121-458: The Proterozoic ( IPA : / ˌ p r oʊ t ər ə ˈ z oʊ ɪ k , ˌ p r ɒ t -, - ər oʊ -, - t r ə -, - t r oʊ -/ PROH -tər-ə- ZOH -ik, PROT-, -ər-oh-, -trə-, -troh- ) is the third of the four geologic eons of Earth's history , spanning the time interval from 2500 to 538.8 Mya , the longest eon of the Earth's geologic time scale . It is preceded by
242-535: A continental shelf . This classification has been more widely accepted, but the failure to appreciate that it is strictly based on the characteristics of the depositional basin and not the lithology of the BIF itself has led to confusion, and some geologists have advocated for its abandonment. However, the classification into Algoma versus Lake Superior types continues to be used. Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to
363-445: A photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase ) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized
484-483: A pitch trace on a musical scale . Beyond the letters themselves, there are a variety of secondary symbols which aid in transcription. Diacritic marks can be combined with the letters to add tone and phonetic detail such as secondary articulation . There are also special symbols for prosodic features such as stress and intonation. There are two principal types of brackets used to set off (delimit) IPA transcriptions: Less common conventions include: All three of
605-434: A tsunami at least 1,000 m (3,300 ft) high at the point of impact, and 100 m (330 ft) high about 3,000 km (1,900 mi) away. It has been suggested that the immense waves and large underwater landslides triggered by the impact caused the mixing of a previously stratified ocean, oxygenated the deep ocean, and ended BIF deposition shortly after the impact. Although Cloud argued that microbial activity
726-462: A Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 Ma that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron. Banded iron formations of this period are predominantly associated with
847-520: A conflicting use to delimit prosodic transcription within the Voice Quality Symbols , which are an extension of IPA used in extIPA, but are not otherwise used in IPA proper. Other delimiters sometimes seen are pipes and double pipes taken from Americanist phonetic notation . However, these conflict with the pipes used in basic IPA prosodic transcription. Other delimiters are double slashes, –
968-505: A factor of 50 under conditions of low oxygen. Oxygenic photosynthesis is not the only biogenic mechanism for deposition of banded iron formations. Some geochemists have suggested that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs . The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria. Iron isotope ratios in
1089-568: A few billion years in age. It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic . Studies by Condie (2000) and Rino et al. (2004) harvp error: no target: CITEREFRinoKomiyaWindleyet_al2004 ( help ) suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it
1210-657: A few centimeters thick. Many of the chert mesobands contain microbands of iron oxides that are less than a millimeter thick, while the iron mesobands are relatively featureless. BIFs tend to be extremely hard, tough, and dense, making them highly resistant to erosion, and they show fine details of stratification over great distances, suggesting they were deposited in a very low-energy environment; that is, in relatively deep water, undisturbed by wave motion or currents. BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types. Banded iron formations of
1331-428: A few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia). The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first
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#17328373303141452-562: A group of French and English language teachers, led by the French linguist Paul Passy , formed what would be known from 1897 onwards as the International Phonetic Association (in French, l'Association phonétique internationale ). The idea of the alphabet had been suggested to Passy by Otto Jespersen . It was developed by Passy along with other members of the association, principally Daniel Jones . The original IPA alphabet
1573-447: A higher energy depositional environment , in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations. The great majority of banded iron formations are Archean or Paleoproterozoic in age. However, a small number of BIFs are Neoproterozoic in age, and are frequently, if not universally, associated with glacial deposits, often containing glacial dropstones . They also tend to show
1694-721: A higher level of oxidation, with hematite prevailing over magnetite, and they typically contain a small amount of phosphate, about 1% by mass. Mesobanding is often poor to nonexistent and soft-sediment deformation structures are common. This suggests very rapid deposition. However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations. Banded iron formations are distinct from most Phanerozoic ironstones . Ironstones are relatively rare and are thought to have been deposited in marine anoxic events , in which
1815-534: A hydrous silica gel. The conversion of iron hydroxide and silica gels to banded iron formation is an example of diagenesis , the conversion of sediments into solid rock. There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today. The BIFs of the Hamersley Range show great chemical homogeneity and lateral uniformity, with no indication of any precursor rock that might have been altered to
1936-430: A key element of most theories of deposition. The few formations deposited after 1,800 Ma may point to intermittent low levels of free atmospheric oxygen, while the small peak at 750 million years ago may be associated with the hypothetical Snowball Earth. The microbands within chert layers are most likely varves produced by annual variations in oxygen production. Diurnal microbanding would require
