Chapter 8

Precambrian Earth and Life History— The Archean Eon

• Archean Rocks The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses, some of the oldest rocks in the US.

Precambrian

– This large time span is difficult for humans to comprehend • Suppose that a 24-hour clock represented – all 4.6 billion years of geologic time – then the Precambrian would be – slightly more than 21 hours long, – constituting about 88% of all geologic time

Precambrian Time Span

The Precambrian lasted for more than 4 billion years!

If Earth’s history were a 24 hour clock, the Precambrian would use 21 hours!

• 88% of geologic time

Precambrian

• The Precambrian includes – time from Earth’s origin 4.6 billion years ago – to the beginning of the Phanerozoic Eon – 542 million years ago – all rocks below the Cambrian system • No rocks are known for the first – 600 million years of geologic time – The oldest known rocks on Earth – Are ~ 4.0 billion years old

Rocks Difficult to Interpret

• 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, and • the few fossils present are not of any use in biostratigraphy • Two eons for the Precambrian – are the

Archean

and

Proterozoic

– which are based on absolute ages

Eons of the Precambrian

• •

Eoarchean refers to all time

–

from Earth’s origin to the Paleoarchean

–

3.6 billion years ago Earth’s oldest body of rocks

–

the Acasta Gneiss in Canada

–

is about 4.0 billion years old*

•

We have no geologic record

–

for much of the Archaen

What Happenedwas Earth like During the Eoarchean?

• Although no rocks of Eoarchean age are present on Earth, – except for meteorites, • we do know some events that took place then – Earth accreted from planetesimals – and differentiated into a core and mantle • and at least some crust was present – Earth was bombarded by meteorites – Volcanic activity was occurring globally – An atmosphere formed, quite different from today’s – Oceans began to accumulate

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 meteorites and comets – with no continents, intense cosmic radiation – and widespread volcanism

Oldest Rocks

• Continental crust was present by 4.0 billion years ago – Sedimentary rocks in Australia contain detrital zircons (ZrSiO 4 ) dated at 4.4 billion years old – so source rocks at least that old existed • The Eoarchean Earth probably rotated in as little as 10 hours – and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate

Eoarchean Crust

• Early crust formed as upwelling mantle currents – of mafic and ultramafic magma, – and numerous subduction zones developed – to form the first island arcs • Eoarchean continental crust may have formed – by collisions between island arcs – as silica-rich materials were metamorphosed. – Larger groups of merged island arcs • protocontinents – grew faster by accretion along their margins

Origin of Continental Crust

• Andesitic island arcs – form by subduction – and partial melting of oceanic crust • The island arc collides with another

Continental Foundations

• Continents have composition similar to that of granite • Continental crust is thicker Ocean crust is more dense – Composition 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

Cratons

• A shield and its platform make up a craton, – a continent’s ancient nucleus • Along the margins of cratons, – more continental crust was added • Cratons have experienced little deformation – since the Precambrian

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

Canadian Shield

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

Evolution of North America

• North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added.

Archean Rocks

• Archean rock associations – are granite-gneiss complexes • Other rocks range from ultramafic peridotite – To sedimentary rocks – which have been metamorphosed • Greenstone belts are subordinate in quantity, – account for only 10% of Archean rocks – but are important in unraveling Archean tectonic events

Archean Rocks

• Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada

Archean Rocks

• Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite

Greenstone Belts

•

A 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 green – Because they contain chlorite, actinolite, and epidote

Greenstone Belts and Granite Gneiss Complexes

• Two adjacent greenstone belts showing synclinal structure • They are underlain by granite-gneiss complexes • and intruded by granite

Greenstone Belt Volcanics

• Pillow lavas in greenstone belts – indicate that much of the volcanism was – subaqueous Pillow lavas in Ispheming greenstone belt at Marquette, Michigan

Ultramafic Lava Flows

• Ultramafic magma (< 45% silica) – requires near surface magma temperatures – of more than 1600°C – 250°C hotter than any recent flows • This is evidence that • During Earth’s early history, – radiogenic heating was greater – and the mantle was as much as 300 °C hotter – than it is now • This allowed ultramafic magma – to reach the surface • Earth cooled: They are rare in rocks younger – than Archean and none occur now

Ultramafic Lava Flows

• As Earth’s production – of radiogenic heat decreased, – the mantle cooled – and ultramafic flows no longer occurred

