• Proterozoic sedimentary rocks – in Glacier National Park, Montana

• The angular peaks, ridges and broad valleys – were carved by Pleistocene and Recent glaciers

Proterozoic Rocks, Glacier NP

• 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 Length of the Proterozoic

• 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

The Phanerozoic

• 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

Disparity in Time

• 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

Archean-Proterozoic Boundary

• 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

Style of Crustal Evolution

• 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

Contrasting Metamorphism

– 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

Other Differences

• It was during the Proterozoic – that oxygen-dependent organisms – made their appearance

• and the first cells evolved – that make up most organisms today

Proterozoic Evolution of Oxygen-Dependent

Organisms

• 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

Evolution of Proterozoic Continents

• 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

Proterozoic Greenstone Belts

• 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

Focus on 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

Early Proterozoic History of Laurentia

Proterozoic Evolution of Laurentia

• Laurentia grew along its southern margin – by accretion

• 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

• 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

Craton-Forming Processes

• 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

Wilson Cycle

• Some of the rocks in Wopmay orogen– are sandstone-

carbonate-shale assemblages,

– a suite of rocks typical of passive continental margins

– that first become widespread during the Proterozoic

Wopmay Orogen

• Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

Early Proterozoic Rocks in Great Lakes Region

• The sandstones have a variety of sedimentary structures – such as – ripple

marks – and

cross-beds

– Northern Michigan

Outcrop of Sturgeon Quartzite

• Some of the carbonate rocks, now mostly dolostone, – such as the Kona Dolomite, – contain

abundant bulbous structures known as stromatolites

– NorthernMichigan

Outcrop of Kona Dolomite

• These rocks of northern Michigan – have been only moderately deformed – and are now part

of the Penokean orogen

Penkean Orogen

• 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

Accretion along Laurentia’s Southern Margin

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

• 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

BIF, Red Beds, Glaciers

• 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

Early and Middle Proterozoic 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

Igneous Activity

• 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

Middle Proterozoic Orogeny and Rifting

• 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

Grenville Orogeny

• 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

75% of North America

• 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

Midcontinent Rift

• Rocks filling the rift – are

exposed around Lake Superior

– but are deeply buried elsewhere

Location of the Midcontinent 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

Midcontinental Rift

Michigan

Cooper Harbor Conglomerate

Michigan

Portage Lake Volcanics

• 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

Middle and Late Proterozoic Sedimentation

• Map showing the locations of sedimentary Basins – in the western United

States and Canada• Belt Basin• Uinta Basin• Apache Basin

Sedimentary Basins in the

West

• 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

Sedimentary Rocks

• Outcrop of red mudrock in Glacier National Park, Montana

Proterozoic Mudrock

• Outcrop of limestone with stromatolites in Glacier National Park, Montana

Proterozoic Limestone

• 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

Proterozoic Sandstone

• Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona

Grand Canyon Super-group

• 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

Style of Plate Tectonics

• Reconstruction – of the highly

deformed – Jormua mafic-

ultramafic complex – in Finland

• This sequence of rock – is the oldest known

complete ophiolite – at 1.96 billion

years old

Jormua Complex, Finland

Jormua Complex, Finland

• Metamorphosed basaltic pillow lava

12 cm

• Metamorphosed gabbro between mafic dikes

Jormua Complex, Finland

65 cm

• 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

Proterozoic 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 Supercontinents

• Possible configuration – of the Late

Proterozoic supercontinent Rodinia

– before it began fragmenting about 750 million years ago

Early Supercontinent

• 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

Pannotia

• 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

Ancient Glaciers

• 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

Recognizing Glaciation

• But the extensive geographic distribution – of other conglomerates

– and their associated glacial features

– is distinctive,

– such as striated and polished bedrock

Glacial Evidence

• Bagganjarga tillite in Norway– overlies striated bedrock surface – on sandstone of the Veidnesbotn Formation

Proterozoic Glacial Evidence

• 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

Geologists Convinced

• Deposits in North America– indicate that

Laurentia – had an

extensive ice sheet

– centered southwest of Hudson Bay

Early Proterozoic Glaciers

• 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

One or More Glaciations?

