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Precambrian Earth and Life History—The Proterozoic Eon


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Title: Precambrian Earth and Life History—The Proterozoic Eon

Chapter 9
Precambrian Earth and Life HistoryThe
Proterozoic Eon
Proterozoic Rocks, Glacier NP
  • Mesoproterozoic to Neoproterozoic sedimentary
  • of the Belt Supergroup
  • in Glacier National Park, Montana

The Length of the Proterozoic
  • The Proterozoic Eon alone,
  • at 1.958 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
  • The Phanerozoic,
  • consisting of
  • Paleozoic,
  • Mesozoic,
  • Cenozoic eras,
  • lasted a comparatively brief 542 million years
  • but is the subject of 10 chapters!

Disparity in Time
  • Older parts of the geologic record
  • are more inaccessible
  • and more difficult to interpret.
  • The Proterozoic Eon is subdivided
  • into three eras
  • with prefixes Paleo, Meso, and Neo
  • which are strictly terms denoting time

Archean-Proterozoic Boundary
  • Geologists 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 an unconformity

Other Differences
  • In addition to changes in the style of crustal
  • the Proterozoic is characterized
  • by widespread sedimentary 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
  • by the origin of some important mineral resources

Proterozoic Evolution of Oxygen-Dependent
  • 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
  • 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
  • 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, komatiites,
  • which no doubt resulted from
  • Earth's decreasing amount of radiogenic heat

Focus on Laurentia
  • Our focus here is on the geologic evolution of
  • a large landmass that consisted of what is now
  • North America,
  • Greenland,
  • parts of northwestern Scotland,
  • and perhaps some of the Baltic shield of

Early Proterozoic History of Laurentia
  • Laurentia originated and underwent important
  • 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
  • rifting of a continent,
  • opening and closing of an ocean basin,
  • and finally reassembly of the continent

Wopmay Orogen
  • Some of the rocks in Wopmay orogen
  • are sandstone-carbonate-shale assemblages,
  • a suite of rocks typical of passive continental
  • that first become widespread during the

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
  • such as
  • ripple marks
  • and cross-beds
  • Northern Michigan

Outcrop of Kona Dolomite
  • Some of the carbonate rocks, now mostly
  • such as the Kona Dolomite,
  • contain abundant bulbous structures known as
  • NorthernMichigan

Penokean Orogen
  • These rocks of northern Michigan
  • have been only moderately deformed
  • and are now part of the Penokean orogen

Accretion along Laurentias 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
  • 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
  • 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 Earths 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
  • In addition, some Early Proterozoic rocks
  • and associated features provide excellent
  • for widespread glaciation

Paleo- and Mesoproterozoic 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 Laurentias size
  • because magma was either intruded into
  • or erupted onto already existing continental

Igneous Activity
  • These igneous rocks are exposed
  • in eastern Canada, extend across Greenland,
  • and are also found in the Baltic shield of

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

Mesoproterozoic Orogeny and Rifting
  • The only Mesoproterozoic event in Laurentia
  • was the Grenville orogeny
  • in the eastern part of the continent
  • 1.3 to 1.0 billion years ago
  • Grenville rocks are well exposed
  • in the present-day northern Appalachian Mountains
  • as well as in eastern Canada, Greenland, and

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

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
  • extending from the Lake Superior basin southwest
    into Kansas,
  • and a southeasterly branch extends through
    Michigan into Ohio
  • It cuts through Archean and 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 area,
  • to the great outpourings of lava during the

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

Meso- and Neoproterozoic Sedimentation
  • Remember the Grenville orogeny
  • took place 1.3 and 1.0 billion 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
  • 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
  • Meso- and Neoproterozoic 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 Belt basin in western
    North America

Rocks of the Uinta Mountain Group
  • Utah

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
  • impressions of algae in some of these rocks
  • indicate probable marine deposition

Grand Canyon Super-group
  • Proterozoic sandstone of the Grand Canyon
    Supergroup 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
  • In fact, the oldest known ophiolites
  • providing evidence for an ancient convergent
    plate boundaries
  • Are known from Neoarchean and Paleoproterozoic
    rocks of Russia and China
  • They compare closely with younger,
    well-documented ophiolites,
  • such as the Jormua mafic-ultramafic complex in

Jormua Complex, Finland
  • Reconstruction
  • of the highly deformed
  • Jormua mafic-ultramafic complex
  • in Finland
  • This sequence of rock
  • is one of 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
  • at least two continents merged into one, but
    usually includes
  • all or most of all Earths landmasses
  • The supercontinent Pangaea,
  • which existed at the end of the Paleozoic Era,
  • is familiar,
  • but few people are aware of earlier

Early Supercontinents
  • Supercontinents may have existed
  • as early as the Neoarchean,
  • 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 Neoproterozoic supercontinent Rodinia
  • before it began fragmenting about 750 million
    years ago

  • 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
  • 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 instances 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
  • and two major episodes of Proterozoic glaciation

Recognizing Glaciation
  • How can we be sure that there were Proterozoic
  • After all, their most common deposit,
  • called tillite, is simply a type of conglomerate
  • that may look much like conglomerates
  • originating from other processes
  • Tillite or tillite-like deposits are known
  • from at least 300 Precambrian localities,
  • and some of these are undoubtedly not glacial

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
  • Tillite in Norway
  • overlies striated bedrock surface of sandstone

Geologists Convinced
  • Geologists are now convinced
  • based on this kind of evidence
  • that widespread glaciation
  • took place during the Paleoproterozoic
  • The occurrence of tillites of about the same age
  • in Michigan, Wyoming, and Quebec
  • indicates that North America may have had
  • an 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,
  • may date from 2.7 billion years ago,
  • thus making it Neoarchean

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
  • but was episodic with four major glacial episodes
    so far recognized

