Transcript Document 7212229

The Proterozoic Eon:
Top Ten Significant Events
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Plate tectonics occurred similar to modern rates
Accretion at continental boundaries
Assembly of Laurentia; and two super continents
Rifting and widespread sandstone, carbonate, shale
deposits (continental shelf deposits)
Extensive continental glaciation
Mid-continent rift formed in North America
Widespread occurrence of stromatolites
Formation of banded iron and other mineral
resources (gold, copper, platinum, nickel)
Free oxygen in atmosphere
Evolution of eukaryotic cells
The Length of the Proterozoic
• the
Proterozoic
Eon alone,
– at 1.955
billion years
long,
– accounts for
42.5% of all
geologic time
– yet we review
this long
episode of
Earth and life
history in a
single section
The Phanerozoic
• Yet the
Phanerozoic,
– consisting of
• Paleozoic,
• Mesozoic,
• Cenozoic
eras,
– lasted a
comparatively
brief 545
million years
– is the subject
of the rest of
the course
Style of Crustal Evolution
• Archean:
– granite-gneiss complexes
– and greenstone belts
– cratons
• During the Proterozoic, these formed at a
considerably reduced rate and cooler
temperatures
Contrasting Metamorphism
• Proterozoic rocks
– show little or no effects of metamorphism,
– and in many areas they are separated
– from Archean rocks by a profound
unconformity
Evolution of
Proterozoic Continents
• Archean cratons assembled from collisions
of island arcs and minicontinents,
• Proterozoic accretion at craton margins
– probably took place more rapidly than today
• forming much larger landmasses
• Earth possessed more radiogenic heat,
– but the process continues even now
Proterozoic Greenstone
Belts
• Not as common after the Archean,
– near absence of ultramafic rocks
– WHY would this happen?
Focus on Laurentia
• Geologic evolution of Laurentia,
– a large landmass that consisted of what is now
• North America,
• Greenland,
• parts of northwestern Scotland,
• and perhaps some of the Baltic shield of
Scandinavia
Early Proterozoic History of
Laurentia
• Laurentia originated ~ 2.0 billion years ago
• collisions called orogens
formed linear deformation belts
– rocks have been
• metamorphosed
• and intruded by magma
• thus forming plutons, especially
batholiths
Proterozoic Evolution of Laurentia
• Archean cratons were sutured
– along deformation belts called orogens,
• By 1.8 billion years ago,
– much of what is now Greenland, central
Canada, and the north-central United
States existed
• Laurentia grew along its southern margin
– by accretion
What is the evidence?
Craton-Forming Processes
– Recorded in
rocks
– In northwestern
Canada
• where the Slave
and Rae cratons
collided
Craton-Forming Processes
• the Trans Hudson orogen
• in Canada and the
United States,
– where the Superior,
Hearne, and Wyoming
cratons
– were sutured
• The southern margin of
Laurentia
– is the site of the
Penokian orogen
Wilson Cycle
• A complete Wilson cycle,
• named for the Canadian geologist J. Tuzo Wilson,
– involves
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fragmentation of a continent (rifting)
opening of an ocean basin
followed by closing of an ocean basin,
and finally reassembly of the continent
• Example: Wopmay Orogeny (Canada) =
complete Wilson Cycle
Wopmay Orogen
• Some of the
rocks in
Wopmay orogen
– are sandstonecarbonateshale
assemblages,
– a suite of rocks
typical of passive
continental
margins
– that first become
widespread during
the Proterozoic
Early Proterozoic Rocks in Great Lakes
Region: Evidence of continental shelf
• Early Proterozoic sandstone-carbonate-shale
assemblages are widespread near the Great
Lakes
Where? N. Michigan
Outcrop of Sturgeon Quartzite
• The sandstones have a variety of
sedimentary structures
– such as
– ripple
marks
– and
crossbeds
– Northern
Michigan
Outcrop of Kona Dolomite
“warm shallow marine”
• Some of the carbonate rocks, now mostly
dolostone,
– such as the Kona Dolomite,
– contain
abundant
bulbous
structures
known as
stromatolites
– Northern
Michigan
Penokean Orogen
• These rocks of northern Michigan
– have been only moderately deformed
– and are now part
of the Penokean
orogen
