Precambrian Time

1. Precambrian Time • “Precambian” is the informal term for the interval of time prior to the evolutionary radiation of skeletonized animals at 543 mybp • “Precambrian” is subdivided into: • Archean Eon, from the origin of the Earth (4.6 bybp) to the stabilization of Earth’s basic structure (core/mantle/crust) (2.5 bybp) • Proterozoic Eon, from 2.5 bybp to the beginning of Cambrian time (543 mybp) Earth History, Ch. 11

2. Geologic time Earth History, Ch. 11

3. Precambrian rocks • Although Precambrian time accounts for 88% of Earth’s history, Precambrian rock exposures make up only about 20% of Earth’s land surface • Most Precambrian rocks have been destroyed in the course of plate tectonic cycles (and most remaining ones are buried beneath the veneer of Phanerozoic rocks) Earth History, Ch. 11

4. Earth History, Ch. 11

5. Precambrian rocks • Cratons are the large, stable, interior regions of continents that have not undergone major deformation since Precambrian or early Phanerozoic time • Most Precambrian rocks are confined to cratons, where they may be exposed in a “Precambrian shield” Earth History, Ch. 11

6. Precambrianshield area inNW Canada Earth History, Ch. 11

7. Archean Time:From the very beginning…. • Age of universe is estimated at ~15 billion years (redshift evidence) • Oldest radiometrically dated rocks on Earth are ~4.1 billion years old • But, meteorites and lunar rocks have been dated at 4.6 billion years, suggesting that our solar system is about that old Earth History, Ch. 11

8. Origin of our galaxy and solar system Solar nebula forms (remains of supernova) Rotation and contraction to disk Central concentration of matter Formation of discrete rings of matter Condensation of matter into planets Earth History, Ch. 11

9. Origin of our galaxy and solar system (cont.) • Outer planets are composed largely of volatile compounds • Denser, less volatile compounds make up the inner planets • Asteroid belt is a ring of debris that has not coalesced into a planet Earth History, Ch. 11

10. Origin of Earth • Primordial Earthaccreted from successive impacts of hot, giant asteroids (some the size of Mars) • Early Earth was molten because of heat from energy of impacts and radioactive decay • Dense materials sank to center of planet, with less dense materials rising toward surface • “Magma ocean” at surface eventually cooled to form oceanic crust Earth History, Ch. 11

11. Origin of Earth (cont.) Homogeneous molten Earth Segregation of materials by density Final differentiation of core/mantle/crust Earth History, Ch. 11

12. Earth’s early heat flow • Earth had greater heat flow in the Archean Eon than today, because Earth’s radioactive “furnace” was hotter • “Hot spots” were numerous; lithosphere was fragmented into many small plates • Felsic crust was partitioned into small “protocontinents” Earth History, Ch. 11

13. Earth’s internal heat Earth History, Ch. 11

14. Origin of the Moon • Moon originated when a large (Mars-size) body collided with Earth (“glancing blow”) • Core of impacting body was absorbed into Earth’s core • Remaining mantle of impacting body and was then captured in Earth’s gravitational field • Collision caused Earth’s rotation to increase • Moon has no water; a metallic core and feldspar-rich outer layer; relative abundance of iron and magnesium differ from that in Earth’s mantle Earth History, Ch. 11

15. Earth’s early atmosphere • Earth did not inherit its atmosphere from the initial asteroids that coalesced to form it • Earliest atmosphere was generated by emission of internal gases (similar to those emitted today from volcanoes): • Water vapor, hydrogen, hydrogen chloride, carbon monoxide, carbon dioxide, nitrogen • Note absence of oxygen, which was rare prior to the advent of photosynthetic organisms! Earth History, Ch. 11

16. Earth’s early oceans • Ocean water originated partly from emitted water vapor and partly from icy comets as they melted upon entry into Earth’s atmosphere • 15 million small comets (~12 meters in diameter) enter Earth’s atmosphere every year! • Salts were added to the oceans from rivers carrying by-products of chemically weathered rocks • Salinity stabilized very early in Archean time because salt is removed from the oceans by precipitation of salt minerals Earth History, Ch. 11

17. Origin of continents • Earth’s early crust was entirely oceanic crust of mafic composition • Earliest continental (felsic) crust must have originated from a mafic parent,but how? • When mafic crust is subducted and melted, the resulting extrusive volcanics still possess a mafic or intermediate composition • Igneous activity associated with hot spots can produce felsic volcanics!! Earth History, Ch. 11

18. Origin of continents: Iceland example • Iceland is a volcanic island situated over a hot spot along the mid-Atlantic ridge • Here, lower oceanic crust contains isolated “pods” of felsic material that have segregated from igneous material in the mantle • Mafic magma flows to the surface along faults; in doing so it melts felsic bodies along the wayàfelsic volcanics • As volcanics pile up, isostatic sinking of Iceland causes partial melting and further segregation of felsicsàmore felsic volcanics Earth History, Ch. 11

19. Origin of continents:Iceland example About 10% of Iceland’s crust is felsic in composition Earth History, Ch. 11

20. Origin of continents:Iceland example • Iceland’s crust is 8–10 km thick, about twice the average thickness of oceanic crust • Iceland is only about 16 million years old and still growing—it’s a protocontinent! • Archean continents remained small: lithospheric plates were all small because of Earth’s high heat flow • In Proterozoic time, once the pace of plate tectonics slowed, protocontinents were sutured together to form larger continents Earth History, Ch. 11

