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Cambrian page

I've added sections and edited others on the Cambrian page and I've 2 requests: One, can someone with more coding nous than me add a ICS (global) subdivisions table cf. Ordovician or Carboniferous please. Gives the info nice and concisely; and two, there are at least two other Cambrian pages that are now obsolete Stratigraphy of the Cambrian and Early Cambrian geochemical fluctuations. Again, could someone with more knowledge of Wikipedian ways delete them. Just to tidy things up. Thx

Most of the continents lay in the southern hemisphere surrounded by the vast Panthalassic Ocean.  (Torsvik and Cocks) The assembly of Gondwana during the Ediacaran and early Cambrian led to the development of new convergent plate margins and continental-margin arc magmatism along its margins that helped drive global warming. (43) Myrow et al, 2024 Laurentia lay across the equator, separated from Gondwana by the opening Iapetus Ocean. (33) Torsvik and Cocks

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 lead to extinction events, whilst periods of raised oxygenation led to increased biodiversity. (43) Pruss and Gill, 2024

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 organisms with mineralised hard parts are found in the rock record, and the rapid diversification of lifeforms, known as the Cambrian explosion, produced the first representatives of most modern animal phyla. 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.

By the end of the Cambrian, myriapods, arachnids, and hexapods started adapting to the land, along with the first plants.

T pedum at base of Cambrian

Geochemistry

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. (5) 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. (1, 2, 11)

Isotope excursions

Base of Cambrian

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. (1, 5)

Ediacaran-Cambrian boundary section at Fortune Head, Newfoundland, GSSP

Cambrian Stages 2 and 3

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. (12) 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. (5, 10) These pulses of oxygen are associated with the radiation of the small shelly fossils and the Cambrian arthropod radiation isotope excursion (CARE). (12) 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. (5, 10)

Cambrian Stage 4 to early Miaolingian

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. (10, 11) 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. (5, 12)

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. (11)

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. (12)

Early Furongian

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). (11) 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. (12) 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. (1, 5)

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. (5, 12)

Magnesium/calcium isotope ratios in seawater

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. (9) The shells and skeletons of biomineralising organisms reflect the dominant form of calcite. (8)

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. (9) Mixed aragonite–calcite seas continued through the middle and late Cambrian, with fully calcite seas not established until the early Ordovician. (8)

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. (9)

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. (1)

Etymology and history

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. (1) 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.(3)

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 which 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. 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. (3)

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. (1)

Stratigraphy

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.

Terreneuvian

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. (1) 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. (4)

Cambrian Series 2

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. (1) 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. (4)

Miaolingian

The Miaolingian is the third series/epoch of the Cambrian, lasting from c. 509 Ma to c. 497 Ma. 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. (1) The name replaces Cambrian Series 3 and was ratified by the IUGS in 2018. (2) 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. (2) Unlike the Terreneuvian and Series 2, all the stages of the Miaolingian are defined by GSSPs. (1)

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. (4)

Furongian

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. (4)

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. (1)

Cambrian–Ordovician boundary

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. (5)

Paleogeography

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. (10) 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. (10)

Most models show Gondwana stretching from the south polar region to north of the equator. (1) 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. (10)

Laurentia lay across the equator, separated from Gondwana by the Iapetus Ocean. (1) 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. (11) 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. (1)

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. (1,9) 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. (1)

Laurentia

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. (1, 8)

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. (1)

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. (8) 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. (8)

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. (8)

Gondwana

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. (1)

The cratons that formed Gondwana came together during the Neoproterozoic to early Cambrian. A narrow ocean separated Amazonia from Gondwana until c. 530 Ma (7) and the Arequipa-Antofalla block united with the South American sector of Gondwana in the early Cambrian. (1) 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. (4)

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. (1)

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. (8)

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. (2, 6) This subduction extended west along the Gondwanan margin and by c. 530 Ma may have evolved into a major transform fault system. (6)

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. (6)

Ganderia, East and West Avalonia, Carolinia and Meguma Terranes

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. (8,10) 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. (8) 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. (10,13)

Baltica

During the Cambrian, Baltica rotated more than 60° anti-clockwise and began to drift northwards. (8) This rotation was accommodated by major strike-slip movements in the Ran Ocean between it and Gondwana. (1)

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. (1,8) 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. (1)

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. (3, 12)

Its southeastern margin was also a convergent boundary, with the accretion of island arcs and microcontinents to the craton, although the details are unclear. (1)

Siberia

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. (1,9) 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. (1)

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. (1, 12) 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. (10)

Along the then western margin, Late Neoproterozoic to early Cambrian rifting was followed by the development of a passive margin. (12)

