PRECAMBRIAN PART 1

1. PRECAMBRIANPART 1 Chapter 8 HADEAN & ARCHEAN EONS

2. TOPICS • Formation & Evolution of: • Rocks • Atmosphere • Hydrosphere • Life Forms

3. LECTURE NOTES • Topic • Event • Date • Description

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6. Defining Characteristics of 3 Eons • Hadean: 4.6–4.0 bya • formation of Earth’s crust and main bombardment • Archean: 4.0–2.5 bya • first life appears • plate tectonics established • oxygen-poor atmosphere • Proterozoic: 2.5 bya–542 mya • first multicellular animals at end of interval • 4 major mountain-building episodes • • oldest known glaciation

7. t08_03_pg226 PRECAMBRIAN EONS PROTEROZOIC EON ARCHEAN EON HADEAN EON t08_03_pg226

8. Formation of the Earth • Cold Acretion or Hot Acretion Models resulting in differentiationof layers • Formed from the coalescing and bombardment by objects from space ranging in size from dust particles to small planets (planetismals). • The impacts of large bodies and the decay of radioactive elements generated heat that melted the materials of the young Earth, creating the “hellish” conditions for which the Hadean Eon was named. • Evidence of bombardment is the craters on the moon that occurred at the same time (4.0 to 3.8 bya). • Heat on the Earth was generated through decay of radioactive elements and continued frequent bombardment by asteroids, • Earth lost heat to space and slowly cooled. • Earth eventually segregated into an iron core and silicate mantle..

9. Earth’s Crust • Once differentiation occurred, Earth's crust was dominated by iron and magnesium silicate minerals. • Covered by an extensive magma ocean in the Archean. • Magma cooled to form rocks called komatiites. • form at temperatures greater than those at which basalt forms (greater than 1100oC). • ultramafic rocks composed mainly of olivine and pyroxene.This rock formed Earth's Archean crust. • The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of rocks in the upper mantle.

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11. Earth's Crust Today • Earth has two types of crust today: • Oceaninc Crust - dense, mafic (magnesium- and iron-rich) dominated by basalt. • Continental Crust - less dense, sialic (silicon- and aluminum-rich) dominated by granite. • Formed about 4 bya.

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14. Evolution of the 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. • Volatiles = substances easily driven off by heating. • Primitive Atmosphere • Earth's gases were originally derived from impacts of comets and meteorites.

15. Volcanic Outgassing • Outgassing = the release of water vapor and other gases from the Earth through volcanism. • Analysis of samples from Hawaiian eruptions include the following gases: • 70% water vapor (H2O) • 15% carbon dioxide (CO2) • 5% nitrogen (N2) • 5% sulfur (in H2S) • chlorine (in HCl) • hydrogen • argon • Most of the water on the surface of the Earth and in the atmosphere was outgassed in the first billion years of Earth history. We know this because there are 3.8 billion-year-old marine sedimentary rocks, indicating the presence of an ocean by 3.8 billion years ago.

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17. Formation of the Hydrosphere • Hydrologic Cycle initiated • Rain water accumulated in low places to form seas. The seas were originally freshwater (rain). • Water more acidic than today. • Ions accumulated in the water, increasing the salinity. Ocean salinity is relatively constant today because surplus salts are precipitated at about the same rate at which they are supplied to the sea. • Much later, when the seas became less acidic, calcium ions bonded with carbon dioxide to form shells of marine organisms and limestones. • The presence of marine fossils suggests that sodium has not varied appreciably in sea water for at least the past 600 million years. • Today Earth's water is continuously recirculated through the hydrologic cycle (evaporation and precipitation, powered by the sun and by gravity).

18. The Early Anoxic Atmosphere • Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen or O2 gas) and probably consisted primarily of: • Water vapor (H2O) • Carbon dioxide (CO2) • Nitrogen (N2) • Carbon monoxide (CO) • Hydrogen sulfide (H2S) • Hydrogen chloride (HCl) • The composition would have been similar to that of modern volcanoes, but probably with more hydrogen, and possibly traces of methane (CH4) 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.

19. Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere • Lack of oxidized iron in the oldest sedimentary rocks. • Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sedimentary rocks. • Rocks are commonly dark due to the presence of carbon, which would have been oxidized if oxygen had been present. • 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.

20. Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere – “Continued” • Banded iron formations (BIF) appear in the Precambrian (1.8 - about 3 by). • Represented by cherts with alternating laminations of red oxidized iron and gray unoxidized iron. • BIF formed as precipitates on the floors of shallow seas.Some of the iron probably came from weathering of iron-bearing rocks on the continents, but most was probably from submarine volcanoes and hydrothermal vents (hot springs) on the sea floor. Great economic importance; major source of iron mined in the world. • 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. • Chemical building blocks of life (such as amino acids, DNA) could not have formed in the presence of O2.

