Precambrian Earth and Life History—The Eoarchean and Archean

1. Precambrian Earth and Life History—The Eoarchean and Archean

2. Ch 8 Precambrian Time Span How many years Are represented By the Precambrian? How many years are Represented by the Archean Eon? What per cent of time Is represented by the Eoarchean (Hadean)? What per cent of time Is represented by the Entire Precambrian?

3. Time check If a 24-hour clock represented all 4.6 billion years of geologic time • the Precambrian would be slightly more than 21 hours long, • It constitutes about 88% of all geologic time How big is a billion? 1. If you received $1,000 a day, seven days a week, how long would it take to reach a billion? • If you received $1 every second for your entire life, and you just reached $1 billion dollars, when were you born? • Scientists believe the earth to be approximately 4.5 billion years old. If someone had stashed $155 under a rock each year since the Big Bang or Creation, whichever is your cup of tea, we’d have about $700 billion just in time to solve the current financial crisis!

4. Precambrian No rocks are known for the first 640 million years of geologic time • The oldest known rocks on Earth are 3.96 billion years old • Updated: • http://www.sciencedaily.com/releases/2008/09/080925144624.htm • Ancient rocks such as the 3.9 billion year old Acasta Gneiss in Canada are metamorphic. What does that reveal about earth’s crust? • What do detrital zircons reveal about earth’s crust?

5. Rocks of the Precambrian • Why is it difficult to interpret the geologic past of the Precambrian period? • buried deep beneath younger rocks • altered by metamorphism and deformed • fossils are rare and not much help

6. The Eoarchean: What is known? • Except for meteorites no rocks of Hadean age are present on Earth, however we do know some events that took place during this period • Earth was accreted • Differentiation occurred, creating a core and mantle and at least some crust

7. Earth beautiful Earth…. about 4.6 billion years ago • Shortly after accretion, Earth was a rapidly rotating, hot, barren, waterless planet • bombarded by comets and meteorites • There were no continents, • intense cosmic radiation • widespread volcanism

8. Earth’s earliest crust formed • Eoarchean crust was probably thin, unstable and made up of ultramafic rock • rock with comparatively little silica Less than 40% SiO2 – ultramafic 40% - 60% SiO2 – mafic 60% to 70% SiO2 – intermediate Greater than 70% -- felsic • This ultramafic crust was disrupted by upwelling basaltic magma at ridges and consumed at subduction zones • Eoarchean continental crust may have formed by evolution of silica-rich material • Sialic crust contains considerable silicon, oxygen and aluminum as in present day continental crust • Only sialic-rich crust, because of its lower density, is immune to destruction by subduction

9. Crustal Evolution • A second stage in crustal evolution began as Earth’s production of radiogenic heat decreased • Subduction and partial melting of earlier-formed basaltic crust resulted in the origin of andesitic island arcs • Partial melting of lower crustal andesites, in turn, yielded silica-rich granitic magmas that were emplaced in the andesitic arcs

10. Dynamic Processes • During the Eoarchean, various dynamic systems similar to ones we see today, became operative, • Once Earth differentiated into core, mantle and crust, • internal heat caused interactions among plates • they diverged, converged and slid past each other • Continents began to grow by accretion along convergent plate boundaries • Density of Continental Crust ~ 2.7g/cm3 Density of Oceanic Crust ~ 3.0 g/cm3

11. Continental Foundations:Precambrian Cratons Precambrian shields found on every continent Broadplatforms of buried Precambrian rocks that underlie much of each continent

12. Distribution of Precambrian Rocks • Areas of exposed Precambrian rocks constitute the shields • Platforms consist of buried Precambrian rocks • Shields and adjoining platforms make up cratons

13. Canadian Shield • The craton in North America is the Canadian shield • Occupies most of northeastern Canada, a large part of Greenland, parts of the Lake Superior region in Minnesota, Wisconsin, Michigan, and the Adirondack Mountains of New York

