Transcript PreCambrian - Kean University

The Precambrian Record
Key Events of Precambrian time
Acasta Gneiss is dated at
3.96 bya. It is near Yellowknife Lake , NWT Canada
Zircons possibly a bit older in Australia
Precambrian
•4.6 billion years to, say, 548 or 544 million years (depending on method).
•Represents 88% of all of the history of the earth.
•Referred to as the Cryptozoic Eon.
–“hidden life”
Proterozoic
(no more BIFs)
Archean
Hadean (oldest)
(prokaryotes)
Early Hadean Highlights 1
• Earth formed about 4.6 billion years ago from
coalescing interstellar dust.
• Earth was bombarded by large
planetesimals adding to earth’s mass (adds
heat)
• Hot spinning pre-earth mass melted, caused
differentiation of materials according to density.
• Distinct earth layers begin to form
– Dense iron and nickel sink to center forms core.
– silicate material floats up, forms mantle
Early Hadean Highlights 2
• Huge impact from a Mars-sized
planetessimal created the moon.
– Caused earth to spin faster.
– Possible Tilt change
– Moon controls earth’s spin and creates tidal
forces.
– Moon’s orbit at an angle to planets around
Sun
– Earth got most of the core – outer part molten.
Earth rotates. We have magnetic field and,
therefore, an atmosphere
Moon Origin hypotheses -1
Speed and approach angle unlikely.
Moon Origin hypotheses - 2
Does not explain the depletion of metallic iron
in the Moon
Moon Origin hypotheses - 3
Precambrian Early Atmosphere
•First earth atmosphere H He. Lost to solar wind. No magnetic field.
•Post-differentiation start of liquid core induced magnetic field
•Early permanent earth atmosphere mostly N2 CO2 H2O
gasses from volcanic outgassing. Not lost-protected by magnetic field
•Liquid water is required to remove CO2 from atmosphere.
–Mars is too cold to have liquid water.
–Venus is too hot to have liquid water.
–So both have CO2 atmospheres.
•On Earth, most of the world’s CO2 was converted to O2 by photosynthesis.
•Enough by 2.0 bya
•CO2 is locked up in life, limestones, dolomites!
Mars
Venus
Earth
Early Permanent Atmosphere
• Gasses from cooling magmas formed early
atmosphere mostly N2, CO2, with CH4, H2O
•Earth not conducive to modern oxygen breathing
organisms: too much UV.
• Little oxygen O2 occurred in the atmosphere
until the evolution of photosynthetic organisms
(Eubacteria) 3.5 billion years ago. Fully
oxygenated about 1.9 billion years ago.
Sulphur Dioxide from
Kilauea
Precambrian
Early Oceans from 4 bya
OCEANS
•Much water vapor from volcanic degassing.
•Salt in oceans is derived from weathering and
carried to the oceans by rivers.
•Blood of most animals has chemistry of seawater.
•Part of the earth’s water probably came from comets.
–Comets are literally large dirty snowballs.
–Provide fresh water.
Archean To Proterozoic Sedimentary Rocks
• Archean
•3.8 bya: mostly deep water clastic deposits such as mudstones and muddy
sandstones.
–high concentration of eroded volcanic minerals (Sandstones called Graywackes).
• 3 bya: absence of shallow water shelf carbonates.
–increasing chert.
– low oxygen levels, free iron was much more common in the Archean.
–Iron formed “chemical sinks” that consumed much of the early planetary oxygen.
–Formed banded ironstones, commonly with interbedded chert.
•Proterozoic – 2 bya Carbonates become important
- Non-marine sediments turn red – iron is oxidized by the oxygen in AIR
Precambrian Hadean
Formation of Continents
•Early earth surface was magma sea, gradually cooled to form the
crust.
•Continents did not always exist but grew from the chemical
differentiation of early, mafic magmas in the young hot earth.
Floating “Volcanic Islands” of less dense higher silica magmas.
Precambrian: Hadean and Archean
Formation of Felsic Islands
Convection fast due high temperatures – ultramafic melts.
