Chapter 8 Earth's Formative Stages and the Archean Eon

Transcription

1 Chapter 8 Earth's Formative Stages and the Archean Eon

2 Archean Eon Archean (Archean Eon) is the oldest unit on the geologic time scale. It began 4.56 billion years ago and ended 2.5 billion years ago. Archean lasted for 2.1 billion years (2,100,000,000 years).

3 Earth's Oldest Rocks Earth's oldest rocks are found in Canada. They are about 4.04 billion years old. But there are even older mineral grains. Sand-sized zircon grains in metamorphosed sedimentary rocks from Australia are 4.4 billion years old.

4 Earth's Oldest Rocks There are no rocks on Earth that date back to 4.6 billion years ago because the Earth is geologically active, and the oldest rocks have been recycled by plate tectonics or by weathering and erosion as part of the rock cycle. Much of our knowledge of the Earth's earliest history comes from indirect evidence - meteorites.

5 Archean and Proterozoic Eons comprise in interval of time informally called Precambrian, which spans 87% of the geologic time scale. Precambrian

6 The Big Bang Calculations indicate that the Big Bang occurred billion years ago. The Big Bang marked the instantaneous creation of all matter in the Universe.

7 The Solar System The Sun and the planets, moons, asteroids, comets and other objects that orbit it, comprise the Solar System.

8 Lines of evidence that must be considered for any hypothesis on the origin of the Solar System 1. Planets revolve around sun in same direction - counterclockwise (CCW) 2. Planets lie roughly within sun's equatorial plane (plane of sun's rotation) 3. Solar System is disk-like in shape

9 4. Planets rotate CCW on their axes, except for: a. Venus - slowly clockwise b. Uranus - on its side c. Pluto - on its side 5. Moons go CCW around planets (with a few exceptions)

10 6. Distribution of planet densities and compositions is related to their distance from sun a. Inner, terrestrial planets have high density b. Outer, Jovian planets have low density 7. Age - Meteorites are as old as 4.56 billion years

11 Solar Nebula Hypothesis or Nebular Hypothesis 1. Cold cloud of gas and dust contracts, rotates, and flattens into a disk-like shape. 2. Roughly 90% of mass becomes concentrated in the center, due to gravitational attraction. 3. Turbulence in cloud caused matter to collect in certain locations.

12 Solar Nebula Hypothesis or Nebular Hypothesis 4. Clumps of matter begin to form in the disk. 5. Accretion of matter (gas and dust) around clumps by gravitational attraction. Clumps develop into protoplanets. 6. Solar nebula cloud condenses, shrinks, and becomes heated by gravitational compression to form Sun.

13 Solar Nebula Hypothesis or Nebular Hypothesis 7. Ultimately hydrogen (H) atoms begin to fuse to form helium (He) atoms, releasing energy (heat and light). The Sun "ignites." 8. The Sun's solar wind drives lighter elements outward, causing observed distribution of masses and densities in the Solar System.

14 Solar Nebula Hypothesis or Nebular Hypothesis 9. Planets nearest Sun lose large amounts of lighter elements (H, He), leaving them with smaller sizes and masses, but greater densities than the outer planets. Inner planets are dominated by rock and metal. 10. Outer planets retain light elements such as H and He around inner cores of rock and metal. Outer planets have large sizes and masses, but low densities.

15 Solar Nebula Hypothesis or Nebular Hypothesis

16 How old is the Solar System? Based on radiometric dates of moon rocks and meteorites, the Solar System is about 4.56 billion years old.

17 Meteorites: Samples of the Solar System Meteors = "shooting stars." The glow comes from small particles of rock from space being heated as they enter Earth's atmosphere. Meteorites = chunks of rock from the Solar System that reach Earth's surface. They include fragments of: Asteroids Moon rock Planets, such as Mars (i.e., "Martian meteorites")

18 Types of Meteorites 1. Ordinary chondrites 2. Carbonaceous chondrites 3. Iron meteorites 4. Stony-iron meteorites

19 Meteorites: Ordinary Chondrites Most abundant type of meteorite About 4.6 billion years old, May contain chondrules - spherical bodies that solidified from molten droplets thrown into space during Solar System impacts

20 Meteorites: Carbonaceous Chondrites Contain about 5% organic compounds, including amino acids the building blocks of proteins, DNA, and RNA May have supplied basic building blocks of life to Earth Contain chondrules

21 Meteorites: Iron Meteorites Iron-nickel alloy Coarse-grained intergrown crystal structure About 5% of all meteorites

22 Meteorites: Stony-iron Meteorites Composed partly of Fe, Ni and partly of silicate minerals, including olivine (like Earth's mantle). About 1% of all meteorites. Least abundant type.

