Transcript Chapter 9 - LSU Geology & Geophysics

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