The Precambrian

General information

From 4.6 billion years ago to the beginning of the Cambrian Period (about 570 mya)

Covers approximately 4 billion years of Earth History

Includes 3 Eons:

1. Hadean 4.6 - 3.8 bya (or 4.6 - 3.96 bya) - no record
2. Archean 3.8-2.5 bya
3. Proterozoic 2.5-0.57 bya

The Precambrian world was almost certainly as diverse and complex a place as today's world.

The Precambrian is not well known or completely understood. Why?

• Rocks are poorly exposed
• many rocks have been eroded or metamorphosed
• most are deeply buried beneath younger rocks
• fossils seldom found

Most information is from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic time.

Where exposed, cratons are called Precambrian Shields.

Example = Canadian Shield. Mostly igneous and metamorphic rocks; few sedimentary rocks. Scraped by glaciers.

The Hadean

1. Origin of the Earth and solar system
   nebular hypothesis or solar nebula theory
   meteoritic bombardment

2. Differentiation of the Earth to form crust, mantle and core
   
   • Cold accretion model, heating from impacts and radioactivity lead to molten Earth and gravitational differentiation
   • Hot accretion model, material accretes sequentially beginning with iron (core), similar to Bowen's Reaction Series crystallization

3. Origin of the atmosphere
   Volcanic outgassing (or degassing)
   H2O, H2, HCl, CO, CO2, N2, Sulfur gases
   Little or no free oxygen (O2); would lead to rapid oxidation of iron minerals

4. Condensation of water vapor
   rain; runoff leads to lakes, rivers, oceans
   originally freshwater (rain); may have been acidic from sulfurous gases
   slow accumulation of salts due to weathering

5. Origin of continental crust
   most of the early crust was mafic
   continental crust developed secondarily
   several models proposed involving partial melting and weathering

   Continental crust was probably present prior to 4 billion years ago.

   Oldest dated Earth rocks are 3.96 by old (Canada)

Evidence for a lack of free oxygen in the Earth's early atmosphere

1. Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sediments

2. There are no early PC iron oxides (no red beds)

3. Banded iron formations appear in stratigraphic record in PC
   1.8 - 2.5 bya

4. Evidence from Precambrian soils shows O2 was only about 2% of modern levels

5. Chemical building blocks of life could not have formed in the presence of O2
   amino acids
   DNA

6. The simplest living organisms have an anaerobic metabolism
   They are killed by oxygen
   Includes some bacteria (such as botulism)
   Includes some or all Archaebacteria or Archaea which inhabit unusual conditions

Archean Rocks

1. Granulites - high grade metamorphic rocks (gneiss and anorthosite)
2. Greenstone belts - volcanic and sedimentary rocks commonly metamorphosed
   chlorite produces green color
   Sedimentary rocks altered to metasedimentary rocks
   metagraywackes, slates, schists, metaconglomerates, diamictites
   some relict sedimentary structures
   
3. Banded Iron Formations red chert (jasper) and unoxidized iron-rich sedimentary rocks

Banded Iron Formation, Alternating bands of red jasper and black hematite, about 2250 million years old (2.55 billion years old) Jasper Knob, Ishpeming, Michigan

Archean Life

1. Stromatolites (cyanobacteria or BGA - blue-green algae)
   in carbonate sediment
   oldest are 3.4 - 3.5 by old
   also in rocks 2.8 - 3 by old more abundant in Proterozoic rocks

2. Algal filament fossils sometimes found
   3.5 b.y. at North Pole, western Australia

3. Spheroidal bacterial structures (Monera)
   Fig Tree Group, South Africa
   3.0 - 3.1 by
   prokaryotic cells; cell division?

Chemical Evidence for the Origin of Life

1. Simulation experiments
   1950's Miller and Urey formed amino acids
   H2, CH4 (methane), NH3 (ammonia), H2O (steam) gases and sparks (simulate lightning)

   Model of Miller-Urey apparatus Denver Museum of Natural History

   Click here to see diagram of Miller-Urey apparatus

2. Oparin and Fox formed protobionts, proteinoids, and microspheres
   also called coacervate droplets
   mimic cell behavior, but non-living
3. Evidence from meteorites
   Carbonaceous chondrites
   Murchison Meteorite, Australia
   yielded organic compounds
   amino acids of extra-terrestrial origin

Requirements to be Life

1. self-replicating (DNA)
2. metabolism (chemical processes that convert food into energy)

Primitive metabolisms

• fermentative anaerobes produce CO2 and alcohol
• sulfate-reducing bacteria use SO4 and produce H2S
• methanogenic bacteria produce methane
• archaea

Forming more complex molecules needed for life

From a mural at the Smithsonian Institution Museum of Natural History. Color of photograph is off due to lighting conditions and film, but this coloration is reminiscent of pictures from Mars (which has a pinkish or orangish color due to airborne dust).

