita embayment with a new ocean established that continued to widen through the Cambrian and Early Ordovician.[35]

Gondwana

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Gondwana was a massive continent, three times the size of any of the other Cambrian continents. Its continental land area extended from the south pole to north of the equator. Around it were extensive shallow seas and numerous smaller land areas.[7]

The cratons that formed Gondwana came together during the Neoproterozoic to early Cambrian. A narrow ocean separated Amazonia from Gondwana until c. 530 Ma[36] and the Arequipa-Antofalla block united with the South American sector of Gondwana in the early Cambrian.[7] The Kuunga Orogeny between northern (Congo Craton, Madagascar and India) and southern Gondwana (Kalahari Craton and East Antarctica), which began c. 570 Ma, continued with parts of northern Gondwana over-riding southern Gondwana and was accompanied by metamorphism and the intrusion of granites.[37]

Subduction zones, active since the Neoproterozoic, extended around much of Gondwana's margins, from northwest Africa southwards round South America, South Africa, East Antarctica, and the eastern edge of West Australia. Shorter subduction zones existed north of Arabia and India.[7]

The Famatinian continental arc stretched from central Peru in the north to central Argentina in the south. Subduction beneath this proto-Andean margin began by the late Cambrian.[35]

Along the northern margin of Gondwana, between northern Africa and the Armorican Terranes of southern Europe, the continental arc of the Cadomian Orogeny continued from the Neoproterozoic in response to the oblique subduction of the Iapetus Ocean.[38] This subduction extended west along the Gondwanan margin and by c. 530 Ma may have evolved into a major transform fault system.[38]

At c. 511 Ma the continental flood basalts of the Kalkarindji large igneous province (LIP) began to erupt. These covered an area of > 2.1 × 106 km2 across northern, central and Western Australia regions of Gondwana making it one of the largest, as well as the earliest, LIPs of the Phanerozoic. The timing of the eruptions suggests they played a role in the early to middle Cambrian mass extinction.[38]

Ganderia, East and West Avalonia, Carolinia and Meguma Terranes

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The terranes of Ganderia, East and West Avalonia, Carolinia and Meguma lay in polar regions during the early Cambrian, and high-to-mid southern latitudes by the mid to late Cambrian.[35][31] They are commonly shown as an island arc-transform fault system along the northwestern margin of Gondwana north of northwest Africa and Amazonia, which rifted from Gondwana during the Ordovician.[35] However, some models show these terranes as part of a single independent microcontinent, Greater Avalonia, lying to the west of Baltica and aligned with its eastern (Timanide) margin, with the Iapetus to the north and the Ran Ocean to the south.[31]

Paleogeographic map showing Gondwana close to the south pole, Siberia, North and South China near the equator and Baltica to the south of Siberia.
Approximate positions of Siberia, Gondwana, North and South China, Baltica and smaller terranes in the middle Cambrian (c. 500 Ma). AN: Annamia, CM: Central Mongolian terrane, JA: Japan arc, KHT: Kazakhstania terranes, MOO: Mongol-Okhotsk Ocean, NC: North China, QT: Qinling terrane, SC: South China, TA: Tarim microcontinent, VT: Variscan terranes. Plate boundaries: red - subduction; white - ridges; yellow - transform.[7][39]

Baltica

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During the Cambrian, Baltica rotated more than 60° anti-clockwise and began to drift northwards.[35] This rotation was accommodated by major strike-slip movements in the Ran Ocean between it and Gondwana.[7]

Baltica lay at mid-to-high southerly latitudes, separated from Laurentia by the Iapetus and from Gondwana by the Ran Ocean. It was composed of two continents, Fennoscandia and Sarmatia, separated by shallow seas.[7][35] The sediments deposited in these unconformably overlay Precambrian basement rocks. The lack of coarse-grained sediments indicates low lying topography across the centre of the craton.[7]

Along Baltica's northeastern margin subduction and arc magmatism associated with the Ediacaran Timanian Orogeny was coming to an end. In this region the early to middle Cambrian was a time of non-deposition and followed by late Cambrian rifting and sedimentation.[40]

Its southeastern margin was also a convergent boundary, with the accretion of island arcs and microcontinents to the craton, although the details are unclear.[7]

