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THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Anoxia
Lack of oxidized iron in the oldest sedimentary rocks.
Urananite and pyrite are readily oxidized today,
but are found unoxidized in Precambrian
sediments
Archean sedimentary rocks are commonly dark due to
the presence of carbon, which would have been
oxidized if oxygen had been present.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Anoxia
Archean sedimentary sequences lack carbonate rocks
but contain abundant chert, presumably due to
the presence of an acidic, carbon dioxide-rich
atmosphere.
Carbon dioxide and water combine to form
carbonic acid.
In such an acidic environment, alkaline rocks such as
limestone do not develop.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Anoxia
Banded iron formations (BIF) appear in stratigraphic
record in the Precambrian (1.8 - about 3 by).
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Anoxia
Banded iron formations (BIF) appear in stratigraphic
record in the Precambrian (1.8 - about 3 by).
They are cherts with alternating laminations of red
oxidized iron and gray unoxidized iron.
Origin of these BIFs is puzzling, and several possible
explanations exist.
May be related to hydrothermal vents (hot springs)
in the sea floor.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Anoxia
Bacteria may have played a role in the origin of BIFs.
The simplest living organisms have an anaerobic
metabolism. They are killed by oxygen.
Includes some bacteria (such as botulism).
Includes some or all Archaea, which inhabit
unusual conditions
Chemical building blocks of life could not have
formed in the presence of O2 (amino acids, DNA)
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evolution of the Hydrosphere
Ocean Formation - As the Earth cooled, H2O
produced by out gassing could exist as liquid
in the Early Archean, allowing oceans to form.
Evidence - pillow basalts, deep marine sediments in
greenstone belts.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evolution of the Hydrosphere
Oceans were originally freshwater (rain); may have
been acidic from carbon dioxide and sulfurous
gases in the atmosphere.
Slow accumulation of salts derived from weathering
(dissolution of soluble minerals).
Ocean salinity is relatively constant today because
surplus salts are precipitated at about the same
rate at which they are supplied to the sea.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evolution of the Hydrosphere
Sodium remains in sea water due to its high solubility.
Today Earth's water is continuously re-circulated
through the hydrologic cycle
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Formation of Atmosphere with Oxygen
The development of an oxygen-rich atmosphere
is the result of:
1. Photochemical dissociation - breaking up of water
molecules into hydrogen and oxygen in the upper
atmosphere caused by ultraviolet radiation from
the sun (minor process)
2. Photosynthesis - the process by which plants
produce oxygen (major process)
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Free Oxygen
Red sediments with iron oxide (red beds) appear
in the stratigraphic record after the last
appearance of the BIF (younger than 1.8 by).
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Evidence of Free Oxygen
Carbonate rocks (limestones and dolostones) appear
in the stratigraphic record at about the same
time that red beds appear.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Rock Types
Granulites
Highly metamorphosed gneisses (metamorphosed
tonalites, granodiorites, and granites) and
anorthosites (layered intrusive gabbroic rocks)
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Rock Types
Greenstones
Low-grade metamorphic minerals chlorite and
hornblende produce green color.
Mostly in trough-like or synclinal belts.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Rock Types
Volcanics
Volcanic (basaltic, andesitic, and rhyolitic) rocks
with pillow structures (pillow basalts), indicating
extrusion under water.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Rock Types
Meta-sedimentary rocks
Metamorphosed sedimentary rocks derived from the
weathering and erosion of the volcanics.
Metagraywackes, slates, schists, metaconglomerates
(with granite pebbles), diamictites.
Mostly deep water deposits.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Life
The earliest evidence of life occurs in Archean
sedimentary rocks.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Life
Oldest direct evidence of life is in 3.5 by old
Chert bed associated with Warrawoona Group
western Australia
Similar to cyanobacteria living today, which produce O2.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Life
Stromatolites (cyanobacteria - blue-green algae)
Also in rocks 2.8 - 3 by old - Pongola Group of southern
Africa, and Bulawayan Group of Australia.
More abundant later in Proterozoic rocks, but they are rare
today.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Life
Algal filament fossils (filamentous prokaryotes)
3.5 b.y. at North Pole, western Australia
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Archean Life
Spheroidal bacterial structures (Monera)
Fig Tree Group, South Africa 3.0 - 3.1 by
cherts, slates, ironstones, and sandstones
prokaryotic cells, showing possible cell division
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Life
Most likely developed under anaerobic conditions
O2 is poisonous to the construction of organic
molecules
No O3 layer to serve as protection from ultraviolet
radiation
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Life
Consequently life may have developed
in water at depth
below the surface of rocks
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Life
Consequently life may have developed
in the vicinity of black smokers
(deep sea hydrothermal vents)
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Life
Elements necessary to produce life:
Carbon
Hydrogen
Oxygen
Nitrogen
Phosphorus
Sulfur
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Life
Four essential components of life:
Proteins (chains of amino acids linked together),
used to build living materials and as catalysts in
chemical reactions in organisms.
Nucleic acids
DNA
RNA
Organic phosphorus, used to transform light or
chemical fuel into energy required for cell activities.
A cell membrane to enclose the components within
the cell.
THE PRIMORDIAL EARTH
Hadean and Archean Eons
The Origin of Amino Acids
Lab experiments by Miller and Urey in the 1950's
Formed amino acids from:
H2
CH4 (methane)
NH3 (ammonia)
H2O (steam) and
sparks
(to simulate lightning)
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Making Proteins
Amino acids join together to make proteins.
For them to join it requires:
Input of energy
Removal of water
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Making Proteins
Where do we get the energy and remove water?
Heating from volcanic activity
At lower temperatures in the presence of phosphoric acid.
Evaporation
Freezing
Involve water in a dehydration chemical reaction
On clays, which have charged surfaces, and to which
polar molecules could attach
On pyrite, which has a positively charged surface to
which simple organic compounds can become bonded.
Formation of pyrite yields energy which could be used to
link amino acids into proteins
THE PRIMORDIAL EARTH
Hadean and Archean Eons
Making Proteins
Proteinoids produced experimentally
Film-like outer wall
Capable of osmotic shrinking and swelling
Budding similar to yeast
Divide into daughter microspheres
Aggregate into lines to form filaments, as in some bacteria
Streaming movement of internal particles, as in living cells