Southern Margin Accretion
• Laurentia grew along its southern margin
– Central Plains, Yavapai, and Mazatzal orogens
• Also notice that the
Midcontinental
Rift had formed in the
Great Lakes region by
this time
BIF, Red Beds, Glaciers
• This was also the time during which
– most of Earth’s banded iron formations (BIF)
– were deposited
• The first continental red beds
– sandstone and shale with oxidized iron
– were deposited about 1.8 billion years ago
• We will have more to say about BIF
– and red beds in the section on “The Evolving
Atmosphere”
• In addition, some Early Proterozoic rocks
– provide excellent evidence for widespread
glaciation
Middle Proterozoic
Orogeny and Rifting
• The only Middle Proterozoic event in
Laurentia
– 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
Grenville Orogeny
• A final episode of Proterozoic accretion
– occurred during the Grenville orogeny
75% of North America formed
by 900 million years ago
• By this final stage, about 75%
– of present-day North America existed
• The remaining 25%
– accreted along its margins,
– particularly its eastern and western margins,
– during the Phanerozoic Eon
Midcontinent Rift
• Grenville deformation in Laurentia
– was accompanied by the origin
– of the Midcontinent rift,
• a long narrow continental trough bounded by faults,
• extending from the Lake Superior basin southwest
into Kansas,
• and a southeasterly branch extends through
Michigan into Ohio
• It cuts through Archean and Early
Proterozoic rocks
– and terminates in the east against rocks
– of the Grenville orogen
What is the relative age?
Location of the Midcontinent Rift
• Rocks
filling the
rift
– are
exposed
around
Lake
Superior
– but are
deeply
buried
elsewher
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Midcontinental Rift
• Most of the rift is buried beneath younger
rocks
– except in the Lake Superior region
– with various igneous and sedimentary rocks
exposed
• The Evidence:
• numerous overlapping basalt lava flows
– forming a volcanic pile several kilometers
thick
Portage Lake Volcanics
Michigan
Proterozoic Sedimentary Rocks,
Glacier NP
• Proterozoic sedimentary rocks
– in Glacier National Park, Montana
• The angular peaks, ridges and broad
valleys
– were carved by Pleistocene and Recent
Proterozoic Mudrock
• Outcrop of red mudrock in Glacier
National Park, Montana
Proterozoic Limestone
• Outcrop of limestone with stromatolites in
Glacier National Park, Montana
Grand Canyon Super-group
• Proterozoic Sandstone of the Grand
Canyon Super-group in the Grand Canyon
Arizona
Proterozoic Supercontinents
• A supercontinent consists of all
– Or much of the present-day continents,
– so other than size it is the same as a continent
• The supercontinent Pangaea,
– existed MUCH LATER but few people are
aware of earlier supercontinents
Early Supercontinents
• Rodinia
– assembled between 1.3 and 1.0 billion years
ago
– and then began fragmenting (rifting apart) 750
million years ago (THE Proterozoic ends at
545my ago)
Early Supercontinent
• Possible
configuration
– of the Late
Proterozoic
supercontinent
Rodinia
– before it began
fragmenting
about 750
million years
ago
• Rodinia's separate pieces reassembled
– and formed another supercontinent
– Pannotia
– about 650 million years ago
• Fragmentation was underway again,
– about 550 million years ago,
– giving rise to the continental configuration
– that existed at the onset of the Phanerozoic Eon – the Cambrian
Recognizing Glaciation
• extensive geographic distribution
– conglomerates and tillites
– and their associated glacial features
– striated and polished bedrock
Proterozoic Glacial Evidence
• Bagganjarga Tillite in Norway
Over bedrock
Geologists Convinced
• The occurrence of tillites
– in Michigan, Wyoming, and Quebec
– indicates that North America may have had
– an Early Proterozoic ice sheet centered
southwest of Hudson Bay
Early Proterozoic Glaciers
• Deposits in
North
America
– indicate that
Laurentia
– had an
extensive ice
sheet
– centered
southwest of
Hudson Bay
Late Proterozoic Glaciers
• The approximate distribution of Late
Proterozoic glaciers
• Late Proterozoic glaciers
– seem to have been present even
– in near-equatorial areas!!
