Precambrian Earth and Life History

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Precambrian Earth and Life History

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Title: Precambrian Earth and Life History

1
Chapter 8
Precambrian Earth and Life HistoryThe Hadean and
Archean
2
Archean Rocks

The Teton Range
is largely Archean
gneiss, schist, and granite
Younger rocks are also present
but not visible

Grand Teton National Park, Wyoming
3
Precambrian 4 Billion Years

The Precambrian lasted for more than 4 billion
years!
Such a time span is difficult for humans to
comprehend

4
Precambrian Time Span

88 of geologic time

5
Precambrian

The Precambrian includes
time from Earths origin 4.6 billion years ago
to the beginning of the Phanerozoic Eon
545 million years ago
No rocks are known for the first
640 million years of geologic time

6
Rocks Difficult to Interpret

Precambrian rocks have been
altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare
the few fossils present are of little use in
stratigraphy
Eon Subdivisions
Archean and Proterozoic

7
Eons of the Precambrian

The Archean Eon
Start coincides with the age of Earths oldest
known rocks
4 billion years old
lasted until 2.5 billion years ago
the beginning of the Proterozoic Eon
Hadean is an informal designation
for time preceding the Archean Eon

8
What Happened During the Hadean?

Earth accreted from planetesimals
differentiated into a core and mantle
and at least some crust
was bombarded by meteorites
ubiquitous volcanism
atmosphere formed
Ocean waters accumulate

9
Hot, Barren, Waterless Early Earth

about 4.6 billion years ago

Shortly after accretion, Earth was
a rapidly rotating, hot, barren, waterless planet
bombarded by comets and meteorites
with no continents, intense cosmic radiation
and widespread volcanism

10
Oldest Rocks

3.96-billion-year-old Acasta Gneiss in Canada and
other rocks in Montana
Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.2 billion years old
so continental source rocks at least that old
existed during the Hadean

11
Hadean Crust

Early Hadean crust was probably thin, unstable
and made up of ultramafic rock
those with comparatively little silica
This ultramafic crust was disrupted
by upwelling basaltic magma at ridges
and consumed at subduction zones
Hadean continental crust may have formed
by evolution of sialic material
Sialic crust contains considerable silicon,
oxygen
and aluminum as in present day continental crust
Only sialic-rich crust, because of its lower
density,
is immune to destruction by subduction

12
Second Crustal Evolution Stage

Subduction and partial melting
of earlier-formed basaltic crust
resulted in the origin of andesitic island arcs
Partial melting of lower crustal andesites,
in turn, yielded silica-rich granitic magmas
that were emplaced in the andesitic arcs

13
Second Crustal Evolution Stage

Several sialic continental nuclei
had formed by the beginning of Archean time
by subduction and collisions
between island arcs

14
Continental Foundations

Continents consist of rocks
with composition similar to that of granite
Continental crust is thicker
and less dense than oceanic crust
which is made up of basalt and gabbro
Precambrian shields
consist of vast areas of exposed ancient rocks
and are found on all continents
Outward from the shields are broad platforms
of buried Precambrian rocks
that underlie much of each continent

15
Cratons

A shield and platform make up a craton,
a continents ancient nucleus and its foundations
Along the margins of cratons,
more continental crust was added
as the continents took their present sizes and
shapes
Both Archean and Proterozoic rocks
are present in cratons and show evidence of
episodes of deformation accompanied by
metamorphism, igneous activity
and mountain building
Cratons have experienced little deformation
since the Precambrian

16
Distribution of Precambrian Rocks

Areas of exposed
Precam-brian rocks
constitute the shields
Platforms consist of
buried Pre-cambrian rocks

Shields and adjoining platforms make up cratons

17
Canadian Shield

The craton in North America is the Canadian
shield
which occupies most of northeastern Canada
a large part of Greenland
parts of the Lake Superior region
in Minnesota, Wisconsin and Michigan
and the Adirondack Mountains of New York

18
Canadian Shield Rocks

Gneiss, a metamorphic rock, Georgian Bay Ontario,
Canada

19
Canadian Shield Rocks

Basalt (dark, volcanic) and granite (light,
plutonic) on the Chippewa River, Ontario

