Transcript Document

Chapter 11
Effect of Life on the Atmosphere: The
Rise of Oxygen and Ozone
Some preliminary chemistry
Chemical reactions – involve the giving and taking of electrons
between atoms. the nucleus is not affected in chemical reactions.
e.g. when C is combined with O2, oxygen gives electrons.
isotopes – same number of protons and electrons, different number of
neutrons. Affects the atomic weight, but not the electrons, and
therefore not the types of chemical reactions it undergoes.
Oxidation states – associated with the number of electrons either
given or taken in a chemical bond. Oxygen is considered to have an
oxidation state of -2, so when a mineral is oxidized it gives 2 electrons
to the oxygen molecule. Each element can be found on earth in one or
more oxidation states, but usually only certain states are stable and are
more frequent.
How did early organisms affect the atmosphere?
Speculative – these paleo-studies of the origin of the earth and life
(chapter 10) and the rise of oxygen and ozone (chapter 11) are highly
speculative.
How did organisms affect oxygen, methane, nitrogen
The answer depends on the metabolic functions of the organisms:
oxygenic photosynthesis
anoxygenic photosynthesis
methanogenic chemosynthesis
How did early organisms survive?
Stromatolites – 3.5 b.y. old (?), fossil remains of bacteria today found
in shallow-salty environment in Western Australia. Layers  sunlight
and that they can photosynthesize! But produce O2? Not necessarily.
Recall:
oxygenic photosynthesis  sunlight + CO2 + H2O  organic Ccompounds + O2
anoxygenic photosynthesis  sunlight + CO2 + H2N or H2 (instead of
H2O)  organic C-compounds + no O2!
And there is cyanobacteria that can perform both!! and can switch
from one to the other depending on whether H2N is there or not!!
But there is more to the rise of O2 in the atmosphere than O2 ; we need
to look into the production of CH4 and the cycling of atmospheric N2
We need some other evidence to indicate which form of photosynthesis
these bacteria performed, and therefore how they affected oxygen
levels, or how they affected any other atmospheric constituent.
How did early organisms survive?
Methanogenic bacteria (methanogens)
hyperthermophiles, which live at high temperatures (deep sea vents),
get energy from chemical reactions, not sunlight
CO2 + 4H2  CH4 + 2H2O
Both CO2 and hydrogen were probably present in the deep sea vents
as well as the atmosphere, in high enough concentrations.
So, the early atmosphere may have had a lot of methane (CH4), like
Titan (Saturn’s moon) which looks orange.
Sequencing of
ribosomal RNA
results in these
categories of
organisms –
Bacteria and
Archaea are
single-celled
organisms
(Bacteria is used
for both) and
Eucarya include
some single-cell
but all the rest are
there too (this
includes humans)
Ratio of 13C to 12C – definition:
 C / Csample

  13 12
 1 x1000
 C/ C

PDB


13
 Csample
13
12
standard from K marine fossil
CH4 produced by methanogens is depleted in 13C in relation to 12C so
the their ration can be used as evidence of methanogenic activity. But
this requires a little bit extra: IF the CH4 thus produced is used by
other organisms, which died and eventually made it to the sediments
(the ‘preserved geological record’ which is analyzed), then the record
will show low 13C content (again, in relation to 12C) – see figure next.
Evidence in the geologic record: particularly low 13C levels in ancient
sediments at around 2.7 b. y. ago may be associated with
methanogens, because they preferentially incorporate light C (12C).
is taken up faster during photosynthesis than 13C – so organic C
formed by photosynthesis is depleted in 13C but methane is even
more depleted in 13C – photosynthesis alone cannot explain the
extreme low concentrations of 13C in the record around 2.7 b. y. ago.
12C
Effect of organisms (life) on the early atmosphere:
1. The production of CH4 by methanogenic bacteria (simple
autotrophic)
CO2 + 4H2
CH4 + 2H2O
2. The cycling of N2 through the atmosphere-ocean system organisms need fixed nitrogen compounds (ammonia NH3 and
nitrate NO3- )
Today, Nitrogen may be fixed abiotically
(lightening)
N2 + O2