– Geologists have recently named this
phenomenon
– “SNOWBALL EARTH”
The Evolving Atmosphere
– Archean: little or no free oxygen
– At beginning of the Proterozoic was probably
no more than 1% of that present now
– Stromatolites—not common until:
– 2.3 billion years ago,
• that is, during the Early Proterozoic
• There is evidence of increasing oxygen….
Early Proterozoic
Banded Iron Formation
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At this outcrop in Ishpeming, Michigan
the rocks are alternating layers of
red chert
and
silvercolored
iron
minerals
Banded Iron Formations (BIF)
• Banded iron formations (BIFs),
– consist of alternating layers of
• iron-rich minerals
• and chert
– about 92% of all BIFs
• formed during the interval
• from 2.5 to 2.0 billion years ago
BIFs and the Atmosphere
• How are these rocks related to the atmosphere?
• Their iron is in iron oxides, especially
– hematite (Fe2O3)
– and magnetite (Fe3O4)
• Iron combines with oxygen in an oxidizing atmosphere
– to from rustlike oxides
– that are not readily soluble in water
• If oxygen is absent in the atmosphere, though,
– iron easily dissolves
– so that large quantities accumulate in the world's oceans,
– which it undoubtedly did during the Archean
Formation of BIFs
• The Archean atmosphere was deficient in free oxygen
• so that little oxygen was dissolved in seawater
• However, as photosynthesizing organisms
– increased in abundance,
• as indicated by stromatolites,
– free oxygen,
• released as a metabolic waste product into the oceans,
– caused the precipitation of iron oxides along with silica
– and thus created BIFs
Formation of BIFs
• Depositional model for the origin of banded
iron formation
Source of Iron and Silica
• 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
• iron and silica combined with oxygen
– thus resulting in the precipitation
– of huge amounts of banded iron formation
• Precipitation continued until
– the iron in seawater was largely used up
Continental Red Beds
• Obviously continental red beds refers
– to red rocks on the continents,
– but more specifically it means red sandstone or shale
– colored by
iron oxides,
– especially
hematite
(Fe2O3)
Red mudrock in
Glacier
National Park,
Montana
Red Beds
• Red beds first appear
– in the geologic records about 1.8 billion years
ago,
– and are quite common in rocks of Phanerozoic
age
– coincides with the introduction of free oxygen
– into the Proterozoic atmosphere
– may have had only 1% - 2% of present levels
Red Beds
• Is this percentage sufficient to account
– for oxidized iron in sediment?
