UNIVERSITY OF SOUTH ALABAMA
Last time we covered…
Proterozoic Geology
MAS 603: Geological Oceanography
1) The Proterozoic time frame
2) Paleogeography
3) Tectonics
4) Paleoclimatology
Lecture 8: BIFs
The Proterozoic Eon
Proterozoic Paleogeography
Neoproterozoic 650 MA
Eon
Time
Phanerozoic
550 MA to 0 MA
Proterozoic
2.5 GA to 550 MA
Archean
4.1 GA to 2.5 GA
Hadean
4.6 GA to 4.1 GA
Division
Age
Neoproterozoic
900 MA to 550 MA
Mesoproterozoic
1.6 GA to 900 MA
Paleoproterozoic
2.5 GA to 1.6 GA
• Paleo- old
• Meso-middle
• Neo-new
• This is about as far back as we can go with detailed paleogeography
Proterozoic Glaciations
Proterozoic Tectonics
The Varangian glaciation was
weird; many of the best
glaciated sites were near the
paleoequator.
• The culprit that hit us was Australia seen here fleeing the scene
of the accident about 500 MA after the incident
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Today’s Agenda
Banded Iron Formations
(Evolution of the Earth’s atmosphere and hydrosphere)
The Archean (4.1 to 2.5 GA)
• As far as we can tell, there was no modern plate tectonics
during this interval (crustal differentiation?)
1) The Early Atmosphere
2) The Oceans and Hydrosphere
3) The Change
• As far as we can tell, there was no modern plate tectonics
during this interval (crustal differentiation?)
• But oceans were around, even during the Late Heavy
Bombardment period (4.1 to 3.8 GA)
Earth’s Atmosphere/Hydrosphere
• Water and atmospheric gases on the Earth probably came
from volcanoes.
The Archean (4.1 to 2.5 GA)
• Evidence for early oceans consists of old zircons
http://serc.carleton.edu/images/earthscoperockies/Vervoort.jpg
The Archean (4.1 to 2.5 GA)
Earth’s Atmosphere/Hydrosphere
• Water and atmospheric gases on the Earth probably came
from volcanoes.
• Some have suggested that comets were also a possible water
source
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Earth’s Early Atmosphere
At 4.0 GA:
N2; HCl; SO2; CO2; CH4; NH3; NO2; H2O
Ozone
Without ozone, ultraviolet
radiation from the sun
would strike the Earth and
most life would die off.
No…
No…. O2 and No…
No…. O3 (Ozone)
Ozone
Ozone
When? - Hadean to Archean
(4 to 3 billion years ago)
So, when and how did
ozone first appear in the
Earth’s atmosphere?
Based on the rise of life
forms in the world’s oceans.
Earth 4.0 GA
Ozone
Ozone
How? – Via the same
UV radiation that ozone
today protects us from
Ozone destruction is far
more rapid than ozone
formation
2H2O + UV → H2 + O2
O3 + Cl- → ClO- + O2
ClO- + O3 → Cl- + 2O2
2O2 + UV → O3 + O
O + O2 → O3
Which is why CFCs are
generally banned around
the world.
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Now the other big
question(s)…
How and when did oxygen
enter the atmosphere and
hydrosphere?
http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/Images/Archean.jpg
Earth’s Hydrosphere
• Like the early atmosphere, the early oceans were likely
anaerobic (no free O2)
http://cache.eb.com/eb/image?id=24020&rendTypeId=4
Oxygen
Oxygen
The how is easy:
Cyanobacteria (e.g., the
microorganisms comprising
stromatolites)
and photosynthesis
Photosynthesis
Oxygen
The when component
requires geology and
geochemistry (i.e., rocks)
Ferric iron-bearing
sedimentary rocks
deposited in marine
environments date
from 3.1 GA.
Mount Bruce, Hamersley Iron formation with Banded Iron Formations (BIF).
http://www.geo.vu.nl/~smit/hamersley/hamersley.htm
6CO2 + 6H2O → C6H12O6 (“sugar”) + 6O2
Oxygen
Of particular interest
are banded iron
formations (BIFs)
that are composed of
alternating layers of
hematite (Fe2O3) and
silica.
Iron Oxide Minerals
Hematite (Fe2O3)
Crystallography: Hexagonal
─
Pt. Group: 32/m
Habit: platy, massive
SG: 5.25
H: 5-6
Luster: metallic to earthy
Colour: red, brown-red, steel grey
Streak: brown-red
Cleavage: none; parting on {001}
and {101} due to twinning
Uses: ore of iron, jewlery
Name derivation: After "haimatos" (Greek) blood - because one form is the colour of blood
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Iron Formation
Three major types of iron deposits.
Iron Formation
Three major types of iron deposits.
Mt. Whaleback open pit iron mine at Newman, Western Australia.