21. Formation of an Oxygen-rich Atmosphere • The change from an oxygen-poor to an oxygen-rich atmosphere occurred by the Proterozoic, which began 2.5 billion years ago at the end of the Archean. • The development of an oxygen-rich atmosphere is the result of: • Photochemical dissociation - The breaking up of water molecules into hydrogen and oxygen in the upper atmosphere caused by ultraviolet radiation from the Sun (a minor process today) • Photosynthesis - The process by which photosynthetic bacteria and plants produce oxygen (major process).

22. Evidence for Free Oxygen in the Proterozoic Atmosphere • Red beds, or sedimentary rocks with iron oxide cements, including shales, siltstones, and sandstones, appear in rocks younger than 1.8 billion years old. This is in the Proterozoic Eon, after the disappearance of the BIF. • Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear. This indicates that carbon dioxide was less abundant in the atmosphere and oceans so that the water was no longer acidic.

23. Rock Deposits Precambrian Rocks

24. Precambrian Rocks • Precambrian rocks are 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.

25. Precambrian Rock Deposits • Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata. • 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. • 3 types of Precambrian Deposits • Shields – Base precambrian rock. • Platforms – Layer of sedimentary rock that covers the shields. • Cratons – Combination of the Shields and Platforms.

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29. 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. • 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.

30. Granulites and Greenstones • The major types of Archean rocks on the cratons are: • Granulites – Formed from highly metamorphosed crust and sediments in subduction zones. • Greenstones - Metamorphosed volcanic rocks and sediments derived from the weathering and erosion of the volcanic rocks. Greenstone volcanic rocks commonly have pillow structures, indicating extrusion under water.There is a specific sequence of rock types in greenstone belts. These include: • 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. • Granulites are present between greenstone belts.

31. 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.

32. Life of the Archean - The Fossil Record • The earliest evidence of life occurs in Archean sedimentary rocks. • Evidence of Archean life consists of: • Stromatolites - An organo-sedimentary structure built by photosynthetic cyanobacteria or blue-green algae. They are not true fossils. • 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.Modern stromatolites are found today in isolated environments with high salinity, such as Shark Bay, western Australia.

33. Other evidence of Archean life: • Oldest direct evidence of lifeMicroscopic cells and filaments of prokaryotes. Found in Warrawoona Group, Pilbara Supergroup, western Australia3.460-3.465 b.y. • Indirect evidence of life in older rocksFound in banded iron deposits in Greenland.Carbon-13 to carbon-14 ratios are similar to those in present-day organisms.3.8 b.y. • Algal filament fossils Filamentous prokaryotes preserved in stromatolites.Found at North Pole, western Australia.3.4-3.5 b.y. • Spheroidal bacterial structuresFound in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones).Prokaryotic cells, showing possible cell division.3.0 - 3.1 b.y. • Molecular fossilsPreserved 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.

34. The Origin of Life • Creation 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.) • Lab simulation experiments by Miller and Urey in the 1950's. Formed amino acids from gases present in Earth's early atmosphere:H2, CH4 (methane), NH3 (ammonia), and H2O (water vapor or steam), along with electrical sparks (to simulate lightning).

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36. 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. • Evidence for life beginning in the sea near hydrothermal vents:

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38. Evolution of early life and the transition from prokaryotes to eukaryotes • 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. • Photosynthesis was an adaptive advantage. • Oxygen was a WASTE PRODUCT of photosynthesis. • Consequences of oxygen buildup in the atmosphere: • Development of ozone layer which absorbs harmful UV radiation, and protected primitive and vulnerable life forms. • End of banded iron formations which only formed in low, fluctuating O2 conditions • Oxidation of iron, leading to the beginning of red beds - iron oxides (hematite).

39. Evolution of early life and the transition from prokaryotes to eukaryotes • Prokaryotes • No Nucleus or organelles • reproduce asexually by simple cell division. This restricts their genetic variability. For this reason, prokaryotes have shown little evolutionary change for more than 2 billion years. • Eukaryotes • contain a nucleus and organelles • Aerobic metabolism developed. Uses oxygen to convert food into energy. • could cope with the oxygen in the atmosphere. • reproduce sexually leading to genetic recombination and increased variability and rate of evolution. • Origin of eukaryotic life was probably around 2.7 b.y., based on molecular fossils. • appeared in the fossil record about 1.6 - 1.4 billion years ago (in the 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.

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41. Endosymbiotic Theory • The Endosymbiotic Theory for the Origin of Eukaryotes proposes that 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) + spirochaete-like organelle (flagellum for motility).

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43. THE END