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Chapter 9 Precambrian Earth and Life History—The Proterozoic Eon Proterozoic Rocks, Glacier NP • Proterozoic sedimentary rocks – in Glacier National Park, Montana • The angular peaks, ridges and broad valleys – were carved by Pleistocene and Recent glaciers The Length of the Proterozoic • the Proterozoic Eon alone, – at 1.955 billion years long, – accounts for 42.5% of all geologic time – yet we review this long episode of Earth and life history in a single section The Phanerozoic • Yet the Phanerozoic, – consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, – lasted a comparatively brief 545 million years – is the subject of the rest of the course Disparity in Time • Perhaps this disparity – between the coverage of the Proterozoic and the Phanerozoic – seems disproportionate, • but we know far more – about Phanerozoic events – than we do for either of the Precambrian eons Archean-Proterozoic Boundary • Geologist have rather arbitrarily placed – – – – the Archean-Proterozoic boundary at 2.5 billion years ago because it marks the approximate time of changes in the style of crustal evolution • However, we must emphasize "approximate," – – – – – because Archean-type crustal evolution was largely completed in South Africa nearly 3.0 billion years ago, whereas in North America the change took place from 2.95 to 2.45 billion years ago Style of Crustal Evolution • Archean crust-forming processes generated – granite-gneiss complexes – and greenstone belts – that were shaped into cratons • Although these same rock associations – continued to form during the Proterozoic, – they did so at a considerably reduced rate Contrasting Metamorphism • In addition, Archean and Proterozoic rocks – contrast in metamorphism • Many Archean rocks have been metamorphosed, – although their degree of metamorphism – varies and some are completely unaltered • However, vast exposures of Proterozoic rocks – show little or no effects of metamorphism, – and in many areas they are separated – from Archean rocks by a profound unconformity Other Differences – In addition to changes in the style of crustal evolution, • the Proterozoic is characterized – by widespread rock assemblages • that are rare or absent in the Archean, – by a plate tectonic style essentially the same as that of the present – by important evolution of the atmosphere and biosphere – by the origin of some important mineral resources Proterozoic Evolution of Oxygen-Dependent Organisms • It was during the Proterozoic – that oxygen-dependent organisms – made their appearance • and the first cells evolved – that make up most organisms today Evolution of Proterozoic Continents • Archean cratons assembled during collisions – – – – of island arcs and minicontinents, providing the nuclei around which Proterozoic crust accreted, thereby forming much larger landmasses • Proterozoic accretion at craton margins – probably took place more rapidly than today – because Earth possessed more radiogenic heat, – but the process continues even now Proterozoic Greenstone Belts • Most greenstone belts formed – during the Archean – between 2.7 and 2.5 billion years ago • They also continued to form – during the Proterozoic and at least one is known – from Cambrian-aged rocks in Australia • They were not as common after the Archean, – and differed in one important detail • the near absence of ultramafic rocks • which no doubt resulted from • Earth's decreasing amount of radiogenic heat Focus on Laurentia • Our focus here is on the geologic evolution of Laurentia, – a large landmass that consisted of what is now • North America, • Greenland, • parts of northwestern Scotland, • and perhaps some of the Baltic shield of Scandinavia Early Proterozoic History of Laurentia • Laurentia originated and underwent important growth – between 2.0 and 1.8 billion years ago • During this time, collisions – among various plates formed several orogens, – which are linear or arcuate deformation belts – in which many of the rocks have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths Proterozoic Evolution of Laurentia • Archean cratons were sutured – along deformation belts called orogens, – thereby forming a larger landmass • By 1.8 billion years ago, – much of what is now Greenland, central Canada, – and the north-central United States existed • Laurentia grew along its southern margin – by accretion Craton-Forming Processes • Examples of these craton-forming processes – are recorded in rocks – in the Thelon orogen in northwestern Canada • where the Slave and Rae cratons collided, Craton-Forming Processes • the Trans Hudson orogen • in Canada and the United States, – where the Superior, Hearne, and Wyoming cratons – were sutured • The southern margin of Laurentia – is the site of the Penokian orogen Wilson Cycle • Rocks of the Wopmay orogen – – – – in northwestern Canada are important because they record the opening and closing of an ocean basin or what is called a Wilson cycle • A complete Wilson cycle, • named for the Canadian geologist J. Tuzo Wilson, – involves • • • • fragmentation