1. 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 Length of the Proterozoic

2. 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 The Phanerozoic

3. The Proterozoic Eon: Top Ten Significant Events Plate tectonics occurred similar to modern rates Accretion at continental boundaries Assembly of Laurentia; and two super continents Widespread sandstone, carbonate, shale deposits (continental shelf deposits) Extensive continental glaciation Mid-continent rift formed in North America Widespread occurrence of stromatolites Formation of banded iron and other mineral resources (gold, copper, platinum, nickel) Free oxygen in atmosphere Evolution of eukaryotic cells

4. Archean crust-forming processes generated –granite-gneiss complexes –and greenstone belts –that were shaped into cratons During the Proterozoic, these formed at a considerably reduced rate and cooler temperatures Style of Crustal Evolution

5. Many Archean rocks have been metamorphosed, 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 Contrasting Metamorphism

6. Archean cratons assembled during collisions of island arcs and minicontinents, –providing the center of today’s continents. –Proterozoic crust accreted, at edges forming much larger landmasses Proterozoic accretion at craton margins –probably took place more rapidly than today Earth possessed more radiogenic heat, –but the process continues even now Evolution of Proterozoic Continents

7. They were not as common after the Archean, –near absence of ultramafic rocks –WHY would this happen? Proterozoic Greenstone Belts

8. 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 Focus on Laurentia

9. Laurentia originated ~ 2.0 billion years ago collisions called orogens formed linear or arcuate deformation belts –in which many of the rocks have been metamorphosed and intruded by magma thus forming plutons, especially batholiths Early Proterozoic History of Laurentia

10. Proterozoic Evolution of Laurentia Laurentia grew along its southern margin –by accretion Archean cratons were sutured –along deformation belts called orogens, By 1.8 billion years ago, –much of what is now Greenland, central Canada, and the north-central United States existed

11. –Recorded in rocks –In northwestern Canada where the Slave and Rae cratons collided What is the evidence? Craton-Forming Processes

12. the Trans Hudson orogen in Canada and the United States, –where the Superior, Hearne, and Wyoming cratons –were sutured The southern margin of Laurentia –is the site of the Penokian orogen Craton-Forming Processes

13. Rocks of the Wopmay orogen –in northwestern Canada are important –because they record the Wilson cycle opening and closing of an ocean basin A complete Wilson cycle, named for the Canadian geologist J. Tuzo Wilson, –involves fragmentation of a continent (rifting) opening of an ocean basin followed by closing of an ocean basin, and finally reassembly of the continent Wilson Cycle

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

15. Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes Early Proterozoic Rocks in Great Lakes Region: Evidence of continental shelf

16. The sandstones have a variety of sedimentary structures –such as –ripple marks –and cross- beds –Northern Michigan Where? N. Michigan Outcrop of Sturgeon Quartzite

17. Some of the carbonate rocks, now mostly dolostone, –such as the Kona Dolomite, –contain abundant bulbous structures known as stromatolites –Northern Michigan Outcrop of Kona Dolomite “warm shallow marine”

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

19. 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

20. This was also the time during which –most of Earth’s banded iron formations (BIF) –were deposited The first continental red beds –sandstone and shale with oxidized iron –were deposited about 1.8 billion years ago We will have more to say about BIF –and red beds in the section on “The Evolving Atmosphere” In addition, some Early Proterozoic rocks –provide excellent evidence for widespread glaciation BIF, Red Beds, Glaciers

21. These igneous rocks are exposed –in eastern Canada, extend across Greenland, –and are also found in the Baltic shield of Scandinavia Proterozoic Igneous Activity

22. However, the igneous rocks are deeply buried –by younger rocks in most areas The origin of these –are the subject of debate According to one hypothesis –large-scale upwelling of magma –beneath a Proterozoic supercontinent –produced the rocks Igneous Activity Why? How do we know ?

