Precambrian Precambrian Earth and Life Earth and Life

HistoryHistory

88% of 88% of geologic timegeologic time

Precambrian Time SpanPrecambrian Time Span

The Precambrian lasted for more than 4 billion years!

The term The term PrecambrianPrecambrian is informal is informal but widely used, referring to both time but widely used, referring to both time

and rocksand rocks The Precambrian: 4.6 bya to 542 The Precambrian: 4.6 bya to 542

myamya Oldest known rocks are 3.96 by oldOldest known rocks are 3.96 by old Other information about pC is Other information about pC is

inferred using what we know about inferred using what we know about planet formationplanet formation

PrecambrianPrecambrian

Key Events of Key Events of Precambrian timePrecambrian time

Acasta Gneiss is dated at3.96 bya. It is near Yellowknife Lake , NWT CanadaZircons possibly a bit older in Australia

Shortly after accretion, Earth was Shortly after accretion, Earth was a rapidly rotating, hot, barren, waterless planeta rapidly rotating, hot, barren, waterless planet bombarded by comets and meteoritesbombarded by comets and meteorites with no continents, intense cosmic radiation with no continents, intense cosmic radiation and widespread volcanismand widespread volcanism

Hot, Barren, Waterless Early Hot, Barren, Waterless Early EarthEarth

about 4.6 billion years agoabout 4.6 billion years ago

Judging from the oldest known rocks on Earth, Judging from the oldest known rocks on Earth, the 3.96-billion-year-old Acasta Gneiss in the 3.96-billion-year-old Acasta Gneiss in

Canada and other rocks in Montana and Canada and other rocks in Montana and GreenlandGreenland

some continental crust had evolved by early some continental crust had evolved by early Archean time (3.8 bya)Archean time (3.8 bya)

Sedimentary rocks in Australia contain Sedimentary rocks in Australia contain detrital zircons (ZrSiOdetrital zircons (ZrSiO44) dated at 4.4 billion ) dated at 4.4 billion years oldyears old

so source rocks at least that old existedso source rocks at least that old existed These rocks indicte that some kind These rocks indicte that some kind

of Eoarchean crust was certainly present, of Eoarchean crust was certainly present, but its distribution is unknownbut its distribution is unknown

Oldest RocksOldest Rocks

Early Archean crust was probably thin Early Archean crust was probably thin and made up of ultramafic rockand made up of ultramafic rock

igneous rock with less than 45% silicaigneous rock with less than 45% silica This ultramafic crust was disrupted This ultramafic crust was disrupted

by upwelling mafic magma at ridges, by upwelling mafic magma at ridges, and the first island arcs formed at subduction and the first island arcs formed at subduction

zoneszones Early Archean continental crust may Early Archean continental crust may

have formed have formed by collisions between island arcs as silica-rich by collisions between island arcs as silica-rich

materials were metamorphosed. materials were metamorphosed. Larger groups of merged island arcs Larger groups of merged island arcs

(protocontinents) grew faster by accretion (protocontinents) grew faster by accretion along their marginsalong their margins

Early Archean CrustEarly Archean Crust

Origin of Continental Origin of Continental CrustCrust

Andesitic Andesitic island arcs island arcs form by form by

subduction subduction and partial and partial melting of melting of oceanic oceanic crustcrust

The island The island arc collides arc collides with with another another (accretion)(accretion)

M&W, Fig. 7.1

• Each continent has an ancient, relatively flat interior with very little tectonic or mountain-building activity. These large “tracts” of exposed metamorphic rocks in continental interiors are called Precambrian shields, and are some of the oldest crustal rocks.