Sedimentary Rocks of Greenstone Belts

•

Many of these rocks are successions of

–

graywacke

•

sandstone with abundant clay and rock fragments

–

and argillite

•

slightly metamorphosed mudrock

Sedimentary Rocks of Greenstone Belts

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

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

• Greenstone belts formed in several tectonic settings • Models for the formation of greenstone belts – involve Archean plate movement • In one model, greenstone belts formed – in back-arc marginal basins

Evolution of Greenstone Belts

• According to this model, – There was an early stage of extension as the back arc marginal basin formed – volcanism and sediment deposition followed

Evolution of Greenstone Belts

• Then during closure, – the rocks were compressed, – metamorphosed, – and intruded by granitic magma • The Sea of Japan – is a modern example – of a back-arc basin

Another Model

• In another model accepted by some geologists, – greenstone belts formed over a hot spot == – in intracontinental rifts • As the plume rises beneath continental crust – it spreads and generates tensional forces – The mantle plume is the source – of the volcanic rocks in the lower and middle units – of the greenstone belt – and erosion of volcanic rocks and flanks for the rift – supply the sediment to the upper unit • , metamorphism and plutonism followed

Greenstone Belts—Intracontinental Rift Model

• Ascending mantle plume – causes rifting – and volcanism

Greenstone Belts—Intracontinental Rift Model

• Erosion of the rift flanks – accounts for sediments

Greenstone Belts—Intracontinental Rift Model

• Closure of rift – causes compression – and deformation

Archean Plate Tectonics

• Plates must have moved faster – with more residual heat from Earth’s origin – and more radiogenic heat, – and magma was generated more rapidly • As a result of the rapid movement of plates, – continents grew more rapidly along their margins – a process called continental accretion – as plates collided with island arcs and other plates

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

Atmosphere and Hydrosphere

• Earth’s early atmosphere and hydrosphere – were quite different than they are now • Today’s atmosphere is mostly – nitrogen (N 2 ) – free oxygen (O 2 ), • or oxygen not combined with other elements • such as in carbon dioxide (CO 2 ) – water vapor (H 2 O) – other gases, like ozone (O ultraviolet radiation 3 ) • in the upper atmosphere block most of the Sun’s

Present-day Atmosphere Composition

• Nonvariable gases Nitrogen Oxygen N O 2 2 • Variable gases 78.08% Water vapor Carbon dioxide 20.95

Ozone H 2 O CO 2 O 3 Argon Ar 0.93

Other gases Neon Others Ne 0.002

0.001

0.1 to 4.0

0.038

0.000006

Trace • Particulates normally trace in percentage by volume

Earth’s Very Early Atmosphere

• composed of – hydrogen and helium, • the most abundant gases in the universe • • Earth’s gravity is insufficient to retain them –

because Earth had no magnetic field until its core formed (magnetosphere) Without a magnetic field,

– –

the solar wind would have swept away any atmospheric gases

Outgassing

• •

Once a magnetosphere was present

–

Atmosphere began accumulating as a result of outgassing

–

released during volcanism Water vapor

–

is the most common volcanic gas today

– – – –

but volcanoes also emit carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen

Archean Atmosphere

• Archean volcanoes probably – emitted the same gases, – and thus an atmosphere developed – but one lacking free oxygen and an ozone layer • CO2 ammonia (NH 3 ) methane (CH 4 )

Evidence for an Oxygen-Free Atmosphere

• detrital deposits – containing minerals that oxidize rapidly – in the presence of oxygen – These minerals are NOT bound to typical abundances of oxygen • pyrite (FeS 2 ) • uraninite (UO 2 )

Introduction of Free Oxygen

• Two processes account for – introducing free oxygen into the atmosphere, • one or both of which began during the Eoarchean.

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 organisms that practiced photosynthesis

Photosynthesis

• Photosynthesis is a metabolic process – in which carbon dioxide and water – to make organic molecules – and oxygen is released as a waste product CO 2 + H 2 O ==> organic compounds + O 2 • 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 some of Earth’s surface water • the hydrosphere – most of which is in the oceans • more than 97% • Another source of our surface water – was meteorites and icy comets • Numerous erupting volcanoes, – and an early episode of intense meteorite and comet bombardment • accounted for rapid rate of surface water accumulation

Ocean Water

• 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 – -- heat needed to generate magma has diminished

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 3 to 6 times as great as it is now • With less heat outgassing decreased

First Organisms

• • •

Only bacteria and archea are found in Archean rocks We have fossils from Archean rocks

–

3.5 billion years old Chemical evidence in rocks in Greenland

– –

that are 3.8 billion years old convince some investigators that organisms were present then

What Is Life?

