Co-evolution of life and
atmosphere
Moving toward a stable Earth
Green plant
photosynthesis
Autotrophs
"CH 2O"
CO2
H 2O
O2
Oxic respiration

Bacterial
photosynthesis
Heterotrophs

"CH 2O"
CO2
H2S
SO42
Sulfate reduction

Each autotrophic process has counterbalancing heterotrophic process
Autotrophy vs. heterotrophy
• Describes where an organism get its C
• Autotrophs – get C from CO2 fixation
– Requires external energy source
• Photosynthesis – energy from light
• Chemosynthesis – energy from chemical bonds
– Autotrophs generally the base of the food chain
• Heterotrophs – get C from organic C
– Eat their C
– Energy from chemical bonds
Energy Metabolism in Bacteria
light energy
chemical energy
phototrophy
photoautotrophs
use inorganics
e.g. cyanobacteria
Synechococcus
Trichodesmium (Nfixation)
20-80% of C fixation
due to cyanobacteria
photoheterotrophs
use complex organics
chemotrophy
chemoautotrophs
use inorganics
Oxidize inorganic,
reduced compounds
H2, CO, NH3, Fe2+
-generate ATP
nitrifiers
sulfate oxidizers
H2 S + O2 + CO2
CH2O+ 4S + 3 H2 O
chemoheterotrophs
use complex organics
decomposers
CO2
NH3
other inorganics
most abundant in sea
Energy metabolism vs. electron donor
• Phototrophs – light is energy
• Chemotrophs – chemical reactions for energy
•
•
•
•
Photoautotrophs – light is energy and C from CO2
Photoheterotrophs – light is energy and C from organic C
Chemoautotrophs – chemical rxns for energy and CO2
Chemoheterotrophs - ….
• Lithotrophs – inorganic electron donors
(chemolithotrophs and photolithotrophs)
• Organotrophs – organic electron donors
(chemoorganotrophs and photoorganitrophs)
Heterotrophs
• Use organic carbon as both carbon and energy sources
• Aerobic heterotrophs – use O2 as terminal e- acceptor
• Anaerobic heterotrophs – use nitrate, sulfate, carbon
dioxide, etc. as terminal e- acceptors (many
biogeochemically important processes including
denitrification, sulfate reduction, acetogenesis)
• Non-respiratory anaerobes – fermentation to generate
energy and reducing power; oxidize organic compounds
using other organic compounds as both the terminal
electron acceptor
• Faculative anaerobes – switch between fermentation and
anaerobic respiration
Respiration
• Done by plants and animals
– Process to convert biochemical energy to ATP which a
cell can use and waste products
– Involves oxidation of one compound and reduction of
another
• Aerobic – oxidized compound is O2 (terminal
electron acceptor); reduced compound is glucose,
some other sugar or amino and fatty acids, etc.
• Anaerobic – oxidized compound is something else
Aerobic respiration
• Heterotrophs need some sort of external
electron acceptor to oxidize organic C and
extract energy
• Oxygen is the most efficient electron
acceptor
– Yields the most energy
– Reverse of photosynthesis
Anaerobic respiration
• Once O2 is depleted, bacteria use other ways to
extract energy from organic matter oxidation
– Less efficient than aerobic respiration
– Store some energy in reduced end-products
– From evolutionary standpoint, we started at least
efficient and worked our way up
• Respiration and fermentation are often coupled
together in the decomposition of complex organic
matter
Respiration
Aerobic
Anaerobic/fermentative
ATP is produced – cellular energy
Oxidation of organic compounds
Oxygen is the terminal electron acceptor
ATP is produced
Oxidation of organic compounds
Other compounds are the terminal
electron acceptors – nitrate, sulfate,
carbon dioxide, Fe and Mn oxides
Distribution of metabolic traits has been used to define them
taxonomically
evolution (?)
energetics
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Fermentation is important to microbes …
… and people too !!!
