Chapter 11: The Archean Eon of Precambrian Time 4.6 to 2.5 BYA

Origins: Universe  Observations red shift: expansion looking back in time –stars & galaxies –quasars –cosmic background laws of physics  explanation: big bang formation of all matter from energy elemental composition: 75% H and 25% He  age methods –date the cosmic background (using red shift) –run the expansion backwards –estimate the mass 10 to 20 billion years

Origins: solar system  observations galaxies: hot, new stars in nebulae other star systems & nebulae –composition  old stars: mostly H and He  newer stars: mostly H and He with other, heavier elements –activity  collapsing nebulae  protostars  planets

Origins: solar system  more observations our solar system –composition: H and He with other, heavier elements –distribution  sun at center with most of mass  planetary composition all are different most dense element nearest sun least dense elements farthest from sun  uniform rotation and revolution  comets and asteroids

Origins: Solar system  explanation: nebular hypothesis (fig p 293) nebula formed of dust and gas {of previous star(s)} collapse due to disturbance slow rotation increases as nebula collapses mass collects at center of system –hot, dense gas begins fusion (sun ignites) additional material collects around smaller centers of mass (planetesimals) –higher density elements condense near primary center of mass –lower density material cleared from center by solar wind planetesimals coalesce into planets

Origins: Solar System  age methods –solar fuel use –radiometric dating –Xe and Pu isotope studies 4.5 to 5 BYO time to form 50 to 100 MY

Origins: Earth  observations layered interior asteroid & comet compositions other planets other star systems  explanation: planetary accretion homogeneous (fig p 294) –(1)accretion of planetesimals –(2)melting –(3)differentiation into layers heterogeneous –(1)accretion of most dense material while the nebula was hot and less dense stuff as the nebula cooled  Ni & Fe first  peridotite later –(2)limited differentiation later –(3)atmosphere still accreting

Origins: Moon  observations almost no water small metallic core feldspar-rich outer layer fast earth rotation compositionally differs from Earth  explanation: glancing blow planetesimal sideswiped earth shortly after Earth’s accretion

Early Archean conditions  no rocks  heavy impacting very large impacts: alter rotation large impacts –disrupt surface –extinguish life –vaporize oceans  internal heat production - 2 to 3 X modern rate

late Archean rocks  Sedimentary (most are similar to modern types) deep water marine (graywacke, BIFs, volcanic seds) terrestrial/shallow marine some quartz sandstone some carbonates examples: Witwatersand sequence/Pongola Supergroup  greenstone belts located in bands between felsic gneisses low-grade metamorphic –mafic and ultra-mafic meta-volcanics (inc. pillow basalts) –some felsic volcanics –turbidites and mudstones BIFs - interlayered chert and iron interpretation: old ocean crust caught between colliding continents

late Archean: crust forms  oceanic crust (mafic) forms from mantle material differentiates as it cools may have melted and reformed several times  continental crust (intermediate-felsic) hot spots –segregation of molten rock –partial remelting of roots subduction zones –water from subducting crust enters mantle –partial melting produces intermediate-felsic magmas original differentiation –intermediate and felsic material floated to the top of the molten earth

Archean tectonics  early Archean thin crust? small continents mostly mafic crust? vigorous movement disruption by impact  later Archean movement and impacts slow cratons form –2.7 to 2.3 BYA –continents accrete as island arcs coalesce (greenstone belts)  plate core: shield & platform (oldest rocks) –mountains form and weather (sedimentary rocks)

Archean air and water  atmosphere origin –(1)outgassing –(2)accretion of comets composition –(1)water vapor –(2)H, HCl, CO, CO2, N –(3)no oxygen (very reactive, combines with iron in water)  oceans origin –(1)outgassing & comets –(2)earth cooled & water condensed –(3)salts from volcanoes and weathered rocks composition appx. same as today

