THE PRIMORDIAL EARTH Hadean and Archean Eons Evidence of Anoxia Lack of oxidized iron in the oldest...

THE PRIMORDIAL EARTHHadean and Archean EonsEvidence 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.


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.


Banded iron formations (BIF) appear in stratigraphic record in the Precambrian (1.8 - about 3 by).


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.


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)

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 EARTHHadean and Archean EonsEvolution 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.


Sodium remains in sea water due to its high solubility.

Today Earth's water is continuously re-circulated through the hydrologic cycle


THE PRIMORDIAL EARTHHadean and Archean EonsFormation 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 EARTHHadean and Archean EonsEvidence 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 EARTHHadean and Archean EonsEvidence of Free Oxygen

Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear.

THE PRIMORDIAL EARTHHadean and Archean EonsArchean Rock Types

Granulites

Highly metamorphosed gneisses (metamorphosed tonalites, granodiorites, and granites) and anorthosites (layered intrusive gabbroic rocks)


Greenstones

Low-grade metamorphic minerals chlorite and hornblende produce green color. Mostly in trough-like or synclinal belts.


Volcanics

Volcanic (basaltic, andesitic, and rhyolitic) rocks with pillow structures (pillow basalts), indicating extrusion under water.


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 EARTHHadean and Archean EonsArchean Life

The earliest evidence of life occurs in Archean sedimentary rocks.


Oldest direct evidence of life is in 3.5 by old Chert bed associated with Warrawoona Groupwestern Australia

Similar to cyanobacteria living today, which produce O2.


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.


Algal filament fossils (filamentous prokaryotes)3.5 b.y. at North Pole, western Australia


Spheroidal bacterial structures (Monera)Fig Tree Group, South Africa 3.0 - 3.1 by

cherts, slates, ironstones, and sandstonesprokaryotic cells, showing possible cell division

THE PRIMORDIAL EARTHHadean and Archean EonsThe 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 ultravioletradiation


Consequently life may have developedin water at depthbelow the surface of rocks


Consequently life may have developed

in the vicinity of black smokers(deep sea hydrothermal vents)


Elements necessary to produce life:

Carbon Hydrogen Oxygen Nitrogen Phosphorus Sulfur


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 EARTHHadean and Archean EonsThe Origin of Amino Acids

Lab experiments by Miller and Urey in the 1950'sFormed amino acids from:H2

CH4 (methane)NH3 (ammonia)H2O (steam) and sparks (to simulate lightning)

THE PRIMORDIAL EARTHHadean and Archean EonsMaking Proteins

Amino acids join together to make proteins.

For them to join it requires:

Input of energy Removal of water


Where do we get the energy and remove water?

Heating from volcanic activityAt lower temperatures in the presence of phosphoric acid. Evaporation FreezingInvolve 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


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

THE PRIMORDIAL EARTH Hadean and Archean Eons Evidence of Anoxia Lack of oxidized iron in the oldest...

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