Siberia

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Siberia began the Cambrian close to western Gondwana and north of Baltica. It drifted northwestwards to close to the equator as the Ægir Ocean opened between it and Baltica.[7][34] Much of the continent was covered by shallow seas with extensive archaeocyathan reefs. The then northern third of the continent (present day south; Siberia has rotated 180° since the Cambrian) adjacent to its convergent margin was mountainous.[7]

From the Late Neoproterozoic to the Ordovician, a series of island arcs accreted to Siberia's then northeastern margin, accompanied by extensive arc and back-arc volcanism. These now form the Altai-Sayan terranes.[7][40] Some models show a convergent plate margin extending from Greater Avalonia, through the Timanide margin of Baltica, forming the Kipchak island arc offshore of southeastern Siberia and curving round to become part of the Altai-Sayan convergent margin.[31]

Along the then western margin, Late Neoproterozoic to early Cambrian rifting was followed by the development of a passive margin.[40]

To the then north, Siberia was separated from the Central Mongolian terrane by the narrow and slowly opening Mongol-Okhotsk Ocean. The Central Mongolian terrane's northern margin with the Panthalassa was convergent, whilst its southern margin facing the Mongol-Okhotsk Ocean was passive.[7]

Central Asia

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During the Cambrian, the terranes that would form Kazakhstania later in the Paleozoic were a series of island arc and accretionary complexes that lay along an intra-oceanic convergent plate margin to the south of North China.[40]

To the south of these the Tarim microcontinent lay between Gondwana and Siberia.[7] Its northern margin was passive for much of the Paleozoic, with thick sequences of platform carbonates and fluvial to marine sediments resting unconformably on Precambrian basement. Along its southeast margin was the Altyn Cambro–Ordovician accretionary complex, whilst to the southwest a subduction zone was closing the narrow seaway between the North West Kunlun region of Tarim and the South West Kunlun terrane.[40]

North China

[edit]
Life reconstruction of the Linyi Lagerstätte in Northern China

North China lay at equatorial to tropical latitudes during the early Cambrian, although its exact position is unknown.[34] Much of the craton was covered by shallow seas, with land in the northwest and southeast.[7]

Northern North China was a passive margin until the onset of subduction and the development of the Bainaimiao arc in the late Cambrian. To its south was a convergent margin with a southwest dipping subduction zone, beyond which lay the North Qinling terrane (now part of the Qinling Orogenic Belt).[40]

South China and Annamia

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South China and Annamia formed a single continent. Strike-slip movement between it and Gondwana accommodated its steady drift northwards from offshore the Indian sector of Gondwana to near the western Australian sector. This northward drift is evidenced by the progressive increase in limestones and increasing faunal diversity.[7]

The northern margin South China, including the South Qinling block, was a passive margin.[7]

Along the southeastern margin, lower Cambrian volcanics indicate the accretion of an island arc along the Song Ma suture zone. Also, early in the Cambrian, the eastern margin of South China changed from passive to active, with the development of oceanic volcanic island arcs that now form part of the Japanese terrane.[7]

Climate

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The distribution of climate-indicating sediments, including the wide latitudinal distribution of tropical carbonate platforms, archaeocyathan reefs and bauxites, and arid zone evaporites and calcrete deposits, show the Cambrian was a time of greenhouse climate conditions.[41][42] During the late Cambrian the distribution of trilobite provinces also indicate only a moderate pole-to-equator temperature gradient.[42] There is evidence of glaciation at high latitudes on Avalonia. However, it is unclear whether these sediments are early Cambrian or actually late Neoproterozoic in age.[41]

Calculations of global average temperatures (GAT) vary depending on which techniques are used. Whilst some measurements show GAT over c. 40 °C (104 °F) models that combine multiple sources give GAT of c. 20–22 °C (68–72 °F) in the Terreneuvian increasing to c. 23–25 °C (73–77 °F) for the rest of the Cambrian.[42][9] The warm climate was linked to elevated atmospheric carbon dioxide levels. Assembly of Gondwana led to the reorganisation of the tectonic plates with the development of new convergent plate margins and continental-margin arc magmatism that helped drive climatic warming.[9][8] The eruptions of the Kalkarindji LIP basalts during Stage 4 and into the early Miaolingian, also released large quantities of carbon dioxide, methane and sulphur dioxide into the atmosphere leading to rapid climatic changes and elevated sea surface temperatures.[8]