2057-452: A mix of IPA with Americanist phonetic notation or Sinological phonetic notation or otherwise use nonstandard symbols for various reasons. Authors who employ such nonstandard use are encouraged to include a chart or other explanation of their choices, which is good practice in general, as linguists differ in their understanding of the exact meaning of IPA symbols and common conventions change over time. Many British dictionaries, including
2178-613: A mode of formation does not require a global anoxic ocean, but is consistent with either a Snowball Earth or Slushball Earth model. Banded iron formations provide most of the iron ore presently mined. More than 60% of global iron reserves are in the form of banded iron formation, most of which can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States. Different mining districts coined their own names for BIFs. The term "banded iron formation"
2299-472: A peculiar kind of Precambrian evaporite . Other proposed abiogenic processes include radiolysis by the radioactive isotope of potassium , K, or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean. Another abiogenic mechanism is photooxidation of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight. However, if
2420-519: A period of increasing crustal recycling, suggesting subduction . Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6 Ga . The occurrence of eclogite (a type of metamorphic rock created by high pressure, > 1 GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor
2541-408: A process that did not produce great quantities of biomass, so that little carbon was present to reduce hematite to magnetite. However, it is possible that BIF was altered from carbonate rock or from hydrothermal mud during late stages of diagenesis. A 2018 study found no evidence that magnetite in BIF formed by decarbonization, and suggests that it formed from thermal decomposition of siderite via
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#17328373303142662-695: A row left out to save space. In the table below, a slightly different arrangement is made: All pulmonic consonants are included in the pulmonic-consonant table, and the vibrants and laterals are separated out so that the rows reflect the common lenition pathway of stop → fricative → approximant , as well as the fact that several letters pull double duty as both fricative and approximant; affricates may then be created by joining stops and fricatives from adjacent cells. Shaded cells represent articulations that are judged to be impossible or not distinctive. Vowel letters are also grouped in pairs—of unrounded and rounded vowel sounds—with these pairs also arranged from front on
2783-800: A thickness of 60 meters (200 feet). Other examples of early Archean BIFs are found in the Abitibi greenstone belts , the greenstone belts of the Yilgarn and Pilbara cratons , the Baltic shield , and the cratons of the Amazon , north China , and south and west Africa. The most extensive banded iron formations belong to what A.F. Trendall calls the Great Gondwana BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus, such as
2904-492: A thin layer on the ocean floor. Each band is similar to a varve , resulting from cyclic variations in oxygen production. Banded iron formations were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia , Brazil , Canada , India , Russia , South Africa , Ukraine , and
3025-485: A twofold division of BIFs into an Algoma type and a Lake Superior type, based on the character of the depositional basin. Algoma BIFs are found in relatively small basins in association with greywackes and other volcanic rocks and are assumed to be associated with volcanic centers. Lake Superior BIFs are found in larger basins in association with black shales, quartzites , and dolomites , with relatively minor tuffs or other volcanic rocks, and are assumed to have formed on
3146-448: A variety of foreign languages. They are also taught by vocal coaches to perfect diction and improve tone quality and tuning. Opera librettos are authoritatively transcribed in IPA, such as Nico Castel 's volumes and Timothy Cheek's book Singing in Czech . Opera singers' ability to read IPA was used by the site Visual Thesaurus , which employed several opera singers "to make recordings for
3267-429: A very high rate of deposition of 2 meters per year or 5 km/Ma. Estimates of deposition rate based on various models of deposition and sensitive high-resolution ion microprobe (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or rhythmites produced by tidal cycles. Preston Cloud proposed that mesobanding
3388-489: Is an alphabetic system of phonetic notation based primarily on the Latin script . It was devised by the International Phonetic Association in the late 19th century as a standard written representation for the sounds of speech . The IPA is used by lexicographers , foreign language students and teachers, linguists , speech–language pathologists , singers, actors, constructed language creators, and translators . The IPA
3509-421: Is designed for transcribing sounds (phones), not phonemes , though it is used for phonemic transcription as well. A few letters that did not indicate specific sounds have been retired (⟨ ˇ ⟩, once used for the "compound" tone of Swedish and Norwegian, and ⟨ ƞ ⟩, once used for the moraic nasal of Japanese), though one remains: ⟨ ɧ ⟩, used for the sj-sound of Swedish. When
