Transcript Ch 8 Archean

Precambrian Earth and Life
History—The Eoarchean and
Archean
Ch 8
Precambrian Time Span
How many years
Are represented
By the Precambrian?
How many years are
Represented by the
Archean Eon?
What per cent of time
Is represented by the
Eoarchean (Hadean)?
What per cent of time
Is represented by the
Entire Precambrian?
Time check
If a 24-hour clock represented all 4.6 billion
years of geologic time
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the Precambrian would be slightly more than 21
hours long,
It constitutes about 88% of all geologic time
How big is a billion?
1. If you received $1,000 a day, seven days a week, how long
would it take to reach a billion?
2.
If you received $1 every second for your entire life, and you
just reached $1 billion dollars, when were you born?
3.
Scientists believe the earth to be approximately 4.5 billion
years old. If someone had stashed $155 under a rock each
year since the Big Bang or Creation, whichever is your cup of
tea, we’d have about $700 billion just in time to solve the
current financial crisis!
Precambrian
No rocks are known for the first 640 million years of
geologic time
 The oldest known rocks on Earth are 3.96 billion years old
 Updated:
 http://www.sciencedaily.com/releases/2008
/09/080925144624.htm
 Ancient rocks such as the 3.9 billion year
old Acasta Gneiss in Canada are
metamorphic. What does that reveal about
earth’s crust?
 What do detrital zircons reveal about
earth’s crust?
Rocks of the Precambrian
 Why is it difficult to interpret the
geologic past of the Precambrian
period?
 buried deep beneath younger rocks
 altered by metamorphism and deformed
 fossils are rare and not much help
The Eoarchean: What is
known?
 Except for meteorites no rocks of Hadean age
are present on Earth, however we do know
some events that took place during this period
 Earth was accreted
 Differentiation occurred, creating a core and
mantle and at least some crust
Earth beautiful
Earth….
about 4.6 billion years ago
• Shortly after accretion, Earth was a rapidly rotating, hot,
barren, waterless planet
–
–
–
–
bombarded by comets and meteorites
There were no continents,
intense cosmic radiation
widespread volcanism
Earth’s earliest crust formed
 Eoarchean crust was probably thin, unstable and made up of
ultramafic rock
 rock with comparatively little silica
Less than 40% SiO2 – ultramafic
40% - 60% SiO2 – mafic
60% to 70% SiO2 – intermediate
Greater than 70% -- felsic
 This ultramafic crust was disrupted by upwelling basaltic
magma at ridges and consumed at subduction zones
 Eoarchean continental crust may have formed by evolution
of silica-rich 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
Crustal Evolution
 A second stage in crustal evolution began as Earth’s
production of radiogenic heat decreased
 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
Dynamic Processes
 During the Eoarchean, various dynamic systems similar
to ones we see today, became operative,
 Once Earth differentiated into core, mantle and crust,
 internal heat caused interactions among plates
 they diverged, converged and slid past each other
 Continents began to grow by accretion along
convergent plate boundaries
 Density of Continental Crust ~ 2.7g/cm3
Density of Oceanic Crust ~ 3.0 g/cm3
Continental Foundations:
Precambrian Cratons
Precambrian shields found on every
continent
Broad platforms of buried Precambrian
rocks that underlie much of each
continent
Distribution of Precambrian Rocks
• Areas of exposed
Precambrian rocks
constitute the shields
• Platforms consist of
buried Precambrian
rocks
Shields and adjoining platforms make up cratons
Canadian Shield
 The craton in North America is the
Canadian shield
 Occupies most of northeastern Canada,
a large part of Greenland, parts of the
Lake Superior region in Minnesota,
Wisconsin, Michigan, and the
Adirondack Mountains of New York
Canadian Shield Rocks
• Gneiss, a metamorphic rock, Georgian Bay
Ontario, Canada
Canadian Shield Rocks
• Basalt (dark, volcanic) and granite (light,
plutonic) on the Chippewa River, Ontario
Archean Rocks
 The most common Archean Rock associations
are granite-gneiss complexes
 The rocks vary from granite (felsic) to
peridotite (ultramafic) to various sedimentary
rocks all of which have been metamorphosed
 Greenstone belts are subordinate in quantity
but are important in unraveling Archean tectonism
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)
Greenstone Belt Volcanics
 Abundant pillow lavas in greenstone belts
indicate that much of the volcanism was under
water
 Pyroclastic
materials probably
erupted where
large volcanic
centers built above
sea level
Pillow lavas in Ispheming greenstone
at Marquette, Michigan
Ultramafic Lava Flows
 The most interesting rocks in greenstone belts cooled
from ultramafic lava flows
 Ultramafic magma has less than 40% silica
 requires near surface magma temperatures of
more than 1600°C—250°C
 hotter than any recent flows
 During Earth’s early history, radiogenic heating was
higher and the mantle was as much as 300 °C hotter
than it is now
 This allowed ultramafic magma to reach the surface
Sedimentary Rocks of
Greenstone Belts
 Sedimentary rocks are found
throughout the greenstone belts
 Mostly found 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
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
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
Canadian Greenstone Belts
 In North America,
 most greenstone
belts (dark green)
occur in the
Superior and
Slave cratons of
the Canadian
shield
Evolution of Greenstone Belts
 Models for the formation of greenstone
belts involve Archean plate movement
 In one model, plates formed volcanic
arcs by subduction
 the greenstone belts
formed in back-arc
marginal basins
Evolution of Greenstone Belts
 According to this model,
 volcanism and sediment deposition took place as
the basins opened
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
Archean Plate Tectonics
 Plates must have moved faster
 residual heat from Earth’s origin
 more radiogenic heat
 magma was generated more
rapidly
Archean Plate Tectonics
 continental accretion: Continents grew quickly
as plates collided with island arcs and other plates
 Also, ultramafic extrusive igneous rocks were
more common due to the higher temperatures
Southern Superior Craton Evolution
Geologic map
• Plate tectonic model for
evolution of the southern
Superior craton
• North-south cross
section
• Greenstone belts (dark
green)
• Granite-gneiss
complexes (light green
Atmosphere and Hydrosphere
 Earth’s early atmosphere and hydrosphere were quite
different than they are now
 They also played an important role in the development of
the biosphere
 Today’s atmosphere
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is mostly nitrogen (N2)
abundant free oxygen (O2)
 oxygen not combined with other elements
such as in carbon dioxide (CO2)
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
water vapor (H2O)
ozone (O3)
which is common enough in the upper atmosphere to block most of
the Sun’s ultraviolet radiation
Present-day Atmosphere
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
Carbon dioxide CO2
0.034
Ozone
O3
0.0006
Other gases
Trace
 Particulates
normally trace
Earth’s Very Early Atmosphere
Earth’s very early atmosphere was
probably composed of hydrogen and
helium, the most abundant gases in the
universe
What are two reasons that Earth’s early
atmosphere was most likely lost to space?