2 NO (nitric oxide, a radical)
NO is oxidized to HNO3 highly soluble
(dissociation)
HNO3
 H+ + NO3- (can be used)
However, in the early atmosphere there would have been little O2.
How did early organisms affect the atmosphere
In the ancient atmosphere, Nitrogen may be fixed abiotically, in the
absence of O2, the most likely reaction to get fixed nitrogen would
have been
(lightening)
N2 + 2CO2  2 NO + 2 CO
NO is oxidized to HNO3 highly soluble
(dissociation)
HNO3
 H+ + NO3- (can be used)
Nitrogen may also be fixed biologically – cyanobacteria in the
oceans, prokaryotes (believed to have been first) such as
methanogens, simpler, more primitive organisms that resist UV well
How did early organisms affect the atmosphere
Nitrogen – if Nitrogen was being fixed, there had to have some
reverse process to balance this, otherwise nitrogen would have been
depleted from the atmosphere in 20 million years
Denitrification some bacteria derive energy by reacting dissolved
nitrate with CH2O. They release either N2 or N20. N20 is then
converted to N2 by photolysis. Particularly in anoxic environments.
The rise of O2
Some evidence suggests that oxygenic cyanobacteria first produced
O2 about 2.7 b. y. ago (organic chemicals in sediments)
chloroplasts – prokaryotes, perform oxygenic photosynthesis. They
were later incorporated into eukaryotes by endosymbiosis
The rise of O2
But, evidence also suggests that atmospheric oxygen did not start
rising until 400 my later, 2.3 bya. Perhaps oxygen was reacting with
something at the surface, and not remaining in the atmosphere.
Evidence for the rise of oxygen comes from geological sources:
1. banded-iron formations
2. detrital uraninite and pyrite
3. paleosols and redbeds
4. mass-independent sulfur isotope ratios - most conclusive
The rise of O2
banded-iron formations (BIFs)
“banded” because of alternating layers with/without iron: at certain
times iron precipitated out of the water, and at other times it did not.
Iron in different oxidation states is more or less soluble. The two
common oxidation states of iron are:
Fe2+ is soluble in water
Fe3+ is not soluble
During times when O2 is present, iron will preferentially be in the
Fe3+ state, which will not dissolve in water, not be transported by the
oceans large distances, and therefore not end up in sediments.
So, at times when the sediments have iron (the iron bands), oxygen
was not present.
BIFs occur prior to 1.9 bya, but almost no other time
The rise of O2
detrital uraninite
Detrital refers to minerals that were transported from their origin to
the site of deposition in its original solid form, not in solution.
Geologists can usually identify such rocks.
For uranium, the more oxidized (U6+) is soluble, while the more
reduced (U4+) is not soluble. Thus, the presence of detrital uranium in
the 4+ oxidation state indicates low atmospheric oxygen levels, while
oxidized uranium in the 6+ oxidation state indicates the presence of
atmospheric oxygen
Sediments older than ~2.2 bya contain detrital uraninite in the 4+
oxidation state, indicating low atmospheric O2 levels.
The rise of O2
detrital pyrite
Pyrite (FeS2), when weathered in the presence of oxygen, is oxidized
to SO4 2- and Fe3+. When conditions are anoxic, the detrital form is
carried all the way to the sea and deposited.
Sediments older than ~2.2 bya contain detrital pyrite, indicating little
or no oxygen in the atmosphere.
The rise of O2
paleosols
Paleosols (ancient soils) are found to be depleted in iron prior to
around 2.2 bya, while since 1.9 bya they have more iron.
If atmospheric oxygen is low, iron released during weathering
remains as soluble Fe2+, and gets washed out of the soil.
If atmospheric oxygen is higher, iron released during weathering is
oxidized to Fe3+, which is insoluble and does not get washed out of
the soil.
The rise of O2
redbeds
Redbeds are sandy sediments with reddish color, such as in the
southwest US (dry conditions). The color is associated with the
presence of hematite (Fe2O3), which is oxidized iron.
The presence of hematite indicates the presence of oxygen in the
atmosphere.
Earliest known redbeds are ~2.2 bya.
NOTE: oxidized iron? RUST!
The rise of O2
sulfur isotope ratios – considered most conclusive evidence of low
oxygen levels prior to 2.3 bya.