There is uncertainty around the maximum sea surface temperatures. These are calculated using δ18O values from marine rocks, and there is an ongoing debate about the levels δ18O in Cambrian seawater relative to the rest of the Phanerozoic.[42][43] Estimates for tropical sea surface temperatures vary from c. 28–32 °C (82–90 °F),[42][43] to c. 29–38 °C (84–100 °F).[44][41] Modern average tropical sea surface temperatures are 26 °C (79 °F).[42]

Atmospheric oxygen levels rose steadily rising from the Neoproterozoic due to the increase in photosynthesising organisms. Cambrian levels varied between c. 3% and 14% (present day levels are c. 21%). Low levels of atmospheric oxygen and the warm climate resulted in lower dissolved oxygen concentrations in marine waters and widespread anoxia in deep ocean waters.[9][45]

There is a complex relationship between oxygen levels, the biogeochemistry of ocean waters, and the evolution of life. Newly evolved burrowing organisms exposed anoxic sediments to the overlying oxygenated seawater. This bioturbation decreased the burial rates of organic carbon and sulphur, which over time reduced atmospheric and oceanic oxygen levels, leading to widespread anoxic conditions.[46] Periods of higher rates of continental weathering led to increased delivery of nutrients to the oceans, boosting productivity of phytoplankton and stimulating metazoan evolution. However, rapid increases in nutrient supply led to eutrophication, where rapid growth in phytoplankton numbers result in the depletion of oxygen in the surrounding waters.[9][47]

Pulses of increased oxygen levels are linked to increased biodiversity; raised oxygen levels supported the increasing metabolic demands of organisms, and increased ecological niches by expanding habitable areas of seafloor. Conversely, incursions of oxygen-deficient water, due to changes in sea level, ocean circulation, upwellings from deeper waters and/or biological productivity, produced anoxic conditions that limited habitable areas, reduced ecological niches and resulted in extinction events both regional and global.[45][46][47]

Overall, these dynamic, fluctuating environments, with global and regional anoxic incursions resulting in extinction events, and periods of increased oceanic oxygenation stimulating biodiversity, drove evolutionary innovation.[46][9][47]

Geochemistry

[edit]

During the Cambrian, variations in isotope ratios were more frequent and more pronounced than later in the Phanerozoic, with at least 10 carbon isotope (δ13C) excursions (significant variations in global isotope ratios) recognised.[18] These excursions record changes in the biogeochemistry of the oceans and atmosphere, which are due to processes such as the global rates of continental arc magmatism, rates of weathering and nutrients levels entering the marine environment, sea level changes, and biological factors including the impact of burrowing fauna on oxygen levels.[9][47][8]

Isotope excursions

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Base of Cambrian

[edit]

The basal Cambrian δ13C excursion (BACE), together with low δ238U and raised δ34S indicates a period of widespread shallow marine anoxia, which occurs at the same time as the extinction off the Ediacaran acritarchs. It was followed by the rapid appearance and diversification of bilaterian animals.[18][9]

Cambrian Stages 2 and 3

[edit]

During the early Cambrian, 87Sr/86Sr rose in response to enhanced continental weathering. This increased the input of nutrients into the oceans and led to higher burial rates of organic matter.[48] Over long timescales, the extra oxygen released by organic carbon burial is balanced by a decrease in the rates of pyrite (FeS2) burial (a process which also releases oxygen), leading to stable levels of oxygen in the atmosphere. However, during the early Cambrian, a series of linked δ13C and δ34S excursions indicate high burial rates of both organic carbon and pyrite in biologically productive yet anoxic ocean floor waters. The oxygen-rich waters produced by these processes spread from the deep ocean into shallow marine environments, extending the habitable regions of the seafloor.[18][49] These pulses of oxygen are associated with the radiation of the small shelly fossils and the Cambrian arthropod radiation isotope excursion (CARE).[48] The increase in oxygenated waters in the deep ocean ultimately reduced the levels of organic carbon and pyrite burial, leading to a decrease in oxygen production and the re-establishment of anoxic conditions. This cycle was repeated several times during the early Cambrian.[18][49]

