View
216
Download
2
Tags:
Embed Size (px)
Citation preview
• 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