3630-483: Is designed to represent those qualities of speech that are part of lexical (and, to a limited extent, prosodic ) sounds in oral language : phones , intonation and the separation of syllables . To represent additional qualities of speech—such as tooth gnashing , lisping , and sounds made with a cleft palate —an extended set of symbols may be used. Segments are transcribed by one or more IPA symbols of two basic types: letters and diacritics . For example,
3751-489: Is elected by the membership – for further discussion and a formal vote. Many users of the alphabet, including the leadership of the Association itself, deviate from its standardized usage. The Journal of the IPA finds it acceptable to mix IPA and extIPA symbols in consonant charts in their articles. (For instance, including the extIPA letter ⟨ 𝼆 ⟩ , rather than ⟨ ʎ̝̊ ⟩, in an illustration of
Proterozoic - Misplaced Pages Continue
3872-411: Is more precisely defined as chemically precipitated sedimentary rock containing greater than 15% iron . However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica. The iron in BIFs is divided roughly equally between the more oxidized ferric form, Fe(III), and the more reduced ferrous form, Fe(II), so that
3993-528: Is not always accessible to sight-impaired readers who rely on screen reader technology. Double angle brackets may occasionally be useful to distinguish original orthography from transliteration, or the idiosyncratic spelling of a manuscript from the normalized orthography of the language. Pipes are sometimes used instead of double angle brackets to denote the distinct allographs of a grapheme that are known as glyphs . For example, print | g | and script | ɡ | are two glyph variants of
4114-425: Is oxidation by anaerobic denitrifying bacteria . This requires that nitrogen fixation by microorganisms is also active. The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition. On the other hand, there is fossil evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition and of hydrocarbon markers in shales within banded iron formation of
4235-427: Is the official chart as posted at the website of the IPA. The letters chosen for the IPA are meant to harmonize with the Latin alphabet . For this reason, most letters are either Latin or Greek , or modifications thereof. Some letters are neither: for example, the letter denoting the glottal stop , ⟨ ʔ ⟩, originally had the form of a question mark with the dot removed. A few letters, such as that of
4356-617: The Archean and followed by the Phanerozoic , and is the most recent part of the Precambrian "supereon". The Proterozoic is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic , Mesoproterozoic and Neoproterozoic . It covers the time from the appearance of free oxygen in Earth's atmosphere to just before the proliferation of complex life on the Earth during
4477-555: The Cambrian Explosion . The name Proterozoic combines two words of Greek origin: protero- meaning "former, earlier", and -zoic , meaning "of life". Well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; the evolution of eukaryotes via symbiogenesis ; several global glaciations , which produced the 300 million years-long Huronian glaciation (during
4598-821: The Great Lakes region and the Frere Formation of western Australia are somewhat different in character and are sometimes described as granular iron formations or GIFs . Their iron sediments are granular to oolitic in character, forming discrete grains about a millimeter in diameter, and they lack microbanding in their chert mesobands. They also show more irregular mesobanding, with indications of ripples and other sedimentary structures , and their mesobands cannot be traced out any great distance. Though they form well-defined, discrete units, these are commonly interbedded with coarse to medium-grained epiclastic sediments (sediments formed by weathering of rock). These features suggest
4719-754: The Hamersley Range . The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world, with a maximum thickness in excess of 900 meters (3,000 feet). Similar BIFs are found in the Carajás Formation of the Amazon craton, the Cauê Itabirite of the São Francisco craton , the Kuruman Iron Formation and Penge Iron Formation of South Africa, and
4840-461: The Handbook recommended against their use, as cursive IPA is "harder for most people to decipher". A braille representation of the IPA for blind or visually impaired professionals and students has also been developed. The International Phonetic Alphabet is occasionally modified by the Association. After each modification, the Association provides an updated simplified presentation of the alphabet in
4961-456: The Hebrew alphabet for transcription of foreign words. Bilingual dictionaries that translate from foreign languages into Russian usually employ the IPA, but monolingual Russian dictionaries occasionally use pronunciation respelling for foreign words. The IPA is more common in bilingual dictionaries, but there are exceptions here too. Mass-market bilingual Czech dictionaries, for instance, tend to use
Proterozoic - Misplaced Pages Continue
5082-567: The IPA extensions . In the IPA itself, however, only lower-case letters are used. The 1949 edition of the IPA handbook indicated that an asterisk ⟨*⟩ might be prefixed to indicate that a word was a proper name, but this convention was not included in the 1999 Handbook , which notes the contrary use of the asterisk as a placeholder for a sound or feature that does not have a symbol. The IPA has widespread use among classical singers during preparation as they are frequently required to sing in
5203-929: The Mulaingiri Formation of India . Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield . The Iron Range is a group of four major deposits: the Mesabi Range , the Vermilion Range , the Gunflint Range , and the Cuyuna Range . All are part of the Animikie Group and were deposited between 2500 and 1800 Ma. These BIFs are predominantly granular iron formations. Neoproterozoic banded iron formations include
