The Archean Eon(4000 ma - 2500 ma)

Modern Geologic Time Scale

Hadean

Archean

Proterozoic

Phanerozoic

4000 Ma

4600 Ma

2500 Ma

540 Ma

0 MaM

C

P

Four Eons of Geologic Time

DecemberNovemberOctoberSeptemberAugustJulyJuneMayAprilMarchFebruaryJanuary

oldest surviving rocks

first large continents

Oldest surviving continental crust = 3.8-4.0 Ga•stable continents, atmosphere, and oceans•plate tectonics?

Acasta Gneiss, CanadaWorld’s Oldest Rock!!

Early Archean Rocks

Gneiss - granite plutons

Greenstone - volcanic island arcs, ocean crust

Banded Iron Formation - deep ocean sediments

Isua Supercrustal Rocksand Akia Terrane

• volcano-sedimentary rocks

• BIFs• Cherts• Greywacke• Pillow Basalts• No terrestrial sediments.• No shallow shelf

sediments.• Steep-sided volcanic

islands.

Banded Iron Formation (BIF) - 3.8 Ga

• Chert interlayered with iron oxide (magnetite).• Deposition on deep ocean floor.• Must be a source of oxygen - photosynthesis?• Mechanism for depositing BIF is unknown

(why the alternating layers of silica and iron?

3.5 Ga Pillow Basalts, Komati Greenstone Belt, South Africa

Pillow Basalts - formed by the eruption of lava underwater.

Warrawoona Group, Australia

Archean Plate Tectonics• Many small plates - oceanic crust and volcanic island arcs.• Mantle is hotter than today. Crust is recycled rapidly.• Island arcs collide, forming many small proto-continents.

Archean Crust• Continental crust develops in island arcs over subduction

zones.• Granite plutons are emplaced within the arcs by rising

magma.• Erosion of volcanic islands produces greywacke.

island arcLava flows and greywacke

Archean Crust• Collision of island arcs creates larger masses of continental

crust (cratons).• Archean cratons consist of pods of gneiss (metamorphosed

granite) surrounded by greenstone belts (regions of metamorphosed basalt and greywacke.

gneissgreenstone belt

Pilbara Archean Shield, Northwestern Australia

Archean greenstone belts and cratons

gneiss

gneiss

gneiss

Greenstone belt

gneiss

Archean Cratons, North America

gneiss

Continental Cores of Archean Rock

What about life?Let’s start with the evidence.

What are the characteristics of early life? • Assume earliest life is simplest - Archaea / Bacteria.• First organisms appear to have been hyperthermophiles.

Characteristics of Early Life• Hyperthermophiles - thrive at high temperatures .• Most are anaerobic methane producers

Obsidian Hole, Yellowstone Pk.

Octopus Spring, Yellowstone Pk.

What can we look for?• Assume earliest life is simple - Archaea / Bacteria.

• Microfossils - organic remains in chert.

• Sedimentary structures produced by bacterial mats.

• Chemical signatures of metabolism.

–Carbon isotope fractionation

Carbon isotope fractionation• CO2 can contain either isotope of carbon- 12C or 13C• Most metabolic chemical reactions prefer 12C

•Chemoautotrophism•Photosynthesis•Methanogenesis

• Organic carbon becomes enriched in 12C - it is isotopically light relative to inorganic carbon.

• Few known inorganic processes produce light carbon.• However - some hydrothermal processes might!!!!• Isotopic ratios can be measured.• Ratio not affected by metamorphism!

Archean BIF

Graphite (carbon) in chert Isua Banded Iron Formation (BIF) - 3.8 Ga

• Carbon in chert is isotopically light (enriched in C12).• May have been produced by life.• Circumstantial evidence for life if found in rock with a

sedimentary origin.

Isua Chert hand sample

Pilbara SupergroupWarrawoona Gp.Western Australia

3.45 Ga

Apex Chert microfossils?

North Pole stromatolites?

Apex ChertMicrofossils3.45 Ga

Brasier et al. (2002)• NOT microfossils!• NOT organic in

origin!• Hydrothermal

springs can produce similar structures in chert from inorganic carbon.

Apex Chert cyanobacterial fossils?

Living Cyanobacteria (also called “blue-green algae”)

Fig Tree Chert, South Africa, 3.0 GaMicrofossils in various stages of cell division?

Buck Reef ChertBuck Reef ChertSouth Africa3.4 Ga• carbonaceous

filaments• 12C enrichment

consistent with photosynthesis

• interpreted to be the remains of microbial mats

• Photosynthesis under anaerobic conditions.

banded chert deposits

carbonaceous filamentsPhotosynthetic microbial mats in the 3,416-Myr-old oceanMichael M. Tice and Donald R. LoweNature 431, 549-552 (30 September 2004)

Stromatolites• Bacteria form mat-like colonies.• Sediment particles settle on the mat.• Bacterial grow upward, trapping sediment.• Process repeats, forming stacked laminae of mud.• Stacks take on a variety of shapes and sizes.• Laminated sediment is preserved in rock.• Problem: similar structures can be produced

inorganically.

Modern Stromatolites, Shark Bay, Australia

Fossil Stromatolite

Pilbara Supergroup, Warrawoona Gp.

Late Archean Stromatolites, South Africa

Life in the Archean• When did it evolve - Hadean, Archean?• Methanogens - anerobic, methane-producing

bacteria• Cyanobacteria? - photosynthesis - oxygen

producing.• Bacterial mat communities - stromatolites?• We really don’t know much about Archean life -

very little good fossil evidence preserved in unmetamorphosed rock.

Late Archean: Evidence for oldest large continent with river systems

• Pongola Group and Witwatersrand Supergroup, South Africa.

• 3.0 to 2.7 Ga.

• Tidal flat sedimentary rocks.

• Large area of terrestrial sedimentary rocks - conglomerates, sandstones, shales.

• Oldest well-preserved subaerial environments.

Witwatersrand Conglomerate, South Africa, 3.0 Ga

Witwatersrand Conglomerate, South Africa, 3.0 Ga

Archean Atmosphere• Similar to Hadean - high CO2, N2, low O2

• Oxygen must have been produced by cyanobacteria in the oceans - quickly combined with iron (BIFs).

• Evidence in sedimentary rocks for < 1% present levels of oxygen in the atmosphere.

•Detrital pyrite and uraninite in conglomerates.

•Lack of red beds - sediments with oxidized iron.

Pyrite in Witwatersrand Conglomerate

Pyrite is destroyed by exposure to oxygen - it is not found as a detrital mineral in terrestrial sedimentary rocks after the Archean.

Archean Atmosphere• What kept the Archean Earth warm? No evidence

of glaciation, yet Sun was 80% as bright as today.

• Need a Greenhouse Gas like CO2.

• Not enough CO2 in the Archean to warm the Earth.

• CH4 (methane) is produced by anaerobic bacteria that metabolize hydrogen.

• Very little methane in the modern atmosphere - reacts quickly with oxygen.

Methane haze in the atmosphere of Saturn’s moon Titan

Archean Atmosphere• 1000X present level of methane?• Warming Earth favorable to methanogenic bacteria -

more methane - more greenhouse.• Did the Earth become very hot?• Methane reacts with sunlight to form a smog-like

haze.• Too much methane = too much haze, blocks

sunlight, cooling the Earth.• Negative feedback - tends toward equilibrium.

Earth in the Archean - N2, CO2, CH4 atmosphere

No blue skies!