Archeocyathids from the Poleta formation in the Death Valley area

Cambrian Stage 4 to early Miaolingian

[edit]

The beginning of the eruptions of the Kalkarindji LIP basalts during Stage 4 and the early Miaolingian released large quantities of carbon dioxide, methane and sulphur dioxide into the atmosphere. The changes these wrought are reflected by three large and rapid δ13C excursions. Increased temperatures led to a global sea level rise that flooded continental shelves and interiors with anoxic waters from the deeper ocean and drowned carbonate platforms of archaeocyathan reefs, resulting in the widespread accumulation of black organic-rich shales. Known as the Sinsk anoxic extinction event, this triggered the first major extinction of the Phanerozoic, the 513 - 508 Ma Botoman-Toyonian Extinction (BTE), which included the loss of the archaeocyathids and hyoliths and saw a major drop in biodiversity.[8][49] The rise in sea levels is also evidenced by a global decrease in 87Sr/86Sr. The flooding of continental areas decreased the rates of continental weathering, reducing the input of 87Sr to the oceans and lowering the 87Sr/86Sr of seawater.[48][18]

The base of the Miaolingian is marked by the Redlichiid–Olenellid extinction carbon isotope event (ROECE), which coincides with the main phase of Kalkarindji volcanism.[8]

During the Miaolingian, orogenic events along the Australian-Antarctic margin of Gondwana led to an increase in weathering and an influx of nutrients into the ocean, raising the level of productivity and organic carbon burial. These can be seen in the steady increase in 87Sr/86Sr and δ13C.[48]

Early Furongian

[edit]

Continued erosion of the deeper levels of the Gondwanan mountain belts led to a peak in 87Sr/86Sr and linked positive δ13C and δ34S excursions, known as the Steptoean positive carbon isotope excursion (SPICE).[8] This indicates similar geochemical conditions to Stages 2 and 3 of the early Cambrian existed, with the expansion of seafloor anoxia enhancing the burial rates of organic matter and pyrite.[48] This increase in the extent of anoxic seafloor conditions led to the extinction of the marjumiid and damesellid trilobites, whilst the increase in oxygen levels that followed helped drive the radiation of plankton.[18][9]

87Sr/86Sr fell sharply near the top of the Jiangshanian Stage, and through Stage 10 as the Gondwanan mountains were eroded down and rates of weathering decreased.[18][48]

Magnesium/calcium isotope ratios in seawater

[edit]

The mineralogy of inorganic marine carbonates has varied through the Phanerozoic, controlled by the Mg2+/Ca2+ values of seawater. High Mg2+/Ca2+ result in calcium carbonate precipitation dominated by aragonite and high-magnesium calcite, known as aragonite seas, and low ratios result in calcite seas where low-magnesium calcite is the primary calcium carbonate precipitate.[50] The shells and skeletons of biomineralising organisms reflect the dominant form of calcite.[51]

During the late Ediacaran to early Cambrian increasing oxygen levels led to a decrease in ocean acidity and an increase in the concentration of calcium in sea water. However, there was not a simple transition from aragonite to calcite seas, rather a protracted and variable change through the Cambrian. Aragonite and high-magnesium precipitation continued from the Ediacaran into Cambrian Stage 2. Low-magnesium calcite skeletal hard parts appear in Cambrian Age 2, but inorganic precipitation of aragonite also occurred at this time.[51] Mixed aragonite–calcite seas continued through the middle and late Cambrian, with fully calcite seas not established until the early Ordovician.[51]

These variations and slow decrease in Mg2+/Ca2+ of seawater were due to low oxygen levels, high continental weathering rates and the geochemistry of the Cambrian seas. In conditions of low oxygen and high iron levels, iron substitutes for magnesium in authigenic clay minerals deposited on the ocean floor, slowing the removal rates of magnesium from seawater. The enrichment of ocean waters in silica, prior to the radiation of siliceous organisms, and the limited bioturbation of the anoxic ocean floor increased the rates of deposition, relative to the rest of the Phanerozoic, of these clays. This, together with the high input of magnesium into the oceans via enhanced continental weathering, delayed the reduction in Mg2+/Ca2+ and facilitated continued aragonite precipitation.[50]

