Reconstructing Earth’s Surface Oxidation

8/9/2019 Reconstructing Earths Surface Oxidation

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Reconstructing Earths

Surface Oxidation

with special reference to the

Great Oxygenation Event


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Geological Evidences for variation in the oceanicand atmospheric redox states

Mritunjay Kumar

Geochemical Evidences in the rock record to

track the rise of free oxygenSulfur and Chromium : Sumitra KCarbon : Deepak Kumar Jha

Oxidative mechanisms and survival of oxygenSoumyaditya Mondal


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Faint young sun paradox

In Archean-Proterozoic period the solar

luminosity was 20 - 30% lower than

today. But scientific studies show the evidence

ofliquid water on Earths surface.

So, what happened ?


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Green house hypothesis

Earth's atmosphere may have contained more

greenhouse gases early on. Carbon

dioxide concentrations may have been higher

without plant photosynthesis converting it tooxygen. Methane, a very active greenhouse gas

which reacts with oxygen to produce carbon

dioxide, may have been more prevalent as well.

So for methane to be abundant Oxygen levelshave to be very low.


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BANDED IRON FORMATION

What is banded iron formation ?

Distinctive type of rocks often found in Precambrian Sedimentary

rocks, consisting of repeated thin layers of iron oxides, either

magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of

iron-poor shale and chert.


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BIFs contains about 20 times as muchoxygen as todays atmosphere does

Chemical composition of BIF in wt%

Fe-20-40%

SiO2-43-56%

CaO-1.75

MgO-9.0

Al2O3-very low


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Snowball Earth

It has been proposed that rise of free oxygen

that occurred during this part of

the Paleoproteozoic removed methane in the

atmosphere through oxidation. As the Sun wasnotably weaker at the time, the Earth's climate

may have relied on methane, a powerful

greenhouse gas, to maintain surface

temperatures above freezing. In the absence ofthis methane greenhouse, temperatures plunged

and a snowball event could have occurred.


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Environment of Archean-

Proterozoic transitionArchean-Proterozoic transition contents low

O2

Rich in methane. Solar luminosity was low.

Thereafter the Earths surface environment

became irreversibly oxidized.


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Geochemical Evidences


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Sulfur


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33S=0.515 x 34S

Terrestrial Fractionation Line


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Earths early Sulfur cycle


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General trend with geologic time in 33S values

for sedimentary sulfides and sulfates33

S = 33

S-0.515 x 34

S


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Chromium in BIF


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Schematic of the surface chemistry of

chromium


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Robert Frei et al.

History of chromium in

sea water


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Carbon Anomaly


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What is 13C isotope anomaly?

Carbonate sediments during archean -proterozoic shows high value of13C value during archean -proterozoic boundary compared from present

and phanerozoic.

Carbonate sediments shows enriched (13C ~+5 to +14 PDB) 13C

isotopes which is higher than the present and phanerozoic eon.

13C anomaly during Archean - Proterozoic

Measurement of 13C isotopes:

Carbon has two stable isotopes, 12C and 13C . Enrichment in 13C can be expressed

as 13C, in the same way that an excess of33S can be expressed as 33S


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(Del) HC13 in

(C13/ C12) sample - (C13/ C12) standard

= --------------------------------------------------- X 1000

(C13/ C12) standard

(C13/ C12) standard is the ratio in a standard sample of the fossil invertebrate Belemnitella

americana (Cretaceous Peedee formation in South Carolina)

Enrichement of the sample in12

C relative to standard produces anegative values for 13C and vice versa.The standard used is the

equivalent of carbonates made from Belemenite americana

collected from the Pedee formation,USA.


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Outline of carbon isotope, tectonic, and biospheric changes through the Proterozoic. After Lindsay and Brasier (2004). (Circles)

Supercontinents; (black bars) supercontinent events; (hachured bars) intracratonic basins; (black triangles) glaciations.

CO2+H

2O=CH

2O+O

2 {Incorporation of 13C in sediments}


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Data recorded from various craton:

The data from carbonate sediments deposited between 2.6 and 1.6 Ga indicate that

the isotopic composition of marine carbon underwent a very large positive change

between approximately 2.22 and 2.06 Ga.(as reported from Africa (LomagundiFormation; values as high as +13 PDB; the Fennoscandian Shield , Scotland and

North America.)

Carbon isotope measurements, carried out on carbonate samples from different

localities of the early Proterozoic Aravalli Craton, gives positive 13

C values up to+11.20 PDB.

Why 18O cant be used?

18O

compositions of Precambrian carbonates from different parts of world showsvalues between 5 and 8 to be normal for Precambrian carbonates .

Most of the studied carbonate samples with heavy carbon also preserve oxygen

isotope compositions . This is good evidence for a pre-metamorphic origin of the

heavy carbon isotope .