20
Amalgamated Cratons

Actually the Canadian shield and adjacent
platform
is made up of numerous units or smaller cratons
that amalgamated along deformation belts
during the Early Proterozoic
Absolute ages and structural trends
help geologists differentiate
among these various smaller cratons
Drilling and geophysical evidence indicate
that Precambrian rocks underlie much
of North America,
in places exposed by deep erosion or uplift

21
Archean Rocks Beyond the Shield
Rocky Mountains, Colorado

Archean metamorphic rocks found
in areas of uplift in the Rocky Mtns

22
Archean Rocks Beyond the Shield

Archean Brahma Schist in the deeply eroded parts
of the Grand Canyon, Arizona

23
Archean Rocks

The most common Archean Rock associations
are granite-gneiss complexes
The rocks vary from granite to peridotite
to various sedimentary rocks
all of which have been metamorphosed
Greenstone belts are subordinate in quantity
but are important in unraveling Archean tectonism

24
Greenstone Belts

An ideal greenstone belt has 3 major rock units

volcanic rocks are most common
in the lower and middle units
the upper units are mostly sedimentary
The belts typically have synclinal structure
Most were intruded by granitic magma
and cut by thrust faults
Low-grade metamorphism
makes many of the igneous rocks
greenish (chlorite)

25
Greenstone Belt Volcanics

Abundant pillow lavas in greenstone belts
indicate that much of the volcanism was
under water, probably at or near a spreading ridge

Pyroclastic materials probably erupted
where large volcanic centers built above sea
level

Pillow lavas in Ispheming greenstone at
Marquette, Michigan
26
Ultramafic Lava Flows

The most interesting rocks
in greenstone belts cooled
from ultramafic lava flows
Ultramafic magma has less than 45 silica
and requires near surface magma temperatures
of more than 1600C250C hotter
than any recent flows
During Earths early history,
radiogenic heating was higher
and the mantle was as much as 300 C hotter
This allowed ultramafic magma
to reach the surface

27
Ultramafic Lava Flows

As Earths production
of radiogenic heat decreased,
the mantle cooled
and ultramafic flows no longer occurred
They are rare in rocks younger
than Archean and none occur now

28
Sedimentary Rocks of Greenstone Belts

Sedimentary rocks are found
throughout the greenstone belts
although they predominate
in the upper unit
Many of these rocks are successions of
graywacke
a sandstone with abundant clay and rock fragments
and argillite
a slightly metamorphosed mudrock

29
Sedimentary Rocks of Greenstone Belts

Small-scale cross-bedding and
graded bedding indicate an origin
as turbidity current deposits

Quartz sandstone and shale,
indicate delta, tidal-flat,
barrier-island and shallow marine deposition

30
Relationship of Greenstone Belts to
Granite-Gneiss Complexes

Two adjacent greenstone belts showing synclinal
structure

They are underlain by granite-gneiss complexes

and intruded by granite

31
Canadian Greenstone Belts

In North America,
most greenstone belts
(dark green)
occur in the Superior and Slave cratons
of the Canadian shield

32
Evolution of Greenstone Belts

Models for the formation of greenstone belts
involve Archean plate movement
In one model, plates formed volcanic arcs
by subduction

and the greenstone belts formed
in back-arc marginal basins

33
Evolution of Greenstone Belts

According to this model,
volcanism and sediment deposition
took place as the basins opened

34
Evolution of Greenstone Belts

Then during closure,
the rocks were compressed, deformed,
cut by faults,
and intruded by rising magma

The Sea of Japan
is a modern example
of a back-arc basin

35
Archean Plate Tectonics

Most geologists are convinced
that some kind of plate tectonics
took place during the Archean
BUT, Plates must have moved faster
with more residual heat from Earths origin
and more radiogenic heat,
Thus, magma was generated more rapidly

36
Archean Plate Tectonics

As a result of the rapid movement of plates,
continents, no doubt,
grew more rapidly along their margins
a process called continental accretion
as plates collided with island arcs and other
plates
Also, ultramafic extrusive igneous rocks
were more common
due to the higher temperatures

