Chapter 8

Precambrian Earth and Life History—The Eoarchean and

Archean

• 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%88% of all geologic time

Time check

Precambrian Time Span

• 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 No rocks are known for the first 640 million years of geologic timegeologic time– The oldest known rocks on Earth are 3.96 billion years old

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 ArcheanArchean and ProterozoicProterozoic

Rocks 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

Eons of the Precambrian

• 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

US Geologic Survey Terms

• 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

The Hadean?

• 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 beautiful Earth….about 4.6 billion years ago

• 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

Oldest Rocks

• 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 Only sialic-rich crust, because of its lower density, is immune to destruction by subductiondestruction by subduction

Hadean Crust

• 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

Crustal Evolution

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

Crustal Evolution

• 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

Dynamic Processes

• 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 shieldsPrecambrian shields – consist of vast areas of exposed ancient rocks and are

found on all continents

• Outward from the shields are broad platformsplatforms of buried Precambrian rocks that underlie much of each continent

Continental Foundations

• A shield and platform make up a cratoncraton– 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

Cratons

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

• The craton in North America is the Canadian shieldCanadian 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

Canadian Shield

• Gneiss, a metamorphic rock, Georgian Bay Ontario, Canada

Canadian Shield Rocks

• Basalt (dark, volcanic) and granite (light,

plutonic) on the Chippewa River, Ontario

Canadian Shield Rocks

• The Canadian shield and adjacent platform consists of numerous units or smaller cratons that were weldedwelded together along deformation beltsdeformation belts during the Early Proterozoic– Absolute ages and structural trends help geologists

differentiate among these various smaller cratons

Amalgamated Cratons

• 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

Archean Rocks

– 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 Belts

• An ideal greenstone belt has 3 major rock units

• Abundant pillow lavas in greenstone belts indicate that much of the volcanism was under water

Greenstone Belt Volcanics

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

Pillow lavas in Ispheming greenstone at Marquette, Michigan

• 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

Ultramafic Lava Flows

• Sedimentary rocks are found throughout the greenstone belts – Mostly found in the upper unit

• Many of these rocks are successions of– graywackegraywacke

• a sandstone with abundant clay and rock fragments

– and argilliteargillite• a slightly metamorphosed mudrock

Sedimentary Rocks of Greenstone Belts

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

Sedimentary Rocks of Greenstone Belts

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

• Two adjacent greenstone belts showing synclinal structure

Relationship of Greenstone Belts to Granite-Gneiss Complexes

• They are underlain by granite-gneiss complexes

• and intruded by granite

• In North America,– most greenstone

belts (dark green) occur in the Superior and Slave cratons of the Canadian shield

Canadian Greenstone Belts

• Models for the formation of greenstone belts involve Archean plate movement

• In one model, plates formed volcanic arcs by subduction

Evolution of Greenstone Belts

– the greenstone belts formed in back-arc marginal basins

• According to this model, – volcanism and sediment deposition took place as

the basins opened

Evolution of Greenstone Belts

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

• 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

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 accretioncontinental accretion as plates collided with island arcs and other plates

• Also, ultramafic extrusive igneous rocks were more common due to the higher temperatures

Archean Plate Tectonics

Archean World Differences

– We have little evidence of Archean rocks

– deposited on broad, passive continental margins

– but associations of passive continental margin sediments

– are widespread in Proterozoic terrains

– 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

• 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

The Origin of Cratons

Southern Superior Craton Evolution

Geologic map • Greenstone belts (dark green)

• Granite-gneiss complexes (light green

• Plate tectonic model for evolution of the southern Superior craton

• North-south cross section

• 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

Atmosphere and Hydrosphere

Nonvariable gases

Nitrogen N2 78.08%

Oxygen O2 20.95

Argon Ar 0.93

Neon Ne 0.002

Others 0.001in percentage by volume

Present-day Atmosphere

• 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

• 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

Earth’s Very Early Atmosphere

• Once a core-generated magnetic field protected Earth, gases released during volcanism began to accumulate – CalledCalled outgassing outgassing

• Water vapor is the most common volcanic gas today– also emitted

• carbon dioxide• sulfur dioxide• Hydrogen Sulfide• carbon monoxide• Hydrogen• Chlorine• nitrogen

Outgassing

• 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

Hadean-Archean 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

Evidence for an Oxygen-Free Atmosphere

• Two processes account for introducing free oxygen into the atmosphere, 1. Photochemical dissociationPhotochemical 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 photosynthesisphotosynthesis

Introduction of Free Oxygen

• PhotosynthesisPhotosynthesis 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

Photosynthesis

• 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

Earth’s Surface Waters

• 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

Ocean water

• Today, Earth’s biosphere consists of millions of species of bacteria, fungi, protistans, plants, and animals,– only bacteria are found in Archean rocksonly 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

First Organisms

• 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

What Is Life?

– 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

• 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

How Did Life First Originate?

• 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)

Elements 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

Basic Building Blocks of Life

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

• 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

Polymerization

• 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

Proteinoids

• Proteinoid microspheres produced in experiments

Proteinoid Microspheres

• Proteinoids grow and divide much as bacteria do

• 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

Protobionts

• 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

Monomer and Proteinoid Soup

• 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

Next Critical Step

• 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

Azoic (“without life”)

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

algae

Oldest Know Organisms

• Now called stromatolites – not until 1954 were

they shown to be products of organic activity

Present-day stromatolites Shark Bay, Australia

• 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 blue-green algae (cyanobacteria)(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

Stromatolites

• 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

Other Evidence of Early Life

• The earliest organisms must have resembled tiny anaerobicanaerobic bacteria– meaning they required no oxygen– They must have totally depended on an external source of

nutrients that is, they were heterotrophicheterotrophic• as opposed to autotrophicautotrophic organisms that make their own nutrients,

as in photosynthesis

• They all had prokaryotic cellsprokaryotic cells– meaning they lacked a cell nucleus – and lacked other internal cell structures typical of eukaryotic

cells (to be discussed later in the term)

Earliest Organisms

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

Fossil Prokaryotes