C6 H12O6  2CH 3CH 2OH  2CO2
Metabolisms
• Important coupling between elements
• Changes in reactants and byproducts
• Important for balancing elemental cycles
Organic matter production
• Organic matter produced by autotrophs is more
than CH2O
– Polysaccharides, proteins, lipids
– Include N (amino acids, proteins, nucleotides)
– Includes P (phospholipids, energy compounds)
• Marine versus terrestrial organic matter
– Marine OM rich in N and P
• Protein very important
• Redfield ratio 106:16:1 (C:N:P)
• Production and consumption of 1 mole of this material
produces/consumes 138 moles of O2
– Terrestrial OM rich in C (lignocellulose)
• Different degrees of reactivity – some is recycled
and some is buried and its not random usually
Timing of evolution of metabolisms
Zircon proof of water
• Common in granites
• Rare in mafic rocks
• Resistant to mechanical and chemical weathering
so persists in sediments
• Resistant even to metamorphism!
• Contains uranium and thorium so important for
radiometric dating
• Used as protolith indicators
• Oldest zircons are 4.4 bybp (Australia)
• Oxygen isotopes indicate presence of liquid water
Oxygen in early atm (Hadean &
early Archaen)
• Small amounts of O2 from photolysis
– Early oxic ps possible?
• Photodissociation of water vapor to produce O2
and H2
– H2O + hv  H2 + O2
– H2 lost to space
• Photolysis could lead to loss of all water
– Dead oxidized planet? E.g., Venus, more later
• Retention of water on earth crucial and related to
CO2 retention
Where did the oxygen go?
•
•
•
•
Biological O2 production began ~ 2.7 bybp
No accumulation of O2 until ~ 2.3 bybp
Reasons not well understood
Requires a change in relationship between sources
and sinks (inputs and exports)
– If sinks > sources then O2 does NOT accumulate
• Sinks consume all O2 produced
– Once source > sink, O2 can accumulate
• Sinks decrease over time (sources constant)
• Sources increase over time
O2 consumption in Archaen
• Oxidation of reduced substances
• Large O2 sinks early on
– Reduced Fe and S
– Reduced mantle components and gasses
• Swamp out oxygen sources
• Leads to no net accumulation at first
• Likely that O2 production by PS also
increased over time during this period
Evidence of O2 sinks
• Banded Fe formations (BIFs)
–
–
–
–
–
–
Alternating layers of silica and Fe-rich minerals
Fe(II) – reduced and soluble
Fe(III) – oxidized and insoluble (ppt out of solution)
Almost exclusively formed prior to 1.9 bybp
Source of O2 not well-understood
Can be taken as evidence of changing atm (can’t infer O2 content
of atm)
• Detrital uraninite and pyrite
– Reduced minerals oxidized during weathering so see oxidized
forms today
– Disappear around 2.2 bybp due to weathering
– Consistent with rise of atm O2
– Atm O2 must have crossed some threshold (0.005 PAL) around
time of their disappearance
More evidence
• Paleosols (ancient soils) and redbeds – Australia
– Reduced Fe is soluble and so paleosols older than 2.2 by are Fe
depleted
– Requires higher O2 than for BIFs
• Sulfur isotopes fractionation in oxic versus anoxic
atm (see previous)
– Change in patter of fractionation ~2.3 – 2.45 bybp
– Rocks older than ~2.45 bybp show mass-independent fractionation
– Younger rocks have well-defined and predicted mass-dependent
fractionation
– Related to presence of atm O2
– Mass independent fractionation only in atm
– Reactions in liquids or solids are mass-dependant
– In oxic atm, S is oxidized and rains out
Evolution of Ozone
• Accumulation of free O2 in the atm also led to the
accumulation of ozone
– Ozone important for blocking incoming UV radiation
– Catalytic cycles produce and consume ozone in the atm
– Attenuates solar energy flux between 180 – 320 nm
• Even small amounts of atm O2 leads to enough
ozone to provide some protection against UV
– Partial screen likely to have formed ~ 1.9 bybp
– Presence of this UV filter allowed life to move out of
the oceans and onto land
– Consistent with the timing of evolution of eukaryotes
and higher plants
Incoming radiation
O3 absorption of short 
  240 nm

Ozone prdn.