• Probably not,
– but no ozone (O3) layer existed in the upper
atmosphere
– before free oxygen (O2) was present
• As photosynthesizing organisms released
– free oxygen into the atmosphere,
– ultraviolet radiation converted some of it
– to elemental oxygen (O) and ozone (O3),
– both of which oxidize minerals more effectively
than O2
Red Beds
• Once an ozone layer became established,
– most ultraviolet radiation failed
– to penetrate to the surface,
– and O2 became the primary agent
– for oxidizing minerals
Important Events in Life History
• Archean fossils are not very common,
– and all of those known are varieties
– of bacteria and cyanobacteria (blue-green
algae),
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• Little diversification
– all organisms were single-celled
prokaryotes,
– until about 2.1 billion years ago
– when more complex eukaryotic cells
evolved
Gunflint Microfossils
• Even in well-known Early Proterozoic
fossils assemblages, only fossils of
bacteria are recognized
Photomicrograph
of spheroidal
and filamentous
microfossils
from the Gunflint
Chert of Ontario
Canada
Prokaryote and Eukaryotes
• An organism made up of prokaryotic cells
is called a prokaryote
– whereas those composed of eukaryotic cells
are eukaryotes
– is the basis for the most profound distinction
between all living things
Lack of Organic Diversity
– 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
Sexual Reproduction Increased the Pace of
Evolution
• Prior to the appearance of cells capable of
sexual reproduction,
– evolution was a comparatively slow
process,
– thus accounting for the low organic diversity
• This situation did not persist
• Sexually reproducing cells probably
– evolved by Early Proterozoic time,
– and the tempo of evolution increased
Eukaryotic Cells Evolve
• The appearance of eukaryotic cells
– marks a milestone in evolution
– comparable to the development
• of complex metabolic mechanisms
• such as photosynthesis during the Archean
• How do they differ from their predecessors,
– the prokaryotic cells?
• All prokaryotes are single-celled,
– but most eukaryotes are multicelled,
Eukaryotes
• Most eukaryotes reproduce sexually,
– in marked contrast to prokaryotes,
• and nearly all are aerobic,
– that is, they depend on free oxygen
– to carry out their metabolic processes
• Accordingly, they could not have evolved
– before at least some free oxygen was present
in the atmosphere
Prokaryotic Cell
• Prokaryotic cells
– do not have a cell nucleus
– do not have organelles
– are smaller and not nearly as complex as
eukaryotic cells
Eukaryotic Cell
• Eukaryotic cells
have
– a cell nucleus
containing
– the genetic material
– and organelles
– such as mitochondria
– and plastids,
– as well as
chloroplasts in plant
cells
Eukaryotic Fossil Cells
• The Negaunee Iron Formation in Michigan
– 2.1 billion years old
– Fossils are oldest known eukaryotic cells
• Bitter Springs Formation
– of Australia --1 billion yrs old
– fossils of single-celled eukaryotes
– evidence of meiosis and mitosis,
– processes carried out only by eukaryotic cells
Evidence for Eukaryotes
• Prokaryotic cells are mostly rather simple
– spherical or platelike structures
• Eukaryotic cells
– are larger
– much more complex
– have a well-defined, membrane-bounded cell
nucleus, which is lacking in prokaryotes
– have several internal structures
– called organelles such as plastids and mitochondria
– their organizational complexity
– is much greater than it is for prokaryotes
Acritarchs
• These common Late Proterozoic
microfossils
– are probably from eukaryotic organisms
• Acritarchs are very likely the cysts of algae
Late Proterozoic Microfossil
• Numerous
microfossils of
organisms
– with vase-shaped skeletons
– have been found
– in Late Proterozoic rocks
– in the Grand Canyon
• These too have tentatively been
identified as
– cysts of some kind of algae
Endosymbiosis and the
Origin of Eukaryotic Cells
• Formed from several prokaryotic cells
– In 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
Evidence for Endosymbiosis
• Supporting evidence for endosymbiosis
– comes from studies of living eukaryotic
cells
– containing internal structures called
organelles,
• such as mitochondria and plastics,
– which contain their own genetic material
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In addition, prokaryotic cells
– synthesize proteins as a single system,
whereas eukaryotic cells
– are a combination of protein-synthesizing systems
Organelles Capable of
Protein Synthesis
• That is, some of the organelles
– within eukaryotic cells are capable of protein
synthesis
• These organelles
• with their own genetic material
• and protein-synthesizing capabilities
– are thought to have been free-living bacteria
• that entered into a symbiotic relationship,
• eventually giving rise to eukaryotic cells
Multicelled Organisms
• Obviously multicelled organisms
– are made up of many cells,
– perhaps billions,
– as opposed to a single cell as in prokaryotes
• In addition, multicelled organisms
– have cells specialized to perform specific
functions
– such as respiration,
– food gathering,
– and reproduction
Dawn of Multicelled
Organisms
• We know from the fossil record
– that multicelled organisms were present during the Proterozoic,
– but we do not know exactly when they appeared
• What seem to be some kind of multicelled
algae appear
– in the 2.1-billion-year-old fossils
• from the Negaunee Iron Formation in Michigan
– as carbonaceous filaments
• from 1.8 billion-year-old rocks in China
– as somewhat younger carbonaceous impressions
– of filaments and spherical forms
Multicelled Algae?