of a continent, opening followed by closing of an ocean basin, and finally reassembly of the continent Wopmay Orogen • Some of the rocks in Wopmay orogen – are sandstonecarbonate-shale assemblages, – a suite of rocks typical of passive continental margins – that first become widespread during the Proterozoic Early Proterozoic Rocks in Great Lakes Region • Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes Outcrop of Sturgeon Quartzite • The sandstones have a variety of sedimentary structures – such as – ripple marks – and crossbeds – Northern Michigan Outcrop of Kona Dolomite • Some of the carbonate rocks, now mostly dolostone, – such as the Kona Dolomite, – contain abundant bulbous structures known as stromatolites – Northern Michigan Penkean Orogen • These rocks of northern Michigan – have been only moderately deformed – and are now part of the Penokean orogen Accretion along Laurentia’s Southern Margin • Following the initial episode – of amalgamation of Archean cratons • 2.0 to 1.8 billion years ago – accretion took place along Laurentia's southern margin • From 1.8 to 1.6 billion years ago, – continental accretion continued • in what is now the southwestern and central United States – as successively younger belts were sutured to Laurentia, – forming the Yavapai and Mazatzal-Pecos orogens Southern Margin Accretion • Laurentia grew along its southern margin – by accretion of the Central Plains, Yavapai, and Mazatzal orogens • Also notice that the Midcontinental Rift – had formed in the Great Lakes region by this time BIF, Red Beds, Glaciers • This was also the time during which – most of Earth’s banded iron formations (BIF) – were deposited • The first continental red beds – sandstone and shale with oxidized iron – were deposited about 1.8 billion years ago • We will have more to say about BIF – and red beds in the section on “The Evolving Atmosphere” • In addition, some Early Proterozoic rocks – and associated features provide excellent evidence – for widespread glaciation Early and Middle Proterozoic Igneous Activity • During the interval – from 1.8 to 1.1 billion years ago, – extensive igneous activity took place – that seems to be unrelated to orogenic activity • Although quite widespread, – this activity did not add to Laurentia’s size – because magma was either intruded into – or erupted onto already existing continental crust Igneous Activity • These igneous rocks are exposed – in eastern Canada, extend across Greenland, – and are also found in the Baltic shield of Scandinavia Igneous Activity • However, the igneous rocks are deeply buried – by younger rocks in most areas • The origin of these – granitic and anorthosite plutons, • Anorthosite is a plutonic rock composed • almost entirely of plagioclase feldspars – calderas and their fill, – and vast sheets of rhyolite and ash flows – are the subject of debate • According to one hypothesis – large-scale upwelling of magma – beneath a Proterozoic supercontinent – produced the rocks Middle Proterozoic Orogeny and Rifting • The only Middle Proterozoic event in Laurentia – was the Grenville orogeny – in the eastern part of the continent – 1.3 to 1.0 billion years old • Grenville rocks are well exposed – in the present-day northern Appalachian Mountains – as well as in eastern Canada, Greenland, and Scandinavia Grenville Orogeny • A final episode of Proterozoic accretion – occurred during the Grenville orogeny Grenville Orogeny • Many geologists think the Grenville orogen – resulted from closure of an ocean basin, • the final stage in a Wilson cycle • Others disagree and think – intracontinental deformation or major shearing – was responsible for deformation • Whatever the cause of the Grenville orogeny, – it was the final stage – in the Proterozoic continental accretion of Laurentia 75% of North America • By this final stage, about 75% – of present-day North America existed • The remaining 25% – accreted along its margins, – particularly its eastern and western margins, – during the Phanerozoic Eon Midcontinent Rift • Grenville deformation in Laurentia – was accompanied by the origin – of the Midcontinent rift, • a long narrow continental trough bounded by faults, • extending from the Lake Superior basin southwest into Kansas, • and a southeasterly branch extends through Michigan into Ohio • It cuts through Archean and Early Proterozoic rocks – and terminates in the east against rocks – of the Grenville orogen Location of the Midcontinent Rift • Rocks filling the rift – are exposed around Lake Superior – but are deeply buried elsewhere Midcontinental Rift • Most of the rift is buried beneath younger rocks – except in the Lake Superior region – where various igneous and sedimentary rocks – are well exposed • The central part of the rift contains – numerous overlapping basalt lava flows – forming a volcanic pile several kilometers thick • In fact, the volume of volcanic rocks, – between 300,000 and 1,000,000 km3, – is comparable in volume although not areal extent – to the great outpourings of lava during the Cenozoic Midcontinental Rift • Along the rift's margins – coarse-grained sediments were deposited – in large alluvial fans – that grade into sandstone and shale – with increasing distance – from the sediment source • In the vertical section – Freda Sandstone overlies – Cooper Harbor conglomerate, – which overlies Portage Lake Volcanics Cooper Harbor Conglomerate Michigan Portage Lake Volcanics Michigan Middle and Late Proterozoic Sedimentation • Remember the Grenville orogeny – took place 1.2 billion – 900 million years ago, – the final episode of continental accretion – in Laurentia until the Ordovician Period • Nevertheless, important geologic events – – – – were taking place, such as sediment deposition in what is now the eastern United States and Canada, in the Death Valley region of California and Nevada, – and in three huge basins in the west Sedimentary Basins in the West • Map showing the locations of sedimentary Basins – in the western United States and Canada • Belt Basin • Uinta Basin • Apache Basin Sedimentary Rocks • Middle to Late Proterozoic sedimentary rocks – are exceptionally well exposed – in the northern Rocky Mountains – of Montana and Alberta, Canada • Indeed, their colors, deformation features, – and erosion by Pleistocene and recent glaciers – have yielded some fantastic scenery • Like the rocks in the Great Lakes region – and the Grand Canyon, – they are mostly sandstones, shales, – and stromatolite-bearing carbonates Proterozoic Mudrock • Outcrop of red mudrock in Glacier National Park, Montana Proterozoic Limestone • Outcrop of limestone with stromatolites in Glacier National Park, Montana Proterozoic Sandstone • Proterozoic rocks – – – – of the Grand Canyon Super-group lie unconformably upon Archean rocks and in turn are overlain unconformably by Phanerozoic-age rocks • The rocks, consisting mostly – of sandstone, shale, and dolostone, – were deposited in shallow-water marine – and fluvial environments • The presence of stromatolites and carbonaceous – impression of algae in some of these rocks – indicate probable marine deposition Grand Canyon Super-group • Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona Style of Plate Tectonics • The present style of plate tectonics – involving opening and then closing ocean basins – had almost certainly been established by the Early Proterozoic • In fact, the oldest known complete ophiolite – providing evidence for an ancient convergent plate boundary – is the Jormua mafic-ultramafic complex in Finland • It is about 1.96 billion years old, – but nevertheless compares closely in detail – with younger well-documented ophiolites Jormua Complex, Finland • Reconstruction – of the highly deformed – Jormua maficultramafic complex – in Finland • This sequence of rock – is the oldest known complete ophiolite – at 1.96 billion years old Jormua Complex, Finland • Metamorphosed basaltic pillow lava 12 cm Jormua Complex, Finland • Metamorphosed gabbro between mafic dikes 65 cm Proterozoic Supercontinents • You already know that a continent – is one of Earth's landmasses – consisting of granitic crust – with most of its surface above sea level • A supercontinent consists of all – or at least much of the present-day continents, – so other than size it is the same as a continent • The supercontinent Pangaea, – which existed at the end of the Paleozoic Era, – is familiar, – but few people are aware of earlier supercontinents Early Supercontinents • Supercontinents may have existed – as early as the Late Archean, – but if so we have little evidence of them • The first that geologists recognize – with some certainty, known as Rodinia – assembled between 1.3 and 1.0 billion years ago – and then began fragmenting 750 million years ago Early Supercontinent • Possible configuration – of the Late Proterozoic supercontinent Rodinia – before it began fragmenting about 750 million years ago Pannotia • Rodinia's separate pieces reassembled – – – – and formed another supercontinent this one known as Pannotia about 650 million years ago judging by the Pan-African orogeny • the large-scale deformation that took place • in what are now the Southern Hemisphere continents • Fragmentation was underway again, – by the latest Proterozoic, about 550 million years ago, – giving rise to the continental configuration – that existed at the onset of the Phanerozoic Eon Ancient Glaciers • Very few times of widespread glacial activity – have occurred during Earth history • The most recent one during the Pleistocene – 1.6 million to 10,000 years ago – is certainly the best known, – but we also have evidence for Pennsylvanian glaciers – and two major episodes of Proterozoic glaciation Recognizing Glaciation • How can we be sure that there were Proterozoic glaciers? – – – – After all, their most common deposit called tillite is simply a type of conglomerate that may look much like conglomerate that originated by other processes • Tillite or tillite-like deposits are known – from at least 300 Precambrian localities, – and some of these are undoubtedly not glacial deposits Glacial Evidence • But the extensive geographic distribution – of other conglomerates – and their