Continents consist of rocks with Continents consist of rocks with composition similar to that of granitecomposition similar to that of granite

Continental crust is thicker and less Continental crust is thicker and less dense than oceanic crust dense than oceanic crust

Precambrian shieldsPrecambrian shields consist of vast consist of vast areas of exposed ancient rocks and areas of exposed ancient rocks and are found on all continentsare found on all continents

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

Continental Continental FoundationsFoundations

A shield and platform make up a A shield and platform make up a craton, craton, a continent’s ancient nucleusa continent’s ancient nucleus

Along the margins of cratons, more Along the margins of cratons, more continental crust was added as the continental crust was added as the continents took their present sizes continents took their present sizes and shapesand shapes

Both Archean and Proterozoic rocks Both Archean and Proterozoic rocks show evidence of episodes of show evidence of episodes of deformation accompanied by deformation accompanied by metamorphism, igneous activity, and metamorphism, igneous activity, and mountain buildingmountain building

Cratons have experienced little Cratons have experienced little deformation since the Precambriandeformation since the Precambrian

CratonsCratons

The exposed part of the craton in The exposed part of the craton in North America is the North America is the Canadian shield Canadian shield which occupies most of northeastern which occupies most of northeastern Canada a large part of Greenland parts Canada a large part of Greenland parts of the Lake Superior region in of the Lake Superior region in Minnesota, Wisconsin, and Michigan Minnesota, Wisconsin, and Michigan and the Adirondack Mountains of New and the Adirondack Mountains of New YorkYork

Its topography is subdued, with Its topography is subdued, with numerous lakes and exposed Archean numerous lakes and exposed Archean and Proterozoic rocks thinly covered in and Proterozoic rocks thinly covered in places by Pleistocene glacial depositsplaces by Pleistocene glacial deposits

Canadian ShieldCanadian Shield

Outcrop of Archean gneiss in the Outcrop of Archean gneiss in the Canadian Shield in Ontario, CanadaCanadian Shield in Ontario, Canada

Canadian Shield RocksCanadian Shield Rocks

Archean Brahma Schist in the deeply Archean Brahma Schist in the deeply eroded parts of the Grand Canyon, Arizonaeroded parts of the Grand Canyon, Arizona

Archean Rocks Beyond the Archean Rocks Beyond the ShieldShield

Early Continents (Cratons) ArcheanEarly Continents (Cratons) Archean•Archean cratons consist of regions of light-colored felsic rock (granulite gneisses)• surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks).

–Pilbara Shield, Australia–Canadian Shield–South African Shield

Mafic Greenstone

Belts

Felsic Islands

40km40km

Plate tectonic activity has operated Plate tectonic activity has operated since the early Proterozoic (or since the early Proterozoic (or perhaps late Archean)perhaps late Archean)

Most geologists are convinced that Most geologists are convinced that some kind of plate tectonic activity some kind of plate tectonic activity took place during the Archean as well took place during the Archean as well but it differed in detail from todaybut it differed in detail from today

Plates must have moved faster with Plates must have moved faster with more residual heat from Earth’s origin more residual heat from Earth’s origin and more radiogenic heat, and magma and more radiogenic heat, and magma was generated more rapidlywas generated more rapidly

Archean Plate Archean Plate TectonicsTectonics

Formation of RodiniaFormation of Rodinia

Grenville Orogeny 1.3-1.0 byaGrenville Orogeny 1.3-1.0 bya Collisions between N. Am, S. Am, Collisions between N. Am, S. Am,

Africa and Antartica creates Africa and Antartica creates supercontinentsupercontinent

Climate change!Climate change!

Snowball EarthSnowball Earth Rodinia: abundant basalts with easily Rodinia: abundant basalts with easily

weathered Ca feldspars. Ocean gets Caweathered Ca feldspars. Ocean gets Ca+ ++ + . . COCO2 2 tied up in extensive limestones. Less tied up in extensive limestones. Less greenhouse effect. Atmosphere can’t trap greenhouse effect. Atmosphere can’t trap heat – Earth gets colderheat – Earth gets colder

Grenville Orogeny left extensive highlands Grenville Orogeny left extensive highlands from high latitudes to equatorfrom high latitudes to equator

About 635 mya glacial deposits found in low About 635 mya glacial deposits found in low latitudes and elevationslatitudes and elevations

Huge Ice sheet reflects solar radiation Huge Ice sheet reflects solar radiation “Albedo”“Albedo”

Some workers believe oceans frozeSome workers believe oceans froze

Break up of Rodinia

Hypothesis: Ice an insulator, heat Hypothesis: Ice an insulator, heat builds upbuilds up

Heavy volcanic activity poured COHeavy volcanic activity poured CO22 into atmosphere – greenhouse effectinto atmosphere – greenhouse effect