23 The Sun The Sun is a star Composition: 70% hydrogen 27% helium 3% heavier elements Size: About 1.5 million km in diameter Contains about 98.8% of the matter in the Solar System.

24 The Sun Temperature: may exceed 20 million o C in the interior. Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen atoms are fused together to form helium, releasing energy. The Sun's gravity holds the planets in their orbits.

25 Sun's energy is the force behind many geologic processes on Earth Evaporation of water to produce clouds, which cause precipitation, which causes erosion. Uneven heating of the Earth's atmosphere causes winds and ocean currents. Variations in heat from Sun may trigger continental glaciations or change forests to deserts. Sun and moon influence tides which affect the shoreline.

26 The Planets 1. Mercury 2. Venus 3. Earth 4. Mars 5. Jupiter 6. Saturn 7. Uranus 8. Neptune

27 The Planets Terrestrial planets: Small Dense (4-5.5 g/cm 3 ) Rocky + Metals Mercury, Venus, Earth, Mars Jovian planets: Large Low density ( g/cm 3 ) Gaseous Jupiter, Saturn, Uranus, Neptune

28 Mercury Smallest of the terrestrial planets Revolves rapidly around the sun; its year is 88 Earth days Densely cratered Thin atmosphere of sodium and lesser amounts of helium, oxygen, potassium and hydrogen Weak magnetic field and high density suggest an iron core No moons

29 Venus Similar to Earth in size, mass, volume, density and gravity No oceans or liquid water Very high atmospheric pressure Atmosphere is 98% carbon dioxide Dense clouds of sulfuric acid droplets in atmosphere Greenhouse effect causes temperature on planet's surface to reach 470 C, hot enough to melt lead

30 Venus Rotates once on its axis (one day on Venus) in 243 Earth days Rotates on axis in opposite direction to other planets, possibly due to collision with other object Has volcanoes Has craters Surface rocks resemble basalt No moons

31 Earth Diameter = nearly 13,000 km (8000 mi) Oceans cover 71% of surface Atmosphere = 78% nitrogen and 21% oxygen Surface temperature approx. -50 and +50 o C Average density = 5.5 g/cm 3 Surface rock density = g/cm 3 Core about 7000 km in diameter; Mantle surrounds core. Extends from base of crust to depth of 2900 km. Geologically active. Plate tectonics. Only body in the Universe known to support life.

32 Earth's Internal Layered Structure The Earth is internally layered, with a basic structure consisting of: Crust Mantle Inner and outer core The Earth's internal structure may be primary (formed initially as the Earth formed), or secondary due to later heating.

33 Factors that make Earth hospitable for life Distance from Sun maintains temperatures in the range where water is liquid. Temperature relatively constant for billions of years. Rotation allows all sides of Earth to have light and heat. Atmosphere absorbs some heat from the Sun and reflects some solar radiation back to space. Magnetic field protects life from dangerous high energy particles and radiation in the solar wind.

34 Earth's Moon Diameter = about 1/4 that of Earth. Density = about 3.3 g/cm3 (similar to Earth's mantle). Rotates on its axis at same rate as it revolves around Earth (29.5 days). Results in same side of Moon always facing Earth. Far side of moon is more densely cratered No atmosphere. Ice is present at the poles.

35 Geology of the Moon Dominant rock type is anorthosite (related to gabbro; rich in Ca plagioclase feldspar). Basalt is also present. Two types of terrane Lunar highlands Maria (singular = mare)

36 Lunar highlands Light-colored Rough topography Highly cratered Rocks more than 4.2 billion years old

37 Lunar maria Large, dark areas Immense basins covered with basaltic lava flows Age of basalt is 3.8 to 3.2 billion years Maria have few craters. This indicates a decrease in meteorite bombardment after about 3.8 billion years ago.

38 Origin of the Moon Moon may have formed as a result of an impact of a large body with Earth about 4.4 billion years ago. Debris from the impact was thrown into orbit around Earth and collected to form the Moon. Heat from impacts led to melting and differentiation (or segregation of materials of different density; low density materials rose and high density materials sank).