• 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

Glaciers of the Late Proterozoic

• The approximate distribution of Late Proterozoic glaciers

Late Proterozoic Glaciers

• 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

Most Extensive Glaciation in Earth History

• 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

The Evolving Atmosphere

• 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

Cyanobacteria and Stromatolites

• 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?

Oxygen Versus Carbon Dioxide

• 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

Evidence from Rocks

• 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

Banded Iron Formations (BIF)

• At this outcrop in Ishpeming, Michigan • the rocks are alternating layers of • red chert • and

silver-colorediron minerals

Early Proterozoic Banded Iron Formation

• A more typical outcrop of BIF near Nagaunee, Michigan

Typical BIF

• 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

BIFs and the Atmosphere

• 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

Formation of BIFs

• 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

Source of Iron and Silica

• 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)

Continental Red Beds

Red mudrock in Glacier National

Park, Montana

• 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

Red Beds

• 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

Important Events in Life History

• Even in well-known Early Proterozoic fossils assemblages, only fossils of bacteria are recognized

Gunflint Microfossils

Photomicrograph of spheroidal

and filamentous microfossils

from the Gunflint Chert of Ontario

Canada

• 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

Prokaryote and Eukaryotes

• 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

Lack of Organic Diversity

• 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

Genetic Variation in Bacteria

• 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 the tempo of evolution increased

Sexual Reproduction Increased the Pace of Evolution

• 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

Eukaryotic Cells Evolve

• 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

Eukaryotes

• Prokaryotic cells – do not have a cell nucleus– do not have organelles – are smaller and not nearly as complex as

eukaryotic cells

Prokaryotic Cell

• Eukaryotic cells have – a cell nucleus

containing – the genetic material – and organelles

Eukaryotic Cell

– such as mitochondria – and plastids, – as well as

chloroplasts in plant 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 yrs old– it has some remarkable fossils of single-celled

eukaryotes – that show evidence of meiosis and mitosis, – processes carried out only by eukaryotic cells

Eukaryotic Fossil Cells

• Prokaryotic cells are mostly rather simple – spherical or platelike structures

• Eukaryotic cells– are 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

Evidence for Eukaryotes

• 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

Acritarchs

• 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

Late Proterozoic Microfossil

• 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 and the Origin of Eukaryotic Cells

• 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

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

Evidence for Endosymbiosis

• 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

Organelles Capable of Protein Synthesis

• 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

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

Dawn of Multicelled Organisms

• Carbonaceous impressions – in Proterozoic rocks, Montana

• These may be impressions of multicelled algae– Skip next slide

Multicelled Algae?

• 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

The Multicelled Advantage?

• 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

Late Proterozoic Animals

• In 1947, an Australian geologist, R.C. Sprigg, – in the Pound Quartzite in the Ediacara Hills of South Australia

• Additional discoveries by others turned up what appeared to be – discovered impressions of soft-bodied animals – 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

The Ediacaran Fauna

• The Ediacaran fauna of AustraliaTribrachidium heraldicum, a possible primitive

echinoderm

Ediacaran Fauna

Spriggina floundersi, a possible ancestor of trilobites

Pavancorina minchami

Ediacaran Fauna

• Restoration of the Ediacaran Environment

• 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 pre-Phanerozoic animals,

– but they have certainly increased our knowledge

– about this chapter in the history of life

Ediacaran Fauna

• 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

Represented Phyla

• 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, --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

Distinct Evolutionary Group

• 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

Other Proterozoic Animal Fossils

• Wormlike fossils from Late Proterozoic rocks in China

Wormlike Fossils from China

• All known Proterozoic animals were soft-bodied, – 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

Soft Bodies

• Kimberella, an animal from latest Proterozoic rocks in Russia

Latest Proterozoic Kimberella

– Exactly what Kimberella was remains uncertain

– Some think it was a sluglike creature

– whereas others think it was more like a mollusk

• 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

Durable Skeletons

• 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

Proterozoic Mineral Resources

• The Empire Mine at Palmer, Michigan – where iron ore from the Early Proterozoic

Negaunee Iron Formation is mined

Iron Mine

• 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

Nickel

• 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

Sudbury Basin

• 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

Platinum and Chromium

• 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

Oil and Gas

• 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

Proterozoic 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