5324-747: The Oxford English Dictionary and some learner's dictionaries such as the Oxford Advanced Learner's Dictionary and the Cambridge Advanced Learner's Dictionary , now use the International Phonetic Alphabet to represent the pronunciation of words. However, most American (and some British) volumes use one of a variety of pronunciation respelling systems, intended to be more comfortable for readers of English and to be more acceptable across dialects, without
5445-668: The Siderian and Rhyacian periods of the Paleoproterozoic) and the hypothesized Snowball Earth (during the Cryogenian period in the late Neoproterozoic); and the Ediacaran period (635–538.8 Ma ), which is characterized by the evolution of abundant soft-bodied multicellular organisms such as sponges , algae , cnidarians , bilaterians and the sessile Ediacaran biota (some of which had evolved sexual reproduction ) and provides
5566-563: The Sturtian glaciation . An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones due to glacially-driven thermal overturn. The limited extent of these BIFs compared with the associated glacial deposits, their association with volcanic formations, and variation in thickness and facies favor this hypothesis. Such
5687-546: The United States . A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides , either magnetite (Fe 3 O 4 ) or hematite (Fe 2 O 3 ), alternating with bands of iron-poor chert , often red in color, of similar thickness. A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Banded iron formation
5808-452: The oxygenation of the Earth's oceans . Some of the Earth's oldest rock formations, which formed about 3,700 million years ago ( Ma ), are associated with banded iron formations. Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria . The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming
5929-558: The voiced pharyngeal fricative , ⟨ ʕ ⟩, were inspired by other writing systems (in this case, the Arabic letter ⟨ ﻉ ⟩, ʿayn , via the reversed apostrophe). Some letter forms derive from existing letters: The International Phonetic Alphabet is based on the Latin script , and uses as few non-Latin letters as possible. The Association created the IPA so that the sound values of most letters would correspond to "international usage" (approximately Classical Latin ). Hence,
6050-539: The 150,000 words and phrases in VT's lexical database ... for their vocal stamina, attention to the details of enunciation, and most of all, knowledge of IPA". The International Phonetic Association organizes the letters of the IPA into three categories: pulmonic consonants, non-pulmonic consonants, and vowels. Pulmonic consonant letters are arranged singly or in pairs of voiceless ( tenuis ) and voiced sounds, with these then grouped in columns from front (labial) sounds on
6171-471: The Archean. These older BIFs tend to show a positive europium anomaly consistent with a hydrothermal source of iron. By contrast, Lake Superior-type banded iron formations primarily formed during the Paleoproterozoic era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents. The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into
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#17328373303146292-573: The Ediacaran, proving that multicellular life had already become widespread tens of millions of years before the Cambrian Explosion in what is known as the Avalon Explosion . Nonetheless, the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian , which is currently placed at 538.8 Ma. International Phonetic Alphabet The International Phonetic Alphabet ( IPA )
6413-454: The Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor. Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to free radical damage by oxygen. This also explained the relatively limited extent of early Archean deposits. The great peak in BIF deposition at the end of the Archean
6534-514: The Great Oxygenation Event. Prior to 2.45 billion years ago, the high degree of mass-independent fractionation of sulfur (MIF-S) indicates an extremely oxygen-poor atmosphere. The peak of banded iron formation deposition coincides with the disappearance of the MIF-S signal, which is interpreted as the permanent appearance of oxygen in the atmosphere between 2.41 and 2.35 billion years ago. This
6655-724: The Greek alphabet, though their sound values may differ from Greek. For most Greek letters, subtly different glyph shapes have been devised for the IPA, specifically ⟨ ɑ ⟩, ⟨ ꞵ ⟩, ⟨ ɣ ⟩, ⟨ ɛ ⟩, ⟨ ɸ ⟩, ⟨ ꭓ ⟩ and ⟨ ʋ ⟩, which are encoded in Unicode separately from their parent Greek letters. One, however – ⟨ θ ⟩ – has only its Greek form, while for ⟨ ꞵ ~ β ⟩ and ⟨ ꭓ ~ χ ⟩, both Greek and Latin forms are in common use. The tone letters are not derived from an alphabet, but from
6776-461: The IPA Handbook , the letters ⟨ c ⟩ and ⟨ ɟ ⟩ are used for /t͡ʃ/ and /d͡ʒ/ . Among the symbols of the IPA, 107 letters represent consonants and vowels , 31 diacritics are used to modify these, and 17 additional signs indicate suprasegmental qualities such as length , tone , stress , and intonation . These are organized into a chart; the chart displayed here
6897-566: The IPA have consisted largely of renaming symbols and categories and in modifying typefaces . Extensions to the International Phonetic Alphabet for speech pathology (extIPA) were created in 1990 and were officially adopted by the International Clinical Phonetics and Linguistics Association in 1994. They were substantially revised in 2015. The general principle of the IPA is to provide one letter for each distinctive sound ( speech segment ). This means that: The alphabet