23. 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 Middle Proterozoic Orogeny and Rifting

24. A final episode of Proterozoic accretion –occurred during the Grenville orogeny Grenville Orogeny

25. 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 75% of North America

26. 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 Midcontinent Rift

27. Rocks filling the rift –are exposed around Lake Superior –but are deeply buried elsewher e Location of the Midcontinent Rift

28. Most of the rift is buried beneath younger rocks –except in the Lake Superior region –with various igneous and sedimentary rocks exposed The Evidence: numerous overlapping basalt lava flows –forming a volcanic pile several kilometers thick Midcontinental Rift

29. Michigan Portage Lake Volcanics

30. Middle to Late Proterozoic sedimentary rocks –are exceptionally well exposed –in the northern Rocky Mountains –of Montana and Alberta, Canada Glacier National Park Sedimentary Rocks

31. Proterozoic sedimentary rocks –in Glacier National Park, Montana The angular peaks, ridges and broad valleys –were carved by Pleistocene and Recent glaciers Proterozoic Rocks, Glacier NP

32. Outcrop of red mudrock in Glacier National Park, Montana Proterozoic Mudrock

33. Outcrop of limestone with stromatolites in Glacier National Park, Montana Proterozoic Limestone

34. Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona Grand Canyon Super-group

35. A supercontinent consists of all –Or much of the present-day continents, –so other than size it is the same as a continent The supercontinent Pangaea, –existed MUCH LATER but few people are aware of earlier supercontinents Proterozoic Supercontinents

36. Rodinia –assembled between 1.3 and 1.0 billion years ago –and then began fragmenting (rifting apart) 750 million years ago (THE Proterozoic ends at 545my ago) Early Supercontinents

37. Possible configuration –of the Late Proterozoic supercontinent Rodinia –before it began fragmenting about 750 million years ago Early Supercontinent

38. Rodinia's separate pieces reassembled –and formed another supercontinent –Pannotia –about 650 million years ago Fragmentation was underway again, –about 550 million years ago, –giving rise to the continental configuration –that existed at the onset of the Phanerozoic Eon – the Cambrian

39. How can we be sure that there were Proterozoic glaciers? the extensive geographic distribution –of other conglomerates and tillites –and their associated glacial features –is distinctive, –such as striated and polished bedrock Recognizing Glaciation

40. Bagganjarga Tillite in Norway Over bedrock Proterozoic Glacial Evidence

41. The occurrence of tillites –in Michigan, Wyoming, and Quebec –indicates that North America may have had –an Early Proterozoic ice sheet centered southwest of Hudson Bay Geologists Convinced

42. Deposits in North America –indicate that Laurentia –had an extensive ice sheet –centered southwest of Hudson Bay Early Proterozoic Glaciers

43. The approximate distribution of Late Proterozoic glaciers Late Proterozoic Glaciers

44. Late Proterozoic glaciers –seem to have been present even –in near-equatorial areas!! –Geologists have recently named this phenomenon –“SNOWBALL EARTH”

45. –Archean: little or no free oxygen –the amount present –at the beginning of the Proterozoic was probably no more than 1% of that present now –Stromatolites—not common until: – 2.3 billion years ago, that is, during the Early Proterozoic There is evidence of increasing oxygen…. The Evolving Atmosphere

46. At this outcrop in Ishpeming, Michigan the rocks are alternating layers of red chert and silver- colored iron minerals Early Proterozoic Banded Iron Formation

47. Banded iron formations (BIFs), –consist of alternating layers of iron-rich minerals and chert –about 92% of all BIFs formed during the interval from 2.5 to 2.0 billion years ago Banded Iron Formations (BIF)

48. How are these rocks related to the atmosphere? Their iron is in iron oxides, especially –hematite (Fe 2 O 3 ) –and magnetite (Fe 3 O 4 ) Iron combines with oxygen in an oxidizing atmosphere –to from rustlike oxides –that are not readily soluble in water If oxygen is absent in the atmosphere, though, –iron easily dissolves –so that large quantities accumulate in the world's oceans, –which it undoubtedly did during the Archean BIFs and the Atmosphere

49. 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

50. Depositional model for the origin of banded iron formation Formation of BIFs

51. A likely source of the iron and silica –was submarine volcanism, –similar to that now talking place –at or near spreading ridges Huge quantities of dissolved minerals are –also discharged at submarine hydrothermal vents In any case, the iron and silica –combined with oxygen –thus resulting in the precipitation –of huge amounts of banded iron formation Precipitation continued until –the iron in seawater was largely used up Source of Iron and Silica

