Transcript Hadean and Archean

Precambrian Earth and Life
History—The Hadean and Archean
Grand Teton National Park,
Wyoming
Precambrian 4 Billion Years
• 88% of
geologic
time
Precambrian
• The term Precambrian is informal
– widely used for reference to time and rocks
• The Precambrian includes
– time from Earth’s 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
– the oldest known rocks on Earth are 3.96 billion
years old
Rocks Difficult to Interpret
• The earliest record of geologic time preserved in
rocks is difficult to interpret
– many Precambrian rocks have been
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altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare
• Subdivisions of the Precambrian have been
difficult to establish
http://wwwrohan.sdsu.edu/~rhmiller/fossilrecord/FossilRecord.htm
Eons of the Precambrian
• Two eons for the Precambrian
– are the Archean and Proterozoic
• Hadean is an informal designation
– for the time preceding the Archean Eon
– period for which we don't have a rock record
• Precambrian eons have no stratotypes
– eons strictly denote times
– unlike the Cambrian Period
• based on the Cambrian System, a time-stratigraphic unit
with a stratotype in Wales
What Happened
During the Hadean?
• No rocks of Hadean age present on Earth
– except for meteorites
• We do know some events that took place
– Earth accreted from planetesimals
– differentiated into a core and mantle
and some crust (how much, how thin/thick?)
– Earth was bombarded by meteorites
– volcanic activity was ubiquitous
– atmosphere formed, quite different from today’s
– oceans began to accumulate
Hot, Barren, Waterless Early Earth
• Shortly after accretion, Earth was
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a rapidly rotating, hot, barren, waterless planet
bombarded by comets and meteorites
with no continents, intense cosmic radiation
widespread volcanism
Oldest Rocks
• Judging from the oldest known rocks on
Earth
• the 3.96-billion-year-old Acasta Gneiss in Canada
and other rocks in Montana
– some continental crust had evolved by 4 billion
years ago
• Sedimentary rocks in Australia contain
detrital zircons dated at 4.2 billion years
old
• These rocks indicted that some kind of
Hadean crust was certainly present
– distribution is unknown
Hadean Crust
• Early Hadean crust was probably thin, unstable
and made up of ultramafic rock
• rock with comparatively little silica
•This ultramafic crust was disrupted
•by upwelling basaltic magma at ridges
•and consumed at subduction zones
•Later Hadean continental crust may have formed
by evolution of felsic material
•only felsic crust, because of its lower density, is immune to
destruction by subduction
Second Crustal Evolution Stage
• 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,
– yielded silica-rich granitic magmas
• Several sialic continental nuclei had formed by
the beginning of Archean time
Dynamic Processes
• During the Hadean, various dynamic systems
became operative,
– but not all at the same time nor in their present
forms
• Once Earth differentiated
• into core, mantle and crust,
- internal heat caused
interactions among plates as
they diverged, converged, and
slid past each other
•Continents began to grow
by accretion along
convergent plate
boundaries
http://www.geosc.psu.edu/Peopl
e/Faculty/FacultyPages/Fisher/
Web/Taiwan.htm
Continental Foundations
• Continental crust
– has composition similar to that of granite
– is thicker than oceanic crust
– and less dense than oceanic crust
• which is made up of basalt and gabbro
• Precambrian shields
– consist of vast areas of exposed ancient rocks
– found on all continents
• Outward from the shields are broad platforms
– buried Precambrian rocks
– underlie much of each continent
Cratons
• A shield and platform make up a craton
– a continent’s ancient nucleus and its foundations
• Along the margins of cratons,
– more continental crust was added
– as the continents took their present sizes and
shapes
• Cratons have experienced little deformation
since the Precambrian
Distribution of Precambrian Rocks
Canadian Shield
• The craton in North America is the
Canadian shield
– occupies most of northeastern Canada, large
part of Greenland, parts of the Lake Superior
region (Minnesota, Wisconsin and Michigan),
and the Adirondack Mountains of New York
• Its topography is subdued,
– with numerous lakes and exposed Archean
and Proterozoic rocks
– thinly covered in places by Pleistocene glacial
deposits
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 Beyond the Shield
• Archean metamorphic rocks found
– in areas of uplift in the Rocky Mtns
Archean Rocks Beyond the Shield
• Archean Brahma Schist in the deeply
eroded parts of the Grand Canyon, Arizona
The Archean
• Plate tectonic activity has operated since
the Early Proterozoic or earlier
• Most geologists are convinced that some
kind of plate tectonics took place during
the Archean as well
– but it differed in detail from today
• Plates must have moved faster
– more residual heat from Earth’s origin
– more radiogenic heat,
– magma was generated more rapidly
The Origin of Cratons
• Certainly several small cratons existed by the
beginning of the Archean
– and grew by periodic continental accretion during
the rest of that eon
• They amalgamated into a larger unit during the
Early Proterozoic
•By the end of the
Archean, 30-40% of the
present volume of
continental crust existed
More like today? Why?
