Chapter 8 Precambrian Earth and Life History—The Archean Eon

1. Chapter 8Precambrian Earth and Life History—The Archean Eon • Archean Rocks The Beartooth Mountains on the Wyoming and Montana border some of the oldest rocks in the US. gneiss

2. Precambrian Time Span The Precambrian lasted for more than 4 billion years! If Earth’s history were a 24 hour clock, the Precambrian would use 21 hours! • 88% of geologic time

3. Precambrian • from Earth’s origin 4.6 billion years ago • to the beginning of the Phanerozoic Eon • 542 million years ago • all rocks older than the Cambrian system • No rocks are known for the first 600 million years of geologic time • The oldest known rocks on Earth • 4.0 billion years old Two eons for the Precambrian • Archean and Proterozoic • which are based on absolute ages from igneous and metamorphic rocks

4. Eoarchean? • What we know: • Earth accreted from planetesimals • differentiated into a core and mantle • some crust was present • bombarded by meteorites • Volcanic activity was occurring globally • An atmosphere formed, quite different from today’s • Oceans began to accumulate

5. Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was • a rapidly rotating, hot, barren, waterless planet • bombarded by meteorites and comets • with no continents, intense cosmic radiation • and widespread volcanism

6. Eoarchean evidence shows: • Continental crust was present by 4.0 billion years ago • Evidence:Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old • so source rocks at least that old existed • Eoarchean Earth probably rotated in as little as 10 hours • and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface watersto accumulate

7. Eoarchean Crust • mafic and ultramafic magma: Temperatures > 1600C • and numerous subduction zones developed • Small island arcs • Eoarchean continental crust may have formed • by collisions between island arcs • as silica-rich materials were metamorphosed. • Larger groups of merged island arcs • protocontinents • grew faster by accretion along their margins

8. Origin of Continental Crust • Intermediate • (Andesitic)island arcs • form by subduction • and partial melting of oceanic crust • The island arc collides with another

9. Continental Foundations • Continents: composition similar to granite (same density as silica or quartz) • Continental crust is thicker • Ocean crust: Composition of basalt and gabbro (higher density )Ocean Crust is thinner

10. Cratons • Cratons have experienced little deformation • since the Precambrian • Precambrian shields are exposed ancient rocks on all continents • Covered by platforms • Sedimentary rocks that overlie the shield • Shield + Platform = Craton

11. Distribution of Precambrian Rocks • The exposed craton in North America is the Canadian shield • northeastern Canada • large part of Greenland • parts of the Lake Superior region • in Minnesota, Wisconsin, and Michigan • and the Adirondack Mountains of New York

12. Evolution of North America • North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added.

13. Archean Rocks • granite-gneiss complexes • ultramafic igneous peridotite • sedimentary rocks had been metamorphosed • Greenstone belts are 10% of Archean rocks • Help unravel Archean tectonic events

14. Archean Rocks • Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada

15. Archean Rocks • Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite

16. Greenstone Belts • A greenstone belt has distinct rock units • volcanic rocks are most common • Sedimentary • Intruded by granitic magma • Low-grade metamorphism • Makes igneous rocks green • chlorite, actinolite, and epidote

17. Greenstone Belts and Granite-Gneiss Complexes • Two adjacent greenstone belts showing synclinal structure • They are underlain by granite-gneiss complexes • and intruded by granite

18. Greenstone Belt Volcanics • Pillow lavas in greenstone belts • indicate that much of the volcanism was • subaqueous Pillow lavas in Ispheming greenstone belt at Marquette, Michigan

19. Ultramafic Lava Flows • Ultramafic magma (< 45% silica) • requires near surface magma temperatures of more than 1600°C • 250°C hotter than any recent flows • Why then, not now?Early Earth: • radiogenic heating was greater • and the mantle was as much as 300 °C hotter • Earth cooled: They are rare in rocks younger • than Archean and none occur now

20. To summarize…Ultramafic Lava Flows • As Earth’s production • of radiogenic heat decreased, • the mantle cooled • and ultramafic flows no longer occurred

21. Sedimentary Rocks of Greenstone Belts • Many of these rocks are successions of • graywacke • sandstone with abundant clay and rock fragments • and argillite = shale

22. Sedimentary Rocks of Greenstone Belts • Small-scale cross-bedding and graded bedding • indicate an origin as turbidity current deposits

23. Canadian Greenstone Belts • In North America, • most greenstone belts • (dark green) • occur in the Superior and Slave cratons • of the Canadian shield

24. Evolution of Greenstone Belts • Greenstone belts formed in several tectonic settings • Models for the formation of greenstone belts • involve Archean plate movement • In one model, greenstone belts formed • in back-arc marginal basins

25. Evolution of Greenstone Belts • According to this model, • There was an early stage of extension as the back-arc marginal basin formed • volcanism and sediment deposition followed

26. Evolution of Greenstone Belts • Then during closure, • the rocks were compressed, • metamorphosed, • and intruded by granitic magma • The Sea of Japan • is a modern example • of a back-arc basin

27. Archean Plate Tectonics • Plates moved faster • heat from Earth’s origin • more radiogenic heat (radioactive decay), • Magma generated more rapidly • Therefore… • continents grew rapidly • By continental collision and accretion with other plates and islands