Amino acids are monomers and have to be joined together to form proteins, whch are polymers.
This requires:

1. Input of energy
2. Removal of water

How could this occur?

1. evaporation?
2. freezing?
3. involve water in a dehydration chemical reaction
4. clays, which have charged surfaces, and to which polar molecules could attach
5. 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

Early organisms and their contributions

Probably chemosynthetic, producing H2S or CO2

Heterotrophs. Consumed simple organic compounds. Abundantly available in primordial soup

Some of the early organisms became photosynthetic possibly due to a shortage of raw materials for energy.
Photosynthesis was an adaptive advantage.

Produced their own raw materials. Autotrophs.

Examples = cyanobacteria (stromatolites)

Oxygen was a WASTE PRODUCT.

Oxygen begins to build up

1. Development of ozone layer which absorbs harmful UV radiation

2. End of banded iron formations which only formed in low, fluctuation O2

3. Beginning of red beds - iron oxides

4. Development of the eukaryotic cell
   Endosymbiotic or Symbiotic theory (p. 240 Levin, 5th edition)
   Host cell (fermentative anaerobe) + aerobic organelle (mitochondrion) + spirochaete-like organelle (flagellum for motility)

   Aerobic metabolism developed.
   Larger than prokaryotes
   Reprooduce through mitosis and meiosis
   

Precambrian Fossil Record

Prokaryotes

1. Onverwacht Series, South Africa
   3.2 - 3.7 by
2. North Pole, W. Australia
   3.5 by
   algal filaments, among the oldest cells known
   
3. Fig Tree Group
   3.0 by
   spheroidal bacterial structures
   may show cell division
   
4. Gunflint Fm, Ontario, Canada 2.0 by
   bacteria and stromatolites with algal filaments

1. oldest are 3.4 - 3.5 by
2. also in rocks 2.8 - 3 by
3. Bulawayan Gp, S. Rhodesia
   3.1 - 2.7 by
   
4. Pongola System, N. Natal Province, S. Africa
   3.1 by
   

Eukaryotes

1. First fossil cells with what appear to be organelles
   1.8 - 1.2 by
   
2. Beck Spring Dolomite, California
   1.3 by
   oldest convincing eukaryotes
   branched filaments
   
3. Bitter Springs Fm, chert, Australia
   0.8 - 0.9 by
   impressive eukaryote fossils

Metazoans (multicellular)

1. Trace fossils (or Ichnofossils)
   oldest are about 0.7 by (700 my)

   

   Trace fossils are relatively uncommon until 0.57 by (570 my)
   Trace fossils increase in diversity through time

   Examples of Precambrian trace fossils:

   1. Brooksella, Grand Canyon
      1 by
      jellyfish-like; organic?
      
   2. questionable trace fossils even older
      a. Medicine Peak Qtzt, Wyoming
      2.0 - 2.5 by
      tube-like structures (found 1976, 1983)
      older than oldest eukaryote cells

   Oldest Metazoan Body Fossils = EDIACARA FAUNA

   Originally discovered in Pound Qtzt, Ediacara Hills, S. Australia; later found worldwide (including Piedmont area of NC) at low paleolatitudes.
   0.59 - 0.7 by (590 - 700 my)
   impressions and molds of animals (associated with trace fossils)

   Mawsonites, similar to jellyfish, Australia Smithsonian Institution, Museum of Natural History	Dickensonia costata, segmented worm, from Australia Smithsonian Institution, Museum of Natural History

   Tribrachidium heraldicum, Echinoderm?, from Australia Smithsonian Institution, Museum of Natural History	Unnamed "spindle-shaped organism" from Newfoundland Smithsonian Institution, Museum of Natural History

   Oldest diversified and relatively abundant marine fauna known. No skeletons.
   All soft-bodied, jellyfish-like animals. 26 species, 18 genera, 4 or more phyla.
   67% Cnidaria
   25% Annelids (worms)
   5% Arthropods
   

   Reconstruction of the Ediacara sea floor, Smithsonian Institution, Museum of Natural History

   The Ediacara fauna marks the beginning of visible life (and therefore, the real, if not the official beginning of the Phanerozoic). The Phanerozoic really begins in the Late Precambrian, doesn't it? Should this fauna be included in a new geologic period at the base of the Phanerozoic? Various researchers have referred to it as the Vendian Period or the Ediacaran Period.

   Introduction to the Vendian Period (600-540 mya)

   Vendian Life

   Learning about Vendian animals

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   Precambrian skeletonized faunas

   First hard parts appear after Ediacaran but before Cambrian officially begins.
   580 - 590 my.
   Minute scraps, denticles, and plates of unknown affinity. Calcium- phosphate tubes, cones, opercula, chitinoid tubes.

   Calcareous algae also appeared at this time (plants with hard parts).

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   This page created by Pamela J. W. Gore
   October 15, 1995.
   Last modified November 10, 1997