7018-412: The IPA is used for broad phonetic or for phonemic transcription, the letter–sound correspondence can be rather loose. The IPA has recommended that more 'familiar' letters be used when that would not cause ambiguity. For example, ⟨ e ⟩ and ⟨ o ⟩ for [ɛ] and [ɔ] , ⟨ t ⟩ for [t̪] or [ʈ] , ⟨ f ⟩ for [ɸ] , etc. Indeed, in the illustration of Hindi in
7139-667: The IPA only for sounds not found in Czech . IPA letters have been incorporated into the alphabets of various languages, notably via the Africa Alphabet in many sub-Saharan languages such as Hausa , Fula , Akan , Gbe languages , Manding languages , Lingala , etc. Capital case variants have been created for use in these languages. For example, Kabiyè of northern Togo has Ɖ ɖ , Ŋ ŋ , Ɣ ɣ , Ɔ ɔ , Ɛ ɛ , Ʋ ʋ . These, and others, are supported by Unicode , but appear in Latin ranges other than
7260-527: The IPA remained nearly static until the Kiel Convention in 1989, which substantially revamped the alphabet. A smaller revision took place in 1993 with the resurrection of letters for mid central vowels and the retirement of letters for voiceless implosives . The alphabet was last revised in May 2005 with the addition of a letter for a labiodental flap . Apart from the addition and removal of symbols, changes to
7381-532: The IPA.) Of more than 160 IPA symbols, relatively few will be used to transcribe speech in any one language, with various levels of precision. A precise phonetic transcription, in which sounds are specified in detail, is known as a narrow transcription . A coarser transcription with less detail is called a broad transcription. Both are relative terms, and both are generally enclosed in square brackets. Broad phonetic transcriptions may restrict themselves to easily heard details, or only to details that are relevant to
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#17328373303147502-485: The International Phonetic Association. As of the most recent change in 2005, there are 107 segmental letters, an indefinitely large number of suprasegmental letters, 44 diacritics (not counting composites), and four extra-lexical prosodic marks in the IPA. These are illustrated in the current IPA chart , posted below in this article and on the International Phonetic Association's website. In 1886,
7623-665: The Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations. One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen
7744-576: The Palaeoproterozoic or Mesoproterozoic, according to molecular data. Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids , as well as the animal-like Caveasphaera , appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from
7865-481: The Pilbara craton. The carbon that is present in banded iron formations is enriched in the light isotope, C, an indicator of a biological origin. If a substantial part of the original iron oxides was in the form of hematite, then any carbon in the sediments might have been oxidized by the decarbonization reaction: Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be
7986-570: The Precambrian world, they have been intensively studied by geologists. Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with greenstone belts and include the BIFs of the Isua Greenstone Belt , the oldest known, which have an estimated age of 3700 to 3800 Ma. The Temagami banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached
8107-613: The Urucum in Brazil, Rapitan in the Yukon , and the Damara Belt in southern Africa. They are relatively limited in size, with horizontal extents not more than a few tens of kilometers and thicknesses not more than about 10 meters (33 feet). These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the " Snowball Earth ." Banded iron formation provided some of
8228-424: The above are provided by the IPA Handbook . The following are not, but may be seen in IPA transcription or in associated material (especially angle brackets): Also commonly seen are the braces of set theory , especially when enclosing the set of phonemes that constitute the morphophoneme, e.g. {t d} or {t|d} or {/t/, /d/} for a conflated /t/ and /d/ . Braces have
8349-598: The alphabet or the chart is to propose the change in the Journal of the IPA . (See, for example, December 2008 on an open central unrounded vowel and August 2011 on central approximants.) Reactions to the proposal may be published in the same or subsequent issues of the Journal (as in August 2009 on the open central vowel). A formal proposal is then put to the Council of the IPA – which
8470-424: The availability of reduced iron on time scales of decades. In the case of granular iron formations, the mesobands are attributed to winnowing of sediments in shallow water, in which wave action tended to segregate particles of different size and composition. For banded iron formations to be deposited, several preconditions must be met. There must be an ample source of reduced iron that can circulate freely into
8591-478: The breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny . Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are
8712-453: The conceptual counterparts of spoken sounds, are usually enclosed in slashes (/ /) and tend to use simpler letters with few diacritics. The choice of IPA letters may reflect theoretical claims of how speakers conceptualize sounds as phonemes or they may be merely a convenience for typesetting. Phonemic approximations between slashes do not have absolute sound values. For instance, in English, either
8833-934: The consonant is produced. The main chart includes only consonants with a single place of articulation. Notes Non-pulmonic consonants are sounds whose airflow is not dependent on the lungs. These include clicks (found in the Khoisan languages and some neighboring Bantu languages of Africa), implosives (found in languages such as Sindhi , Hausa , Swahili and Vietnamese ), and ejectives (found in many Amerindian and Caucasian languages ). Banded iron formation Banded iron formations ( BIFs ; also called banded ironstone formations ) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert . They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record