39 Mars Has white polar caps made of frozen carbon dioxide ice Has seasonal changes. Polar ice caps expand and contract Rusty orange color due to iron oxides on surface Heavily cratered due to early bombardment by meteorites and asteroids

40 Mars Diameter is about half that of Earth Mass is only about 10% of Earth's mass, so gravity is much less Thin atmosphere (less than 1% as dense as Earth's). Dominant gas is carbon dioxide; small amounts of nitrogen, oxygen and carbon monoxide. No greenhouse effect.

41 Mars Previously had a denser atmosphere Evidence of abundant liquid water in the past. An ocean once existed, at least 0.5 km deep and larger than all 5 U.S. Great Lakes. Temperatures range from -85 o C to 21 o C (21 o C is about room temperature)

42 Mars Lower density than other terrestrial planets Little to no magnetic field, suggesting only a small iron-rich core. Lack of magnetic field exposed planet to solar winds which swept away atmosphere and liquid water. Two small moons, Phobos and Deimos

43 Asteroid Belt Thousands of asteroids, primarily between Mars and Jupiter. Asteroids are composed of rocks and metal (Fe & Ni). Size of asteroids ranges from a few km in diameter to about 1/10 the size of Earth.

44 Jupiter Largest planet in the Solar System. (Diameter 11 times greater than Earth) Low density. (Density is about 1/4 that of Earth) Most of planet's interior is probably liquid metallic hydrogen. Rotates on axis rapidly. One day on Jupiter is 10 hours on Earth. Rotation causes bands in atmosphere

45 Jupiter Note Great Red Spot, a cyclonic storm Atmosphere composed of H, He, with lesser amounts of methane and ammonia Has a faint ring of debris which encircles the planet

46 Jupiter Has more than 60 moons Four largest moons are: Io- covered by sulfur volcanoes Europa - has sea of liquid water beneath an icy surface Ganymede - planet-sized body larger than Mercury. Cratered with sinuous ridges; has sea of liquid water beneath an icy surface Callisto - highly cratered; has sea of liquid water beneath an icy surface

47 Saturn Second largest planet Has prominent rings of debris encircling planet in equatorial plane

48 Saturn Density is less than that of water; it could float. (Density = 0.7g/cm 3 ) Mostly H and He; also contains methane, ammonia, and water; may have iron core Has magnetic field, radiation belts, and internal heat source Has more than 30 moons.

49 Uranus About 4 x larger than Earth Low density (density = 1.3 g/cm 3 ) Axis of rotation is tipped on its side, possibly due to collision with another Solar System object Has more than two dozen moons Atmosphere of hydrogen, helium, and methane Has planetary ring system

50 Neptune Similar in size and color to Uranus Low density (density = 1.6 g/cm3) Atmosphere = H, He, and methane Has more than a dozen moons Has Great Dark Spot, a cyclonic storm Has planetary ring system

51 Solar Nebula Hypothesis or Cold Accretion Model (Secondary Differentiation) Earth formed by accretion of dust and larger particles of metals and silicates. Earth was originally homogeneous throughout - a random mixture of space debris. Origin of layering requires a process of differentiation. Differentiation is the result of heating and at least partial melting.

52 Possible sources of heat for melting: 1. Accretionary heat from bombardment (meteorite impacts) 2. Heat from gravitational compression as material accumulated 3. Radioactive decay

53 Differentiation after Accretion Iron and nickel sink to form core. Less dense material (silicon and oxygen combined with remaining iron and other metals) forms mantle and lighter crust (dominated by silicon and oxygen). Presence of volatile gases on Earth indicates that complete melting did not occur. Earth was repeatedly partly melted by great impacts, such as the Moon-forming impact.

54 An alternative model: Hot Accretion (Primary Differentiation) Internal zonation of planets is a result of hot heterogeneous accretion. Hot solar nebula (over 1000 o C). Initial crystallization of iron-rich materials forms planet's core. With continued cooling, lower density silicate materials crystallized.

55 Which Model? Solar Nebula Hypothesis also known as the Cold Accretion Model (secondary differentiation) OR Hot Accretion Model (primary differentiation)??? Parts of both models may have been in operation.