37
Archean World Differences

The Archean world was markedly different than
later

but associations of passive continental margin
sediments
are widespread in Proterozoic terrains

We have little evidence of Archean rocks
deposited on broad, passive continental margins

but the ophiolites so typical of younger
convergent plate boundaries are rare,
although Late Archean ophiolites are known

Deformation belts between colliding cratons
indicate that Archean plate tectonics was active

38
The Origin of Cratons

Certainly several small cratons
existed by the beginning of the Archean
and grew by periodic continental accretion
By the end of the Archean,
30-40 of the present volume
of continental crust existed
Archean crust probably evolved similarly
to the evolution of the southern Superior craton
of Canada

39
Southern Superior Craton Evolution

Greenstone belts (dark green)
Granite-gneiss complexes (light green

Geologic map

Plate tectonic model for evolution of the
southern Superior craton
North-south cross section

40
Canadian Shield

This deformation was
the last major Late Archean event in North
America
and resulted in the formation of several sizable
cratons now in the older parts of the Canadian
shield

41
Present-day Atmosphere Composition

Nonvariable gases
Nitrogen N2 78.08
Oxygen O2 20.95
Argon Ar 0.93
Neon Ne 0.002
Others 0.001
in percentage by volume

Variable gases
Water vapor H2O 0.1 to 4.0
Carbon dioxide CO2 0.034
Ozone O3 0.0006
Other gases Trace
Particulates normally trace

42
Earths Very Early Atmosphere

Earths very early atmosphere was probably
composed of
hydrogen and helium,
the most abundant gases in the universe
If so, it would have quickly been lost into space
because Earths gravity is insufficient to retain
them
because Earth had no magnetic field until its
core formed
Wthout a magnetic field,
the solar wind would have swept away
any atmospheric gases

43
Outgassing

Once a core-generated magnetic field
protected the gases released during volcanism
called outgassing
they began to accumulate to form a new atmosphere
Water vapor
is the most common volcanic gas today
but volcanoes also emit
carbon dioxide, sulfur dioxide,

carbon monoxide, sulfur, hydrogen, chlorine, and
nitrogen

44
Hadean-Archean Atmosphere

Hadean volcanoes probably
emitted the same gases,
and thus an atmosphere developed
but one lacking free oxygen and an ozone layer
It was rich in carbon dioxide,
and gases reacting in this early atmosphere
probably formed
ammonia (NH3)
methane (CH4)
This early atmosphere persisted
throughout the Archean

45
Evidence for an Oxygen-Free Atmosphere

The atmosphere was chemically reducing
rather than an oxidizing one
Some of the evidence for this conclusion
comes from detrital deposits
containing minerals that oxidize rapidly
in the presence of oxygen
pyrite (FeS2)
uraninite (UO2)
But oxidized iron becomes
increasingly common in Proterozoic rocks
indicating that at least some free oxygen
was present then

46
Introduction of Free Oxygen

Two processes account for
introducing free oxygen into the atmosphere,
one or both of which began during the Hadean
1. Photochemical dissociation involves
ultraviolet radiation in the upper atmosphere
The radiation disrupts water molecules and
releases their oxygen and hydrogen
This could account for 2 of present-day oxygen
but with 2 oxygen, ozone forms, creating a
barrier against ultraviolet radiation
2. More important were the activities of organism
that practiced photosynthesis

47
Photosynthesis

Photosynthesis is a metabolic process
in which carbon dioxide and water
combine into organic molecules
and oxygen is released as a waste product
CO2 H2O gt organic compounds O2
Even with photochemical dissociation
and photosynthesis,
probably no more than 1 of the free oxygen level
of today was present by the end of the Archean

48
Oxygen Forming Processes

Photochemical dissociation and photosynthesis
added free oxygen to the atmosphere

Once free oxygen was present
an ozone layer formed
and blocked incoming ultraviolet radiation

49
Earths Surface Waters

Outgassing was responsible
for the early atmosphere
and also for Earths surface water
the hydrosphere
most of which is in the oceans
more than 97
However, some but probably not much
of our surface water was derived from icy comets
Probably at some time during the Hadean,
the Earth had cooled sufficiently
so that the abundant volcanic water vapor
condensed and began to accumulate in oceans
Oceans were present by Early Archean times