Ozone destr.
  320 nm ( visible light)

O2  h  O  O
slow)
(
O  O2  M  O3  M
(fast)
O3  h  O2  O
fast)
(
O  O3  2O2
slow)
(
~1.9 bybp
Fig. 11-16 Ozone column depth at different atmospheric O2 levels.
Rapid rise in O2 ~2.3 – 2.4 bybp – Great Oxidation Event
Cambrian
(21%)
1%
More gradual increase in O2 after GOE, well below present atm
Levels (PAL) until Cambrian and variable since then
Different O2 requirements for different processes
>0.15 PAL
Banded iron formations
Redbeds
1.9
Ozone screen “established”
2.2
2.3
~0.01 - 0.005 PAL
Detrital pyrite and uraninite
<10-5 PAL
S isotopes
2.7
Evolution of oxygenic photsynthesis
- biological O2 production begins
Evolution of anoxygenic photosynthesis
3.5
Life originates (perhaps earlier)
- hyperthermophiles, methanogens
Atmosphere likely CO2 rich;
Oceans begin to form
Fig. 10-1
Paradox of the faint young sun
How was the planet not frozen?
• Initial sun was likely ~75% as bright as today
– This solar luminosity with present atm composition
would have led to a frozen earth until ~1.9 bybp
– Ancient metamorphosed sediments back to 3.8 bybp
imply running water (so couldn’t have been frozen)
– Zircon data pushes date of running water to 4.4 bybp
• Interior Earth heat from radioactive decay? Not
enough to make up the difference
• Suggests there must have been “super-greenhouse
to keep temperatures warm
– CO2 and CH4 are likely candidates
Note: Te and Ts based on present-day atmospheric composition
Ts below freezing!
Solar luminosity curve
Tg
Fig. 12-2
Assume constant CO2
Assume constant albedo
Fig. 12-2 The paradox of the faint young Sun.
Changes in CO2 with time
• CO2 initially important
• Methane increasingly important in the
Archaen after life forms
– Methane production from microbes
– Production by methanogens greater than abiotic
production
– Could have been 1000 ppm or more
– Oxidation by O2 not significant in early atm
Move C to rock reservoir
Onset of weathering, widespread
CaCO3 ppt., origin of life
}
•
•
•Hadean •
Archean
Present day
CO2 concentrations necessary to compensate for changes in solar
Fig. 12.3
Luminosity (with only H2O and CO2 greenhouse gases)
Temp curves
shift up with
increasing methane
More CO2 not necessary to maintain habitable surface temps if there
Was more CH4 Atmos CO upper limit from paleosol data
2
Need to stay above
this so as not to freeze
Freezing point of water
Hadean/early Achaean
(up to ~1-10 bar)
present-day CO2
Fig. 12-4 Average surface temperature as a function of atmospheric CO2 and CH4
concentrations.
