The other volatile: O 2 What is the mantle/surface/biology connection? Charles H. Langmuir Harvard University

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 chondritePallasite

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! CO 2 +4, CO +2, C, CH 2 O 0 CH 4 -4 Organic molecules all have reduced carbon and hydrogen bonds

Current upper mantle has about 3% Fe 3+ Not in equilibrium with metallic Fe Did photosynthesis do it?

Life produces an Electric Current that makes reduced molecules and oxidized complements CO 2 + electron donor + hydrogen → CH 2 O + 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 O 2 permitted Eukaryotic Cells and Multicellular Life: Aerobic Respiration The full potential of aerobic respiration requires high O 2

PROKARYOTES EUKARYOTES Anaerobic Aerobic (1-2% O 2 ?)

One trillion of these working together with active oxygen transport: 21% O 2

Hydrogen Fuel Cell

Modern Earth’s Fuel Cell Reduced C, Fe, S Aerobic Life, Weathering

+ Modern Earth as Planetary Fuel Cell + + O2O2 C and CO 2 Fe, Ni FeO FeS 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 O 2 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 O 2 production

Oxygenic photosynthesis does not lead to any rise in O 2 unless it leads to increased burial of organic matter

Development of oxidized planetary surface Sources (removal of electrons): –Burial of organic matter –Loss of H 2 from compounds with H + from top of atmosphere Sinks: –oxidation of organic matter –oxidation of Fe –oxidation of Sulfur

O 2 is actively consumed 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?

From Farquhar Mass Independent Sulfur Isotope Fractionation Some atmospheric oxygen beginning at 2.4Ga

Pyrite Sulfur Isotopes From Lyons (2010) Canfield (2004) Seawater sulfate?

Mo abundances From Scott et al 2008

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 *10 18 moles

Oxidized Reservoirs

2% of oxidizing power produced by organic life resides as O 2 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

Mass balance problem = * 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 *10 18 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 Fe 3+ /Fe 2+ by 1% requires 2 billion years of present Fe 3+ subduction. –Data suggest deep ocean not oxidized prior to 700Ma –Even small increase of mantle Fe 3+ 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 Delano 2001

Oxidation State of Upper Mantle Source Regions Has Not Changed Since Archean Delano 2001

Changing the Oxidation State Requires Electron Transport 3FeO → Fe + Fe 2 O 3 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. Ferric iron increases by about 1 wt%. Subduction flux is 8 * moles per year, which is four times the estimated organic carbon burial rate. Is atmospheric O 2 decreasing? Essential feedback on O 2 ? Photo from Alt et al. ODP hole 504b.

Present Earth Is Out of Balance Current burial rate of organic carbon (= O 2 production) is 0.68 * moles/yr Current flux of subducting Fe 3+ is 2*10 12 equivalent moles Suggests plate tectonic feedback on O 2

Modern Convergent Margins Kelley and Cottrell (2009)

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