56 Archean Crust Once differentiation occurred, Earth's crust was dominated by Fe and Mg silicate minerals. If Earth experienced heating and partial melting, it may have been covered by an extensive magma ocean during Archean. Magma cooled to form rocks called komatiites.

57 Komatiites Komatiites are ultramafic rocks composed mainly of olivine and pyroxene. Komatiites form at temperatures greater than those at which basalt forms (greater than 1100 o C). This rock formed Earth's Archean crust.

58 Origin of Mafic Crust The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of rocks in the upper mantle.

59 Earth's Crust Today Earth has two types of crust today: 1. Dense, mafic (Mg- and Fe-rich) oceanic crust dominated by basalt. 2. Less dense, silicic (Si- and Al-rich) continental crust dominated by granite.

60 Origin of Continental Crust Continental crust developed after the initial mafic to ultramafic crust. Continental crust is silicic or felsic (such as granite). Dominated by light-colored minerals such as quartz and feldspar. Felsic crust began forming around 4.4 billion years ago.

61 Origin of Continental Crust Felsic crust formed in subduction zones where descending slabs of crust partially melted. The early-melting, less dense components of the melt rose to the surface where they cooled to form continental crust.

62 Earth's Oldest Rocks One of the oldest dated felsic Earth rocks is the 4.04 billion year old Acasta Gneiss from northwestern Canada. Dates are from zircon grains in tonalite gneisses. (Tonalite gneiss is metamorphosed tonalite, a rock similar to diorite, with at least 10% quartz).

63 Earth's Oldest Rocks The Amitsoq Gneiss from Greenland is another old tonalite gneiss (3.8 b.y. old). Patches of old felsic crust have also been found in Antarctica (3.9 b.y. old).

64 Earth's Oldest Land Surface A 3.46 b.y. old fossil soil zone (or paleosol) associated with an unconformity in the Pilbara region of Australia indicates that Archean continents stood above sea level. This paleosol represents the oldest land surface known, and provides evidence that subaerial weathering, erosion, and soil formation processes were at work during Archean.

65 The Oldest Mineral Grains The oldest zircon grains are 4.4 b.y. old. Found in quartzite in western Australia. Sedimentary structures in the quartzite resemble those in modern stream deposits. Interpreted as fluvial (river) deposits. Derived from weathering of granitic rocks (some of the earliest continental crust), and deposited above sea level, indicating the presence of both liquid water and continental crust by 4.4 b.y. ago.

66 Oceanic Crust Continental Crust First appearance About 4.5 b.y. ago About 4.4 b.y. ago Where formed Mid-ocean ridges Subduction zones Composition Komatiite & basalt Tonalite & granodiorite, and later, granites Lateral extent Widespread Local (few 100 km or mi) How formed Partial melting of ultramafic rocks in upper mantle Partial melting of wet, sedimentcovered mafic rocks in subduction zones

67 Evolution of Earth's Atmosphere and Hydrosphere Earth's first, primitive atmosphere lacked free oxygen. The primitive atmosphere was derived from gases associated with the comets and meteorites which formed the Earth during accretion. The gases reached the Earth's surface through a process called outgassing.

68 Gases Associated with Comets Comets are made of frozen gases, ice and dust. Halley's comet is composed of: 80% water ice Frozen carbon dioxide (dry ice) Hydrogen cloud surrounds comet Dust near the nucleus contains iron, oxygen, silicon, magnesium, sodium, sulfur, and carbon

69 Gases Associated with Meteorites Carbonaceous chondrites are mainly composed of silicate minerals, but also contain: Nitrogen Hydrogen Water Carbon in the form of complex organic molecules (proteins and amino acids)

70 Water and gaseous elements would have been released from the newly accreted Earth by the heat associated with bombardment and accretion, or by melting and volcanism accompanying later differentiation.

71 Volcanic Outgassing Outgassing = release of water vapor and other gases from Earth through volcanism. Gases from Hawaiian eruptions consist of: 70% water vapor (H 2 O) 15% carbon dioxide (CO 2 ) 5% nitrogen (N 2 ) 5% sulfur (in H 2 S) chlorine (in HCl) hydrogen argon

72 Volcanic Outgassing Most of the water on the surface of the Earth and in the atmosphere was outgassed during the first billion years of Earth history. We know this because there are 3.8 b.y.-old marine sedimentary rocks, indicating the presence of an ocean by 3.8 billion years ago.

