Transcript Chapter 11: The Archean Eon of Precambrian Time

Chapter 11:
The Archean Eon of
Precambrian Time
4.6 to 2.5 BYA
Origins: Universe


formation of all matter
from energy
elemental composition:
75% H and 25% He
Observations
red shift: expansion
looking back in time
– stars & galaxies
– quasars
–
cosmic background
laws of physics
explanation: big bang

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)
– (2)
– (3)
accretion of planetesimals
melting
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)
– (3)
limited differentiation later
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)
– (2)
outgassing
accretion of comets
composition
– (1)
– (2)
– (3)

water vapor
H, HCl, CO, CO2, N
no oxygen (very reactive, combines with iron in water)
oceans
origin
– (1)
– (2)
– (3)
outgassing & comets
earth cooled & water condensed
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?
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
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buildup of ice
change in C isotope ratios
deposition of BIFs
effect on life?