associated glacial features – is distinctive, – such as striated and polished bedrock Proterozoic Glacial Evidence • Bagganjarga tillite in Norway – overlies striated bedrock surface – on sandstone of the Veidnesbotn Formation Geologists Convinced • Geologists are now convinced • based on this kind of evidence – that widespread glaciation – took place during the Early Proterozoic • The occurrence of tillites of about the same age – in Michigan, Wyoming, and Quebec – indicates that North America may have had – an Early Proterozoic ice sheet centered southwest of Hudson Bay Early Proterozoic Glaciers • Deposits in North America – indicate that Laurentia – had an extensive ice sheet – centered southwest of Hudson Bay One or More Glaciations? • Tillites of about this age are also found – – – – in Australia and South Africa, but dating is not precise enough to determine if there was a single widespread glacial episode or a number of glacial events at different times in different areas • One tillite in the Bruce Formation in Ontario, Canada – may date from 2.7 billion years ago, – thus making it Late Archean Glaciers of the Late Proterozoic • Tillites and other glacial features – dating from between 900 and 600 million years ago – are found on all continents except Antarctica • Glaciation was not continuous during this entire time – but was episodic with four major glacial episodes so far recognized Late Proterozoic Glaciers • The approximate distribution of Late Proterozoic glaciers Most Extensive Glaciation in Earth History • The map shows only approximate distribution – of Late Proterozoic glaciers – The actual extent of glaciers is unknown • Not all the glaciers were present at the same time • Despite these uncertainties, – this Late Proterozoic glaciation – was the most extensive in Earth history • In fact, Late Proterozoic glaciers – seem to have been present even – in near-equatorial areas The Evolving Atmosphere • Geologists agree that the Archean atmosphere – contained little or no free oxygen so the atmosphere – was not strongly oxidizing as it is now • Even though processes were underway – – – – that added free oxygen to the atmosphere, the amount present at the beginning of the Proterozoic was probably no more than 1% of that present now • In fact, it might not have exceeded – 10% of present levels even – at the end of the Proterozoic Cyanobacteria and Stromatolites • Remember from our previous discussions – that cyanobacteria, • also known as blue-green algae, – were present during the Archean, – but stromatolites • the structures they formed, – did not become common until about 2.3 billion years ago, • that is, during the Early Proterozoic • These photosynthesizing organisms – and to a lesser degree photochemical dissociation • added free oxygen to the evolving atmosphere Oxygen Versus Carbon Dioxide • Earth's early atmosphere – had abundant carbon dioxide • More oxygen became available – whereas the amount of carbon dioxide decreased • Only a small amount of CO2 – still exists in the atmosphere today • It is one of the greenhouse gases – partly responsible for global warming • What evidence indicates – that the atmosphere became oxidizing? • Where is all that additional the carbon dioxide now? Evidence from Rocks • Much carbon dioxide is now tied up – in various minerals and rocks • especially the carbonate rocks – limestone and dolostone, – and in the biosphere • For evidence that the Proterozoic atmosphere was evolving – from a chemically reducing one – to an oxidizing one • we must discuss types – of Proterozoic sedimentary rocks, in particular – banded iron formations – and red beds Banded Iron Formations (BIF) • Banded iron formations (BIFs), – consist of alternating layers of • iron-rich minerals • and chert – Some are found in Archean rocks, – but about 92% of all BIFs • formed during the interval • from 2.5 to 2.0 billion years ago Early Proterozoic Banded Iron Formation • • • • At this outcrop in Ishpeming, Michigan the rocks are alternating layers of red chert and silvercolored iron minerals Typical BIF • A more typical outcrop of BIF near Nagaunee, Michigan BIFs and the Atmosphere • How are these rocks related to the atmosphere? • Their iron is in iron oxides, especially – hematite (Fe2O3) – and magnetite (Fe3O4) • Iron combines with oxygen in an oxidizing atmosphere – to from rustlike oxides – that are not readily soluble in water • If oxygen is absent in the atmosphere, though, – iron easily dissolves – so that large quantities accumulate in the world's oceans, – which it undoubtedly did during the Archean Formation of BIFs • The Archean atmosphere was deficient in free oxygen • so that little oxygen was dissolved in seawater • However, as photosynthesizing organisms – increased in abundance, • as indicated by stromatolites, – free oxygen, • released as a metabolic waste product into the oceans, – caused the precipitation of iron oxides along with silica – and thus created BIFs Formation of BIFs • One model accounting for the details – of BIF precipitation involves – a Precambrian ocean with an upper oxygenated