The Archean Eon

The Archean Eon
(4000 ma - 2500 ma)
December
November
October
September
August
July
June
May
April
March
February
January
0 Ma
Phanerozoic
C
M
P
540 Ma
Proterozoic
2500 Ma
Modern
Geologic
Time Scale
Four Eons of
Geologic Time
first large continents
Archean
4000 Ma
oldest surviving rocks
Hadean
4600 Ma
Oldest surviving continental crust = 3.8-4.0 Ga
• stable continents, atmosphere, and oceans
• plate tectonics?
Acasta Gneiss, Canada
World’s Oldest Rock!!
Early Archean Rocks
Gneiss - granite plutons
Greenstone - volcanic island
arcs, ocean crust
Banded Iron Formation deep ocean sediments
Isua Supercrustal Rocks
and 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.
Lava flows and greywacke
island arc
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.
greenstone belt
gneiss
Pilbara Archean Shield, Northwestern Australia
Archean greenstone belts and cratons
gneiss
gneiss
gneiss
gneiss
Greenstone belt
gneiss
Archean Cratons,
North America
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!
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.
Archean BIF
Isua Chert hand sample
Pilbara Supergroup
Warrawoona Gp.
Western Australia
3.45 Ga
Apex Chert microfossils?
North Pole
stromatolites?
Apex Chert
Microfossils
3.45 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 Ga
Microfossils in various stages of cell division?
Buck Reef Chert
South Africa
3.4 Ga
Buck Reef Chert
• carbonaceous
filaments
• 12C enrichment
consistent with
photosynthesis
banded chert deposits
• interpreted to be
the remains of
microbial mats
• Photosynthesis
under anaerobic
conditions.
Photosynthetic microbial mats in the 3,416-Myr-old ocean
Michael M. Tice and Donald R. Lowe
Nature 431, 549-552 (30 September 2004)
carbonaceous filaments
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 is destroyed by exposure to oxygen - it is not found as
a detrital mineral in terrestrial sedimentary rocks after the
Archean.
Pyrite in Witwatersrand Conglomerate
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!