52. 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 (Fe 2 O 3 ) Continental Red Beds Red mudrock in Glacier National Park, Montana

53. 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

54. Is this percentage sufficient to account –for oxidized iron in sediment? Probably not, –but no ozone (O 3 ) layer existed in the upper atmosphere –before free oxygen (O 2 ) was present As photosynthesizing organisms released –free oxygen into the atmosphere, –ultraviolet radiation converted some of it –to elemental oxygen (O) and ozone (O 3 ), –both of which oxidize minerals more effectively than O 2 Red Beds

55. Once an ozone layer became established, –most ultraviolet radiation failed –to penetrate to the surface, –and O 2 became the primary agent –for oxidizing minerals Red Beds

56. Archean fossils are not very common, –and all of those known are varieties –of bacteria and cyanobacteria (blue-green algae), –although they undoubtedly existed in profusion Likewise, the Early Proterozoic fossil record –has mostly bacteria and cyanobacteria Apparently little diversification –had taken place; –all organisms were single-celled prokaryotes, –until about 2.1 billion years ago –when more complex eukaryotic cells evolved Important Events in Life History

57. Even in well-known Early Proterozoic fossils assemblages, only fossils of bacteria are recognized Gunflint Microfossils Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada

58. An organism made up of prokaryotic cells is called a prokaryote –whereas those composed of eukaryotic cells are eukaryotes In fact, the distinction between prokaryotes and eukaryotes –is the basis for the most profound distinction between all living things Prokaryote and Eukaryotes

59. Actually, the lack of organic diversity –during this early time in life history –is not too surprising –because prokaryotic cells reproduce asexually Most variation in –sexually reproducing populations comes from –the shuffling of genes, –and their alleles, –from generation to generation Mutations introduce new variation into a population, –but their effects are limited in prokaryotes Lack of Organic Diversity

60. A beneficial mutation would spread rapidly –in sexually reproducing organism, –but have a limited impact in bacteria –because they do not share their genes with other bacteria Bacteria usually reproduce by binary fission –and give rise to two cells –having the same genetic makeup Under some conditions, –they engage in conjugation during –which some genetic material is transferred Genetic Variation in Bacteria

61. Prior to the appearance of cells capable of sexual reproduction, –evolution was a comparatively slow process, –thus accounting for the low organic diversity This situation did not persist Sexually reproducing cells probably –evolved by Early Proterozoic time, –and the tempo of evolution increased Sexual Reproduction Increased the Pace of Evolution

62. The appearance of eukaryotic cells –marks a milestone in evolution –comparable to the development of complex metabolic mechanisms such as photosynthesis during the Archean Where did these cells come from? How do they differ from their predecessors, –the prokaryotic cells? All prokaryotes are single-celled, –but most eukaryotes are multicelled, –the notable exception being the protistans Eukaryotic Cells Evolve

63. Most eukaryotes reproduce sexually, –in marked contrast to prokaryotes, and nearly all are aerobic, –that is, they depend on free oxygen –to carry out their metabolic processes Accordingly, they could not have evolved –before at least some free oxygen was present in the atmosphere Eukaryotes

64. Prokaryotic cells –do not have a cell nucleus –do not have organelles –are smaller and not nearly as complex as eukaryotic cells Prokaryotic Cell

65. Eukaryotic cells have –a cell nucleus containing –the genetic material –and organelles Eukaryotic Cell –such as mitochondria –and plastids, –as well as chloroplasts in plant cells

66. 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 yrs old –it has some remarkable fossils of single-celled eukaryotes –that show evidence of meiosis and mitosis, –processes carried out only by eukaryotic cells Eukaryotic Fossil Cells

67. Prokaryotic cells are mostly rather simple –spherical or platelike structures Eukaryotic cells –are larger –much more complex –have a well-defined, membrane-bounded cell nucleus, which is lacking in prokaryotes –have several internal structures –called organelles such as plastids and mitochondria –their organizational complexity –is much greater than it is for prokaryotes Evidence for Eukaryotes

68. 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

69. These common Late Proterozoic microfossils –are probably from eukaryotic organisms Acritarchs are very likely the cysts of algae Acritarchs