Outgassing
 Once a core-generated
magnetic field protected Earth,
gases released during
volcanism began to accumulate
 Called outgassing
 Water vapor is the most common volcanic
gas today
 also emitted
 carbon dioxide
 sulfur dioxide
 Hydrogen Sulfide
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carbon monoxide
Hydrogen
Chlorine
nitrogen
Early Precambrian Atmosphere
 Eoarchean 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
Evidence for an
Oxygen-Free Atmosphere
 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)
Oxidized iron becomes increasingly common in
Proterozoic rocks
Introduction of Free Oxygen
 Two processes account for introducing free oxygen
into the atmosphere,
1. Photochemical dissociation involves ultraviolet
radiation in the upper atmosphere
The radiation breaks up water molecules and releases
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
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 ==> organic compounds + O2
“autotrophic food”
 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
Earth’s Surface Waters
 Outgassing was responsible for the early atmosphere
and also for Earth’s surface water
 the hydrosphere
Some but probably not much of our surface water was
derived from icy comets
 At some point during the Eoarchean, 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
Ocean water
 The volume and geographic extent of the Early Archean
oceans cannot be determined
Volcanoes still erupt and release water vapor
 Is the volume of ocean water still increasing?
 Much of volcanic water vapor today is recycled surface
water
First Organisms
 Today, Earth’s biosphere consists of millions of
species found in the “five kingdoms”
 bacteria, fungi, protists, plants, and animals
 only bacteria are found in Archean rocks
 We have fossils from Archean rocks
3.3 to 3.5 billion years old
(ancient stromatolites)
 Carbon isotope ratios in rocks in Greenland that are
3.85 billion years old convince some investigators that
life was present then
But first, 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
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
How Did Life First Originate?
 To originate by natural processes, life must have
passed through a prebiotic stage
it showed signs of living organisms but was not truly living
 In 1924 A.I. Oparin postulated that life originated
when Earth’s atmosphere had little or no free
oxygen
 Oxygen is damaging to Earth’s most primitive living
organisms
 Some types of bacteria must live where free oxygen is not
present
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
Elements of Life
 All organisms are composed mostly of
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carbon (C)
hydrogen (H)
nitrogen (N)
oxygen (O)
All of which were present in Earth’s early
atmosphere as:
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Carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)
Basic Building Blocks of Life
 Energy from
lightning
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
Experiment on the Origin of Life
 During the late 1950s
 Stanley Miller
synthesized several
amino acids by
circulating gases
approximating the early
atmosphere in a closed
glass vessel
Polymerization
 The molecules of organisms are polymers
 proteins
 nucleic acids
RNA-ribonucleic acid and DNA-deoxyribonucleic acid
 consist 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
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
 are bounded by cell-like membranes
 grow and divide much as bacteria do
Proteinoid Microspheres
 Proteinoid
microspheres
produced in
experiments
 Proteinoids grow and
divide much as
bacteria do
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
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
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 today’s 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
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
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
Stromatolites
Different types of stromatolites include
 irregular mats, columns, and columns linked
by mats
Stromatolites
 Present-day stromatolites form and grow as sediment
grains are trapped on sticky mats of photosynthesizing
blue-green algae (cyanobacteria)
 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
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
Earliest Organisms
 The earliest organisms must have resembled tiny
anaerobic bacteria
 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
 they lacked a cell nucleus
 and lacked other internal cell structures typical of
eukaryotic cells (to be discussed later in the term)
 An alternative to the ocean/land model of
earliest life forms:
 Hydrothermal vents, high in metals and
sulfides, may have contained the materials
and energy (earth’s heat) to cause
polymerization of monomers.
 Previously unknown life communities are
being observed today in these volcanic
vents under the oceans.
Fossil Prokaryotes
 Photomicrographs from western Australia’s 3.3- to 3.5billion-year-old Warrawoona Group
 with schematic restoration shown at the right of each
Iron
 Banded Iron formations are sedimentary rocks
consisting of alternating layers of silica (chert) and
iron minerals
 About 6% of the world’s banded iron formations were
deposited during the Archean Eon
 Although Archean iron ores are mined in some areas
they are neither as thick nor as extensive as those of
the Proterozoic Eon, which constitute the world’s
major source of iron