reconstructing earth’s surface oxidation
TRANSCRIPT
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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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