70. Numerous microfossils of organisms –with vase-shaped skeletons –have been found –in Late Proterozoic rocks –in the Grand Canyon These too have tentatively been identified as –cysts of some kind of algae Late Proterozoic Microfossil

71. Eukaryotic cells probably formed –from several prokaryotic cells –that entered into a symbiotic relationship –Symbiosis, involving a prolonged association of two or more dissimilar organisms, –is quite common today In many cases both symbionts benefit from the association –as occurs in lichens, once thought to be plants but actually symbiotic fungi and algae Endosymbiosis and the Origin of Eukaryotic Cells

72. In a symbiotic relationship, –each symbiont must be capable –of metabolism and reproduction, –but in some cases one symbiont –cannot live independently This may have been the case –with Proterozoic symbiotic prokaryotes –that became increasingly interdependent –until the unit could exist only as a whole In this relationship –one symbiont lived within the other, –which is a special type of symbiosis –called endosymbiosis Endosymbiosis

73. Supporting evidence for endosymbiosis –comes from studies of living eukaryotic cells –containing internal structures called organelles, such as mitochondria and plastics, –which contain their own genetic material In addition, prokaryotic cells –synthesize proteins as a single system, whereas eukaryotic cells –are a combination of protein-synthesizing systems Evidence for Endosymbiosis

74. That is, some of the organelles –within eukaryotic cells are capable of protein synthesis These organelles with their own genetic material and protein-synthesizing capabilities –are thought to have been free-living bacteria that entered into a symbiotic relationship, eventually giving rise to eukaryotic cells Organelles Capable of Protein Synthesis

75. Obviously multicelled organisms –are made up of many cells, –perhaps billions, –as opposed to a single cell as in prokaryotes In addition, multicelled organisms –have cells specialized to perform specific functions –such as respiration, –food gathering, –and reproduction Multicelled Organisms

76. We know from the fossil record –that multicelled organisms were present during the Proterozoic, –but we do not know exactly when they appeared What seem to be some kind of multicelled algae appear –in the 2.1-billion-year-old fossils from the Negaunee Iron Formation in Michigan –as carbonaceous filaments from 1.8 billion-year-old rocks in China –as somewhat younger carbonaceous impressions –of filaments and spherical forms Dawn of Multicelled Organisms

77. Carbonaceous impressions –in Proterozoic rocks, Montana These may be impressions of multicelled algae –Skip next slide Multicelled Algae?

78. 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?

79. 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?

80. 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 The Multicelled Advantage?

81. Biologists set forth criteria such as –method of reproduction –and type of metabolism –to allow us to easily distinguish –between animals and plants Or so it would seem, –but some present-day organisms –blur this distinction and the same is true –for some Proterozoic fossils Nevertheless, the first –relatively controversy-free fossils of animals –come from the Ediacaran fauna of Australia –and similar faunas of similar age elsewhere Late Proterozoic Animals

82. In 1947, an Australian geologist, R.C. Sprigg, –in the Pound Quartzite in the Ediacara Hills of South Australia Additional discoveries by others turned up what appeared to be –discovered impressions of soft-bodied animals –impressions of algae and several animals –many bearing no resemblance to any existing now Before these discoveries, geologists –were perplexed by the apparent absence –of fossil-bearing rocks predating the Phanerozoic The Ediacaran Fauna

83. The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm Ediacaran Fauna Spriggina floundersi, a possible ancestor of trilobites

84. Pavancorina minchami Ediacaran Fauna Restoration of the Ediacaran Environment

85. Geologists had assumed that –the fossils so common in Cambrian rocks –must have had a long previous history –but had little evidence to support this conclusion The discovery of Ediacaran fossils and subsequent discoveries –have not answered all questions about pre-Phanerozoic animals, –but they have certainly increased our knowledge –about this chapter in the history of life Ediacaran Fauna

86. Three present-day phyla may be represented –in the Ediacaran fauna: jellyfish and sea pens (phylum Cnidaria), segmented worms (phylum Annelida), and primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others) One Ediacaran fossil, Spriggina, –has been cited as a possible ancestor of trilobites Another might be a primitive member –of the phylum Echinodermata Represented Phyla