28. Southern Superior Craton Evolution • Greenstone belts (dark green) • Granite-gneiss complexes (light green Geologic map • Plate tectonic model for evolution of the southern Superior craton • North-south cross section

29. Atmosphere and Hydrosphere • Today’s atmosphere: • nitrogen (N2) 78% • free oxygen (O2) 21% • or oxygen in compounds (CO2) • water vapor (H2O) varies 0.1 – 04% • other gases, like ozone (O3) ~ < 2% • block most of the Sun’s ultraviolet radiation

30. Earth’s Very Early Atmosphere • hydrogen and helium, • the most abundant gases in the universe • Earth’s gravity is insufficient to retain them • Earth had no magnetic field until its core formed (magnetosphere) • Without a magnetic field, • the solar wind would have swept gases away

31. Outgassing • After magnetic field forms: • Atmosphere accumulated from outgassing • during volcanism • Water vapor • common volcanic gas • volcanoes also emit • carbon dioxide • sulfur dioxide • Methane • Nitrogen oxide

32. Archean Atmosphere • Archean volcanoes • emitted the same gases, • atmosphere developed • ! lacking free oxygen and an ozone layer • Oxygen in compounds:CO2, ammonia (NH3) methane (CH4)

33. Evidence for a lack ofFree Oxygen Atmosphere • detrital deposits • contain minerals that oxidize rapidly • These minerals are NOT bound to typical abundances of oxygen • pyrite (FeS2) • uraninite (UO2)

34. Introduction of Free Oxygen • Two processes account for 1. Photochemical dissociation • radiation breaks up water molecules upper atmosphere • releases their oxygen and hydrogen • 2% of present-day oxygen • with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis

35. Photosynthesis • Photosynthesis is a metabolic process • carbon dioxide and water make organic molecules • and oxygen is released as a waste product CO2 + H2O ==> organic compounds + O2 • probably no more than 1% of the free oxygen level • of today was present by the end of the Archean

36. Oxygen Forming Processes • Photochemical dissociation and photosynthesis • added free oxygen to the atmosphere • Once free oxygen was present • an ozone layer formed • and blocked incoming ultraviolet radiation

37. Earth’s Surface Waters • Volcanic OutgassingMeteorites and icy cometsRapid rate of accumulation of water • Most of hydrosphere in the oceans -- more than 97% • Today: water vapor still emitted the rate of volcanism has decreased considerably • -- heat needed to generate magma has diminished

38. Decreasing Heat • Ratio of radiogenic heat production in the past to the present • Heat production 4 billion years ago was 3 to 6 times as great as it is now • With less heat outgassing decreased

39. First Organisms • We have fossils from Archean rocks 3.5 billion yrs • Only bacteria and archeaare found in Archean rocks • Chemical evidence in rocks in Greenland • that are 3.8 billion years old • convince some investigators that organisms were present then

40. What Is Life? • living organism must reproduce • and practice some kind of metabolism • The distinction between • living and nonliving things is not always easy • Are viruses living? • When in a host cell they behave like living organisms • but outside they neither reproduce nor metabolize

41. What Is Life? • form spontaneously • can even grow and divide in a somewhat organism-like fashion • but their processes are more like random chemical reactions, so they are not living • Carbon based molecules known as microspheres

42. How Did Life First Originate? • from non-living matter (abiogenesis), life must have passed through a prebiotic stages • it showed signs of living • but was not truly living • The origin of life has 2 requirements • source of appropriate elements for organic molecules • energy sources to promote chemical reactions

43. Elements of Life • All organisms are composed mostly of • carbon (C) • hydrogen (H) • nitrogen (N) • oxygen (O) • S P O N C H • all of which were present in Earth’s early atmosphere as • carbon dioxide (CO2) • water vapor (H2O) • nitrogen (N2) • and possibly methane (CH4) • and ammonia (NH3)

44. Basic Building Blocks of Life • Energy from • Lightning, volcanism, • ultraviolet radiation • C, H, N, and O combined to form monomers • such as amino acids • Monomers are the basic building blocks • of more complex organic molecules

45. Experiment on the Origin of Life • Is it plausible that monomers • originated in the manner postulated? • Experimental evidence indicates that it is • Late 1950s • Stanley Miller • synthesized several amino acids • by circulating gases approximating • the early atmosphere • in a closed glass vessel

46. Experiment on the Origin of Life • This mixture was subjected to an electric spark • to simulate lightning • In a few days • it became cloudy • Produced • several amino acids • typical of organisms • had formed • Since then, • scientists have synthesized • all 20 amino acids • found in organisms

47. Polymerization • The molecules of organisms are polymers • consisting of monomers linked together in a specific sequence • RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) • Problem: • Water usually causes depolymerization, • however, researchers synthesized molecules • known as proteinoids when heating dehydrated concentrated amino acids

48. Proteinoid Microspheres • Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as bacteria do

49. Protobionts • These proteinoid molecules can be referred to asprotobionts • that are intermediate between • inorganic chemical compounds • and living organisms

50. Monomer and Proteinoid Soup:Model for abiogenesis • Monomers likely formed continuously and by the billions • accumulated in the early oceans into a “hot, dilute soup” • The amino acids in the “soup” dried (might have washed up onto a beach or perhaps cinder cones) • they were polymerized by heat • The polymers then washed back into the ocean • where they reacted further