73 Formation of the Hydrosphere Once at the Earth's surface, gases and other volatile elements underwent a variety of changes. 1. Water vapor condensed and fell as rain. 2. Liquid water probably began to fall on the Earth's surface as early as 4.4 billion years ago. 3. Rain water accumulated in low places to form seas. The seas were originally freshwater (rain).

74 Formation of the Hydrosphere 4. Carbon dioxide and other gases dissolved in the rain made the water more acidic than today. Carbon dioxide and water combine to form carbonic acid. 5. Acid waters caused rapid chemical weathering of the exposed rocks, adding Na, Ca, K, and other ions to seawater. 6. A change to more alkaline water may have occurred rapidly as large amounts of Ca, Na, and Fe were introduced by submarine volcanism, neutralizing the acid.

75 Formation of the Hydrosphere 7. Ions accumulated in the seas, increasing the salinity. Sea salinity is relatively constant today because salts are precipitated at about the same rate they are supplied to the sea. Sodium remains in sea water due to its high solubility. 8. Later, when the seas became less acidic, Ca ions bonded with CO 2 to form shells of marine organisms and limestones (CaCO 3 ). 9. The presence of marine fossils suggests that sodium has not varied appreciably in sea water for at least the past 600 million years.

76 Hydrologic Cycle Today Earth's water is continuously recirculated through the hydrologic cycle (evaporation and precipitation, powered by the sun and by gravity).

77 Evolution of the Atmosphere Note - Gases released by volcanoes, condensation of water vapor, precipitation, and accumulation of liquid water, photochemical reactions in the atmosphere, and formation of carbonate rocks (limestones) later, after the seas became less acidic.

78 The Early Anoxic Atmosphere Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen or O 2 gas), and probably consisted primarily of: Water vapor (H 2 O) Carbon dioxide (CO 2 ) Nitrogen (N 2 ) Carbon monoxide (CO) Hydrogen sulfide (H 2 S) Hydrogen chloride (HCl)

79 The Early Anoxic Atmosphere The atmosphere composition would have been similar to that of modern volcanoes, but probably with more hydrogen, and possibly traces of methane (CH 4 ) and ammonia. If any free oxygen had been present, it would have immediately been involved in chemical reactions with easily oxidized metals such as iron.

80 Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere 1. Lack of oxidized iron in the oldest sedimentary rocks. (Instead, iron combined with sulfur to form sulfide minerals like pyrite. This happens only in anoxic environments.) 2. Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sedimentary rocks. 3. Archean sedimentary rocks are commonly dark due to the presence of carbon, which would have been oxidized if oxygen had been present.

81 Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere 4. Archean sedimentary sequences lack carbonate rocks but contain abundant chert, presumably due to the presence of an acidic, carbon dioxide-rich atmosphere. In an acidic environment, alkaline rocks such as limestone do not form.

82 Banded Iron Formation 5. Banded iron formations (BIF) appear during Precambrian (1.8 - about 3 b.y.). Cherts with alternating laminations of red oxidized iron and gray unoxidized iron. Formed as precipitates on shallow sea floor. Some iron probably came from weathering of iron-bearing rocks on continents. Most iron was probably from submarine volcanoes and hydrothermal vents (hot springs). Great economic importance; major source of iron mined in the world.

83 Banded Iron Formations

84 Additional Evidence for an Anoxic Atmosphere 6. The simplest living organisms have an anaerobic metabolism. They are killed by oxygen. Includes some bacteria (such as botulism), and some or all Archaea, which inhabit unusual conditions. 7. Chemical building blocks of life (such as amino acids, DNA) could not have formed in the presence of O 2.

85 Formation of an Oxygen-rich Atmosphere The change from an oxygen-poor to an oxygen-rich atmosphere occurred by Proterozoic, which began 2.5 billion years ago, at the end of Archean.

86 Formation of an Oxygen-rich Atmosphere The development of an oxygen-rich atmosphere is the result of: 1. Photochemical dissociation - Breaking up of water molecules into H and O in the upper atmosphere, caused by ultraviolet radiation from the Sun (a minor process today) 2. Photosynthesis - The process by which photosynthetic bacteria and plants produce oxygen (major process).