Biological sulfate reduction (bacteria use sulfur to oxidize CH20)
preferentially uses “light” sulfur (S33 instead of S34). In sulfur bearing
rocks today, there is a regular ratio of the different isotopes of sulfur
(the mass fractionation line, or MFL). Pyrite (FeS2).
Sulfur isotope concentrations in Archean rocks (3.8 - 2.5 b. y. ago)
The rise of O2
sulfur isotope ratios – considered most conclusive evidence of low
oxygen levels prior to 2.3 bya.
In rocks older than ~2.3 bya, the relative ratios of the different Sulfur
isotopes in Pyrite deviates from more recent values. SOMETHING
CHANGED!
Time series of deviations of S concentrations from the MFL
The rise of O2
sulfur isotope ratios – considered most conclusive evidence of low
oxygen levels prior to 2.3 bya.
In rocks older than ~2.3 bya, the relative ratios of the different sulfur
isotopes in Pyrite deviates from more recent values. SOMETHING
CHANGED!
This indicates that no preferential sulfur isotope was being
incorporated into rocks. The process was probably photolosys of
SO2, which does not occur today because of ozone. This indicates
much lower ozone, and therefore oxygen, values prior to 2.3 bya.
Also, in a low-oxygen atmosphere, sulfur undergoes different
chemical reactions in the atmosphere, and is deposited to the surface
in a variety of molecules (and oxidation states). Today, atmospheric
sulfur is oxidized to sulfuric acid (H2SO4), which is soluble and gets
rained out, which would wash out the effects of photolytic reactions.
The rise of O2
Problem: why does so much geological evidence suggest that
cyanobacteria, which perform photosynthesis and release oxygen
to the atmosphere, existed ~2.7 bya, but oxygen levels did not
begin to rise until at least 2.3 bya?
SOME THEORIES
1. it took that long for the oxygen to oxidize all the iron in the oceans
(researches believes it would have been much quicker)
2. Perhaps the cyanobacteria were not really producing any, or much,
oxygen, but rather they were performing anoxygenic photosynthesis
3. Perhaps volcanic emissions were much more reduced back then,
meaning that the oxygen emitted to the atmosphere would have
reacted with (i.e. oxidized) those gases, and not remained as O2.
The Rise of Ozone
The Rise of Ozone
Variations in Atmospheric O2 Over the Last 2 Billon Years
initial rise of O2 2.7 b. y. ago - conclusive evidence for rise of O2 in
atmosphere 2.3 b. y. ago
multicellular organisms seem to date from 0.5 b. y. ago only (these
need a good amount of O2!)
The Cambrian begins about 0.5 b. y. ago - Phanerozoic - this is when
organisms start to develop hard shells, more detailed fossil records
since then.
Direct evidence for O2 poor for this period (as it was before) but the
estimates come from studies of Carbon isotopes.
Variations in Atmospheric O2 Over the Last 2 Billon Years
Phanerozoic Carbon Isotope Record
1. Burial of organic carbon produces O2 for atmosphere
2. Organic carbon is isotopically “light”: lower C13/C12
3. During periods of more organic carbon burial, the buried organic
sediments are light; the carbon remaining in the atm/ocean
system is heavy; and the inorganic sediments (which are the
same del13C as the ocean) are also heavy.
4. So, del13C is indicator of rate of O2 production
Phanerozoic del13C from carbonate (inorganic) rocks
Indicator of O2 production rate (not atm concentrations)
“carboniferous” coal deposits
Calculated variation in atmospheric O2 during the Phanerozoic (0 0.5 b. y. ago)
Modern controls of atmospheric O2
Atmosphere
Atmospheric O2 “fire window”
Most organic-C burial is Oceanic
(controls atm O2)
Vertical profile of DO in the ocean
(low-lat.)
The “solubility pump”
Modern controls of atmospheric O2
Marine sediments Negative feedback for control of
O2 (?) involving DO and the
C:P ratio
1.
High O2 -> lower C:P (more
P required for every C)
2.
If P is limiting nutrient, then
lower organic C burial
3.
Lower Org-C burial, slower
O2 production rate
Modern controls of atmospheric O2
Forests possible negative feedback for O2 control involving forest fires and the C:P ratio
1.
High O2 -> more forest fires
2.
More forest fires -> Less terrestrial burial (C:P = 1000:1)
3.
More forest fires -> more sediment transport, more marine burial (C:P = 105:1)
4.
If P is limiting, each mol of P
buries less C, and produces less O2