Warming melted snowball earthWarming melted snowball earth

Earth’s very early (Hadean-Archean) Earth’s very early (Hadean-Archean) atmosphere was probably composed atmosphere was probably composed of hydrogen and heliumof hydrogen and helium

If so, it would have quickly been lost If so, it would have quickly been lost into space because Earth’s gravity is into space because Earth’s gravity is insufficient to retain them and insufficient to retain them and because Earth had no magnetic field because Earth had no magnetic field until its core formed until its core formed (magnetosphere)(magnetosphere)

Without a magnetic field the solar Without a magnetic field the solar wind would have swept away any wind would have swept away any atmospheric gasesatmospheric gases

Earth’s Very Early Earth’s Very Early AtmosphereAtmosphere

Once a Once a magnetosphere was magnetosphere was present apresent atmosphere tmosphere began accumulating as began accumulating as a result of a result of outgassingoutgassing

Water vapor Water vapor is the is the most common volcanic most common volcanic gas today but volcanoes gas today but volcanoes also emit carbon also emit carbon dioxide, sulfur dioxide, dioxide, sulfur dioxide, carbon monoxide, sulfur carbon monoxide, sulfur compounds, hydrogen, compounds, hydrogen, chlorine and nitrogenchlorine and nitrogen

OutgassingOutgassing

Archean volcanoes probably emitted Archean volcanoes probably emitted the same gases, thus an atmosphere the same gases, thus an atmosphere developeddeveloped

It was rich in carbon dioxide, and It was rich in carbon dioxide, and gases reacting in this early gases reacting in this early atmosphere probably formed atmosphere probably formed

ammonia (NHammonia (NH33)) methane (CHmethane (CH44))

This early atmosphere persisted This early atmosphere persisted throughout the Archeanthroughout the Archean

Archean AtmosphereArchean Atmosphere

The atmosphere was chemically The atmosphere was chemically reducing reducing rather than an oxidizing onerather than an oxidizing one

Some of the evidence for this Some of the evidence for this conclusion comes from detrital conclusion comes from detrital deposits containing minerals that deposits containing minerals that oxidize rapidly in the presence of oxidize rapidly in the presence of oxygenoxygen

pyrite (FeSpyrite (FeS22)) But oxidized iron becomes But oxidized iron becomes

increasingly common in Proterozoic increasingly common in Proterozoic rocks indicating that at least some rocks indicating that at least some free oxygen was present thenfree oxygen was present then

Evidence for an Evidence for an Oxygen-Free Oxygen-Free AtmosphereAtmosphere

Two processes account for introducing Two processes account for introducing free oxygen into the atmosphere, one or free oxygen into the atmosphere, one or both of which began during the early both of which began during the early Archean.Archean.1. Photochemical dissociation1. Photochemical dissociation involves involves

ultraviolet radiation in the upper atmosphereultraviolet radiation in the upper atmosphere The radiation disrupts water molecules and The radiation disrupts water molecules and

releases their oxygen and hydrogenreleases their oxygen and hydrogen This could account for 2% of present-day oxygen This could account for 2% of present-day oxygen

but with 2% oxygen, ozone forms, creating a but with 2% oxygen, ozone forms, creating a barrier against ultraviolet radiationbarrier against ultraviolet radiation

2. More important were the activities of 2. More important were the activities of organisms that practiced organisms that practiced photosynthesisphotosynthesis

Introduction of Free Introduction of Free OxygenOxygen

PhotosynthesisPhotosynthesis is a process in which is a process in which carbon dioxide and water combine carbon dioxide and water combine into organic molecules and oxygen is into organic molecules and oxygen is released as a waste productreleased as a waste product

6CO6CO22 + 6H + 6H22O + sunlight + O + sunlight + chlorophyll chlorophyll C C66HH1212OO66 + O + O22

Even with photochemical Even with photochemical dissociation and photosynthesis, dissociation and photosynthesis, probably no more than 1% of the free probably no more than 1% of the free oxygen level of today was present by oxygen level of today was present by the end of the Archeanthe end of the Archean