87 Evidence for Free Oxygen in the Proterozoic Atmosphere 1. Red beds - Sedimentary rocks with iron oxide cements (including shales, siltstones, and sandstones), appear in rocks younger than 1.8 billion years old. This occurred during Proterozoic, after the disappearance of the banded iron formations (BIFs).

88 Evidence for Free Oxygen in the Proterozoic Atmosphere 2. Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear. This indicates that CO 2 was less abundant in the atmosphere and oceans so that the water was no longer acidic.

89 Precambrian

90 Precambrian Precambrian covers about 4 billion years (and 87%) of Earth history. Precambrian is divided into 2 eons: Proterozoic Eon billion years ago (or million years ago) Archean Eon billion years ago (lower limit not defined)

91 Table of time divisions of Precambrian

92 Precambrian is not well known or completely understood. Why? Precambrian rocks are often poorly exposed. Many Precambrian rocks have been eroded or metamorphosed. Most Precambrian rocks are deeply buried beneath younger rocks. Many Precambrian rocks are exposed in fairly inaccessible or nearly uninhabited areas. Fossils are seldom found in Precambrian rocks; only way to correlate is by radiometric dating.

93 Areas where Precambrian rocks are exposed are shown in yellow, as well as in the red areas in orogenic belts.

94 Shields and Cratons Most of what we know about Precambrian is based on studies of rocks from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic.

95 Shields and Cratons The most extensive exposures of Precambrian rocks are in geologically stable regions of continents called shields. Example = Canadian shield in North America. Mostly igneous and metamorphic rocks; few sedimentary rocks. Overlying sedimentary rocks were scraped off by glaciers during last Ice Age.

96 Shields and Cratons Stable regions of the craton where shields are covered by sedimentary rocks are called platforms. Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.

97 North American craton, shield, platform, and orogenic belts.

98 Precambrian Provinces Various Precambrian provinces can be delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.

99 Precambrian provinces in North America, with dates Oldest (Archean) rocks are shown in orange. Younger (Proterozoic) rocks are shown in green.

100 Origin of Plate Tectonics By about 4 b.y. ago, the Earth had probably cooled sufficiently for plate formation. Once plate tectonics was in progress, it generated crustal rock that could be partially melted in subduction zones and added to the continental crust.

101 Origin of Plate Tectonics Continents also increased in size by addition of microcontinents along subduction zones. Greater heat in Archean would have caused faster convection in mantle, more extensive volcanism, more midoceanic ridges, more hot spots, etc. Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.

102 Granulites and Greenstones The major types of Archean rocks on the cratons are: Granulites Greenstones

103 Granulites Granulites - Highly metamorphosed gneisses (metamorphosed tonalites, granodiorites, and granites) and anorthosites (layered intrusive gabbroic rocks). Granulites formed from partially melted crust and sediments in subduction zones. Metamorphism altered the rocks to form granulites.

104 Greenstones Greenstones - Metamorphosed volcanic rocks and sediments derived from the weathering and erosion of the volcanic rocks. Greenstone volcanic rocks commonly have pillow structures, (called pillow basalts), indicating extrusion under water. The green color is the result of low-grade metamorphism, producing green minerals such as chlorite and hornblende.

105 Greenstones Mostly found in trough-like or synclinal belts. Sequence of rock types : Ultramafic volcanic rocks near the bottom (komatiites) Mafic volcanic rocks (basalts) Felsic volcanic rocks (andesites and rhyolites) Sedimentary rocks at the top (shales, graywackes, conglomerates, and sometimes BIF), deposited in deep water environments adjacent to mountainous coastlines.

106 Generalized cross-section through two greenstone belts. Note sequence of rock types and relationships between granulites and the greenstones. Granulites are present between greenstone belts.

107 Earth's Earliest Glaciation By 2.8 billion years ago, Earth had cooled sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.

108 Earliest Evidence of Life The earliest evidence of life occurs in Archean sedimentary rocks. Stromatolites Microscopic cells of prokaryotes Algal filaments Molecular fossils

109 Stromatolites An organo-sedimentary structure built by photosynthetic cyanobacteria or bluegreen algae. Stromatolites form through the activity of cyanobacteria in the tidal zone. The sticky, mucilage-like algal filaments of the cyanobacteria trap carbonate sediment during high tides.