8954-496: The consonant letters ⟨ b ⟩, ⟨ d ⟩, ⟨ f ⟩, ⟨ ɡ ⟩, ⟨ h ⟩, ⟨ k ⟩, ⟨ l ⟩, ⟨ m ⟩, ⟨ n ⟩, ⟨ p ⟩, ⟨ s ⟩, ⟨ t ⟩, ⟨ v ⟩, ⟨ w ⟩, and ⟨ z ⟩ have more or less their word-initial values in English ( g as in gill , h as in hill , though p t k are unaspirated as in spill, still, skill ); and
9075-533: The current composition. This suggests that, other than dehydration and decarbonization of the original ferric hydroxide and silica gels, diagenesis likely left the composition unaltered and consisted of crystallization of the original gels. Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations. The relatively high content of hematite in Neoproterozoic BIFs suggests they were deposited very quickly and via
9196-461: The deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles ( Wilson cycle ). In
9317-417: The deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. Heinrich Holland argues that the absence of manganese deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "Canfield ocean" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron
9438-472: The deep ocean or a lack of dissimilatory sulfate reduction (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite. The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that
9559-412: The deposition basin. Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins. The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits. Algoma-type BIFs formed primarily in
9680-551: The depositional basin became depleted in free oxygen . They are composed of iron silicates and oxides without appreciable chert but with significant phosphorus content, which is lacking in BIFs. No classification scheme for banded iron formations has gained complete acceptance. In 1954, Harold Lloyd James advocated a classification based on four lithological facies (oxide, carbonate, silicate, and sulfide) assumed to represent different depths of deposition, but this speculative model did not hold up. In 1980, Gordon A. Gross advocated
9801-709: The discussion at hand, and may differ little if at all from phonemic transcriptions, but they make no theoretical claim that all the distinctions transcribed are necessarily meaningful in the language. For example, the English word little may be transcribed broadly as [ˈlɪtəl] , approximately describing many pronunciations. A narrower transcription may focus on individual or dialectical details: [ˈɫɪɾɫ] in General American , [ˈlɪʔo] in Cockney , or [ˈɫɪːɫ] in Southern US English . Phonemic transcriptions, which express
9922-603: The end of words. the alternations /f/ – /v/ in plural formation in one class of nouns, as in knife /naɪf/ – knives /naɪvz/ , which can be represented morphophonemically as {naɪV } – {naɪV+z }. The morphophoneme {V } stands for the phoneme set {/f/, /v/ }. [ˈf\faɪnəlz ˈhɛld ɪn (.) ⸨knock on door⸩ bɑɹsə{ 𝑝 ˈloʊnə and ˈmədɹɪd 𝑝 }] — f-finals held in Barcelona and Madrid. IPA letters have cursive forms designed for use in manuscripts and when taking field notes, but
10043-423: The eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity ( orogeny ). There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during
10164-492: The first evidence for the timing of the Great Oxidation Event , 2,400 Ma. With his 1968 paper on the early atmosphere and oceans of the Earth, Preston Cloud established the general framework that has been widely, if not universally, accepted for understanding the deposition of BIFs. Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into
10285-658: The first half of the Ediacaran from 0.63–0.55 Ga, and the Belomorian, spanning from 0.55–0.542 Ga. The emergence of advanced single-celled eukaryotes began after the Oxygen Catastrophe . This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria . It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved. By
10406-459: The first obvious fossil evidence of life on Earth . The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas ; furthermore, many of those rocks are less metamorphosed than Archean rocks, and many are unaltered. Studies of these rocks have shown that
10527-403: The form of a chart. (See History of the IPA .) Not all aspects of the alphabet can be accommodated in a chart of the size published by the IPA. The alveolo-palatal and epiglottal consonants, for example, are not included in the consonant chart for reasons of space rather than of theory (two additional columns would be required, one between the retroflex and palatal columns and the other between
10648-418: The formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to
10769-412: The grapheme ⟨ g ⟩ of Latin script. Some examples of contrasting brackets in the literature: In some English accents, the phoneme /l/ , which is usually spelled as ⟨l⟩ or ⟨ll⟩ , is articulated as two distinct allophones: the clear [l] occurs before vowels and the consonant /j/ , whereas the dark [ɫ] / [lˠ] occurs before consonants, except /j/ , and at
10890-438: The implication of a preferred pronunciation that the IPA might convey. For example, the respelling systems in many American dictionaries (such as Merriam-Webster ) use ⟨y⟩ for IPA [ j] and ⟨sh⟩ for IPA [ ʃ ] , reflecting the usual spelling of those sounds in English. (In IPA, [y] represents the sound of the French ⟨u⟩ , as in tu , and [sh] represents
11011-442: The iron came from a shallow hydrothermal source, other laboratory experiments suggest that precipitation of ferrous iron as carbonates or silicates could seriously compete with photooxidation. Regardless of the precise mechanism of oxidation, the oxidation of ferrous to ferric iron likely caused the iron to precipitate out as a ferric hydroxide gel. Similarly, the silica component of the banded iron formations likely precipitated as
