Download Archean sedimentary rocks

PRECAMBRIAN
PART 1
Chapter 8
HADEAN & ARCHEAN EONS
TOPICS
• Formation & Evolution of:
– Rocks
– Atmosphere
– Hydrosphere
– Life Forms
LECTURE NOTES
• Topic
– Event
– Date
– Description
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Defining Characteristics of 3 Eons
• Hadean: 4.6–4.0 bya
– formation of Earth’s crust and main bombardment
• Archean: 4.0–2.5 bya
– first life appears
– plate tectonics established
– oxygen-poor atmosphere
• Proterozoic: 2.5 bya–542 mya
– first multicellular animals at end of interval
– 4 major mountain-building episodes
– • oldest known glaciation
PRECAMBRIAN EONS
PROTEROZOIC EON
ARCHEAN EON
HADEAN EON
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Formation of the Earth
• Cold Acretion or Hot Acretion Models resulting in differentiationof
layers
• Formed from the coalescing and bombardment by objects from
space ranging in size from dust particles to small planets
(planetismals).
• The impacts of large bodies and the decay of radioactive elements
generated heat that melted the materials of the young Earth,
creating the “hellish” conditions for which the Hadean Eon was
named.
• Evidence of bombardment is the craters on the moon that occurred
at the same time (4.0 to 3.8 bya).
• Heat on the Earth was generated through decay of radioactive
elements and continued frequent bombardment by asteroids,
• Earth lost heat to space and slowly cooled.
• Earth eventually segregated into an iron core and silicate mantle..
Earth’s Crust
• Once differentiation occurred, Earth's crust was
dominated by iron and magnesium silicate minerals.
• Covered by an extensive magma ocean in the Archean.
• Magma cooled to form rocks called komatiites.
– form at temperatures greater than those at which basalt forms
(greater than 1100oC).
– ultramafic rocks composed mainly of olivine and pyroxene.
This rock formed Earth's Archean crust.
• The first mafic, oceanic crust formed about 4.5
billion years ago by partial melting of rocks in the upper
mantle.
ARCHEAN
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Earth's Crust Today
• Earth has two types of crust today:
– Oceaninc Crust - dense, mafic (magnesiumand iron-rich) dominated by basalt.
– Continental Crust - less dense, sialic
(silicon- and aluminum-rich) dominated by
granite.
• Formed about 4 bya.
Oceanic Crust
Continental Crust
First
appearance
About 4.5 b.y. ago
About 4.4 b.y. ago
Where
formed
Mid-ocean ridges
Subduction zones
Compositio
n
Komatiite & basalt
Tonalite & granodiorite, and later, granites
Lateral
extent
Widespread
Local (few hundred km or mi)
How
formed
Partial melting of ultramafic rocks in
upper mantle
Partial melting of wet, sediment-covered mafic rocks
descending in subduction zones
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Evolution of the Atmosphere and
Hydrosphere
• Earth's first, primitive atmosphere lacked free oxygen.
• The primitive atmosphere was derived from gases
associated with the comets and meteorites which formed
the Earth during accretion.
The gases reached the Earth's surface through a
process called outgassing.
• Volatiles = substances easily driven off by heating.
• Primitive Atmosphere
• Earth's gases were originally derived from impacts of
comets and meteorites.
Volcanic Outgassing
• Outgassing = the release of water vapor and other gases from the
Earth through volcanism.
• Analysis of samples from Hawaiian eruptions include the following
gases:
–
–
–
–
–
–
–
70% water vapor (H2O)
15% carbon dioxide (CO2)
5% nitrogen (N2)
5% sulfur (in H2S)
chlorine (in HCl)
hydrogen
argon
• Most of the water on the surface of the Earth and in the atmosphere
was outgassed in the first billion years of Earth history. We know this
because there are 3.8 billion-year-old marine sedimentary rocks,
indicating the presence of an ocean by 3.8 billion years ago.
Volcanic Outgassing
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Formation of the Hydrosphere
• Hydrologic Cycle initiated
• Rain water accumulated in low places to form seas. The seas were
originally freshwater (rain).
• Water more acidic than today.
• Ions accumulated in the water, increasing the salinity. Ocean salinity
is relatively constant today because surplus salts are precipitated at
about the same rate at which they are supplied to the sea.
• Much later, when the seas became less acidic, calcium ions bonded
with carbon dioxide to form shells of marine organisms and
limestones.
• The presence of marine fossils suggests that sodium has not varied
appreciably in sea water for at least the past 600 million years.
• Today Earth's water is continuously recirculated through the
hydrologic cycle (evaporation and precipitation, powered by the
sun and by gravity).