110 Stromatolites Modern stromatolites, Shark Bay, western Australia

111 Stromatolites More abundant in Proterozoic rocks than in Archean rocks. Examples: Oldest are 3.5 b.y. old, Warrawoona Group, Australia's Pilbara Shield 3 b.y. old Pongola Group of southern Africa 2.8 b.y. old Bulawayan Group of Australia

112 Stromatolites Stromatolites are scarce today because microorganisms that build them are eaten by marine snails and other grazing invertebrates. Stromatolites survive only in environments that are too saline or otherwise unsuitable for most grazing invertebrates. The decline of stromatolites is associated with the evolutionary appearance of new groups of marine invertebrates during Early Paleozoic.

113 Oldest direct evidence of life Microscopic cells and filaments of prokaryotes. Associated with stromatolites Similar to cyanobacteria living today, which produce oxygen. Fossiliferous chert bed associated with the Apex Basalt Found in Warrawoona Group, Pilbara Supergroup, western Australia billion years old

114 Other evidence of Archean life Indirect evidence of life in older rocks Found in banded iron deposits in Greenland. Carbon-13 to carbon-14 ratios are similar to those in present-day organisms. 3.8 b.y.

115 Other evidence of Archean life Algal filament fossils Filamentous prokaryotes preserved in stromatolites. Found at North Pole, western Australia; b.y. old. Spheroidal bacterial structures Found in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones). Prokaryotic cells, showing possible cell division; b.y. old.

116 Other evidence of Archean life Molecular fossils Preserved organic molecules that only eukaryotic cells produce. Indirect evidence for eukaryotes. In black shales from northwestern Australia; 2.7 b.y. Origin of eukaryotic life is pushed back to 2.7 b.y.

117 The Origin of Life The basic materials from which microbial organisms (i.e., life) could have developed initially. May have arrived on Earth during Archean in meteorites called carbonaceous chondrites, which contain organic compounds.

118 Life requires these elements:»carbon»hydrogen»oxygen»nitrogen»phosphorus»sulfur Each of these is abundant in the Solar System.

119 Four essential components of life: 1. Proteins - Chains of amino acids. Proteins are used to build living materials, and as catalysts in chemical reactions in organisms. 2. Nucleic acids - Large complex molecules in cell nucleus. 1. DNA (carries the genetic code and can replicate itself) 2. RNA 3. Organic phosphorus compounds - Used to transform light or chemical fuel into energy required for cell activities. 4. Cell membrane to enclose the components within the cell.

120 The earliest organisms developed in the presence of an atmosphere which lacked oxygen. The organisms must have been anaerobic (i.e., they did not require oxygen for respiration). Organic molecules could not assemble into larger structures in an oxygenated environment. Oxidation and microbial predators would break down the molecules. Because the atmosphere lacked oxygen, there was no ozone shield to protect the surface of the Earth from harmful ultraviolet (UV) radiation.

121 Origin of amino acids UV radiation can recombine atoms in mixtures of water, ammonia and hydrocarbons, to form amino acids. (The energy in lightning can do the same thing.)

122 Miller Experiment Lab simulation experiments by S. Miller in the 1950's formed amino acids from gases present in Earth's early atmosphere: H 2, CH 4 (methane), NH 3 (ammonia), and H 2 O (water vapor or steam), along with electrical sparks (to simulate lightning).

123 Miller Experiment This was the first laboratory synthesis of amino acids. A liquid was produced that contained a number of amino acids and other complex organic compounds that comprise living organisms. A main requirement was the lack of free oxygen.

124 Joining Amino Acids to Form Proteins Amino acids are monomers and have to be joined together to form proteins, which are polymers (or chains). This requires: Input of energy Removal of water

125 Joining Amino Acids to Form Proteins How could this occur? 1. Heating (volcanic activity) 2. At lower temperatures in the presence of phosphoric acid 3. Evaporation 4. Freezing 5. Involve water in a dehydration chemical reaction

126 Joining Amino Acids to Form Proteins 6. On surface of clay particles, which have charged surfaces, and to which polar molecules could attach. Metallic ions on clays could concentrate organic molecules in an orderly array, causing them to align and link into protein-like chains. 7. On pyrite, which has a positively charged surface to which simple organic compounds can become bonded. Formation of pyrite yields energy which could be used to link amino acids into proteins.