layer – overlying a large volume of oxygen-deficient water – that contained reduced iron and silica • Upwelling, – – – – that is transfer of water from depth to the surface, brought iron- and silica-rich waters onto the shallow continental shelves and resulting in widespread precipitation of BIFs Formation of BIFs • Depositional model for the origin of banded iron formation Source of Iron and Silica • A likely source of the iron and silica – was submarine volcanism, – similar to that now talking place – at or near spreading ridges • Huge quantities of dissolved minerals are – also discharged at submarine hydrothermal vents • In any case, the iron and silica – combined with oxygen – thus resulting in the precipitation – of huge amounts of banded iron formation • Precipitation continued until – the iron in seawater was largely used up Continental Red Beds • Obviously continental red beds refers – to red rocks on the continents, – but more specifically it means red sandstone or shale – colored by iron oxides, – especially hematite (Fe2O3) Red mudrock in Glacier National Park, Montana Red Beds • Red beds first appear – in the geologic records about 1.8 billion years ago, – increase in abundance throughout the rest of the Proterozoic, – and are quite common in rocks of Phanerozoic age • The onset of red bed deposition – coincides with the introduction of free oxygen – into the Proterozoic atmosphere • However, the atmosphere at that time – may have had only 1% – or perhaps 2% of present levels Red Beds • Is this percentage sufficient to account – for oxidized iron in sediment? • Probably not, – but no ozone (O3) layer existed in the upper atmosphere – before free oxygen (O2) was present • As photosynthesizing organisms released – – – – free oxygen into the atmosphere, ultraviolet radiation converted some of it to elemental oxygen (O) and ozone (O3), both of which oxidize minerals more effectively than O2 Red Beds • Once an ozone layer became established, – – – – most ultraviolet radiation failed to penetrate to the surface, and O2 became the primary agent for oxidizing minerals Important Events in Life History • Archean fossils are not very common, – and all of those known are varieties – of bacteria and cyanobacteria (blue-green algae), – although they undoubtedly existed in profusion • Likewise, the Early Proterozoic fossil record – has mostly bacteria and cyanobacteria • Apparently little diversification – – – – had taken place; all organisms were single-celled prokaryotes, until about 2.1 billion years ago when more complex eukaryotic cells evolved Gunflint Microfossils • Even in well-known Early Proterozoic fossils assemblages, – such as the Gunflint Iron Formation of Canada, – only fossils of bacteria are recognized Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada Prokaryote and Eukaryotes • An organism made up of prokaryotic cells is called a prokaryote – whereas those composed of eukaryotic cells are eukaryotes • In fact, the distinction between prokaryotes and eukaryotes – is the basis for the most profound distinction between all living things Lack of Organic Diversity • Actually, the lack of organic diversity – during this early time in life history – is not too surprising – because prokaryotic cells reproduce asexually • Most variation in – – – – sexually reproducing populations comes from the shuffling of genes, and their alleles, from generation to generation • Mutations introduce new variation into a population, – but their effects are limited in prokaryotes Genetic Variation in Bacteria • A beneficial mutation would spread rapidly – in sexually reproducing organism, – but have a limited impact in bacteria – because they do not share their genes with other bacteria • Bacteria usually reproduce by binary fission – and give rise to two cells – having the same genetic makeup • Under some conditions, – they engage in conjugation during – which some genetic material is transferred Sexual Reproduction Increased the Pace of Evolution • Prior to the appearance of cells capable of sexual reproduction, – evolution was a comparatively slow process, – thus accounting for the low organic diversity • This situation did not persist • Sexually reproducing cells probably – evolved by Early Proterozoic time, – and thereafter the tempo of evolution – increased markedly Eukaryotic Cells Evolve • The appearance of eukaryotic cells – marks a milestone in evolution – comparable to the development • of complex metabolic mechanisms • such as photosynthesis during the Archean • Where did these cells come from? • How do they differ from their predecessors, – the prokaryotic cells? • All prokaryotes are single-celled, – but most eukaryotes are multicelled, – the notable exception being the protistans Eukaryotes • Most eukaryotes reproduce sexually, – in marked contrast to prokaryotes, • and nearly all are aerobic, – that is, they depend on free oxygen – to carry out their metabolic processes • Accordingly, they could not have evolved – before at least some free oxygen was present in the atmosphere Prokaryotic Cell • Prokaryotic cells – do not