The Early Anoxic Atmosphere
• Earth's early atmosphere was strongly reducing and anoxic (lacked
free oxygen or O2 gas) and probably consisted primarily of:
–
–
–
–
–
–
Water vapor (H2O)
Carbon dioxide (CO2)
Nitrogen (N2)
Carbon monoxide (CO)
Hydrogen sulfide (H2S)
Hydrogen chloride (HCl)
• The composition would have been similar to that of modern
volcanoes, but probably with more hydrogen, and possibly traces of
methane (CH4) and ammonia. If any free oxygen had been present,
it would have immediately been involved in chemical reactions with
easily oxidized metals such as iron.
Evidence for a Lack of Free Oxygen in Earth's
Early Atmosphere
• Lack of oxidized iron in the oldest sedimentary rocks.
• Urananite and pyrite are readily oxidized today, but are
found unoxidized in Precambrian sedimentary rocks.
• Rocks are commonly dark due to the presence of
carbon, which would have been oxidized if oxygen had
been present.
• Archean sedimentary sequences lack carbonate rocks
but contain abundant chert, presumably due to the
presence of an acidic, carbon dioxide-rich atmosphere.
In an acidic environment, alkaline rocks such as
limestone do not form.
Evidence for a Lack of Free Oxygen in Earth's
Early Atmosphere – “Continued”
• Banded iron formations (BIF) appear in the Precambrian (1.8 about 3 by).
– Represented by cherts with alternating laminations of red oxidized iron
and gray unoxidized iron.
– BIF formed as precipitates on the floors of shallow seas.
Some of the iron probably came from weathering of iron-bearing rocks
on the continents, but most was probably from submarine volcanoes
and hydrothermal vents (hot springs) on the sea floor. Great economic
importance; major source of iron mined in the world.
• The simplest living organisms have an anaerobic metabolism.
– They are killed by oxygen.
– Includes some bacteria (such as botulism), and some or all Archaea,
which inhabit unusual conditions.
• Chemical building blocks of life (such as amino acids, DNA) could
not have formed in the presence of O2.
Formation of an Oxygen-rich Atmosphere
• The change from an oxygen-poor to an oxygen-rich
atmosphere occurred by the Proterozoic, which began
2.5 billion years ago at the end of the Archean.
• The development of an oxygen-rich atmosphere is the
result of:
• Photochemical dissociation - The breaking up of water
molecules into hydrogen and oxygen in the upper
atmosphere caused by ultraviolet radiation from the Sun
(a minor process today)
• Photosynthesis - The process by which photosynthetic
bacteria and plants produce oxygen (major process).
Evidence for Free Oxygen in the Proterozoic
Atmosphere
• Red beds, or sedimentary rocks with iron oxide
cements, including shales, siltstones, and
sandstones, appear in rocks younger than 1.8
billion years old. This is in the Proterozoic Eon,
after the disappearance of the BIF.
• Carbonate rocks (limestones and dolostones)
appear in the stratigraphic record at about the
same time that red beds appear. This indicates
that carbon dioxide was less abundant in the
atmosphere and oceans so that the water was
no longer acidic.
Rock Deposits
Precambrian Rocks
Precambrian Rocks
• Precambrian rocks are poorly exposed.
• Many Precambrian rocks have been eroded or
metamorphosed.
• Most Precambrian rocks are deeply buried
beneath younger rocks.
• Many Precambrian rocks are exposed in fairly
inaccessible or nearly uninhabited areas.
• Fossils are seldom found in Precambrian
rocks; only way to correlate is by radiometric
dating.
Precambrian Rock Deposits
• Precambrian rocks are often called basement rocks
because they lie beneath a covering of fossil-bearing
sedimentary strata.
• Various Precambrian provinces can be delineated
within the North American continent, based on
radiometric ages of rocks, style of folding, and
differences in trends of faults and folds.
• 3 types of Precambrian Deposits
– Shields – Base precambrian rock.
– Platforms – Layer of sedimentary rock that covers the shields.
– Cratons – Combination of the Shields and Platforms.
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Origin of Plate Tectonics
• By about 4 b.y. ago, the Earth had probably cooled sufficiently for
plate formation.
• Once plate tectonics was in progress, it generated crustal rock that
could be partially melted in subduction zones and added to the
continental crust.
• Continents also increased in size by addition of microcontinents
along subduction zones.
• Greater heat in Archean would have caused faster convection in
mantle, more extensive volcanism, more midoceanic ridges, more
hot spots, etc.
• Growth of volcanic arcs next to subduction zones led to formation of
greenstone belts.
Granulites and Greenstones
•
•
•
The major types of Archean rocks on the cratons are:
Granulites – Formed from highly metamorphosed crust and sediments in
subduction zones.
Greenstones - Metamorphosed volcanic rocks and sediments derived from
the weathering and erosion of the volcanic rocks. Greenstone volcanic
rocks commonly have pillow structures, indicating extrusion under water.
There is a specific sequence of rock types in greenstone belts. These
include:
–
–
–
–
•
Ultramafic volcanic rocks near the bottom (komatiites)
Mafic volcanic rocks (basalts)
Felsic volcanic rocks (andesites and rhyolites)
Sedimentary rocks at the top (shales, graywackes, conglomerates, and
sometimes BIF), deposited in deep water environments adjacent to mountainous
coastlines.