• Partial Melting of base makes new melt, fractionates, melt
higher Silica SiO2. Lava piles up, stack thickens. Base deeper,
melts, fractionation leaves melt richer in silica. Silica-rich melts
have a lower density, float up.
•Increasing amounts of Felsic continental material, form
protocontinents.
• Once rocks with different densities exist, subduction of low
silica rocks under higher silica protocontinents is possible.
• Water squeezed from subducted ocean materials partially
melts mantle, magma rises, fractionates and assimilates.
Continents build up, they are too bouyant to be subducted.
•
First continental crust
1.At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite”
First
2. Komatiite partially melts, Basalt gets
to surface, piles up. The stack sinks,
base partially melts when pressure high
enough. Fractionation makes
increasingly silica-rich magmas
Then:
Water out
3.Density differences allow
subduction of mafic rocks. Further
partial melting and fractionation
makes higher silica melt that won’t
subduct
Archean: Growth of the early continents
Magmatism from Subduction Zones causes thickening
Growth of the early continents
Island Arcs and other terranes accrete to edge of small
continents as intervening ocean crust is subducted.
Temps so high that convection is intense, divergence breaks up
protocontinents.
Little Archean ocean crust survives: most was subducted
Growth of the early continents
Sediments extend continental materials seaward
Growth of the early continents
•Continent-Continent collisions result in larger continents
•Again, not very big in Archean; convection cells too small
Archean-Age Surface Rocks
Precambrian
Early Continents (Cratons) Archean
•Archean cratons consist of regions of light-colored felsic rock (granulite gneisses)
• surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks).
–Pilbara Shield, Australia
–Canadian Shield
–South African Shield.
Greenstone Belts
Felsic Islands
40km
Archean Crustal Provinces were once separated
Canadian Shield assembled from small cratons
Intensely folded rocks, now planed off flat, where cratons
were later sutured together in
Early Proterozoic
Longest: Trans-Hudson Orogen
Granulite gneiss and greenstone
Canadian Shield
Exposed by Pleistocene glaciers
Stratigraphic Sequence of a Greenstone belt
Banded Iron Formations
Younger lavas richer in silica
Increasingly Silica-rich extrusives,
some rhyolites with granites
below them.
Komatiites form at very high temps. They
are absent later as Earth cooled
Note similarity to modern
Ophiolite
DEMO: Banded Iron Sample
Archean Formation of greenstone belts
•Early continents formed by collision of felsic proto-continents.
•Greenstone belts represent volcanic rocks and sediments that accumulated
in ocean basins, then were sutured to the protocontinents during collisions.
•Protocontinents small, rapid convection breaks them up
Proterozoic Tectonics:
The Wilson Cycle
• Proterozoic – Convection Slows
• Rift Phase
– Coarse border, valley and lava rocks in normal faulted
basins
• Drift Phase
– Passive margin sediments
• Collision Phase
– Subduction of ocean floor, island arcs form
– Then collision
Crustal provinces: Proterozoic Tectonics
Slave Craton
Rift and Drift
Followed by
Wopmay
Orogen:
remnants of
old
collisional
mountains
Intensely folded rocks where cratons
were sutured together in
Early Proterozoic
Wilson Cycle 1&2 Rift & Drift
Coronation Supergroup
2. Passive Margin sediments
Much later stuff
1. Rift Valley
Proterozoic 2 bya as Slave craton pulled apart
Near-collision phase of the
Wilson Cycle in the Wopmay
Orogen
3. End of Wilson cycle in the
Wopmay Orogeny
Coronation Supergroup thrust faulted eastward over Slave Craton
Note the vertical exaggeration
Key Events of Precambrian time
Proterozoic Assembly of Laurentia
• Trans-Hudson Orogen mostly 2.5 - 2 bya
– Superior, Wyoming, Hearne plates sutured
– Mountain range now eroded away
• Greenland, N. Gr. Brit., Scandinavia by 1.8 bya
• Continued accretion 1.8-1.6 bya of island arcs.