21. Archean continental crust • Oldest dated continental crust minerals are ~4.4 billion years old • Oldest large area of continental crust is ~3.8–4.0 billion years old (NWT Canada) • Geologists believe that by ~3.5 bybp, total volume of continental crust reached its present level • No net gain or loss since then, because as new felsic material is added by igneous activity, old felsic material is consumed at subduction zones Earth History, Ch. 11

22. Archean rocks • Archean sedimentary rocks are mostly of deep-water origin • Sandstones, cherts, shales, banded-iron formations • Very few, if any, limestones or evaporites • No well developed continental shelves for accumulation of shallow-water deposits Earth History, Ch. 11

23. Archean rocks Earth History, Ch. 11

24. Archean rocks (cont.) • Banded iron formations • Alternating bands of iron-rich layers and chert layers • Thought to have precipitated from hot marine water associated with igneous activity • Iron is weakly oxidized (looks like iron), suggesting little or no exposure to oxygen • Very few banded iron formations younger than 1.9 billion years old (when atmospheric O2 increased) • Most iron deposits younger than 1.9 billion are highly oxidized (red beds) • Principal source of world’s iron ore Earth History, Ch. 11

25. Banded iron formations Iron layers Chert layers (red) Earth History, Ch. 11

26. Archean rocks (cont.) • Greenstone belts • Make up large portions of Archean terranes • Age of most greenstone belts is ~2.5–3.0 billion years • Elongate belts of weakly metamorphosed rock separating larger masses of felsic protocontinents • Include metamorphosed mixtures of mafic and felsic volcanics, volcanic sediments, turbidites • Assemblage of precursor rocks is characteristic of forearc basins and subduction zones • Probably formed along subduction zones where protocontinents were sutured together Earth History, Ch. 11

27. Formation of greenstone belts Time 1 Time 2 Earth History, Ch. 11

28. Greenstone belts Satellite view of Archean greenstone belts and felsic protocontinents in western Australia 25 mi Earth History, Ch. 11

29. Life on Earth • Why Earth is well suited for harboring life: • Right size • Gravitational pull of larger planets creates an atmosphere too dense for penetration of sunlight • Gravitational pull of smaller planets is too weak to retain an atmosphere • Right temperature • Most H2O is in the form of liquid water, not water vapor Earth History, Ch. 11

30. The Archean fossil record • All Archean fossils are prokaryotes • Archeobacteria and Eubacteria • The oldest known forms are bacterial filaments like modern cyanobacteria • 3.2 to 3.5 billion years old, from Western Australia • Stromatolites known in rocks 3.4 billion years old and younger Earth History, Ch. 11

31. The Archean fossil record (cont.) 3.5 billion year old bacteria preserved in chert from Western Australia Modern cyanobacterial filaments Earth History, Ch. 11

32. Fossilized bacterial filaments:3.2 billion years old, NW Australia diameter of filaments = 2 µm Earth History, Ch. 11

33. Oldest known stromatolites:3.45 billion years old, W Australia Earth History, Ch. 11

34. The Archean fossil record (cont.) 3.2 billion year old stromatolite from South Africa Growth of cyanobacterial mats Earth History, Ch. 11

35. Origin of life • Basic attributes of life: • Ability to reproduce • Self-regulation (ability to sustain orderly internal chemical reactions) • Proteins are among the compounds required for reproduction and regulation • Amino acids are the building blocks of proteins Earth History, Ch. 11

36. Origin of life • Laboratory synthesis of amino acids from simulated early atmosphere • Stanley Miller Soup(1953) • Hydrogen (H) • Ammonia (NH3) • Methane (CH4) • Water vapor (H2O) • Electrical spark • No O2 Amino acids collected here Earth History, Ch. 11

37. Stanley Miller Earth History, Ch. 11

38. Origin of life • Miller’s assumption was that no O2 existed in Earth’s early atmosphere • Incorrect: at least some was there (but not much) • Experiment did produce many types of amino acids that combined to form simple protein-like compounds • Amino acids later discovered in Murchison meteorite (1969) in the same relative proportions as in Miller’s soup • Thus, amino acids could have been delivered to the Archean Earth from space Earth History, Ch. 11

39. Origin of life • Nucleic acids DNA and RNA—also essential for life • DNA carries genetic code and has ability to replicate itself nucleotide bases sugar phosphate group Earth History, Ch. 11

40. Origin of life • RNA also can replicate itself • Messenger RNAcarries information from DNA to sites where proteins are formed • Transfer RNAferries amino acids to sites where proteins are formed, and serves as a catalyst in protein growth • RNA probably was the nucleic acid in the earliest true form of life, with DNA evolving later • Once RNA and DNA had originated, semipermeable cell membranes evolved that could protect the chemical system of the primitive organism while allowing certain compounds to pass in and out Earth History, Ch. 11

41. Origin of life • Where did life form? • Probably not at the Earth’s surface in shallow pools, as once believed • Presence of oxygen would have inhibited the “cooking” of “Stanley Miller soup” • Most likely in the deep sea, away from O2, and probably near a “vent” of hot water • Modern chemosynthetic bacteria are abundant near vents on mid-ocean ridges • They derive energy by consuming chemical compounds and allowing reactions to occur within their cell membranes Earth History, Ch. 11

42. Mid-ocean ridge “vents” Earth History, Ch. 11

43. Origin of life • Mid-ocean ridges are the most likely sites for origin of life and early bacterial evolution • Enormous sizeà many opportunities for key events to take place • Anoxic (no O2) water with necessary amino acid building blocks present • Other key materials present • Phosphorus, nickel, zinc, clays • Modern “vent” bacteria are genetically the most primitive archeobacteria known Earth History, Ch. 11