have a cell nucleus – do not have organelles – are smaller and not nearly as complex as eukaryotic cells Eukaryotic Cell • Eukaryotic cells have – a cell nucleus containing – the genetic material – and organelles – such as mitochondria – and plastids, – as well as chloroplasts in plant cells Eukaryotic Fossil Cells • The Negaunee Iron Formation in Michigan – which is 2.1 billion years old – has yielded fossils now generally accepted – as the oldest known eukaryotic cells • Even though the Bitter Springs Formation – of Australia is much younger • 1 billion years old – it has some remarkable fossils of single-celled eukaryotes – that show evidence of meiosis and mitosis, – processes carried out only by eukaryotic cells Evidence for Eukaryotes • Prokaryotic cells are mostly rather simple – spherical or platelike structures • Eukaryotic cells – are larger, commonly much larger – much more complex – have a well-defined, membrane-bounded cell nucleus, which is lacking in prokaryotes – have several internal structures – called organelles such as plastids and mitochondria – their organizational complexity – is much greater than it is for prokaryotes Acritarchs • Other organisms that were – – – – almost certainly eukaryotes are the acritarchs that first appeared about 1.4 billion years ago they were very common by Late Proterozoic time and were probably cysts of planktonic (floating) algae Acritarchs • These common Late Proterozoic microfossils – are probably from eukaryotic organisms • Acritarchs are very likely the cysts of algae Late Proterozoic Microfossil • Numerous microfossils of organisms – with vase-shaped skeletons – have been found – in Late Proterozoic rocks – in the Grand Canyon • These too have tentatively been identified as – cysts of some kind of algae Endosymbiosis and the Origin of Eukaryotic Cells • Eukaryotic cells probably formed – from several prokaryotic cells – that entered into a symbiotic relationship – Symbiosis, • involving a prolonged association of two or more dissimilar organisms, – is quite common today • In many cases both symbionts benefit from the association – as occurs in lichens, • once thought to be plants • but actually symbiotic fungi and algae Endosymbiosis • In a symbiotic relationship, – – – – each symbiont must be capable of metabolism and reproduction, but in some cases one symbiont cannot live independently • This may have been the case – with Proterozoic symbiotic prokaryotes – that became increasingly interdependent – until the unit could exist only as a whole • In this relationship – one symbiont lived within the other, – which is a special type of symbiosis – called endosymbiosis Evidence for Endosymbiosis • Supporting evidence for endosymbiosis – comes from studies of living eukaryotic cells – containing internal structures called organelles, • such as mitochondria and plastics, – which contain their own genetic material • In addition, prokaryotic cells – synthesize proteins as a single system, • whereas eukaryotic cells – are a combination of protein-synthesizing systems Organelles Capable of Protein Synthesis • That is, some of the organelles – within eukaryotic cells are capable of protein synthesis • These organelles • with their own genetic material • and protein-synthesizing capabilities – are thought to have been free-living bacteria • that entered into a symbiotic relationship, • eventually giving rise to eukaryotic cells Multicelled Organisms • Obviously multicelled organisms – are made up of many cells, – perhaps billions, – as opposed to a single cell as in prokaryotes • In addition, multicelled organisms – – – – have cells specialized to perform specific functions such as respiration, food gathering, and reproduction Dawn of Multicelled Organisms • We know from the fossil record – that multicelled organisms – were present during the Proterozoic, – but we do not know exactly when they appeared • What seem to be some kind of multicelled algae appear – in the 2.1-billion-year-old fossils • from the Negaunee Iron Formation in Michigan – as carbonaceous filaments • from 1.8 billion-year-old rocks in China – as somewhat younger carbonaceous impressions – of filaments and spherical forms Multicelled Algae? • Carbonaceous impressions – in Proterozoic rocks – in the Little Belt Mountains, Montana • These may be impressions of multicelled algae Studies of Present-Day Organisms • How did this important transition taken place? • Perhaps a single-celled organism divided – but the daughter cells formed – an association as a colony • Each cell would have been capable – of an independent existence, – and some cells might have become somewhat specialized • as are the cells of colonial organisms today • Increased specialization of cells – may have given rise to – comparatively simple multicelled organisms – such as algae and sponges The Multicelled Advantage? • Is there any particular advantage to being multicelled? • For something on the order of 1.5 billion years – all organisms were single-celled – and life seems to have thrived • In fact, single-celled organisms – are quite good at what they