Late Proterozoic Glaciers
  • The approximate distribution of Neoproterozoic

Most Extensive Glaciation in Earth History
  • The map shows only approximate distribution
  • of Neoproterozoic glaciers
  • The actual extent of glaciers is unknown
  • Not all the glaciers were present at the same
  • Despite these uncertainties,
  • this Neoproterozoic glaciation
  • was the most extensive in Earth history
  • In fact, Neoproterozoic 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
  • 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 that cyanobacteria,
  • 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 Paleoproterozoic
  • These photosynthesizing organisms
  • and to a lesser degree, photochemical
  • 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

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
  • 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 silver-colorediron minerals

Typical BIF
  • A more typical outcrop of BIF near Nagaunee,

BIFs and the Atmosphere
  • How are these rocks related to the atmosphere?
  • Their iron consists of iron oxides, especially
  • hematite (Fe2O3)
  • and magnetite (Fe3O4)
  • Iron combines with oxygen in an oxidizing
  • 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
  • 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
  • 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
  • overlying a large volume of oxygen-deficient
  • that contained reduced iron and silica
  • Upwelling,
  • that is transfer of water from depth to the
  • 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
    Formations (BIFs)

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 BIF
  • Precipitation continued until
  • the iron in seawater was largely used up

Continental Red Beds
  • Continental red beds refers
  • to red rocks on the continents,
  • but more specifically it means red sandstone or
  • 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
  • increase in abundance throughout the rest of the
  • 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
  • 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 archea and bacteria,
  • although they undoubtedly existed in profusion
  • Likewise, the Paleoproterozoic fossil record
  • has mostly bacteria and stromatolites
  • Apparently little diversification
  • had taken place
  • all organisms were single-celled prokaryotes

Gunflint Microfossils
  • Proterozoic fossils assemblages,
  • such as the Gunflint Iron Formation of Canada,
  • resemble bacteria living today

Lack of Organic Diversity
  • The lack of organic diversity in the
  • during the Paleoproterozoic
  • 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
  • but their effects are limited in prokaryotes

Genetic Variation in Bacteria
  • A beneficial mutation would spread rapidly
  • in sexually reproducing organisms,
  • but have a limited impact in prokaryotes
  • because they do not share their genes with other

Sexual Reproduction Increased the Pace of
  • 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 Paleoproterozoic 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

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

Oldest Eukaryotes
  • This fossil from the 2.1-billion-year Negaunee
    Iron Formation at Marquette, Michigan, is
    probably some type of multicelled algae.

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
  • Their organizational complexity
  • is much greater than it is for prokaryotes

  • Other organisms that were
  • almost certainly eukaryotes are the acritarchs
  • that first appeared about 1.4 billion years ago
  • they were very common by Neoproterozoic time
  • and were probably cysts of planktonic (floating)

  • These common Proterozoic microfossils
  • are probably from eukaryotic organisms
  • Acritarchs are very likely the cysts of algae

Neoproterozoic Microfossil
  • Numerous microfossils of organisms
  • with vase-shaped skeletons
  • have been found
  • in Neoproterozoic 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
  • as occurs in lichens,
  • once thought to be plants
  • but actually symbiotic fungi and algae

  • 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 plastids,
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • proportionately less of a cell is exposed
  • to the external environment in relation to its
  • 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

Neoproterozoic 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 distinctionand 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 or cnidarian

Spriggina floundersi, a possible ancestor of
Ediacaran Fauna
  • Pavancorina is perhaps related to arthropods
  • 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
  • The discovery of Ediacaran fossils and subsequent
  • have not answered all questions about
    pre-Phanerozoic 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
  • One Ediacaran fossil, Spriggina,
  • has been cited as a possible ancestor of
  • 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 todays invertebrate animals
  • Ediacara-type faunas are known
  • from all continents except Antarctica,
  • are collectively referred to as the Ediacaran
  • were widespread between 545 and 670 million years
  • but their fossils are not common
  • 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
  • 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

Soft Bodies
  • 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
  • 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
  • Neoproterozoic fossils
  • of minute scraps of shell-like material
  • and small toothlike 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 542 million years ago

Proterozoic Mineral Resources
  • Most of the world's iron ore comes from
  • Paleoproterozoic 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 Paleoproterozoic Negaunee
    Iron Formation is mined

  • In the Sudbury mining district in Ontario,
  • 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
  • an elliptical area measuring more than 59 by 27
  • 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 known 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 Neoproterozoic,
  • 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
  • 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

  • The crust-forming processes
  • that yielded Archean granite-gneiss complexes
  • and greenstone belts
  • continued into the Proterozoic
  • but at a considerably reduced rate
  • Paleoproterozoic collisions
  • between Archean cratons formed larger cratons
  • that served as nuclei
  • around which Proterozoic crust accreted

  • One such landmass was Laurentia
  • consisting mostly of North America and Greenland
  • Important events
  • in the evolution of Laurentia were
  • Paleoproterozoic amalgamation of cratons
  • followed by Mesoproterozoic igneous activity,
  • the Grenville orogeny, and the Midcontinent rift
  • Ophiolite sequences
  • marking convergent plate boundaries
  • are first well documented from the Neoarchean and
  • indicating that a plate tectonic style similar
  • to that operating now had been established

  • 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
  • which began fragmenting about 550 million years

  • Glaciers were widespread
  • during both the Paleoproterozoic and the
  • Photosynthesis continued
  • to release free oxygen into the atmosphere
  • which became increasingly oxygen-rich through the
  • 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

  • Most of the known Proterozoic organisms
  • are single-celled prokaryotes (bacteria)
  • When eukaryotic cells first appeared is
  • 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

  • Well-documented multicelled animals
  • are found in several Neoproterozoic 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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