http://spacebio.net/modules/lu_resource/Archean
Landscape.jpeg
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 is mostly
– nitrogen (N2)
– abundant free oxygen (O2)
• oxygen not combined with other elements
• such as in carbon dioxide (CO2)
– water vapor (H2O)
– ozone (O3)
• which is common enough in the upper atmosphere
• to block most of the Sun’s ultraviolet radiation
Earth’s Very Early Atmosphere
• Earth’s 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 Earth’s gravity is insufficient to retain them
– because Earth had no magnetic field until its core
formed
• without a magnetic field, the solar wind would have swept
away any atmospheric gases
Outgassing
• Once a core-generated
magnetic field protected
the gases released during
volcanism
– 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
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
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
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
• radiation disrupts water molecules and releases their
oxygen and hydrogen
• 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
• 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
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
Earth’s Surface Waters
• Outgassing was responsible for the early
atmosphere and also for Earth’s 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
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
Decreasing Heat
• Ratio of radiogenic heat production in the past
to the present
– width of the
colored
band
indicates
variations in
ratios from
different
models
– with less heat
outgassing
decreased
• Heat
production 4
billion years
ago was 4 to
6 times as
great as it is
now
First Organisms
• Today, Earth’s 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
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
Earth’s atmosphere had little or no free
oxygen
– oxygen is damaging to Earth’s
most primitive living organisms
comparatively simple organic (carbon based)
molecules known as microspheres
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
Primordial Soup
• Amino acids in the “ primordial 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
http://www.jmcgowan.com/abscicon.html
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
RNA World?
• Now we know that small RNA molecules can
replicate without the aid of protein enzymes
– 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
http://www.jmcgowan.com/abscicon.html
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
• 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
http://web.uvic.ca/sciweb/pics/hydrothermal-vents.html
Precambrian 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)
– although now they are restricted to environments
where snails cannot live
Shark Bay,
Australia
http://www.mlssa.asn.au/journals/1999Journal.htm
Stromatolites
• 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
http://www.3d-fossils.com/photos/fossils/stromatolites.jpg
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
– 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
Earliest Organisms
• The earliest organisms, then, were
anaerobic, heterotrophic prokaryotes
• Their nutrient source was most likely
adenosine triphosphate (ATP) from their
environment
– which was used to drive the energy-requiring
reactions in cells
• ATP can easily be synthesized from simple
gases and phosphate
– so it was doubtless available in the early Earth
environment
Photosynthesis
• A very important biological event occurring
in the Archean was the development of the
autotrophic process of photosynthesis
• This may have happened as much as 3.5
billion years ago
• These prokaryotic cells were still anaerobic,
– but as autotrophs they were no longer
dependent on preformed organic molecules as
a source of nutrients
• These anaerobic, autotrophic prokaryotes
– belong to the Kingdom Monera,
– represented today by bacteria and
cyanobacteria
Fossil Prokaryotes
• Photomicrographs from western Australia’s3.3to 3.5-billion-year-old Warrawoona Group,
– with schematic restoration shown at the right of
each