do – but what they do is very limited The Multicelled Advantage? • For example, single celled organisms – can not grow very large, because as size increases, – proportionately less of a cell is exposed – to the external environment in relation to its volume – and the proportion of surface area decreases • Transferring materials from the exterior – to the interior becomes less efficient The Multicelled Advantage? • Also, multicelled organisms live longer, – since cells can be replaced and more offspring can be produced • Cells have increased functional efficiency – when they are specialized into organs with specific capabilities Late Proterozoic Animals • Biologists set forth criteria such as – – – – method of reproduction and type of metabolism to allow us to easily distinguish between animals and plants • Or so it would seem, – but some present-day organisms – blur this distinction and the same is true – for some Proterozoic fossils • Nevertheless, the first – relatively controversy-free fossils of animals – come from the Ediacaran fauna of Australia – and similar faunas of similar age elsewhere The Ediacaran Fauna • In 1947, an Australian geologist, R.C. Sprigg, – discovered impressions of soft-bodied animals – in the Pound Quartzite in the Ediacara Hills of South Australia • Additional discoveries by others turned up what appeared to be – impressions of algae and several animals – many bearing no resemblance to any existing now • Before these discoveries, geologists – were perplexed by the apparent absence – of fossil-bearing rocks predating the Phanerozoic Ediacaran Fauna • The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm Spriggina floundersi, a possible ancestor of trilobites Ediacaran Fauna Pavancorina minchami • Restoration of the Ediacaran Environment Ediacaran Fauna • Geologists had assumed that – the fossils so common in Cambrian rocks – must have had a long previous history – but had little evidence to support this conclusion • The discovery of Ediacaran fossils and subsequent discoveries – have not answered all questions about prePhanerozoic animals, – but they have certainly increased our knowledge – about this chapter in the history of life Represented Phyla • Three present-day phyla may be represented – in the Ediacaran fauna: • jellyfish and sea pens (phylum Cnidaria), • segmented worms (phylum Annelida), • and primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others) • One Ediacaran fossil, Spriggina, – has been cited as a possible ancestor of trilobites • Another might be a primitive member – of the phylum Echinodermata Distinct Evolutionary Group • However, some scientists think – these Ediacaran animals represent – an early evolutionary group quite distinct from – the ancestry of today’s invertebrate animals • Ediacara-type faunas are known – from all continents except Antarctica, – are collectively referred to as the Ediacaran fauna – were widespread between 545 and 670 million years ago – but their fossils are rare • Their scarcity should not be surprising, though, – because all lacked durable skeletons Other Proterozoic Animal Fossils • Although scarce, a few animal fossils – older than those of the Ediacaran fauna are known • A jellyfish-like impression is present – in rocks 2000 m below the Ediacara Hills Pound Quartzite, • Burrows, in many areas, – presumably made by worms, – occur in rocks at least 700 million years old • Wormlike and algae fossils come – from 700 to 900 million-year-old rocks in China – but the identity and age of these "fossils" has been questioned Wormlike Fossils from China • Wormlike fossils from Late Proterozoic rocks in China Soft Bodies • All known Proterozoic animals were softbodied, – but there is some evidence that the earliest stages in the origin of skeletons was underway • Even some Ediacaran animals – may have had a chitinous carapace – and others appear to have had areas of calcium carbonate • The odd creature known as Kimberella – from the latest Proterozoic of Russia – had a tough outer covering similar to – that of some present-day marine invertebrates Latest Proterozoic Kimberella • Kimberella, an animal from latest Proterozoic rocks in Russia – Exactly what Kimberella was remains uncertain – Some think it was a sluglike creature – whereas others think it was more like a mollusk Durable Skeletons • Latest Proterozoic fossils – of minute scraps of shell-like material – and small tooth like denticles and spicules, • presumably from sponges • indicate that several animals with skeletons – or at least partial skeletons existed • However, more durable skeletons of • silica, • calcium carbonate, • and chitin (a complex organic substance) – did not appear in abundance until the beginning – of the Phanerozoic Eon 545 million years ago Proterozoic Mineral Resources • Most of the world's iron ore comes from – Proterozoic banded iron formations • Canada and the United States have large deposits of these rocks – in the Lake Superior region – and in eastern Canada • Thus, both countries rank among – the ten leading nations in iron ore production Iron Mine • The Empire