127 Proteinoids Proteinoids are protein-like chains produced in the lab by Fox from a mixture of amino acids. Considered to be possibly like the transitional structures leading to proteins billions of years ago. Similar proteinoids are also found in nature around Hawaiian volcanoes.

128 Hot aqueous solutions of proteinoids will cool to form microspheres, tiny spheres that have many characteristics of living cells: Film-like outer wall Capable of osmotic shrinking and swelling Budding similar to yeast Divide into daughter microspheres Aggregate into lines to form filaments, as in some bacteria Streaming movement of internal particles, as in living cells

129 Where Did Life Originate? Early life may have avoided UV radiation by living: Deep beneath the water Beneath the surface of rocks (or below sediment - such as stromatolites) Life probably began in the sea, perhaps in areas associated with submarine hydrothermal vents or black smokers.

130 Evidence for life beginning in the sea near hydrothermal vents: 1. Sea contains salts needed for health and growth. 2. Water is universal solvent, capable of dissolving organic compounds, producing a "rich organic broth" or primordial soup. 3. Ocean currents mix these compounds, leading to collisions between molecules, leading to combination into larger organic molecules.

131 Evidence for life beginning in the sea near hydrothermal vents: 4. Microbes at vents are hyperthermophiles that thrive in seawater hotter than boiling point (100 o C). 5. These microbes derive energy by chemosynthesis, without light, rather than by photosynthesis (suggests origin in deep water in absence of light). 6. Hyperthermophiles are Archaea, with DNA different from bacteria.

132 Feeding Life on Earth Obtaining Nutrients Examples of types of feeding modes: 1. Fermenters - digest chemicals, such as sugar, in the absence of oxygen, to obtain energy. Produce CO 2 and alcohol. Example: Yeast 2. Autotrophs - manufacture their own food. Examples: sulfur bacteria, nitrifying bacteria, and photoautotrophs (such as plants and photosynthetic bacteria) that use photosynthesis 3. Heterotrophs - can't make their own food, so they must find nutrients in the environment to eat. Example: Animals.

133 Evolution of Early Life The earliest cells had to form and exist in anoxic conditions (in the absence of free oxygen). Likely to have been anaerobic bacteria or Archaea. Some of the early organisms became photosynthetic, possibly due to a shortage of raw materials for energy. Produced their own raw materials. Autotrophs. Photosynthesis was an adaptive advantage. Oxygen was a WASTE PRODUCT of photosynthesis.

134 Consequences of Oxygen Buildup in the Atmosphere 1. Ozone layer which absorbs harmful UV radiation, and protected primitive and vulnerable life forms. 2. End of banded iron formations which only formed in low, fluctuating O 2 conditions 3. Oxidation of iron, leading to the first red beds. 4. Aerobic metabolism developed. Uses oxygen to convert food into energy. 5. Development of eukaryotic cell, which could cope with oxygen in the atmosphere.

135 Prokaryotes vs. Eukaryotes Prokaryotes reproduce asexually by simple cell division. This restricts their genetic variability. Prokaryotes have shown little evolutionary change for more than 2 billion years. Eukaryotes reproduce sexually through the union of an egg and sperm. This combines chromosomes from each parent and leads to genetic recombination and increased variability. Many new genetic combinations. Led to a dramatic increase in the rate of evolution.

136 Prokaryotes vs. Eukaryotes

137 Prokaryotes vs. Eukaryotes

138 The Earliest Eukaryotes Earliest large cells that appear to be eukaryotes appear in the fossil record about b.y. ago (during Proterozoic). Eukaryotes diversified around the time that the banded iron formations disappeared and the red beds appeared, indicating the presence of oxygen in the atmosphere. Origin of eukaryotic life was probably around 2.7 b.y., based on molecular fossils.

139 Endosymbiotic Theory for the Origin of Eukaryotes Billions of years ago, several prokaryotic cells came together to live symbiotically within a host cell as protection from (and adaptation to) an oxygenated environment. These prokaryotes became organelles. Evidence for this includes the fact that mitochondria contain their own DNA. Example - a host cell (fermentative anaerobe) + aerobic organelle (mitochondrion) + spirochaetelike organelle (flagellum for motility).

140 Endosymbiotic Origin of Eukaryotes

141 Eukaryotes The appearance of eukaryotes led to a dramatic increase in the rate of evolution, and was ultimately responsible for the appearance of complex multicellular organisms.