Granulites are present between greenstone belts.
Glaciation
• By 2.8 billion years ago, Earth had cooled
sufficiently for glaciation to occur. Earth's
earliest glaciation is recorded in 2.8
billion year-old sedimentary rocks in
South Africa.
Life of the Archean - The Fossil Record
• The earliest evidence of life occurs in Archean sedimentary
rocks.
• Evidence of Archean life consists of:
• Stromatolites - An organo-sedimentary structure built by
photosynthetic cyanobacteria or blue-green algae. They are not
true fossils.
• Stromatolites form through the activity of cyanobacteria in the tidal
zone. The sticky, mucilage-like algal filaments of the cyanobacteria
trap carbonate sediment during high tides.
Modern stromatolites are found today in isolated environments with
high salinity, such as Shark Bay, western Australia.
Other evidence of Archean life:
•
•
•
•
•
Oldest direct evidence of life
Microscopic cells and filaments of prokaryotes.
Found in Warrawoona Group, Pilbara Supergroup, western Australia
3.460-3.465 b.y.
Indirect evidence of life in older rocks
Found in banded iron deposits in Greenland.
Carbon-13 to carbon-14 ratios are similar to those in present-day organisms.
3.8 b.y.
Algal filament fossils
Filamentous prokaryotes preserved in stromatolites.
Found at North Pole, western Australia.
3.4-3.5 b.y.
Spheroidal bacterial structures
Found in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones).
Prokaryotic cells, showing possible cell division.
3.0 - 3.1 b.y.
Molecular fossils
Preserved organic molecules that only eukaryotic cells produce.
Indirect evidence for eukaryotes.
In black shales from northwestern Australia.
2.7 b.y.
Origin of eukaryotic life is pushed back to 2.7 b.y.
The Origin of Life
• Creation of amino acids
• UV radiation can recombine atoms in mixtures
of water, ammonia and hydrocarbons, to form
amino acids. (The energy in lightning can do
the same thing.)
• Lab simulation experiments by Miller and Urey
in the 1950's.
Formed amino acids from gases present in
Earth's early atmosphere:
H2, CH4 (methane), NH3 (ammonia), and H2O
(water vapor or steam), along with electrical
sparks (to simulate lightning).
Miller and Urey
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Where Did Life Originate?
• Early life may have avoided UV radiation
by living:
– Deep beneath the water
– Beneath the surface of rocks (or below
sediment - such as stromatolites)
– Life probably began in the sea, perhaps in
areas associated with submarine
hydrothermal vents or black smokers.
• Evidence for life beginning in the sea near
hydrothermal vents:
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Evolution of early life and the transition
from prokaryotes to eukaryotes
• The earliest cells had to form and exist in anoxic
conditions (in the absence of free oxygen).
• Likely to have been anaerobic bacteria or Archaea.
• Some of the early organisms became photosynthetic,
possibly due to a shortage of raw materials for energy.
• Photosynthesis was an adaptive advantage.
• Oxygen was a WASTE PRODUCT of photosynthesis.
• Consequences of oxygen buildup in the atmosphere:
– Development of ozone layer which absorbs harmful UV
radiation, and protected primitive and vulnerable life forms.
– End of banded iron formations which only formed in low,
fluctuating O2 conditions
– Oxidation of iron, leading to the beginning of red beds - iron
oxides (hematite).
Evolution of early life and the transition
from prokaryotes to eukaryotes
• Prokaryotes
– No Nucleus or organelles
– reproduce asexually by simple cell division. This restricts their
genetic variability. For this reason, prokaryotes have shown little
evolutionary change for more than 2 billion years.
• Eukaryotes
– contain a nucleus and organelles
– Aerobic metabolism developed. Uses oxygen to convert food into
energy.
– could cope with the oxygen in the atmosphere.
– reproduce sexually leading to genetic recombination and increased
variability and rate of evolution.
– Origin of eukaryotic life was probably around 2.7 b.y., based on
molecular fossils.
– appeared in the fossil record about 1.6 - 1.4 billion years ago (in the
Proterozoic).
– Eukaryotes diversified around the time that the banded iron formations
disappeared and the red beds appeared, indicating the presence of
oxygen in the atmosphere.
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Endosymbiotic Theory
• The Endosymbiotic Theory for the Origin of
Eukaryotes proposes that billions of years ago,
several prokaryotic cells came together to live
symbiotically within a host cell as protection from
(and adaptation to) an oxygenated environment.
These prokaryotes became organelles.
Evidence for this includes the fact that
mitochondria contain their own DNA.
Example - a host cell (fermentative anaerobe) +
aerobic organelle (mitochondrion) + spirochaetelike organelle (flagellum for motility).
Stromatolites
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THE END