14. Canadian Shield Rocks • Gneiss, a metamorphic rock,Georgian Bay Ontario, Canada

15. Canadian Shield Rocks • Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario

16. Archean Rocks • The most common Archean Rock associations are granite-gneiss complexes • The rocks vary from granite (felsic) to peridotite (ultramafic) to various sedimentary rocks all of which have been metamorphosed • Greenstone belts are subordinate in quantity but are important in unraveling Archean tectonism

17. Greenstone Belts • volcanic rocks are most common in the lower and middle units • the upper units are mostly sedimentary • The belts typically have synclinal structure • Most were intruded by granitic magma and cut by thrust faults • Low-grade metamorphism • makes many of the igneous rocks greenish (chlorite) • An ideal greenstone belt has 3 major rock units

18. Greenstone Belt Volcanics • Abundant pillow lavas in greenstone belts indicate that much of the volcanism was under water • Pyroclastic materials probably erupted where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone at Marquette, Michigan

19. Ultramafic Lava Flows • The most interesting rocks in greenstone belts cooled from ultramafic lava flows • Ultramafic magma has less than 40% silica • requires near surface magma temperatures of more than 1600°C—250°C • hotter than any recent flows • During Earth’s early history, radiogenic heating was higher and the mantle was as much as 300 °C hotter than it is now • This allowed ultramafic magma to reach the surface

20. Sedimentary Rocks of Greenstone Belts • Sedimentary rocks are found throughout the greenstone belts • Mostly found in the upper unit • Many of these rocks are successions of • graywacke • a sandstone with abundant clay and rock fragments • and argillite • a slightly metamorphosed mudrock

21. Sedimentary Rocks of Greenstone Belts • Small-scale cross-bedding and graded bedding indicate an origin as turbidity current deposits • Quartz sandstone and shale, indicate delta, tidal-flat, barrier-island and shallow marine deposition

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

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 • Models for the formation of greenstone belts involve Archean plate movement • In one model, plates formed volcanic arcs by subduction • the greenstone belts formed in back-arc marginal basins

25. Evolution of Greenstone Belts • According to this model, • volcanism and sediment deposition took place as the basins opened

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

28. Archean Plate Tectonics • Plates must have moved faster • residual heat from Earth’s origin • more radiogenic heat • magma was generated more rapidly

29. Archean Plate Tectonics • continental accretion: Continents grew quickly as plates collided with island arcs and other plates • Also, ultramafic extrusive igneous rocks were more common due to the higher temperatures

30. 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

31. Atmosphere and Hydrosphere • Earth’s early atmosphere and hydrosphere were quite different than they are now • They also played an important role in the development of the biosphere • Today’s atmosphere • is mostly nitrogen (N2) • abundant free oxygen (O2) • oxygen not combined with other elements such as in carbon dioxide (CO2) • water vapor (H2O) • ozone (O3) which is common enough in the upper atmosphere to block most of the Sun’s ultraviolet radiation

32. Present-day Atmosphere Nonvariable gases Nitrogen N2 78.08% Oxygen O2 20.95 Argon Ar 0.93 Neon Ne 0.002 Others 0.001 in percentage by volume • Variable gases Water vapor H2O 0.1 to .4 Carbon dioxide CO2 0.034 Ozone O3 0.0006 Other gases Trace • Particulates normally trace

33. Earth’s Very Early Atmosphere • Earth’s very early atmosphere was probably composed of hydrogen and helium, the most abundant gases in the universe • What are two reasons that Earth’s early atmosphere was most likely lost to space?

34. Outgassing • Once a core-generated magnetic field protected Earth, gases released during volcanism began to accumulate • Called outgassing • Water vapor is the most common volcanic gas today • also emitted • carbon dioxide • sulfur dioxide • Hydrogen Sulfide • carbon monoxide • Hydrogen • Chlorine • nitrogen

35. Early Precambrian Atmosphere • Eoarchean volcanoes probably emitted the same gases, and thus an atmosphere developed • but one lacking free oxygen and an ozone layer • It was rich in carbon dioxide, and gases reacting in this early atmosphere probably formed • ammonia (NH3) • methane (CH4) • This early atmosphere persisted throughout the Archean