Siderite absent from late Archean
paleosols
• Siderite (FeCO3) should be there if CO2 was
higher
• Set upper limit for atmospheric CO2
• Combined with the freezing point of water,
constrains atmospheric gas content
• Suggests CO2 levels could have dropped
significantly by late Archaen
• Methane and CO2 possibly of equal importance as
atm components that led to the needed “supergreenhouse”
Why the drop in Atm CO2
• Weathering, calcium carbonate ppt, and the origin
of life would have all removed CO2 from the
Archaen atmosphere
– Amount of C in sed rocks as OM and CaCO3 may have
been close to present day value by late Archaen
• Active plate tectonics did not start until early
Proterozoic
– Don’t have a “complete” carbonate-silicate cycle
– Effective CO2 removal from the atm (weathering)
without as efficient replacement (subduction, melting
and return of sedimentary C)
The carbonate/silicate cycle in the early Archaean
Atm. CO2 loss in the Archean
CO2
X
Uptake into organic matter
Weathering of
silicate rocks
CO2
Ions (and silica) carried
by rivers to oceans
Ca2+ + 2HCO3-
+(+SiO
SiO2 2[aq])
Organisms build calcareous
(and siliceous) shells
CaCO3 + CO2 + H2O
(+ SiO2(s)]
CO2
Subduction
(increased P and T)
CaSiO3 + 2CO2 + H2O  Ca2+ + 2HCO3- + SiO2
CaCO3 + SiO2  CaSiO3 + CO2
Archaen Methane
• Production has potential to develop a positive
feedback loop – high temp, more methane
production, etc.
– High methane also leads to an anti-greenhouse effect
avoiding runaway warming
– Due to polymerization of CH4 to hydrocarbons
– Orange haze (Titan) due to Mie scattering when light of
similar  to particle size
– Anti-greenhouse effect as CH4 absorbs red  high in
atm so it doesn’t reach surface
• Feedback mechanisms involving atm CO2 and
methane and Archaen climate control
– Methane production biologically driven (so could be
Gaian in nature)
Most methanogens are hyperthermophiles
Runaway warming
Photochemical polymerization to
form higher hydrocarbons
Fig. 12-5
Sunlight absorbed in upper
atmosphere, re-radiated back
to space as heat (IR)
Fig. 12-6
Breakdown of Archean climate
control
• Evolution of oxygenic ps enhances oxidation of
methane by O2
• Decrease in methane production
• Low methane and CO2 decrease greenhouse effect
• Coincides with first documented glaciation on
Earth (Huronian glaciation)
• Development of plate tectonics “completes”
carbonate-silicate cycle
– Leading to long-term climate regulation by CO2
– Rebound from global glaciation event
“adds” back CO2
Onset of modern plate
tectonics “turns this on”
Maintaining habitable climate
• Low methane levels and the ability to
control CO2 despite increasing solar
luminosity
• Relative contribution of geochemical versus
biological process in maintaining this
balance?
• How do the feedback mechanisms work?
Long-term climate regulation
• Climate stabilization broke down at beginning and end of
Proterozoic
– Huronian glaciation (2.3 bybp) – rise of atm O2 displacing CH4
• Invoke carbonate-silicate cycle negative feedback to end this
– Neoproterozoic “Snowball Earth” – entire oceans may have
frozen (0.8 – 0.6 bybp) – atm CO2 drawn down to low levels…
• Phanerozoic oscillated between hot houses and cold
houses
– Long-term carbonate-silicate system modulated by other factors
•
•
•
•
Biological processes and organic C burial
Changes in tectonic activity
Periods of rapid seafloor spreading – high CO2
Periods of slower seafloor spreading – low CO2 and deeper basins
– Cooling in mid-Cenozoic may be related to changes in
weathering rates
Fig. 10-1
1.9
Ozone screen “established”
2.3
Onset of “modern”
plate tectonics
Atmospheric methane
decreases
2.7
Evolution of oxygenic photsynthesis
- biological O2 production begins
Evolution of
anoxygenic photosynthesis
Atmos. CO2 levels drop,
methane increases
3.5
Life originates (perhaps earlier)
Atmosphere likely CO2 rich
Oceans begin to form
Snowball Earth
Continents clustered in
tropics
CO2 drawdown
Continued weathering
b/c of continent location
More drawdown
Albedo effects from
growing ice sheets
Freezing of earth
Stops weathering
Stops CO2 drawdown ….