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Cross-plot of13C/12C versus 18O/16O for Aravalli craton Iswal (), Bari Talab ( ),

Babarmal area ().


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As any process involving a carbonate during metamorphic decrease its 18O values

significantly. Exchange with other silicates, exchange with an infiltrating fluid and

devolatisation reactions are all likely to cause a decrease in 18

O.

Comparatively low 18O values in Bari Talab carbonates may be due to the associated

Delwara volcanic rocks.

Carbonate strata belonging to the Jhamarkotra Formation, Paleoproterozoic

Aravalli Supergroup, are characterised by 13C enrichment.

It is suggested that increased oxygen contents of the oceans were produced and

accompanied by an increase in organic production.The increased oxygen contents of

the atmosphere may have been the direct result of an increase in the population of

photosynthesising bacteria.

DISCUSSION


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This increased organic productivity, in the absence of a compensating increase in

the rate of organic carbon recycling, would have increased the rate of organic carbon

deposition and resulted in the deposition of high 13C carbonates .

The global presence of13C rich carbonates suggest that the Lomagundi Formation

in Zimbabwe , Kapvaal Craton;Brazil and the Aravalli Craton basin in India are marine in

origin and were deposited under almost similar depositional conditions.

CONCLUSION

According to this model the positive carbon isotopic anomaly of sedimentary

carbonates at about 2.3 Byr ago reflects a very high O2-content of the atmosphere

and the subsequent drop in 13C-values equates with a sharp decrease of the O2-

content of the atmosphere.

The close relationship between C-isotopic anomaly and glaciogenic rocks

suggests that high rates of organic carbon burial facilitated glaciation by reducing

atmospheric greenhouse capacity.


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It has been proposed that rise of free oxygen that occurred during this part of the

Paleoproterozoic removed methane in the atmosphere through oxidation. As the

Sun was notably weaker at the time, the Earth's climate may have relied on

methane, a powerful greenhouse gas, to maintain surface temperatures above

freezing. In the absence of this methane greenhouse, temperatures plunged and a

snowball event could have occurred.

The oxygenation event at the end of the Proterozoic would have resulted in a

decrease of methane flux and could have caused the first Neoproterozoic "snowball"

glaciation.

In addition to evolutionary questions, S and C data can also be used for

stratigraphic correlations. This is particularly important for the Precambrian, which

lacks biostratigraphic framework.


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Methane and the irreversible

oxidation of the early EarthArchean environment was reducing.

Solar luminosity was 20-30% lower than

today.

O2 content was very low.

Methane should have present at the level of

102 to 103 ppmV to counter the solar

luminosity of the early sun.Though we are getting the evidences of the

earth surface oxidation.

HOW ???


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How did the earth surface

oxidize?There are two ways

1) Increase in the atmospheric oxygen

and,

2) Decrease in the atmospheric

hydrogen.


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Coupling of early oxygenic photosynthesis

to the escape of H to space:

CO2 + 2H2O CH4 + 2O2this equation sums the photosynthesis and

methanogenesis.

CH4 + U.V C + 4H(space)

methane decomposed in the upper atmosphereby U.V radiation.


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Diffusion limited H escape rate

Hescape rate = 2.5* 1013 ftotal (H2

molecules cm-2*s-1)

The total concentration of all H-bearing

compounds in the lower stratosphere

ftotal= (5fH2O + fH2+ 2fCH4. . .)(expressed as H2 molecules for these

calculations)


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We can explain oxidation results fromCH4 induced H escape in three ways

(i) When CH4 originates from organic matterproduced by oxygenic photosynthesis,

(ii) When CH4 derives from organic matter

produced by anoxygenic autotrophic

metabolisms,

and,

(iii) When CH4 derives from mantle H.


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When CH4 originates from organic matter

produced by oxygenic photosynthesis

CO2 + H2O = CH2O +O2

2CH2O = CH4 + CO2

CH4 + U.V C + 4HC +O2 = CO2

OVERALL,

2H2O +the biosphere + U.V O2 +4H


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CH4 derives from organic matter producedby anoxygenic autotrophic metabolisms

CH4

originating from anoxygenic

photoautotroph's or

chemoautotroph's

H2S + CO2 + UV radiation

3CH2O +H2O+ S

If CH4 were derived from such

organic matter, H escape would


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CH4 derives from mantle HIt concerns methanogenic CH4 derived from mantle

hydrogen in volcanic gases.

Mantle H can escape directly to space.

In all cases discussed above, Earths overall

oxidation state increases. Case (iii) oxidizes the

mantle. Cases (i) and (ii) oxidize the crust (e.g., asFe2O3 or SO42), which, in the long-term, must shift

kinetics to favor the survival of free O2. Free O2 is

only produced in case.


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Reconstructing Earth’s Surface Oxidation

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