PhotosynthesisPhotosynthesis

Outgassing was responsible for the Outgassing was responsible for the early atmosphere and also for early atmosphere and also for Earth’s surface water the Earth’s surface water the hydrospherehydrosphere

However, some—but probably not However, some—but probably not much— of our surface water was much— of our surface water was derived from icy cometsderived from icy comets

Once Earth had cooled sufficiently, Once Earth had cooled sufficiently, the abundant volcanic water vapor the abundant volcanic water vapor condensed and began to accumulate condensed and began to accumulate in oceansin oceans

Oceans were present during early Oceans were present during early Archean timesArchean times

Earth’s Surface WatersEarth’s Surface Waters

Ratio of radiogenic heat production in Ratio of radiogenic heat production in the past to the presentthe past to the present

Decreasing HeatDecreasing Heat

The width The width of the of the colored colored band band indicates indicates variations variations in ratios in ratios from from different different modelsmodels

Heat Heat production 4 production 4 billion years billion years ago was 3 to ago was 3 to

6 times as 6 times as great as it is great as it is nownow

With less With less heat heat outgassing outgassing decreaseddecreased

Today, Earth’s biosphere consists of Today, Earth’s biosphere consists of millions of species of bacteria, archea, millions of species of bacteria, archea, fungi, protistans, plants, and animals, fungi, protistans, plants, and animals, whereas only bacteria and archea are whereas only bacteria and archea are found in Archean rocksfound in Archean rocks

We have fossils from Archean rocks 3.3 to We have fossils from Archean rocks 3.3 to 3.5 billion years old3.5 billion years old

Chemical evidence in rocks in Greenland Chemical evidence in rocks in Greenland that are 3.85 billion years old convince that are 3.85 billion years old convince some investigators that organisms were some investigators that organisms were present thenpresent then

First OrganismsFirst Organisms

The first organisms were members of The first organisms were members of bacteria and/or archaea, both of bacteria and/or archaea, both of which consist of which consist of prokaryotic cellsprokaryotic cells, , cells that lack an internal, membrane-cells that lack an internal, membrane-bounded nucleus and other structuresbounded nucleus and other structures

Prior to the 1950s, scientists assumed Prior to the 1950s, scientists assumed that life must have had a long early that life must have had a long early historybut the fossil record offered historybut the fossil record offered little to support this idealittle to support this idea

The Precambrian, once called The Precambrian, once called Azoic Azoic (“without life”), seemed devoid of life(“without life”), seemed devoid of life

Oldest Known Oldest Known OrganismsOrganisms

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

Oldest Known Oldest Known OrganismsOrganisms

Now called Now called stromatolitstromatoliteses, , not until not until 1954 were they 1954 were they shown to be shown to be products of products of organic activityorganic activity

Present-day stromatolites (Shark Bay, Present-day stromatolites (Shark Bay, Australia)Australia)

Present-day stromatolites form and Present-day stromatolites form and grow as sediment grains (calcium grow as sediment grains (calcium carbonate) are trapped on sticky mats carbonate) are trapped on sticky mats of photosynthesizing cyanobacteria.of photosynthesizing cyanobacteria.

The oldest known undisputed The oldest known undisputed stromatolites are found in rocks in stromatolites are found in rocks in South Africa that are 3.0 billion years South Africa that are 3.0 billion years old but probable ones are also known old but probable ones are also known from the Warrawoona Group in from the Warrawoona Group in Australia which is 3.3 to 3.5 billion Australia which is 3.3 to 3.5 billion years oldyears old

StromatolitesStromatolites

At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created.

The earliest organisms must have The earliest organisms must have resembled tiny resembled tiny anaerobicanaerobic bacteria bacteria meaning they required no oxygenmeaning they required no oxygen

They must have totally depended on They must have totally depended on an external source of nutrients that an external source of nutrients that is, they were is, they were heterotrophicheterotrophic

They all had They all had prokaryotic cellsprokaryotic cells The earliest organisms, then, were The earliest organisms, then, were

anaerobic, heterotrophic prokaryotesanaerobic, heterotrophic prokaryotes

Earliest OrganismsEarliest Organisms

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

Fossil ProkaryotesFossil Prokaryotes