• Minimally, a living organism must reproduce – and practice some kind of metabolism • 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 – 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?

• – from non-living matter (abiogenesis), life must have passed through a prebiotic stages – it showed signs of living – but was not truly living

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 – carbon (C) – hydrogen (H) – nitrogen (N) – oxygen (O) • all of which were present in Earth’s early atmosphere as – carbon dioxide (CO 2 ) – water vapor (H 2 O) – nitrogen (N 2 ) – and possibly methane (CH 4 ) – and ammonia (NH 3 )

Basic Building Blocks of Life

• Energy from • Lightning, volcanism, • and ultraviolet radiation – C, H, N, and O combined to form monomers • such as amino acids • Monomers are the basic building blocks – of more complex organic molecules

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

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

Polymerization

• The molecules of organisms are sequence polymers • consisting of monomers linked together in a specific • RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) • How did polymerization take place?

• Water usually causes depolymerization, – however, researchers synthesized molecules – known as

proteinoids

when heating dehydrated concentrated amino acids

Proteinoids

• These concentrated amino acids spontaneously polymerized to form proteinoids • Experiments show that proteinoids – spontaneously aggregate into microspheres – which are bounded by cell-like membranes – and grow and divide much as bacteria do

Proteinoid Microspheres

• Proteinoid microspheres produced in experiments • Proteinoids grow and divide much as bacteria do

Protobionts

• These proteinoid molecules can be referred to as

protobionts

– that are intermediate between – inorganic chemical compounds – and living organisms

Monomer and Proteinoid Soup: Model for abiogenesis

• Monomers likely formed continuously and by the billions – and accumulated in the early oceans into a “hot, dilute soup” – 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

• • The microspheres divide – and may represent a protoliving system – but in today’s cells, nucleic acids, • either RNA or DNA – are necessary for reproduction Dead End???

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

Much Remains to Be Learned

• Scientists agree on some basic requirements – for the origin of life, – but the exact steps involved – and significance of results are debated • 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

Submarine Hydrothermal Vents

• • Seawater seeps into the crust near spreading ridges, becomes heated, rises and discharges

Black smokers

– Discharge water saturated with dissolved minerals – Life may have formed near these in the past

Submarine Hydrothermal Vents

• Several minerals containing zinc, copper, and iron precipitate around them • Communities of organisms – previously unknown to science, are supported here.

– Necessary elements, sulfur, and phosphorus are present in seawater – Polymerization can take place on surface of clay minerals – Protocells were deposited on the ocean floor

Oldest Known Organisms

• The first organisms were archaea and bacteria – both of which consist of

prokaryotic cells

, – cells that lack an internal, membrane-bounded nucleus and other structures • Prior to the 1950s, scientists assumed that life – must have had a long early history – but the fossil record offered little to support this idea • The Precambrian, once called

Azoic

– (“without life”), seemed devoid of life

Oldest Know Organisms

• Charles Walcott (early 1900s) described structures – from the Paleoproterozoic 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 cyanobacteria • The oldest known undisputed stromatolites – are found in rocks in South Africa – that are 3.0 billion years old

Other Evidence of Early Life

• Chemical evidence in rocks 3.85 billion years old – in Greenland indicate life was perhaps present then • The oldest known cyanobacteria – were photosynthesizing organisms: COMPLEX!!

• 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 – 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

Earliest Organisms

• The earliest organisms, then, – were anaerobic, heterotrophic prokaryotes

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

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

Archean Mineral Resources

• A variety of mineral deposits are of Archean-age – but

gold

is the most commonly associated, • • jewelry, – monetary standard, – ` glass making, electric circuitry, and chemical industry

About half the world’s gold since 1886

– –

has come from Archean and Proterozoic rocks in South Africa

• Gold mines also exist in Archean rocks – of the Superior craton in

Canad

a

Archean Sulfide Deposits

• Archean sulfide deposits of • zinc, • copper • and nickel – occur in Australia, Zimbabwe, – and in the Abitibi greenstone belt – in Ontario, Canada • Some, at least, formed as mineral deposits – next to hydrothermal vents on the seafloor, – much as they do now around black smokers

Chrome

• About 1/4 of Earth’s chrome reserves – are in Archean rocks, especially in Zimbabwe • Chrome is needed in the steel industry • The United States has very few chrome deposits – so must import most of what it uses

Pegmatites

•

Pegmatites

rocks, are very coarsely crystalline igneous – Archean pegmatites contain minerals mined – for lithium, beryllium, rubidium, and cesium