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The other volatile: O2
What is the mantle/surface/biology
connection?
Charles H. Langmuir
Harvard University
[email protected]
Major Questions
• Why is the mantle slightly oxidized?
• Why isn’t it more oxidized?
• Does oxygenation of the surface oxidize the
mantle?
• Has oxidation state changed through Earth
history?
• What’s is happening today?
Planetary Evolution as Energy
Transformation
From an initial reduced oxidation
state --To a “planetary fuel cell” that
permits greater access to energy
and efficient transfer and
processing of stellar energy
Planets are Initially Reduced
• Solar nebula has excess of hydrogen and Fe
metal, no free oxygen
Meteorites that make up planets or reveal
their interior have reduced minerals
Carbonaceous chondrite
Pallasite
– Fe as Fe, FeO and FeS
– S as FeS
– C in meteorites is reduced, CO2 in ancient
atmosphere
Solar System objects with truncated
evolution remain reduced
Origin of Life
requires reducing conditions
• precursor organic molecules can form and survive
only under reducing conditions
• Carbon can vary from +4 to -4 in its oxidation state!
•
CO2
+4,
• CO
+2,
• C, CH2O
0
• CH4
Organic molecules all have
reduced carbon and
hydrogen bonds
-4
Current upper mantle has about 3% Fe3+
• Not in equilibrium with metallic Fe
• Did photosynthesis do it?
Life produces an Electric Current that
makes reduced molecules
and oxidized complements
• CO2 + electron donor + hydrogen →
CH2O + oxidized by-product
Carbon changes valence from 4+ to neutral or negative:
electron flow
• Over Earth history this current has created larges
masses of organic matter and a complementary
oxidized surface reservoir
Rise of O2 permitted Eukaryotic Cells and
Multicellular Life:
Aerobic Respiration
The full potential of aerobic respiration requires high O2
Aerobic (1-2% O2?)
EUKARYOTES
PROKARYOTES
Anaerobic
• One trillion of these
working together
with active oxygen
transport: 21% O2
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Hydrogen Fuel Cell
Modern Earth’s Fuel Cell
Aerobic Life,
Weathering
Reduced
C, Fe, S
Modern Earth as Planetary Fuel Cell
+O
2
+
C and CO2
FeO
FeS
Fe,
Ni
+
Permits far greater energy flow than earlier in Earth history
Electron mass balance means
every oxidized element must
be matched by a reduced
element.
Net O2 production is the excess of organic matter
production over destruction, and this organic matter has
to end up somewhere, unoxidized
From this perspective the current Earth
has zero net O2 production
O2 is actively consumed
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So the rise of oxygen and the creation of the planetary
fuel cell involves sources and sinks and their evolution
through Earth history.
When and how did it all happen?
Mass Independent Sulfur Isotope Fractionation
Some atmospheric oxygen beginning at 2.4Ga
From Farquhar
Pyrite Sulfur Isotopes
Seawater sulfate?
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From Lyons (2010)
Canfield (2004)
Mo abundances
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From Scott et al 2008
One possibility from Snowball Earth Proponents
www.snowballearth.org
Deep ocean is source of oxidzied
materials for subduction
(a) Did not exist before ~600Ma
(b) Cannot have caused significant
mantle oxidation
There must be mass balance between
oxidized and reduced reservoirs
Reservoirs of Carbon
Total organic carbon is 600 - 1250 *1018 moles
Oxidized Reservoirs
2% of oxidizing power
produced by organic life
resides as O2 in the
atmosphere. 98% is in
oxidized Fe and S.
Most of the story is in rocks.
Mantle carbon output through time?
Implies that
MOST “oxygen
production”
occurred early;
Gobbled up by Fe
and S
From Hayes and Waldbauer (2006)
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Mass balance problem = 500 - 1000 *
1018 moles of reduced carbon
equivalent
Implies a large reduced reservoir
somehere:
(a) Subducted organic carbon
(b) Hydrogen loss to space
Reservoirs of Carbon on the
Mantle
Earth
Total organic carbon is 600 - 1250 *1018 moles
Simple mass balance constraints:
• One possible reduced reservoir is subduction of
organic carbon. Happening today. Earlier oceans
were reduced, permitting organic matter
accumulation.
– Many others propose hydrogen loss from upper
atmosphere. Unobservable and untestable?
•
Simple mass balance constraints:
• To increase upper mantle Fe3+/Fe2+ by 1% requires 2
billion years of present Fe3+ subduction.
– Data suggest deep ocean not oxidized prior to 700Ma
– Even small increase of mantle Fe3+ requires thousands
of examoles of subducted oxidized material-- makes
mass balance problem MUCH worse
• Production of oxidized species can have had only a
negligible impact on mean upper mantle oxidation state
Elements with variable
oxidation states record
mantle conditions
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Delano 2001
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Oxidation State of Upper Mantle Source
Regions Has Not Changed Since Archean
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Delano 2001
Changing the Oxidation State
Requires Electron Transport
• 3FeO → Fe + Fe2O3
Iron changes valence from +2 to neutral and +3:
electron flow occurs if the Fe metal is segregated to the
core
• This process could oxidize the mantle if it were
significant. Might it occur progressively over Earth
history?
Is the solid Earth important today?
Ocean crust is oxidized as it interacts with seawater.
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Photo from Alt et al.
ODP hole 504b.
Ferric iron increases by about 1 wt%. Subduction
flux is 8 * 1012 moles per year, which is four times the
estimated organic carbon burial rate. Is atmospheric
O2 decreasing? Essential feedback on O2?
Present Earth Is Out of
Balance
• Current burial rate of organic carbon (=
O2 production) is 0.68 * 1012 moles/yr
• Current flux of subducting Fe3+ is 2*1012
equivalent moles
• Suggests plate tectonic feedback on O2
Modern Convergent Margins
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• Kelley and Cottrell (2009)
Fe-Mn Differences MORB/Back-Arc
0.21
FeO/MnO= 54+- 1/sqrt(210)
0.2
MnO
0.19
EPR
ELSC
0.18
0.17
FeO/MnO= 57+- 2/sqrt(30)
0.16
0.15
8
9
10
11
12
Fe2O3 Total
13
14
15
Reflections
Mantle was slightly oxidized early and
has maintained that state within tight
bounds
Life did not oxidize the mantle-- it may
have slightly reduced it
Life today is changing mantle oxidation
state
Mantle plays a critical role in the
oxygen story