the other volatile: o 2 what is the mantle/surface/biology connection? charles h. langmuir harvard...

The other volatile: O2

What is the mantle/surface/biology

connection?

Charles H. LangmuirHarvard University

langmuir@eps.harvard.edu

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

– Fe as Fe, FeO and FeS– S as FeS– C in meteorites is reduced, CO2 in ancient

atmosphere

Carbonaceous chondrite Pallasite

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 -4

Organic molecules all havereduced carbon and

hydrogen bonds

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

PROKARYOTES

EUKARYOTES

Anaerobic

Aerobic (1-2% O2?)

• One trillion of these working together with active oxygen transport: 21% O2

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Hydrogen Fuel Cell

Modern Earth’s Fuel Cell

ReducedC, Fe, S

Aerobic Life,Weathering

+

Modern Earth as Planetary Fuel Cell

+

+O2

C and CO2

Fe, Ni

FeOFeS

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 planetaryfuel cell involves sources and sinks and their evolutionthrough Earth history.

When and how did it all happen?

From Farquhar

Mass Independent Sulfur Isotope FractionationSome atmospheric oxygen beginning at 2.4Ga

Pyrite Sulfur Isotopes

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From Lyons (2010)

Canfield (2004)

Seawater sulfate?

Mo abundances

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From Scott et al 2008

www.snowballearth.org

One possibility from Snowball Earth Proponents

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?

From Hayes and Waldbauer (2006)

Implies that MOST “oxygen

production” occurred early;Gobbled up by

Fe and S

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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 Earth Mantle

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

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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Ferric iron increases by about 1 wt%. Subductionflux is 8 * 1012 moles per year, which is four times theestimated organic carbon burial rate. Is atmosphericO2 decreasing? Essential feedback on O2?

Photo from Alt et al.ODP hole 504b.

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

• Kelley and Cottrell (2009)

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Fe-Mn Differences MORB/Back-Arc

0.15

0.16

0.17

0.18

0.19

0.2

0.21

8 9 10 11 12 13 14 15

Fe2O3 Total

MnO

EPRELSC

FeO/MnO= 54+- 1/sqrt(210)

FeO/MnO= 57+- 2/sqrt(30)

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