11132-554: The late Archean (2800–2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrian formation in western China. Because the processes by which BIFs are formed appear to be restricted to early geologic time, and may reflect unique conditions of
11253-441: The late Archean peak of BIF deposition was spread out over tens of millions of years, rather than taking place in a very short interval of time following the evolution of oxygen-coping mechanisms. However, his general concepts continue to shape thinking about the origins of banded iron formations. In particular, the concept of the upwelling of deep ocean water, rich in reduced iron, into an oxygenated surface layer poor in iron remains
11374-652: The late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse. The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria – in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic Era, some 2.4 billion years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis . The Viridiplantae evolved sometime in
11495-601: The late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia . An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after
11616-454: The left to back (glottal) sounds on the right. In official publications by the IPA, two columns are omitted to save space, with the letters listed among "other symbols" even though theoretically they belong in the main chart. They are arranged in rows from full closure (occlusives: stops and nasals) at top, to brief closure (vibrants: trills and taps), to partial closure (fricatives), and finally minimal closure (approximants) at bottom, again with
11737-428: The left to back on the right, and from maximal closure at top to minimal closure at bottom. No vowel letters are omitted from the chart, though in the past some of the mid central vowels were listed among the "other symbols". A pulmonic consonant is a consonant made by obstructing the glottis (the space between the vocal folds) or oral cavity (the mouth) and either simultaneously or subsequently letting out air from
11858-408: The lungs. Pulmonic consonants make up the majority of consonants in the IPA, as well as in human language. All consonants in English fall into this category. The pulmonic consonant table, which includes most consonants, is arranged in rows that designate manner of articulation , meaning how the consonant is produced, and columns that designate place of articulation , meaning where in the vocal tract
11979-592: The oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (<0.001% of modern O 2 levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II): This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread. By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period. An alternate route
12100-469: The pharyngeal and glottal columns), and the lateral flap would require an additional row for that single consonant, so they are listed instead under the catchall block of "other symbols". The indefinitely large number of tone letters would make a full accounting impractical even on a larger page, and only a few examples are shown, and even the tone diacritics are not complete; the reversed tone letters are not illustrated at all. The procedure for modifying
12221-587: The ratio Fe(III)/Fe(II+III) typically varies from 0.3 to 0.6. This indicates a predominance of magnetite, in which the ratio is 0.67, over hematite, for which the ratio is 1. In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates siderite and ankerite , or the iron-rich silicates minnesotaite and greenalite . Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate, though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively. When used in
12342-500: The reaction The iron may have originally precipitated as greenalite and other iron silicates. Macrobanding is then interpreted as a product of compaction of the original iron silicate mud. This produced siderite-rich bands that served as pathways for fluid flow and formation of magnetite. The peak of deposition of banded iron formations in the late Archean, and the end of deposition in the Orosirian, have been interpreted as markers for
12463-427: The same notation as for morphophonology, – exclamation marks, and pipes. For example, ⟨ cot ⟩ would be used for the orthography of the English word cot , as opposed to its pronunciation /ˈkɒt/ . Italics are usual when words are written as themselves (as with cot in the previous sentence) rather than to specifically note their orthography. However, italics are sometimes ambiguous, and italic markup
12584-469: The sequence of consonants in gra ssh opper .) The IPA is also not universal among dictionaries in languages other than English. Monolingual dictionaries of languages with phonemic orthographies generally do not bother with indicating the pronunciation of most words, and tend to use respelling systems for words with unexpected pronunciations. Dictionaries produced in Israel use the IPA rarely and sometimes use
12705-528: The singular, the term banded iron formation refers to the sedimentary lithology just described. The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation. A well-preserved banded iron formation typically consists of macrobands several meters thick that are separated by thin shale beds. The macrobands in turn are composed of characteristic alternating layers of chert and iron oxides, called mesobands , that are several millimeters to
