Co-evolution of life and atmosphere

Moving toward a stable Earth

Bacterial photosynthesis

Sulfate reduction

CO2

H2S

"CH2O"

SO42

Green plantphotosynthesis

Oxic respiration

CO2

H2O

Autotrophs

Heterotrophs

"CH2O"

O2

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

e.g. cyanobacteria

Synechococcus

Trichodesmium (N-fixation)

20-80% of C fixation due to cyanobacteria

phototrophy chemotrophy

photoautotrophs photoheterotrophs chemoautotrophs chemoheterotrophs

light energy chemical energy

use inorganics use complex organics use inorganics use complex organics

Oxidize inorganic, reduced compounds

H2, CO, NH3, Fe2+

-generate ATP

nitrifiers

sulfate oxidizers

H2S + O2 + CO2

CH2O+ 4S + 3 H2O

CO2 NH3

other inorganics

decomposers

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/fermentativeATP is produced – cellular energyOxidation of organic compoundsOxygen is the terminal electron acceptor

ATP is produced Oxidation of organic compoundsOther 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

energetics

evolution (?)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Fermentation is important to microbes …

… and people too !!!

C6H12O6 2CH3CH2OH 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

O2 h OO (slow)

OO2 M O3 M ( fast)

O3 h O2 O (fast)

OO3 2O2 (slow)

240nm

Ozone prdn.

Ozone destr.

320nm ( visible light)

Incoming radiationO3 absorption of short

Fig. 11-16 Ozone column depth at different atmospheric O2 levels.

~1.9 bybp

(21%)

1%

Rapid rise in O2 ~2.3 – 2.4 bybp – Great Oxidation Event

More gradual increase in O2 after GOE, well below present atm Levels (PAL) until Cambrian and variable since then

Cambrian

Atmosphere likely CO2 rich; Oceans begin to form

3.5 Life originates (perhaps earlier) - hyperthermophiles, methanogens

Evolution of anoxygenic photosynthesis

2.7 Evolution of oxygenic photsynthesis - biological O2 production begins

2.3

1.9 Ozone screen “established”

Fig. 10-1

Banded iron formations

Detrital pyrite and uraninite

Redbeds 2.2

<10-5 PAL

~0.01 - 0.005 PAL

>0.15 PAL

S isotopes

Different O2 requirements for different processes

Paradox of the faint young sunHow 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

Fig. 12-2 The paradox of the faint young Sun.

Note: Te and Ts based on present-day atmospheric composition

Fig. 12-2

Tg

Solar luminosity curve

Assume constant CO2

Assume constant albedo

Ts below freezing!

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

•Hadean • Archean

• Move C to rock reservoir• Onset of weathering, widespread CaCO3 ppt., origin of life

}

Fig. 12.3CO2 concentrations necessary to compensate for changes in solar Luminosity (with only H2O and CO2 greenhouse gases)

Present day

Freezing point of water

Atmos CO2 upper limit from paleosol data

Fig. 12-4 Average surface temperature as a function of atmospheric CO2 and CH4 concentrations.

present-day CO2

Hadean/early Achaean (up to ~1-10 bar)

Temp curvesshift up with increasing methane

Need to stay abovethis so as not to freeze

More CO2 not necessary to maintain habitable surface temps if thereWas more CH4

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 “super-greenhouse”

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)

CO2

CO2

CO2

CaSiO3 + 2CO2 + H2O Ca2+ + 2HCO3- + SiO2

Weathering of silicate rocks

+ SiO2

CaCO3 + SiO2 CaSiO3 + CO2

Subduction(increased P and T)

CO 2

Ions (and silica) carried by rivers to oceans

Ca2+ + 2HCO3-

(+ SiO2[aq])

CaCO3 + CO2 + H2O(+ SiO2(s)]

Organisms build calcareous (and siliceous) shells

Uptake into organic matter XAtm. CO2 loss in the Archean

The carbonate/silicate cycle in the early Archaean

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)

Fig. 12-6

Fig. 12-5

Photochemical polymerization to form higher hydrocarbons

Sunlight absorbed in upper atmosphere, re-radiated back to space as heat (IR)

Runaway warming

Most methanogens are hyperthermophiles

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

Onset of modern plate tectonics “turns this on”

“adds” back CO2

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

Atmosphere likely CO2 rich Oceans begin to form

3.5 Life originates (perhaps earlier)

Evolution of anoxygenic photosynthesis

2.7 Evolution of oxygenic photsynthesis - biological O2 production begins

1.9 Ozone screen “established”

2.3

Fig. 10-1

Onset of “modern” plate tectonics

Atmos. CO2 levels drop, methane increases

Atmospheric methanedecreases

Huronian Glaciation(2.5-2.3 bybp)

Neoproterozoic“Snowball” Earth

Snowball EarthContinents clustered in tropicsCO2 drawdownContinued weathering b/c of continent locationMore drawdownAlbedo effects from growing ice sheetsFreezing of earthStops weatheringStops CO2 drawdown ….

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.

1. Positive feedback between water vapor and temperature

2. Warm atmosphere fills with water vapor

3. Photolysis of water in upper atmosphere

4. Loss of water decreases silicate weathering

5. CO2 increases in the atmosphere

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 13-35%• 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 (?)

Cooling related to increased carbon burial

CO2 H2O CH2OO2

Neoproterozoic“Snowball” Earth

Huronian glaciation

Burial

CO2 consumed

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 recoveryof 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 MYA Percent Extinct

• Late Ordovician 435 27

• Late Devonian 365 19

• Late Permian 245 57

• Late Triassic 220 23

• Late Cretaceous 65 17

• Late Eocene 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