The conditions that favoured the deposition of authigenic clays were also ideal for the formation of lagerstätten, with the minerals in the clays replacing the soft body parts of Cambrian organisms.[9]

Flora

[edit]

The Cambrian flora was little different from the Ediacaran. The principal taxa were the marine macroalgae Fuxianospira, Sinocylindra, and Marpolia. No calcareous macroalgae are known from the period.[52]

No land plant (embryophyte) fossils are known from the Cambrian. However, biofilms and microbial mats were well developed on Cambrian tidal flats and beaches 500 mya,[53] and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation.[54][55] Although molecular clock estimates suggest terrestrial plants may have first emerged during the Middle or Late Cambrian, the consequent large-scale removal of the greenhouse gas CO2 from the atmosphere through sequestration did not begin until the Ordovician.[56]

Oceanic life

[edit]
Main article: Cambrian explosion

The Cambrian explosion was a period of rapid multicellular growth. Most animal life during the Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time,[57] but this has proven to be incorrect. Arthropods were by far the most dominant animals in the ocean, but trilobites were only a minor part of the total arthropod diversity. What made them so apparently abundant was their heavy armor reinforced by calcium carbonate (CaCO3), which fossilized far more easily than the fragile chitinous exoskeletons of other arthropods, leaving numerous preserved remains.[58]

The period marked a steep change in the diversity and composition of Earth's biosphere. The Ediacaran biota suffered a mass extinction at the start of the Cambrian Period, which corresponded with an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate which transformed the seabed ecosystems. Before the Cambrian, the sea floor was covered by microbial mats. By the end of the Cambrian, burrowing animals had destroyed the mats in many areas through bioturbation. As a consequence, many of those organisms that were dependent on the mats became extinct, while the other species adapted to the changed environment that now offered new ecological niches.[59] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla, including the Bryozoa,[60] which were once thought to have only appeared in the Lower Ordovician.[61] However, many of those phyla were represented only by stem-group forms; and since mineralized phyla generally have a benthic origin, they may not be a good proxy for (more abundant) non-mineralized phyla.[62]

A reconstruction of Margaretia dorus from the Burgess Shale, which were once believed to be green algae, but are now understood to represent hemichordates[63]

While the early Cambrian showed such diversification that it has been named the Cambrian Explosion, this changed later in the period, when there occurred a sharp drop in biodiversity. About 515 Ma, the number of species going extinct exceeded the number of new species appearing. Five million years later, the number of genera had dropped from an earlier peak of about 600 to just 450. Also, the speciation rate in many groups was reduced to between a fifth and a third of previous levels. 500 Ma, oxygen levels fell dramatically in the oceans, leading to hypoxia, while the level of poisonous hydrogen sulfide simultaneously increased, causing another extinction. The later half of Cambrian was surprisingly barren and showed evidence of several rapid extinction events; the stromatolites which had been replaced by reef building sponges known as Archaeocyatha, returned once more as the archaeocyathids became extinct. This declining trend did not change until the Great Ordovician Biodiversification Event.[64][65]

Artistic reconstruction of Marjum biota, including various arthropods (trilobites, hymenocarines, and radiodonts), sponges, echinoderms, and various other groups

Some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites. Fossil evidence suggests that euthycarcinoids, an extinct group of arthropods, produced at least some of the Protichnites.[66] Fossils of the track-maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusc.[67]

In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;[68] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry.[68]

Many modes of preservation are unique to the Cambrian, and some preserve soft body parts, resulting in an abundance of Lagerstätten. These include Sirius Passet,[69][70] the Sinsk Algal Lens,[71] the Maotianshan Shales,[72] the Emu Bay Shale,[73] and the Burgess Shale.[74][75][76]

Symbol

[edit]

The United States Federal Geographic Data Committee uses a "barred capital C" ⟨Ꞓ⟩ character to represent the Cambrian Period.[77] The Unicode character is U+A792 Ꞓ LATIN CAPITAL LETTER C WITH BAR.[78][79]

[edit]

See also

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References

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Further reading

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Wikisource has original works on the topic: Paleozoic#Cambrian
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