Precambrian Earth and Life History—The Proterozoic Eon

1
Chapter 9
Precambrian Earth and Life HistoryThe
Proterozoic Eon
2
Proterozoic Rocks, Glacier NP

• Mesoproterozoic to Neoproterozoic sedimentary
  rocks
• of the Belt Supergroup
• in Glacier National Park, Montana

3
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

4
The Phanerozoic

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

5
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

6
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

7
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

8
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

9
Other Differences

• In addition to changes in the style of crustal
  evolution,
• 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
  biosphere
• by the origin of some important mineral resources

10
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

11
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

12
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
  production

13
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

14
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

15
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

16
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,

17
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

18
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

19
Wopmay Orogen

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

20
Early Proterozoic Rocks in Great Lakes Region

• Early Proterozoic sandstone-carbonate-shale
  assemblages are widespread near the Great Lakes

21
Outcrop of Sturgeon Quartzite

• The sandstones have a variety of sedimentary
  structures
• such as
• ripple marks
• and cross-beds
• Northern Michigan

22
Outcrop of Kona Dolomite

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

23
Penokean Orogen

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

24
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
  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

25
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

26
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
  evidence
• for widespread glaciation

27
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
  crust

28
Igneous Activity

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

29
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

30
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
  Scandinavia

31
Grenville Orogeny

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

32
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

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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

34
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 Proterozoic rocks
• and terminates in the east against rocks
• of the Grenville orogen

35
Location of the Midcontinent Rift

• Rocks filling the rift
• are exposed around Lake Superior
• but are deeply buried elsewhere

36
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
  Cenozoic

37
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

38
Cooper Harbor Conglomerate

• Michigan

39
Portage Lake Volcanics

• Michigan

40
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
  Nevada,
• and in three huge basins in the west

41
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

42
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

43
Proterozoic Mudrock

• Outcrop of red mudrock in Belt basin in western
  North America

44
Rocks of the Uinta Mountain Group

• Utah

45
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

46
Grand Canyon Super-group

• Proterozoic sandstone of the Grand Canyon
  Supergroup in the Grand Canyon Arizona

47
Style of Plate Tectonics

• The present style of plate tectonics
• involving opening and then closing ocean basins
• had almost certainly been established by the
  Paleoproterozoic
• 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
  Finland

48
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

49
Jormua Complex, Finland

• Metamorphosed basaltic pillow lava

12 cm
50
Jormua Complex, Finland

• Metamorphosed gabbro between mafic dikes

65 cm
51
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
  supercontinents

52
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

53
Early Supercontinent

• Possible configuration
• of the Neoproterozoic supercontinent Rodinia
• before it began fragmenting about 750 million
  years ago

54
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

55
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
  glaciers
• and two major episodes of Proterozoic glaciation

56
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 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
  deposits

57
Glacial Evidence

• But the extensive geographic distribution
• of other conglomerates
• and their associated glacial features
• is distinctive,
• such as striated and polished bedrock

58
Proterozoic Glacial Evidence

• Tillite in Norway
• overlies striated bedrock surface of sandstone

59
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

60
Early Proterozoic Glaciers

• Deposits in North America
• indicate that Laurentia
• had an extensive ice sheet
• centered southwest of Hudson Bay

61
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 Neoarchean

62
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

63
Late Proterozoic Glaciers

• The approximate distribution of Neoproterozoic
  glaciers

64
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
  time
• 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

65
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

66
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
  dissociation
• added free oxygen to the evolving atmosphere

67
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?

68
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

69
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

70
Early Proterozoic Banded Iron Formation

• At this outcrop in Ishpeming, Michigan
• the rocks are alternating layers of
• red chert
• and silver-colorediron minerals

71
Typical BIF

• A more typical outcrop of BIF near Nagaunee,
  Michigan

72
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
  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

73
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

74
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

75
Formation of BIFs

• Depositional model for the origin of Banded Iron
  Formations (BIFs)

76
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

77
Continental Red Beds

• 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
78
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

79
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

80
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

81
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

82
Gunflint Microfossils

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

83
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
  population,
• but their effects are limited in prokaryotes

84
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
  cells

85
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 Paleoproterozoic time,
• and thereafter the tempo of evolution
• increased markedly

86
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

87
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

88
Prokaryotic Cell

• Prokaryotic cells
• do not have a cell nucleus
• do not have organelles
• are smaller and not nearly as complex as
  eukaryotic cells

89
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

90
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

91
Oldest Eukaryotes

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

92
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

93
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 Neoproterozoic time
• and were probably cysts of planktonic (floating)
  algae

94
Acritarchs

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

95
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

96
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

97
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

98
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

99
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

100
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
  functions
• such as respiration,
• food gathering,
• and reproduction

101
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

102
Multicelled Algae?

• Carbonaceous impressions
• in Proterozoic rocks
• in the Little Belt Mountains, Montana
• These may be impressions of multicelled algae

103
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

104
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

105
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

106
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

107
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

108
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

109
Ediacaran Fauna

• The Ediacaran fauna of Australia
• Tribrachidium heraldicum, a possible primitive
  echinoderm or cnidarian

Spriggina floundersi, a possible ancestor of
trilobites
110
Ediacaran Fauna

• Pavancorina is perhaps related to arthropods

• Restoration of the Ediacaran Environment

111
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
  pre-Phanerozoic animals,
• but they have certainly increased our knowledge
• about this chapter in the history of life

112
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

113
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
  fauna
• were widespread between 545 and 670 million years
  ago
• but their fossils are not common
• Their scarcity should not be surprising, though,
• because all lacked durable skeletons

114
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

115
Wormlike Fossils from China

• Wormlike fossils from Late Proterozoic rocks in
  China

116
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
  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

117
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

118
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

119
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

120
Iron Mine

• The Empire Mine at Palmer, Michigan
• where iron ore from the Paleoproterozoic Negaunee
  Iron Formation is mined

121
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

122
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

123
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

124
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

125
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

126
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

127
Summary

• 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

128
Summary

• 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
  Paleoproterozoic,
• indicating that a plate tectonic style similar
• to that operating now had been established

129
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
• which began fragmenting about 550 million years
  ago

130
Summary

• Glaciers were widespread
• during both the Paleoproterozoic and the
  Neoproterozoic
• 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

131
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
  Paleoproterozoic

132
Summary

• 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