Neoproterozoic
“Snowball” Earth
Huronian Glaciation
(2.5-2.3 bybp)
Liquid water/moderate temperature
• Provides the medium for geochemical cycles
– Cycles elements needed for life
– Implies a reasonable ambient temp on the planet (not
Venus)
• May be related to the ability of the Earth system to
initially sequester atm CO2 in crustal rocks
– Development of feedback loops controlling CO2
– Other greenhouse gases of importance (CH4 and N2O)
• Produced by anoxic microbial processes
• Methanogenesis and denitrification
• As Earth evolved from anoxic to oxic environ,
cycles of these gases probably played a role in
fine-tuning climate regulation
Venus
• Runaway greenhouse
• Similar size,density and internal heat flow
– Probably started out with similar amounts of
H2O and CO2
– However on Earth, most of the CO2 is locked
up as limestone or sedimentary OM
• On Venus, it remained in atmosphere
• So surface temperatures of Venus much hotter (>
400oC)
Venus
• Earth’s IR flux/temperature feedback an important
negative feedback controlling climate
• On Venus, early breakdown in that feedback
– Feedback can break down if atm contains too much
H2O
– If you never hit the water vapor line it never rains
• Atm continues to gain H2O (as vapor)
• Greenhouse effect continually increases
• Increasing surface temp does not lead to enhanced IR flux at
top of the atm (loss of radiation from atmosphere)
• Traps heat (radiation) more effectively
– Happened on Venus during early history?
• Closer to the sun
• Solar flux greater than that to present-day Earth (even when
sun was dimmer
Fig. 3-22
Curvature driven by
water vapor feedback
on greenhouse effect
Runaway warming
• Atm becomes warm and full of water vapor
– Negative feedback breaks down (runaway greenhouse)
• Photolysis in upper atm led to loss of water
– H2 lost to space, O2 reacts with reduced Fe in crustal
material or reduced gases in the atm
– Atm on Venus now only has traces of H2O
• Lack of H2O inhibits weathering and volcanic CO2
accumulates
– Volcanic S gases also accumulate as sulfuric acid
– Hot dry planet with a thick, CO2-rich atm
Fig. 19-2 Systems diagram illustrating the runaway greenhouse on Venus.
5. CO2 increases in the atmosphere
4. Loss of water decreases silicate
weathering
3. Photolysis of water in upper atmosphere
2. Warm atmosphere fills with water vapor
1. Positive feedback
between water vapor
and temperature
Mars
• Start colder because smaller solar flux
– CO2 condenses out (but no return mechanism –
no active plate tectonics)
• Small size means smaller internal
radioactive heat source
– Shuts down carbonate-silicate cycle (and return
mechanisms)
Earth – the perfect storm
• Earth’s retention of water and trapping of
CO2 in the crust avoided “runaway
greenhouse”
• Led to rapid decrease in atm CO2 during the
Archaen
– ppt of CaCO3
– OM formation
• Role of life?
Later evolution
• O2 in atm allowed evolution of more complex
organisms
– Eukaryotes, plants, and animals (other half of tree)
• Mass extinction events also important in the
evolutionary process
– Several major mass extinction events
– End of Permian – largest event
– End of Cretaceous (K-T boundary)
• Extinction of dinosaurs
• Allowed for evolution of mammals
• May have been meteor impact
– Caused by variety of factors
– Biological, geological, extra-terrestrial
Later evolution
• Eukaryotes present in fossil record 2.7 bybp
• Multicellular organisms appear in the fossil record
only 560 mybp
• Cambrian explosion – 544 mybp
• O2 in atm allowed evolution of more complex
organisms
– Eukaryotes, plants, and animals (other half of tree)
• Burial of organic C resulted in rapid O2 production
• Large burial event along with rapid rise in O2
• Isotopically light sedimentary organic C due to ps
Fires and Atm O2
• Range of O2 concentrations allow modest fires 1335%
• O2 concentrations stable within this range for
some time
• Have a record of fires (charcoal) since the late
Devonian period (365 mybp)
• Lower bound – O2 concentration of 13% below
which fires can not ignite
• Upper bound – O2 concentration of 35% would
destroy earth’s biota
– After K-T impact event?
