Download Precambrian Era PPT 2

The Precambrian Record
Precambrian Time
•
Timeframe: 4.6 billion
to 570 million years
ago)
•
85% of Earth’s
History!!
•
Simple Life starts in the
oceans
EARTH DURING
PRECAMBRIAN TIME
EARTH DURING
PRECAMBRIAN TIME
Precambrian Was called a
Supereon

The Precambrian Supereon Is divided into
three Precambrian eons, the Proterozoic,
the Archaean, and Hadean.

There were not many fossils during
Precambrian Time, because Precambrian
Time had a lot of soft bodied.
Important Facts
Many
Precambrian fossils
are Trace fossils.
Land (Geological) Features
Important Facts
Earth formed
 Life arose
 First tectonic plates arose and began
to move
 Atmosphere became enriched in
oxygen

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 Era
Simple life starts in the oceans
Primarily covered by the ocean
and
Had a lot of volcanic activity.
4,600,000,000 years ago to 570,000,000 years ago
85% of Earth’s History
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”
(no more BIFs)
(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 migrate to center (core)
 silicate material moves out to 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
Chemical composition of the Moon suggests
that it could not have co-formed with the earth.
Moon Origin hypotheses - 3
The center of Earth was
melted by great pressure
and intense heat.
Oldest rock
found in Western
Australia
4.1BYA The surface of Earth
cooled forming the crust
An atmosphere formed
as Earth cooled.
primitive atmosphere forms
from volcanic eruptions
Did NOT contain oxygen gas
Oceans form from water
vapor released in
eruptions & maybe
comets
The Earth Forms
4.5BYA Rocks in the solar
system condensed to form
Earth
6 BYA The
Solar
system
formed
from
swirling
gases and
dust
4.6 BYA
Precambrian Time
A molten Earth forms
4.3 BYA
Precambrian Time
Surface cools, forming the crust
 Oldest
rock found in Australia & Canada
 primitive
atmosphere forms from volcanic
eruptions (No Free oxygen gas).
 Oceans
form (from water vapor released in
eruptions & comets)
Comparison of Atmospheres
Precambrian
Early Atmosphere
•First earth atmosphere H He lost to solar wind. No magnetic field
•Early permanent earth atmosphere mostly Nitrogen (inert) and CO2
Post-differentiation start of liquid core dynamo
•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 to sustain life.
•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
•Early earth not conducive to modern oxygen
breathing organisms: too much solar UV gets
through atmosphere.
• Little oxygen occurred in the atmosphere until
the evolution of photosynthetic organisms
(Eubacteria) 3.5 billion years ago. Fully
oxygenated about 1.9 billion years ago.
Where did Life Start?

Oceans (nutrient rich, possibly with complex
molecules)

Ice (opportunity for organization)

Meteorites (rich with building blocks of life)
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.
First continental crust
At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite”
First
Komatiite partially melts, Basalt gets
to surface, piles up. The stack sinks,
partially melts when pressure high
enough. Fractionation makes
increasingly silica-rich magmas
Then:
Water out
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
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
SNOWBALL EARTH
SNOWBALL 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
Animals/Organisms(Biological
that First Appeared
Most organisms had soft bodies which
looked like modern jellyfish and worms
 Stromatolites: layered mats formed by
colonies or groups of cyanobacteria
(blue-green algae thought to be the first
or one of the earliest life forms on
Earth.)
 Invertebrates (animals with no backbone)
appeared near the end of the Precambrian
Time

3.5 BYA
Precambrian Time
Oldest fossils of bacteria (prokaryotes).
 Simple and small
 Lack a nucleus
 Do not need oxygen to survive
The Precambrian Era
4.5 billion – 570 million years ago
Banded iron is believed to have formed when oxygen
released by blue green algae combined with iron present
in ancient ocean waters to form iron oxide precipitates
that settled to the ocean floor.
Stromatolites are dome shaped mineral
formations built by microbes.
They continue to survive today in the waters
around Australia.
http://www.eps.mcgill.edu
http://gsc.nrcan.gc.ca/paleochron/03_e.php
http://gsc.nrcan.gc.ca/paleochron/03_e.php
Fossil stromatolites are one of the
most common forms of fossil life
identified in Precambrian rocks.
3.0 BYA
Precambrian Time
Cyanobacteria (blue-green algae)
evolve



creates energy from photosynthesis
gives off oxygen gas (beginning to
form the atmosphere of today)
Form large stromatolites (below)
1.5 BYA

Precambrian Time
Eukaryotes



contain a nucleus
more complex, larger, single
celled organisms
Evolved from prokaryotes
1.5 BYA

Eukaryotes (complex, larger,
single celled organisms)
evolve from prokaryotes
(bacteria).
630 MYA

Precambrian Time
Multicellular organisms evolve with soft bodies
Species include:
jelly fish; coral
stalks;
segmented
worms; algae
WHAT CAUSED THE MASS
EXTINCTION

MASS EXTINCTION IS THE EVENT
THAT ENDED PRECAMBRIAN TIME.
* glaciation event
 Precambrian time were marked by a
prolonged global ice age. This may
have led to widespread extinctions.