Most of S. US “Mazatzal Province”
• Last piece “Grenville Orogeny” 1.3-1 bya
Exposed Adirondacks and Blue Ridge
• Assembly of Rodinia by about 750 mya
Proterozoic Oxygen - Rich Atmosphere
• Eubacteria are photosynthetic
2 bya formed stromatolites along shores
• Free oxygen O2 in atmosphere
• Band Iron Formations (common 3.8 – 2 bya) become
rare, probably depended on disappearing conditions
• 2 bya Redbeds begin forming when iron in freshwater
sediment is exposed to abundant atmosphere oxygen
• Oxygen in atmosphere irradiated - Ozone layer forms,
protecting shallow water and land life forms from UV
Redbeds (also our campus)
Key Events of Precambrian time
Final Assembly of Rodinia
Grenville Orogeny 1.3 – 1.0 BYA
• Eastern US Grenville collided with
Grenville Craton, possibly west coast of
S.America
• Southwest US collided w/ Antarctica
– Grenville Orogeny continues in Antarctica
• South collided with Africa
• Rifted apart by 700 – 600 mya, about the
Time of “Snowball Earth” at 635 mya
Growth of Laurentia
Grenville: Shallow Water sandstones
(lots of graywacke), mudstones and
carbonates subjected to high-grade
metamorphism and igneous intrusion
Grenville Collider
was Western S.
America?
Proterozoic Rifting
• Grenville Time Rifting 1.3 – 1 bya
• Kansas to Ontario to Ohio
• Rift Valley sediments and lavas 15 km
(9 miles) thick!
• Rich in Copper, as are the rift valley
sediments here.
• Why?
Midcontinent rift
1500 km long, exposed near L. Superior
Key Events of Precambrian time
Plenty of highlands,
equator to poles
Grenville
Orogen
What Plate Tectonic
conditions favor
glaciation?
Snowball Earth
• Rodinia: abundant basalts with easily weathered
Ca feldspars. Ocean gets Ca+ + . CO2 tied up in
extensive limestones. Less greenhouse effect.
Atmosphere can’t trap heat – Earth gets colder
• Grenville Orogeny left extensive highlands
– From high latitudes to equator
• About 635 mya glacial deposits found in low
latitudes and elevations
• Huge Ice sheet reflects solar radiation “Albedo”
• Some workers believe oceans froze
Stable isotopes of C and O
d13C and d18O :3 - 4 Proterozoic Glaciations
Earth surface became cold enough to produce glaciations and ice ages
G - Glaciation
BIF - Banded Iron Formation
Cambrian
Snow-ball
Earth
Break up of Rodinia
• Hypothesis: Ice an insulator, heat builds
up
• Heavy volcanic activity poured CO2 into
atmosphere – greenhouse effect
• Warming melted snowball earth
Now, Precambrian Life
Return to the Archean
Origin of Archean
Life
•The origin of life required the organization of
self-replicating organic molecules.
•The basic minimum requirements:
–A membrane-enclosed capsule to contain
the bioactive chemicals.
–Energy-capturing chemical reactions
capable of promoting other reactions.
–Some chemical system for replication (RNA-DNA).
Formation of Enzymes
•1950's and 1960's experiments produced amino acids
by combining atmospheric gases, electrical sparks and
heat.
•Further experiments demonstrated that drying and rewetting of these organic compounds could produce
cell-like membranes and simple proteins.
–Led to shallow water “primordial soup” theory.
–But organic compounds in shallow pools would have
been instantly destroyed by ultraviolet radiation. Need
an Oxygen-rich atmosphere to make an Ozone-Layer
–Modern theory life started at
deep sea vents near “Black smokers”
–2 bya atmosphere has oxygen O2
–and ozone O3 which blocks UV
Stanley L. Miller, working in the laboratory of
Harold C. Urey at the University of Chicago.
DNA => mRNA, TRNAaa bound to mRNA in Ribosomes
Makes chain of amino acids (protein)
The DNA sequence in genes is copied into a messenger RNA (mRNA).
