Chapter 11: The Archean Eon of Precambrian Time

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

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: Universe

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

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: Earth

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) - 1.3 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 - 1.2 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?

Chapter 11: The Archean Eon of Precambrian Time