Mine at Palmer, Michigan – where iron ore from the Early Proterozoic Negaunee Iron Formation is mined Nickel • In the Sudbury mining district in Ontario, Canada, – nickel and platinum are extracted from Proterozoic rocks • Nickel is essential for the production of nickel alloys such as • stainless steel • and Monel metal (nickel plus copper), – which are valued for their strength and resistance to corrosion and heat • The United States must import – more than 50% of all nickel used – mostly from the Sudbury mining district Sudbury Basin • Besides its economic importance, the Sudbury Basin, – an elliptical area measuring more than 59 by 27 km, – is interesting from the geological perspective • One hypothesis for the concentration of ores – is that they were mobilized from metal-rich rocks – beneath the basin – following a high-velocity meteorite impact Platinum and Chromium • Some platinum – – – – for jewelry, surgical instruments, and chemical and electrical equipment is exported to the United States from Canada, but the major exporter is South Africa • The Bushveld Complex of South Africa – is a layered igneous complex containing both • platinum • and chromite – the only ore of chromium, – United States imports much of the chromium – from South Africa – It is used mostly in stainless steel Oil and Gas • Economically recoverable oil and gas – have been discovered in Proterozoic rocks in China and Siberia, – arousing some interest in the Midcontinent rift as a potential source of hydrocarbons • So far, land has been leased for exploration, – and numerous geophysical studies have been done • However, even though some rocks – within the rift are know to contain petroleum, – no producing oil or gas wells are operating Proterozoic Pegmatites • A number of Proterozoic pegmatites – are important economically • The Dunton pegmatite in Maine, – – – – whose age is generally considered to be Late Proterozoic, has yielded magnificent gem-quality specimens of tourmaline and other minerals • Other pegmatites are mined for gemstones as well as for – tin, industrial minerals, such as feldspars, micas, and quartz – and minerals containing such elements – as cesium, rubidium, lithium, and beryllium Proterozoic Pegmatites • Geologists have identified more than 20,000 pegmatites – in the country rocks adjacent – to the Harney Peak Granite – in the Black Hills of South Dakota • These pegmatites formed ~ 1.7 billion years ago – when the granite was emplaced as a complex of dikes and sills • A few have been mined for gemstones, tin, lithium, micas, – and some of the world's largest known – mineral crystals were discovered in these pegmatites Summary • The crust-forming processes – – – – that yielded Archean granite-gneiss complexes and greenstone belts continued into the Proterozoic but at a considerably reduced rate • Archean and Proterozoic greenstone belts – differed in detail • Early Proterozoic collisions – between Archean cratons formed larger cratons – that served as nuclei – around which Proterozoic crust accreted Summary • One such landmass was Laurentia – consisting mostly of North America and Greenland • Important events – in the evolution of Laurentia were • Early Proterozoic amalgamation of cratons • followed by Middle Proterozoic igneous activity, • the Grenville orogeny, and the Midcontinent rift • Ophiolite sequences – – – – marking convergent plate boundaries are first well documented from the Early Proterozoic, indicating that a plate tectonic style similar to that operating now had been established Summary • Sandstone-carbonate-shale assemblages – deposited on passive continental margins – are known from the Archean – but they are very common by Proterozoic time • The supercontinent Rodinia – assembled between 1.3 and 1.0 billion years ago, – fragmented, – and then reassembled to form Pannotia about 650 million years ago • Glaciers were widespread – during both the Early and Late Proterozoic Summary • Photosynthesis continued – to release free oxygen into the atmosphere – which became increasingly oxygen rich through the Proterozoic • Fully 92% of Earth's iron ore deposits – in banded iron formations were deposited – between 2.5 and 2.0 billion years ago • Widespread continental red beds – dating from 1.8 billion years ago indicate – that Earth's atmosphere had enough free oxygen – for oxidation of iron compounds Summary • Most of the known Proterozoic organisms – are single-celled prokaryotes (bacteria) • When eukaryotic cells first appeared is uncertain, – but they may have been present by 2.1 billion years ago • Endosymbiosis is a widely accepted theory for their origin • The oldest known multicelled organisms – are probably algae, – some of which may date back to the Early Proterozoic Summary • Well-documented multicelled animals – are found in several Late Proterozoic localities • Animals were widespread at this time, – but because all lacked durable skeletons – their fossils are not common • Most of the world's iron ore produced – is from Proterozoic banded iron formations • Other important resources – include nickel and platinum