To the then north, Siberia was separated from the Central Mongolian terrane by the narrow and slowly opening Mongolian-Okhotsk Ocean. (1) The Central Mongolian terrane's northern margin with the Panthalassa was convergent, whilst its southern margin facing the Mongolian-Okhotsk Ocean was passive. (1)

Central Asia

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. (12)

To the south of these the Tarim microcontinent lay between Gondwana and Siberia. (1) 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. (12) 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. (12)

North China

North China lay at equatorial to tropical latitudes during the early Cambrian, although its exact position is unknown. (9) Much of the craton was covered by shallow seas, with land in the northwest and southeast. (1)

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). (12)

South China and Annamia

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. (1)

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

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. (1)

Climate

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. (1,4) During the late Cambrian the distribution of trilobite provinces also indicate only a moderate pole-to-equator temperature gradient. (1) 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. (4)

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. (1,10) 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. (7, GC11) 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. (GC11)

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. (1, 3) Estimates for tropical sea surface temperatures vary from c. 28-32°C (82 - 90°F), (1, 3) to c. 29-38°C (84 - 100°F). (4, 9) Modern average tropical sea surface temperatures are 26°C (79°F). (1)

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. (2, 7)

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. (5) 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. (6, 7)

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. (2, 5, 6)

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. (5, 6, 7)

Boundaries

The base of the Paleozoic is one of the major divisions in geological time representing the divide between the Proterozoic and Phanerozoic eons, the Paleozoic and Neoproterozoic eras and the Ediacaran and Cambrian periods (1). When Adam Sedgwick named the Paleozoic in 1835, he defined the base as the first appearance of complex life in the rock record as shown by the presence of trilobite-dominated fauna (x). Since then evidence of complex life in older rock sequences has increased and by the second half of the 20th century, the first appearance of small shelly fauna (SSF), also known as early skeletal fossils, were considered markers for the base of the Paleozoic. However, whilst SSF are well preserved in carbonate sediments, the majority of Ediacaran to Cambrian rock sequences are composed of siliciclastic rocks where skeletal fossils are rarely preserved (1). This led the International Commission on Stratigraphy (ICS) to use trace fossils as an indicator of complex life (2). Unlike later in the fossil record, Cambrian trace fossils are preserved in a wide range of sediments and environments, which aids correlation between different sites around the world. Trace fossils reflect the complexity of the body plan of the organism that made them. Ediacaran trace fossils are simple, sub-horizontal feeding traces.  As more complex organisms evolved, their more complex behaviour was reflected in greater diversity and complexity of the trace fossils they left behind (1). After two decades of deliberation, the ICS chose Fortune Head, Burin Peninsula, Newfoundland as the basal Cambrian Global Stratotype Section and Point (GSSP) at the base of the Treptichnus pedum assemblage of trace fossils and immediately above the last occurrence of the Ediacaran problematica fossils Harlaniella podolica and Palaeopsacichnus (3). The base of the Phanerozoic, Paleozoic and Cambrian is dated at 538.8+/-0.2 Ma and now lies below both the first appearance of trilobites and SSF (1 and 3).

The boundary between the Paleozoic and Mesozoic eras and the Permian and Triassic periods is marked by the first occurrence of the conodont Hindeodus parvus. This is the first biostratigraphic event found worldwide that is associated with the beginning of the recovery following the end-Permian mass extinctions and environmental changes. In non-marine strata, the equivalent level is marked by the disappearance of the Permian Dicynodon tetrapods (4).  This means events previously considered to mark the Permian-Triassic boundary, such as the eruption of the Siberian Traps flood basalts, the onset of greenhouse climate, ocean anoxia and acidification and the resulting mass extinction are now regarded as being of latest Permian in age (4). The GSSP is near Meishan, Zhejiang Province, southern China.  Radiometric dating of volcanic clay layers just above and below the boundary confine its age to a narrow range of 251.902+/-0.024 Ma (4).

The Paleozoic (IPA: /ˌpæli.əˈzoʊ.ɪk,-i.oʊ-, ˌpeɪ-/ pal-ee-ə-ZOH-ik, -⁠ee-oh-, pay-;) (or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of the Mesozoic Era. It is divided into Early and Late sub-Eras: The Early Paleozoic consists of the Cambrian, Ordovician and Silurian; the Late Paleozoic consists of the Devonian, Carboniferous and Permian.

The name Paleozoic was first used by Adam Sedgwick (1785-1873) in 1838 to described the Cambrian and Ordovician periods. It was redefined by John Phillips (1800–1874) in 1840 to in cover the Cambrian to Permian periods. It is derived from the Greek palaiós (παλαιός, "old") and zōḗ (ζωή, "life") meaning "ancient life".