36. Evidence for an Oxygen-Free Atmosphere • Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen • pyrite (FeS2) • uraninite (UO2) • Oxidized iron becomes increasingly common in Proterozoic rocks

37. Introduction of Free Oxygen • Two processes account for introducing free oxygen into the atmosphere, 1.Photochemical dissociation involves ultraviolet radiation in the upper atmosphere • The radiation breaks up water molecules and releases oxygen and hydrogen • This could account for 2% of present-day oxygen • but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiation 2. More important were the activities of organism that practiced photosynthesis

38. Photosynthesis • Photosynthesis is a metabolic process in which carbon dioxide and water combine into organic molecules and oxygen is released as a wasteproduct CO2 + H2O ==> organic compounds + O2 “autotrophic food” • Even with photochemical dissociation and photosynthesis, probably no more than 1% of the free oxygen level of today was present by the end of the Archean

39. Earth’s Surface Waters • Outgassing was responsible for the early atmosphere and also for Earth’s surface water • the hydrosphere • Some but probably not much of our surface water was derived from icy comets • At some point during the Eoarchean, the Earth had cooled sufficiently so that the abundant volcanic water vapor condensed and began to accumulate in oceans • Oceans were present by Early Archean times

40. Ocean water • The volume and geographic extent of the Early Archean oceans cannot be determined Volcanoes still erupt and release water vapor • Is the volume of ocean water still increasing? • Much of volcanic water vapor today is recycled surface water

41. First Organisms • Today, Earth’s biosphere consists of millions of species found in the “five kingdoms” • bacteria, fungi, protists, plants, and animals • only bacteria are found in Archean rocks • We have fossils from Archean rocks • 3.3 to 3.5 billion years old (ancient stromatolites) • Carbon isotope ratios in rocks in Greenland that are 3.85 billion years old convince some investigators that life was present then

42. But first, What Is Life? • Minimally, a living organism must reproduce and practice some kind of metabolism • Reproduction insures the long-term survival of a group of organisms • whereas metabolism such as photosynthesis, for instance insures the short-term survival of an individual • 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

43. What Is Life? • Comparatively simple organic (carbon based) molecules known as microspheres • form spontaneously • show greater organizational complexity than inorganic objects such as rocks • 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

44. How Did Life First Originate? • To originate by natural processes, life must have passed through a prebiotic stage • it showed signs of living organisms but was not truly living • In 1924 A.I. Oparin postulated that life originated when Earth’s atmosphere had little or no free oxygen • Oxygen is damaging to Earth’s most primitive living organisms • Some types of bacteria must live where free oxygen is not present

45. How Did Life First Originate? • With little or no oxygen in the early atmosphere and no ozone layer to block ultraviolet radiation, life could have come into existence from nonliving matter • The origin of life has 2 requirements • a source of appropriate elements for organic molecules • energy sources to promote chemical reactions

46. Elements of Life • All organisms are composed mostly of • carbon (C) • hydrogen (H) • nitrogen (N) • oxygen (O) 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)

47. Basic Building Blocks of Life • Energy from • lightning • ultraviolet radiation • probably promoted chemical reactions • during which C, H, N and O combined • to form monomers • comparatively simple organic molecules • such as amino acids • Monomers are the basic building blocks of more complex organic molecules

48. Experiment on the Origin of Life • During the late 1950s • Stanley Miller synthesized several amino acids by circulating gases approximating the early atmosphere in a closed glass vessel

49. Polymerization • The molecules of organisms are polymers • proteins • nucleic acids • RNA-ribonucleic acid and DNA-deoxyribonucleic acid • consist of monomers linked together in a specific sequence • How did polymerization take place? • Water usually causes depolymerization, however, researchers synthesized molecules known as proteinoidssome of which consist of more than 200 linked amino acids when heating dehydrated concentrated amino acids

50. Proteinoids • The heated dehydrated concentrated amino acids spontaneously polymerized to form proteinoids • Perhaps similar conditions for polymerization existed on early Earth,but the proteinoids needed to be protected by an outer membrane or they would break down • Experiments show that proteinoids • spontaneously aggregate into microspheres • are bounded by cell-like membranes • grow and divide much as bacteria do