Chapter 9

Chapter 9

Precambrian Earth and Life History—The Proterozoic Eon

• 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

– 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

How long was the Proterozoic?

• 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

How is the Archean-Proterozoic Boundary defined?

• Archean crust-forming processes generated – granite-gneiss complexes – and greenstone belts – that were shaped into cratons

• same rock associations – continued during the Proterozoic, – BUT at a considerably reduced rate

Different Style of Crustal Evolution?

• Unlike Archean rocks, vast exposures of Proterozoic rocks show– little or no effects of metamorphism, – and in many areas they are separated – from Archean rocks by a nonconformity

Contrasting Metamorphism?

• 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 with Archean rocks?

• During the Proterozoic – oxygen-dependent organisms – made their appearance

• and the first cells evolved

Proterozoic Evolution of Oxygen-Dependent Organisms!

• Proterozoic accretion at Archean island arcs and minicontinents margins thereby forming much larger landmasses

When did Continents evolve?

• Most greenstone belts formed – during the Archean – between 2.7 and 2.5 billion years ago

• not as common after the Archean, – and differed in one important detail

• the near absence of ultramafic rocks • which resulted from Earth's decreasing amount of

radiogenic heat

Proterozoic Greenstone Belts

– 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

What is Laurentia?

• originated and underwent important growth – between 2.0 and 1.8 billion years ago

• collisions among various plates formed several orogens – linear or arcuate deformation belts in which rocks

have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths

When and how did Laurentia come into existence?

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

• 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

• From 1.8 to 1.6 billion years ago, – as successively younger belts were sutured to

Laurentia, – forming the Yavapai and Mazatzal-Pecos orogens

When did the southern portion of the continent accrete?

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

Deposition of most of Earth’s banded iron formations (BIF)

• First deposition of continental red beds at ~ 1.8 billion years ago– sandstone and shale with oxidized iron

• excellent evidence for widespread glaciation

What else happened during the Proterozoic?

• Extensive igneous activity– from 1.8 to 1.1 billion years ago 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

Other events?

• These igneous rocks are exposed – in eastern Canada, extend across Greenland,

– and are also found in the Baltic shield

of Scandinavia

Igneous Activity

• According to one hypothesis – large-scale upwelling of magma – beneath a Proterozoic supercontinent – produced the rocks

Cause of Igneous Activity?

• The only Middle Proterozoic event in Laurentia– was the Grenville orogeny – in the eastern and southern part of the continent – 1.3 to 1.0 billion years old

• Grenville rocks are well exposed – in the the northern Appalachian Mountains – eastern Canada, Greenland, and Scandinavia

Middle Proterozoic Orogeny and Rifting

• A final episode of Proterozoic accretion – occurred during the Grenville orogeny

Grenville Orogeny

• 1) closure of an ocean basin• the final stage in a Wilson cycle

• 2) intracontinental deformation or major shearing

• Whatever the cause, – it was the final Proterozoic stage of Laurentia

continental accretion

With what was the Grenville Orogeny associated?

• about 75% of present-day North America existed

• The remaining 25% – accreted during the Phanerozoic Eon

How much of North American continent was in existence by the

end of the Proterozoic?

• Grenville-age extension, volcanism and sedimentation in Laurentia 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

What’s the 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– except in the Lake Superior region – 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

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

• 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

• 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

The rocks consist mostly – of sandstone, shale, and dolostone, – deposited in shallow-water marine – and fluvial environments

• The presence of stromatolites and carbonaceous – impression of algae in some of these rocks – also indicate probable marine deposition

Proterozoic Sedimentary Rocks

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

Grand Canyon Super-group

• almost certainly by the Early Proterozoic• the oldest complete ophiolite

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

When did the current style of Style of Plate Tectonics come

into play?

• 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

• A supercontinent consists of all – or at least much of the present-day continents,

• The supercontinent Pangaea, – existed at the end of the Paleozoic Era,

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

Pre-Pangean Supercontinents?

• Possible configuration – of the Late

Proterozoic supercontinent Rodinia

– before it began fragmenting about 750 million years ago

How did Rodinia look?

• Rodinia's separate pieces reassembled – and formed Pannotia– about 650 million years ago

• Fragmentated– by the latest Proterozoic, about 550 million years

ago,

Pannotia: The next supercontinent

• The most recent glacial was during the Pleistocene is certainly the best known.

– BUT several major episodes of Proterozoic glaciation

Was there glaciation during the Proterozoic?

– 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,

How can we be sure that there were Proterozoic glaciers?

• But the extensive geographic distribution – of 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, – such as the Gunflint Iron Formation of Canada, – 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 thereafter the tempo of evolution – increased markedly

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 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

Eukaryotic Fossil Cells

• 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

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 – in the Little Belt Mountains, Montana

• These may be impressions of multicelled algae

Multicelled Algae?

• 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

Studies of Present-Day Organisms

• 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


• 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


• 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, – 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

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, – 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

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

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Chapter 9