• Carbonaceous impressions
– in Proterozoic rocks, Montana
• These may be impressions of
multicelled algae
– Skip next slide
The Multicelled Advantage?
• Is there any particular advantage to
being multicelled?
• For something on the order of 1.5
billion years
– all organisms were single-celled
– and life seems to have thrived
• In fact, single-celled organisms
– are quite good at what they do
– but what they do is very limited
The Multicelled Advantage?
• 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?
• 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
Ediacaran Fauna
• The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive
echinoderm
Spriggina floundersi, a
possible ancestor of
trilobites
Ediacaran Fauna
Pavancorina
minchami
• Restoration of the
Ediacaran Environment
Ediacaran Fauna
• Geologists had assumed that
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– 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 certainly increased our knowledge
– Representatives of jellyfish, corals, worms, insects, spider crabs
Distinct Evolutionary Group
• However, some scientists think
– these Ediacaran animals represent
– an early evolutionary group quite distinct from
– the ancestry of today’s invertebrate animals
• Ediacara-type faunas are known
– from all continents except Antarctica,
--were widespread between 545 and 670 million
years ago
– but their fossils are rare
• Their scarcity should not be surprising,
though,
– because all lacked durable skeletons
Other Proterozoic Animal Fossils
• Although scarce, a few animal fossils
– older than those of the Ediacaran fauna are known
• A jellyfish-like impression is present
– in rocks 2000 m below the Ediacara Hills Pound
Quartzite,
• Burrows, in many areas,
– presumably made by worms,
– occur in rocks at least 700 million years old
• Wormlike and algae fossils come
– from 700 to 900 million-year-old rocks in China
– but the identity and age of these "fossils" has been
questioned
Wormlike Fossils from China
• Wormlike
fossils from
Late
Proterozoic
rocks in China
Soft Bodies
• All known Proterozoic animals were softbodied,
– but there is some evidence that the earliest
stages in the origin of skeletons was underway
• Even some Ediacaran animals
– may have had a chitinous carapace
– and others appear to have had areas of
calcium carbonate
• The odd creature known as Kimberella
– from the latest Proterozoic of Russia
– had a tough outer covering similar to
– that of some present-day marine invertebrates
Latest Proterozoic Kimberella
• Kimberella, an animal from latest
Proterozoic rocks in Russia
– Exactly what
Kimberella was
remains
uncertain
– Some think it
was a sluglike
creature
– whereas others
think it was
more like a
Durable Skeletons
• Latest Proterozoic fossils
– of minute scraps of shell-like material
– and small tooth like denticles and spicules,
• presumably from sponges
• indicate that several animals with skeletons
– or at least partial skeletons existed
• 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 = Cambrian age
Proterozoic Mineral Resources
– Proterozoic banded iron formations
• large deposits of these rocks
– in the US Lake Superior region
– and in eastern Canada
– Nickel, Sudbury basin Canada
– platinum, chromium, S. Africa
Iron Mine
• The Empire Mine at Palmer, Michigan
– where iron ore from the Early Proterozoic
Negaunee Iron Formation is mined
Oil and Gas
• Economically recoverable oil and gas
– have been discovered in Proterozoic rocks in
China and Siberia,
– Dunton pegmatite in Maine:
gemstones: tourmaline, micas, quartz,
aquamarine, feldspars, garnets, topaz, lepidolite
(lithium)
– Also Black Hills, S.D. pegmatite vein