Late Archean life  fossils single-celled small: prokaryotic stromatolites  conditions frequent to occasional bombardment no oxygen no UV protection energy sources: sun, internal heat, bombardment ocean full of chemicals

life begins  steps synthesize amino acids assemble RNA assemble cell  characteristics need energy and building materials location –underwater? –underground? –mid-ocean ridges? life habits –chemosynthetic (1st) –consumers (2nd) –photosynthetic (3rd)

Chapter 12: Proterozoic Eon of preCambrian Time 2.5 BYA to 544 MYA

Proterozoic Plate tectonics  continents assemble, develop primary features central craton –original “microcontinents” –shield - eroding –platform - collecting sediment orogenic belts –mountain ranges  interior (old, now part of craton)  exterior (young, around edge of craton) –orogenies weld large continental masses together

Proterozoic Plate tectonics  history and appearance of typical orogen cross sections p. 319 suite of rocks preserve record –rifting & spreading –passive margin –approching continental mass/island arc 100's of millions of years of erosion - planed off mountains leaving igneous, metamorphic and sedimentary suites exposed on flat land

Proterozoic Plate tectonics  Laurentia (North America), maps p. 331, 332, 335 craton: Canadian Shield, Interior Lowlands –at least six microcontinents assembled between 1.95 and 1.85 BYA orogenic belts –interior (Proterozioc): Wopmay, Trans Hudson, Grenville, et.al. –exterior (Phanerozoic): Cordilleran, Ouachita, Appalacian failed rift –Mid-continent Rift (fig p 335) to 1.0 BYA –Keweenawan Supergroup: mafic intrusions and extrusions>continental seds in grabens

Proterozoic Plate tectonics  supercontinent(s) assemble and break apart Rodinia (figs p 332, 336) –Mesoproterozoic? - complete by 1.0 BYA –continents assemble around Laurentia –collision and orogeny (ie. Grenville Orogeny to 1.0 BYA) rifting and separation of Rodinia –Pacific Ocean opens  extensive deposition, esp. in failed rifts (of triple junctions)  Belt Supergroup et.al. (map p. 335, x-section p. 337) South American & African cratons assemble 2nd supercontinent assembles? –south and east of Laurentia –Neoproterozoic

Proterozoic Life  Fossils micro & macro limited –poorly exposed –missing

Proterozoic Life  chemical evidence early life distinctive organic compounds: indicate types of life atmospheric & oceanic oxygen builds –source: photosynthesis –removal of sinks esp. Fe and C –rocks that contain minerals uraninite & pyrite  pre 2.3 BYA rocks  would break down in presence of free oxygen –banded iron formations (BIFs)  3.5 to 1.9 BYA –continental red beds  after 2 BYA  extensive bioturbation of ocean floor begins

Proterozoic Life  prokayotic bacteria & cyano bacteria (Kingdom Monera) (very limited internal structures, very small) (from Archean) stromatolite colonies seafloor covered with biotic “carpet”

Proterozoic Life  early eukaryotic Kingdom Protista (also from Archean?) key developments –cytoskeleton (flexible cell wall) –assembled from symbiotic Monerans (fig p 321)  “host cells”, “mitochondial bacteria”, cyanobacteria –genetic drift and lateral gene transfer types –acritarchs - single-celled algae

Proterozoic Life  multi-cellular life (metazoans) algae (seaweed) trace fossils –post 570 BYA –animals: moving, feeding, burrowing –oldest - simplest –later - increase in variety and complexity –indicate soft-bodied, multicellular life soft-bodied animals –cnidaria –Ediacaran fauna (may contain unnamed Phyla) –annelida –arthropoda –mollusca skeletal fossil - cloudinia

Proterozoic Ice ages  tillite deposits  record Paleoproterozoic: appx 2.3 BYA Neoproterozoic –4 advances (?) between 850 and 600 MYA –deposits within 30 degrees of EQ –snowball earth?  buildup of ice  change in C isotope ratios  deposition of BIFs  effect on life?