50
Ocean water

The volume and geographic extent
of the Early Archean oceans cannot be determined
Nevertheless, we can envision an early Earth
with considerable volcanism
and a rapid accumulation of surface waters
Volcanoes still erupt and release water vapor
Is the volume of ocean water still increasing?
Perhaps it is, but if so, the rate
has decreased considerably
because the amount of heat needed
to generate magma has diminished
Much of volcanic water vapor today
is recycled surface water

51
Decreasing Heat

Ratio of radiogenic heat production in the past
to the present

The width of the colored band
indicates variations in ratios
from different models

Heat production 4 billion years ago was 4 to 6
times as great as it is now

With less heat outgassing decreased

52
First Organisms

Today, Earths biosphere consists
of millions of species of bacteria, fungi,
protistans, plants, and animals,
whereas only bacteria are found in Archean rocks
We have fossils from Archean rocks
3.3 to 3.5 billion years old
Carbon isotope ratios in rocks in Greenland
that are 3.85 billion years old
convince some investigators that life was present
then

53
What Is Life?

Minimally, a living organism must reproduce
and practice some kind of metabolism
Reproduction insures
the long-term survival of a group of organisms
whereas metabolism
such as photosynthesis, for instance
insures the short-term survival of an individual
The distinction between
living and nonliving things is not always easy
Are viruses living?
When in a host cell they behave like living
organisms
but outside they neither reproduce nor metabolize

54
What Is Life?

Comparatively simple organic (carbon based)
molecules known as microspheres

form spontaneously
show greater organizational complexity
than inorganic objects such as rocks
can even grow and divide in a somewhat
organism-like fashion

but their processes are more like random chemical
reactions, so they are not living

55
How Did Life First Originate?

To originate by natural processes,
life must have passed through a prebiotic stage
in which it showed signs of living organisms
but was not truly living
In 1924, the great Russian biochemist,
A.I. Oparin, postulated that life originated
when Earths atmosphere had little or no free
oxygen
Oxygen is damaging to Earths
most primitive living organisms
Some types of bacteria must live
where free oxygen is not present

56
How Did Life First Originate?

With little or no oxygen in the early atmosphere
and no ozone layer to block ultraviolet
radiation,
life could have come into existence from
nonliving matter
The origin of life has 2 requirements
a source of appropriate elements for organic
molecules
energy sources to promote chemical reactions

57
Elements of Life

All organisms are composed mostly of
carbon (C)
hydrogen (H)
nitrogen (N)
oxygen (O)
all of which were present in Earths early
atmosphere as
Carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)

58
Basic Building Blocks of Life

Energy from
lightning
and ultraviolet radiation
probably promoted chemical reactions
during which C, H, N and O combined
to form monomers
comparatively simple organic molecules
such as amino acids
Monomers are the basic building blocks
of more complex organic molecules

59
Experiment on the Origin of Life

Is it plausible that monomers
originated in the manner postulated?
Experimental evidence indicates that it is

During the late 1950s
Stanley Miller
synthesized several amino acids
by circulating gases approximating
the early atmosphere
in a closed glass vessel

60
Experiment on the Origin of Life

This mixture was subjected to an electric spark
to simulate lightning

In a few days
it became cloudy
Analysis showed that
several amino acids
typical of organisms
had formed
Since then,
scientists have synthesized
all 20 amino acids
found in organisms

61
Polymerization

The molecules of organisms are polymers
such as proteins
and nucleic acids
RNA-ribonucleic acid and DNA-deoxyribonucleic
acid
consisting of monomers linked together in a
specific sequence
How did polymerization take place?
Water usually causes depolymerization,
however, researchers synthesized molecules
known as proteinoids
some of which consist of
more than 200 linked amino acids
when heating dehydrated concentrated amino acids