12826-467: The sound of the English digraph ⟨ch⟩ may be transcribed in IPA with a single letter: [c] , or with multiple letters plus diacritics: [t̠̺͡ʃʰ] , depending on how precise one wishes to be. Slashes are used to signal phonemic transcription ; therefore, /tʃ/ is more abstract than either [t̠̺͡ʃʰ] or [c] and might refer to either, depending on the context and language. Occasionally, letters or diacritics are added, removed, or modified by
12947-546: The time was virtually all obligate anaerobic . A second, later surge in oxygen concentrations is called the Neoproterozoic Oxygenation Event , occurred during the Middle and Late Neoproterozoic and drove the rapid evolution of multicellular life towards the end of the era. The Proterozoic Eon was a very tectonically active period in the Earth's history. The late Archean Eon to Early Proterozoic Eon corresponds to
13068-457: The vowel letters ⟨ a ⟩, ⟨ e ⟩, ⟨ i ⟩, ⟨ o ⟩, ⟨ u ⟩ correspond to the (long) sound values of Latin: [i] is like the vowel in mach i ne , [u] is as in r u le , etc. Other Latin letters, particularly ⟨ j ⟩, ⟨ r ⟩ and ⟨ y ⟩, differ from English, but have their IPA values in Latin or other European languages. This basic Latin inventory
13189-463: The vowel of pick or the vowel of peak may be transcribed as /i/ , so that pick , peak would be transcribed as /ˈpik, ˈpiːk/ or as /ˈpɪk, ˈpik/ ; and neither is identical to the vowel of the French pique , which would also be transcribed /pik/ . By contrast, a narrow phonetic transcription of pick , peak , pique could be: [pʰɪk] , [pʰiːk] , [pikʲ] . IPA is popular for transcription by linguists. Some American linguists, however, use
13310-529: Was a key process in the deposition of banded iron formation, the role of oxygenic versus anoxygenic photosynthesis continues to be debated, and nonbiogenic processes have also been proposed. Cloud's original hypothesis was that ferrous iron was oxidized in a straightforward manner by molecular oxygen present in the water: The oxygen comes from the photosynthetic activities of cyanobacteria. Oxidation of ferrous iron may have been hastened by aerobic iron-oxidizing bacteria, which can increase rates of oxidation by
13431-406: Was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted. Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments. Another theory is that mesobands are primary structures resulting from pulses of activity along mid-ocean ridges that change
13552-565: Was accompanied by the development of a stratified ocean with a deep anoxic layer and a shallow oxidized layer. The end of deposition of BIF at 1.85 billion years ago is attributed to the oxidation of the deep ocean. Until 1992 it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The Snowball Earth hypothesis provided an alternative explanation for these younger deposits. In
13673-602: Was based on the Romic alphabet , an English spelling reform created by Henry Sweet that in turn was based on the Palaeotype alphabet of Alexander John Ellis , but to make it usable for other languages the values of the symbols were allowed to vary from language to language. For example, the sound [ ʃ ] (the sh in shoe ) was originally represented with the letter ⟨c⟩ for English but with ⟨x⟩ for French and German; with German, ⟨c⟩
13794-472: Was blocked by precipitation as pyrite. Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the Sudbury Basin impact. An asteroid (estimated at 10 km (6.2 mi) across) impacted into waters about 1,000 m (3,300 ft) deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated
13915-443: Was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period. Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed
14036-446: Was extended by adding small-capital and cursive forms, diacritics and rotation. The sound values of these letters are related to those of the original letters, and their derivation may be iconic. For example, letters with a rightward-facing hook at the bottom represent retroflex equivalents of the source letters, and small capital letters usually represent uvular equivalents of their source letters. There are also several letters from
14157-499: Was probably due to two factors: Exhaustion of the chemical sinks, and an increase in carbon sequestration , which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the Great Oxygenation Event , or alternately the Oxygen Catastrophe – to reflect the mass extinction of almost all life on Earth, which at
14278-481: Was probably only 1% to 2% of its current level. The banded iron formations , which provide most of the world's iron ore , are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized . Red beds , which are colored by hematite , indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks. The oxygen buildup
14399-725: Was the Labradorian, lasting from 2.0–1.65 Ga . It was followed by the Anabarian, which lasted from 1.65–1.2 Ga and was itself followed by the Turukhanian from 1.2–1.03 Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85 Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63 Ga. The final two zones were the Amadeusian, spanning
14520-432: Was thought to be the result of the evolution of mechanisms for living with oxygen. This ended self-poisoning and produced a population explosion in the cyanobacteria that rapidly depleted the remaining supply of reduced iron and ended most BIF deposition. Oxygen then began to accumulate in the atmosphere. Some details of Cloud's original model were abandoned. For example, improved dating of Precambrian strata has shown that
14641-402: Was used for the [ x ] sound of Bach . With a growing number of transcribed languages this proved impractical, and in 1888 the values of the letters were made uniform across languages. This would provide the base for all future revisions. Since its creation, the IPA has undergone a number of revisions. After relatively frequent revisions and expansions from the 1890s to the 1940s,
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