87. However, some scientists think –these Ediacaran animals represent –an early evolutionary group quite distinct from –the ancestry of today’s invertebrate animals Ediacara-type faunas are known –from all continents except Antarctica, --were widespread between 545 and 670 million years ago –but their fossils are rare Their scarcity should not be surprising, though, –because all lacked durable skeletons Distinct Evolutionary Group

88. Although scarce, a few animal fossils –older than those of the Ediacaran fauna are known A jellyfish-like impression is present –in rocks 2000 m below the Ediacara Hills Pound Quartzite, Burrows, in many areas, –presumably made by worms, –occur in rocks at least 700 million years old Wormlike and algae fossils come –from 700 to 900 million-year-old rocks in China –but the identity and age of these "fossils" has been questioned Other Proterozoic Animal Fossils

89. Wormlike fossils from Late Proterozoic rocks in China Wormlike Fossils from China

90. All known Proterozoic animals were soft- bodied, –but there is some evidence that the earliest stages in the origin of skeletons was underway Even some Ediacaran animals –may have had a chitinous carapace –and others appear to have had areas of calcium carbonate The odd creature known as Kimberella –from the latest Proterozoic of Russia –had a tough outer covering similar to –that of some present-day marine invertebrates Soft Bodies

91. Kimberella, an animal from latest Proterozoic rocks in Russia Latest Proterozoic Kimberella –Exactly what Kimberella was remains uncertain –Some think it was a sluglike creature –whereas others think it was more like a mollusk

92. Latest Proterozoic fossils –of minute scraps of shell-like material –and small tooth like denticles and spicules, presumably from sponges indicate that several animals with skeletons –or at least partial skeletons existed However, more durable skeletons of silica, calcium carbonate, and chitin (a complex organic substance) –did not appear in abundance until the beginning –of the Phanerozoic Eon 545 million years ago Durable Skeletons

93. Most of the world's iron ore comes from –Proterozoic banded iron formations Canada and the United States have large deposits of these rocks –in the Lake Superior region –and in eastern Canada Thus, both countries rank among –the ten leading nations in iron ore production Proterozoic Mineral Resources

94. The Empire Mine at Palmer, Michigan –where iron ore from the Early Proterozoic Negaunee Iron Formation is mined Iron Mine

95. In the Sudbury mining district in Ontario, Canada, –nickel and platinum are extracted from Proterozoic rocks Nickel is essential for the production of nickel alloys such as stainless steel and Monel metal (nickel plus copper), –which are valued for their strength and resistance to corrosion and heat The United States must import –more than 50% of all nickel used –mostly from the Sudbury mining district Nickel

96. Besides its economic importance, the Sudbury Basin, –an elliptical area measuring more than 59 by 27 km, –is interesting from the geological perspective One hypothesis for the concentration of ores –is that they were mobilized from metal- rich rocks –beneath the basin –following a high-velocity meteorite impact Sudbury Basin

97. Some platinum –for jewelry, surgical instruments, –and chemical and electrical equipment –is exported to the United States from Canada, –but the major exporter is South Africa The Bushveld Complex of South Africa –is a layered igneous complex containing both platinum and chromite –the only ore of chromium, –United States imports much of the chromium –from South Africa –It is used mostly in stainless steel Platinum and Chromium

98. Economically recoverable oil and gas –have been discovered in Proterozoic rocks in China and Siberia, –arousing some interest in the Midcontinent rift as a potential source of hydrocarbons So far, land has been leased for exploration, –and numerous geophysical studies have been done However, even though some rocks –within the rift are know to contain petroleum, –no producing oil or gas wells are operating Oil and Gas

99. 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

100. Geologists have identified more than 20,000 pegmatites –in the country rocks adjacent –to the Harney Peak Granite –in the Black Hills of South Dakota These pegmatites formed ~ 1.7 billion years ago –when the granite was emplaced as a complex of dikes and sills A few have been mined for gemstones, tin, lithium, micas, –and some of the world's largest known –mineral crystals were discovered in these pegmatites Proterozoic Pegmatites

101. 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

102. 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

103. 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

104. 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

105. 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

106. 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