Controls on Atm O2?
• Photosynthesis versus burial of organic C
• Negative feedback (purely hypothetical as data
don’t show this)
–
–
–
–
–
–
Cold waters have high O2 and sink to make deep water
This would allow more respiration of sinking C
Consume O2
Deplete oceanic O2
Lower O2 would allow more C burial (less respiration)
Negatively feeding back on and stabilizing atmospheric
O2
Cooling related to changes
(incr.) in weathering rates (?)
CO2 consumed
Cooling related to
increased carbon
burial
Neoproterozoic
“Snowball” Earth
CO2  H 2O  CH 2O  O2
Huronian glaciation

Burial
Rise of atm O2 around 2.3 by would have eliminated the atm CH4 and caused
temporary cooling? Detrital uraninite below indicate low atm O2 above is redbed
which was formed under high O2. Then a jump in CO2 to cause warming or recovery
of atm CH4?
Atmospheric stability
• Mesozoic warming (251 – 65 mybp)
–
–
–
–
Higher atm CO2
Isotopic evidence
Rapid sea floor spreading (magnetic patterns)
CO2 production from carbonate metamorphism &
outgassing at spreading centers
– Low latitudnal heat gradient equator-pole
– Ocean-atm circulation phenomenon?
• Late Cenozoic cooling – 80 mybp
– Decrease in spreading rates
– Perturbation in carbonate-silicate cycle due to collision
of India with Asia?
Back to life Darwin’s main points
• In any population, more offspring are produced
that can survive to reproduction
• Genetic variation occurs in populations
• Some inherited traits increase the probability of
survival
• Bearers of those traits are more likely to leave
offspring to the next generation – those traits
accumulate
• Environmental conditions determines which traits
are favorable
Evolution and the Modern Synthesis
• DNA can be changed by random mutations
• Mutations give rise to different traits
• Traits can be acted upon by natural
selection
• Many unanswered questions
– How did major taxonomic groups arise?
– What was the source of mass extinctions?
Extinctions
A major selective force in evolution?
Six major mass extinctions in Earth
History
•
•
•
•
•
•
Geologic Period
Late Ordovician
Late Devonian
Late Permian
Late Triassic
Late Cretaceous
Late Eocene
MYA Percent Extinct
435
27
365
19
245
57
220
23
65
17
35
2
Cretaceous extinction probably
caused by an impact somewhere near
Yucatan
• Led to
– Extinction of the
dinosaurs
– Extinction of 17% of
marine fauna
– Rise of the mammals
Role of life processes in modern
day global cycles
• Present day controls on O2 in the atmosphere
• Evolution of life has led to an oxidizing environment on
Earth’s surface
• Present day O2 level controlled by balance between PS and
C burial
– CO2 + H2O <-> CH2O + O2
• Bury OM in seds and leave O2 in the atm – not decomposed
– Carboniferous period
• More complex – see book
• Other feedback mechanisms help control large-scale
excursions in O2 and CO2 concentrations
Earth system
• We can think of Earth to have a reducing core and
oxidizing crust
• Without external forcing of continued PS, this
couldn’t exist
• Life harvests solar energy and uses it to maintain
this disequilibrium – between core and crust
• Ability of life to sequester solar input is very
important
The Elements of
Life
• In addition to energy,
life requires certain
material substances
• All organisms require
23 basic elements
• Availability of these
elements can limit
growth and survival
Modern Biogeochemical Cycles
• Elements cycle between organisms, the water, the
sediments and the land
• The maintenance of life requires continued access
to these elements
• Only a few are of biogeochemical significance
• C, N, P, Si, Fe
• Elemental ratios in living organisms are fairly
constant
• Marine systems Redfield Ratio C:N:P 106:16:1
Next time
• Present day global cycles
• Atmosphere
– Chapter 4
• Ocean
– Chapter 5