62
Proteinoids

The heated dehydrated concentrated
amino acids spontaneously polymerized
to form proteinoids
Perhaps similar conditions
for polymerization existed on early Earth,
but the proteinoids needed to be protected
by an outer membrane or they would break down
Experiments show that proteinoids
spontaneously aggregate into microspheres
which are bounded by cell-like membranes
and grow and divide much as bacteria do

63
Proteinoid Microspheres

Proteinoid microspheres produced in experiments

Proteinoids grow and divide much as bacteria do

64
Protobionts

Protobionts are intermediate between
inorganic chemical compounds
and living organisms
Because of their life-like properties
the proteinoid molecules can be referred to
as protobionts

65
Monomer and Proteinoid Soup

The origin-of-life experiments are interesting,
but what is their relationship to early Earth?
Monomers likely formed continuously and by the
billions
and accumulated in the early oceans into a hot,
dilute soup (J.B.S. Haldane, British biochemist)
The amino acids in the soup
might have washed up onto a beach or perhaps
cinder cones
where they were concentrated by evaporation
and polymerized by heat
The polymers then washed back into the ocean
where they reacted further

66
Next Critical Step

Not much is known about the next critical step
in the origin of life
the development of a reproductive mechanism
The microspheres divide
and may represent a protoliving system
but in todays cells nucleic acids,
either RNA or DNA
are necessary for reproduction
The problem is that nucleic acids
cannot replicate without protein enzymes,
and the appropriate enzymes
cannot be made without nucleic acids,
or so it seemed until fairly recently

67
RNA World?

Now we know that small RNA molecules
can replicate without the aid of protein enzymes
Thus, the first replicating systems
may have been RNA molecules
Some researchers propose
an early RNA world
in which these molecules were intermediate
between
inorganic chemical compounds
and the DNA-based molecules of organisms
How RNA was naturally synthesized
remains and unsolved problem

68
Much Remains to Be Learned

The origin of life has not been fully solved
but considering the complexity of the problem
and the fact that scientists have been
experimenting
for only about 50 years
remarkable progress has been made
Debate continues
Many researchers believe that
the earliest organic molecules
were synthesized from atmospheric gases
but some scientist suggest that life arose
instead
near hydrothermal vents on the seafloor

69
Azoic (without life)

Prior to the mid-1950s, scientists
had little knowledge of Precambrian life
They assumed that life of the Cambrian
must have had a long early history
but the fossil record offered little
to support this idea
A few enigmatic Precambrian fossils
had been reported but most were dismissed
as inorganic structures of one kind or another
The Precambrian, once called Azoic
(without life), seemed devoid of life

70
Oldest Know Organisms

Charles Walcott (early 1900s) described
structures
from the Early Proterozoic Gunflint Iron
Formation of Ontario, Canada
that he proposed represented reefs constructed by
algae

Now called stromatolites,
not until 1954 were they shown
to be products of organic activity

Present-day stromatolites Shark Bay, Australia
71
Stromatolites

Different types of stromatolites include
irregular mats, columns, and columns linked by
mats

72
Stromatolites

Present-day stromatolites form and grow
as sediment grains are trapped
on sticky mats
of photosynthesizing blue-green algae
(cyanobacteria)
although now they are restricted
to environments where snails cannot live
The oldest known undisputed stromatolites
are found in rocks in South Africa
that are 3.0 billion years old
but probable ones are also known
from the Warrawoona Group in Australia
which is 3.3 to 3.5 billion years old

73
Other Evidence of Early Life

Carbon isotopes in rocks 3.85 billion years old
in Greenland indicate life was perhaps present
then
The oldest known cyanobacteria
were photosynthesizing organisms
but photosynthesis is a complex metabolic process
A simpler type of metabolism
must have preceded it
No fossils are known of these earliest organisms

74
Earliest Organisms

The earliest organisms must have resembled
tiny anaerobic bacteria
meaning they required no oxygen
They must have totally depended
on an external source of nutrients
that is, they were heterotrophic
as opposed to autotrophic organisms
that make their own nutrients, as in
photosynthesis
They all had prokaryotic cells
meaning they lacked a cell nucleus
and lacked other internal cell structures typical
of eukaryotic cells (to be discussed later in the
term)

75
Earliest Organisms