1) Replacement of limestone (Red Mountain
Formation, Alabama); most are Jurassic in
age (Clinton Type Fe Deposits)
http://www.geo.utexas.edu/courses/381R/Western%20Australia03.html
http://www.msstate.edu/dept/geosciences/
Iron Formation
Iron Formation
Three major types of iron deposits.
Three major types of iron deposits.
1) Replacement of limestone (Red Mountain
Formation, Alabama); most are Jurassic in
age (Clinton Type Fe Deposits)
1) Replacement of limestone (Red Mountain
Formation, Alabama); most are Jurassic in
age (Clinton Type Fe Deposits)
2) Primary hydrothermal deposits (deep sea
vents); most are PreCambrian (Algoma Type
Fe Deposits)
2) Primary hydrothermal deposits (deep sea
vents); most are PreCambrian (Algoma Type
Fe Deposits)
3) Banded Iron Formations (hematite +
jasper); most are Archean in age (Superior
Type Fe Deposits)
http://www.eco-pros.com/images/WaterDwellers/Photos/nur04506.jpg
BIFs
As discussed by Lucie,
BIFs have been linked
to the Snowball Earth
hypothesis, but only
because they returned
after a long absence.
BIFs
As discussed by Lucie,
BIFs have been linked
to the Snowball Earth
hypothesis, but only
because they returned
after a long absence.
The earliest BIFs
formed independantly
of climate.
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BIFs
The first BIFs formed
around 3.1 GA and
peaked in abundance
at 2.7-2.5 GA
BIFs
The amount of iron
available in BIFs is
absolutely staggering.
http://www.ldeo.columbia.edu/edu/dees/U4735/lectures/03.html
http://www.ldeo.columbia.edu/edu/dees/U4735/lectures/03.html
BIFs
BIFs are thought to
have been formed
through oxygenation
of sea water
containing Fe2+
(reduced iron).
Fe2+
Soluble
→
BIFs
The culprits are widely
assumed to be
cyanobacteria (e.g.,
stromatolites).
Fe3+
Fe2+
Insoluble
Soluble
→
Fe3+
Insoluble
http://jersey.uoregon.edu/~mstrick/myimages/Banded%20Iron.gif
BIFs
In an anaerobic ocean, Fe2+ is the stable form of iron and it is
highly soluble in water. The oceans at 4.0 GA were probably
pea green in colour
Fe2+
BIFs
As ozone built up in the atmosphere, stromatolites could
migrate to shallower water where there was more sunlight
penetration.
Fe2+
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BIFs
They grew more numerous and grew faster.
BIFs
Oxygen eventually began to convert the shelf areas into aerobic
environments…
O2
Fe2+
Fe2+
BIFs
BIFs
Oxygen eventually began to convert the shelf areas into aerobic
environments leading to the precipitation of Fe3+-minerals
By 2.0 GA, the oceans were more or less, aerobic. Fe2+ was
now removed via sulfide processes.
Fe2O3
Fe2+
FeS2
BIFs
There is some discussion about the role of microbeasties in the
formation of BIF’s (e.g., are BIFs inorganic or organicallymediated)
BIF REE Distribution
Many geological materials
are characterized by a
“europium anomaly”. In
most cases, Eu content is
lower than expected. This is
because EU tends to be
extracted by feldspar
formation.
Tahahasi et al. (2007)
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BIF REE Distribution
BIFs commonly don’t
show this Eu-anomaly,
but apparently should*.
*the discussion about this is
somewhat beyond Doug’s
abilities.
BIF REE Distribution
Bacteria, like feldspars, tend
to concentrate Eu, leading
some (e.g., Tahahasi et al.)
to suggest microbial
involvement.
http://www.laurentian.ca/NR/rdonlyres/
Tahahasi et al. (2007)
Oxygen in the Atmosphere
What about the
atmosphere?
So the big change
(oxygenation) culminated
in Archean oceans (c.
3.1.GA) courtesy of
cyanobacteria.
Around the world, we
see a shift in color of
river floodplain shale
Green
Fe2+
to Red
→ Fe3+
At 1.8 GA
Oxygen in the Atmosphere
http://www.grisda.org/colorado/images
Oxygen in the Atmosphere
But O2 levels in our
atmosphere
demonstrate
significant variation;
even today.
The atmosphere became
oxidizing by 1.8 GA and
reached near current levels
by the Ordovician.
http://www.biologie.uni-hamburg.de/b-online/ge42/01.jpg
Oxygen in the Atmosphere
But O2 levels in our
atmosphere
demonstrate
significant variation;
even today.
R. A. Berner, PNAS Vol. 96, Issue 20, 10955-10957, September 28, 1999
Some researchers believe O2
levels were closer to 35% in
the Cretaceous
R. A. Berner, PNAS Vol. 96, Issue 20, 10955-10957, September 28, 1999
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Oxygen in the Atmosphere
Next Time
1. Monday: Archean Oceans (Robbie)
2. Wed: Ediacarin beasties
http://www.ldeo.columbia.edu/edu/dees/U4735/lectures/03.html
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