1. Precambrian Earth and Life History—The Eoarchean and Archean
Chapter 8
Precambrian Earth and Life History—The Eoarchean and Archean

2. Time check The Precambrian lasted for more than 4 billion years!
Such a time span is almost impossible for us comprehend
If a 24-hour clock represented all 4.6 billion years of geologic time
the Precambrian would be slightly more than 21 hours long,
It constitutes about 88% of all geologic time

3. Precambrian Time Span

4. Precambrian
The term Precambrian is informal term referring to both time and rocks
It 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

5. Rocks of the Precambrian
The earliest record of geologic time preserved in rocks is difficult to interpret because many 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
Because of this subdivisions of the Precambrian have been difficult to establish
Two eons for the Precambrian
the Archean and Proterozoic

6. Eons of the Precambrian
The onset of the Archean Eon coincides with the age of Earth’s oldest known rocks
approximately 4 billion years old
lasted until 2.5 billion years ago (the beginning of the Proterozoic Eon)
The Eoarchean is an informal designation for the time preceding the Archean Eon
Precambrian eons have no stratotypes
the Cambrian Period, for example, which is based on the Cambrian System, a time-stratigraphic unit with a stratotype in Wales
Precambrian eons are strictly terms denoting time

7. US Geologic Survey Terms
Archean and Proterozoic are used in our discussions of Precambrian history, but the U.S. Geological Survey (USGS) uses different terms
Precambrian W begins within the Early Archean and ends at the end of the Archean
Precambrian X corresponds to the Early Proterozoic, 2500 to 1600 million years ago
Precambrian Y, from 1600 to 800 million years ago, overlaps with the Middle and part of the Late Proterozoic
Precambrian Z is from 800 million years to the end of the Precambrian, within the Late Proterozoic

8. The Hadean?
Except for meteorites no rocks of Eoarchean 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

9. 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

10. Oldest Rocks
Judging from the oldest known rocks on Earth, the 3.96-billion-year-old Acasta Gneiss in Canada some continental crust had evolved by 4 billion years ago
Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.2 billion years old
so source rocks at least that old existed
These rocks indicted that some kind of Hadean crust was certainly present, but its distribution is unknown

11. 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
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. 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

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

14. Dynamic Processes
During the Hadean, various dynamic systems similar to ones we see today, became operative,
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
they diverged, converged and slid past each other
Continents began to grow by accretion along convergent plate boundaries

15. 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

16. 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
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

17. 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

18. 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
It’s topography is subdued, with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits

19. Canadian Shield Rocks
Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada

20. Canadian Shield Rocks
Basalt (dark, volcanic) and granite (light, plutonic) on the Chippewa River, Ontario

21. Amalgamated Cratons
The Canadian shield and adjacent platform consists of numerous units or smaller cratons that were welded together along deformation belts during the Early Proterozoic
Absolute ages and structural trends help geologists differentiate among these various smaller cratons

22. 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

23. 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)

24. 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

25. 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

26. 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

27. 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

28. 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

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

30. 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

31. Evolution of Greenstone Belts
According to this model,
volcanism and sediment deposition took place as the basins opened

32. 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

33. Archean Plate Tectonics
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
residual heat from Earth’s origin
more radiogenic heat
magma was generated more rapidly

34. 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

35. Archean World Differences
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
Deformation belts between colliding cratons
indicate that Archean plate tectonics was active
but the ophiolites so typical of younger convergent plate boundaries are rare,
although Late Archean ophiolites are known

36. The Origin of Cratons
Certainly several small cratons existed by the beginning of the Archean
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
Archean crust probably evolved similarly
to the evolution of the southern Superior craton of Canada

37. 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

38. 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

39. Present-day Atmosphere
Nonvariable gases
Nitrogen N %
Oxygen O
Argon Ar
Neon Ne
Others
in percentage by volume
Variable gases
Water vapor H2O 0.1 to 4.0
Carbon dioxide CO
Ozone O
Other gases Trace
Particulates normally trace

40. 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.
Also because Earth had no magnetic field until its core formed the solar wind would have swept away any atmospheric gases

41. 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
carbon monoxide
Hydrogen
Chlorine
nitrogen

42. 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

43. 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)
Oxidized iron becomes increasingly common in Proterozoic rocks

44. 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

45. 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

46. 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 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

47. 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?
Much of volcanic water vapor today is recycled surface water

48. First Organisms
Today, Earth’s biosphere consists of millions of species of bacteria, fungi, protistans, plants, and animals,
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

49. 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

50. 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

51. 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

52. 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

53. Elements of Life All organisms are composed mostly of
carbon (C)
hydrogen (H)
nitrogen (N)
oxygen (O)
All of which were present in Earth’s early atmosphere as:
Carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)

54. 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

55. 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

56. 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

57. 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

58. Proteinoid Microspheres
Proteinoid microspheres produced in experiments
Proteinoids grow and divide much as bacteria do

59. 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

60. 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 evaporationand polymerized by heat
The polymers then washed back into the ocean where they reacted further

61. 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

62. 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

63. 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

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

65. 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

66. 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

67. 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)

68. Fossil Prokaryotes
Photomicrographs from western Australia’s 3.3- to 3.5-billion-year-old Warrawoona Group
with schematic restoration shown at the right of each