Ribosomes then read the information in this RNA and use it to produce
proteins. Ribosomes do this by binding to a messenger RNA and using it
as a template for the correct sequence of amino acids in a particular
protein. The amino acids are attached to transfer RNA (tRNA)
molecules, which enter one part of the ribosome and bind to the
messenger RNA sequence. The attached amino acids are then joined
together by another part of the ribosome. The ribosome moves along the
mRNA, "reading" its sequence and producing a chain of amino acids.
http://en.wikipedia.org/wiki/Ribosome
http://en.wikipedia.org/wiki/Archaea
Key Events of Precambrian time
Ca+ and CO2 abundant during Rodinia Rifting
Ended Snowball Earth
Origin of Life
Origin of Archaebacteria 3.5 bya
•Archaebacteria are the most primitive fossil life forms
–Likely ancestors of all life.
•Primitive Archaebacteria are hyperthermophiles that thrive near
boiling point of water.
–Modern Archaebacteria live in deep-sea volcanic vents.
•Some Archaebacteria feed directly on sulfur (chemoautotrophs).
–Archean life probably arose in deep oceans hydrothermal
environment; volcanic vents that would have formed near MidOcean Ridges
–Vents provide:
•chemical and heat energy,
•abundant chemical and mineral compounds, including sulfur
•deep water: protection from oxygen and ultraviolet radiation.
Archaebacteria
•They differ from other bacteria (called Eubacteria) because:
• they are mostly anaerobic
• the RNA of their ribosomes is different from that of Eubacteria.
They include the methane forming, the salt loving and the heat loving bacteria.
Example: Methane Forming
The methanogenic bacteria create Adenosine Tri Phosphate ATP by reducing carbon dioxide from the
atmosphere using hydrogen, formate, or methanol. As a result methane is liberated. This can only be
in the absence of free oxygen.
CS: Define Eukaryote
done
Fossil Bacteria
•. About 2 bya Eubacteria (prokaryotes lack membrane bound nucleus)
–Eubacteria form stromatolites (photosynthetic).
–More common in upper Archean as shallow water shelves
began to form along margins of early continents.
–Archean is the age of pond-scum.
•Molds of individual bacterial cells found in Late Archean and
Proterozoic cherts.
850 million years old
Chroococcalean 0.85 bya
Palaeolyngbya 1. bya
Grypania 2.1 bya
2 bya Photosynthesis
Modern Stromatolites
Shark Bay Australia
Formed in areas where grazing gastropods can not thrive.
Used to dominate the landscape in Pre-Cambrian and Early Cambrian.
Also forming today on shores of Rift Valley Lakes in Kenya
Endosymbiosis – origin energy
conversion plastids in Eukaryotes
Food
oxidative reactions
Energy transfer
from sunlight
Evolution of Eukaryotes
• Probably began as a endosymbiotic
relationship between different prokaryotes.
• Early eukaryotes “ate” but could not digest
a cell which became a mitochondria. oxidation
• Plant-like eukaryotic ancestors “ate”
chloroplast-bearing cyanobacteria. photosynthesis
• Once eukaryotes evolved, multi-cellular
forms proliferated.
Evolution of Metazoans
•Multi-cellular organisms appear in the Late
Neoproterozoic (570 million years ago).
•Trace fossils (burrows, etc.) indicate motion of early
multicellular forms.
•Ediacaran (Vendian 580-542 mya) fauna consist of
simple organisms.
•Although originally believed to be related to
Cnidarians or sponges, a closer look reveals they
may represent several unknown early phyla.
•Idea: Early life forms had no competitors and were
highly experimental in form?
Proterozoic Life
•First metazoans evolve 580-542 mya.
Ediacara Fauna
An arthropod?
Jellyfish, Sea Pens?
Not really.
Earliest hard parts Late Ediacaran to base of Cambrian
http://en.wikipedia.org/wiki/Cloudinid
Next week, the Paleozoic