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Interactions between biosphere and atmosphere on earthlike planet

Pekka Janhunen

Finnish Meteorological Institute, Helsinki

(Kumpula Space Centre)

Astrobiology course 2009 “Atmospheres of exoplanets” (53849)University of HelsinkiNov 19, 2009, 16:15-17:45 E204, Kumpula Physicum

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Scope

Earth history from Paleoproterozoic (2000 Ma) up to Phanerozoic (540 Ma), in light of biosphere-climate interactions and evolution of life

Astrobiological viewpoint (keep in mind generalisations to earthlike exoplanets)

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Outline1. Brief history of life

2. Methods of study

3. Physical processes

4. Consistent(?) model

5. Generalisation to exoplanets?

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Brief history of life• Archean: methane-producing bacteria, hot climate

• Older Paleoproterozoic: cyanobacteria, high production, ice age

• Paleoproterozoic-Mesoproterozoic: eucaryotics, modest production, stromatolites, uniform warm climate,

• Neoproterozoic: increasing production, ice ages

• Cambrian radiation of Metazoa: oxygen, end of ice ages

• Phanerozoic: Metazoa, modern climate

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Ediacaran biota (635-542 Ma)• First multicellular animals

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Methods of study• Cold periods recognised from glacial debris (e.g.

dropstones on sediment bed)

– E.g., no glacial deposits during 1000-2000 Ma

– Only possible if climate was warm throughout (since colder periods produced by volcanic aerosols, asteroid impacts, etc. must have existed)

• Production estimated from carbonate C13/C12 ratio

– Organisms prefer C12 because it is more mobile

– High organic productivity==>carbonates enriched in C13

• Stromatolite fossils

• Multicellulars need oxygen

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Stromatolites

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Proterozoic Mechanisms 1• Sun brightens 10% per billion years (a lot!)

• “Walker thermostat” / Silicate-CO2 thermostat: tends to keep equatorial temperature constant and roughly at present value

– Silicate weathering binds CO2 from atmosphere, producing carbonates

– Needs liquid water (CO2 dissolved in water), rate accelerates with increasing temperature

– New un-weathered rocks produced volcanically

– Because rate is strongly dependent on temperature, process occurs at warmest place i.e. at equator. Therefore equalises equatorial, not polar, temperature.

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Proterozoic Mechanisms 2• Walker Thermostat: Because Sun was dimmer during

Proterozoic than nowadays, CO2 greenhouse must have been stronger, for equator to have been equally warm as today

• Because CO2 greenhouse was strong, also polar regions were warmer than today (no glaciers anywhere)

– Paradoxically, polar areas were warmer although Sun was dimmer

– Brightening of the Sun reduces CO2 greenhouse which increases tropical-polar thermal contrast

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Proterozoic Mechanisms 3• Cold polar seas are highly productive, because no

thermal stratification (everything close to 0 C) ==> easy vertical mixing ==> nutrients upwell to surface

• Ocean bottom waters have nearly same temperature as polar waters (close to 0 C nowadays, warmer if no glaciers anywhere) ==> tropical ocean is thermally stratified and thus oligotrophic (low production)

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Proterozoic Mechanisms 4• CO2 production = volcanic activity

• CO2 loss = carbonate sedimentation + organic burial

• Walker thermostat (once more): carbonate sedimentation increases with temperature

• Rate of organic carbon burial decreases a lot, if seafloor is oxygenated and has moving/burrowing animals (Metazoa) [which of course need oxygen to live]

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Proterozoic Mechanisms 5• O2 production = burial rate of organic carbon

– CO2 + H2O → CH2O + O2 → C + H2O + O2

• O2 loss = oxidation of minerals (ground, seafloor)

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Proterozoic Mechanisms 6• Strong greenhouse ==> uniform, steady climate

– Occurred when Sun was weaker (Walker thermostat)

– Main greenhouse gas in tropics is H2O anyway ==> when CO2 or CH4 greenhouse is strong, tropics is not warmer, but larger (so actually Walker thermostat may stabilise more the area of tropics rather than its temperature, since H2O equalises the temperature anyway)

• Weak greenhouse ==> nonuniform, variable climate

– Ice and snow albedo feedback ==> variability, instability

• Nonuniform & variable climate = Engine of evolution!

• Brightening Sun ==> ... ==> evolution !

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Proterozoic: Consequences• High production ==> high organic carbon burial ==>

CO2 removal+oxygen ==> coldness + oxygen

• Brightening Sun ==> lesser CO2 greenhouse ==> polar cooling ==> increased productivity of polar seas ==> carbon burial ==> general cooling, oxygen

• Oxygen = time integral of (photosynthetic) production

• Eventually, mineral oxidation buffer exhausted, after which O2 accumulates in atmosphere ...

• O2 ==> multicellulars ==> less efficient organic carbon burial (burrowing) ==> warming

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The timeline

• C13/C12 ratio in carbonate sediments

• High C13 = organisms have used C12 = high production

• Signs of glaciation marked (strong = white)

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Paleo-Mesoproterozoic static world

• No evidence for glaciations

• Sun 15-20% dimmer than today

• Significant greenhouse (CO2, plus possibly CH

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• Temperature variations smaller than today

• Main carbon burial route is inorganic (small C13 vars.)

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Paleo-Mesoproterozoic

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Meso-Neoproterozoic gradual change

• Sun ~10% dimmer than today

• Increasing temperature variations, polar cooling

• Increasing rate of organic carbon burial

• Increasing modulations of organic carbon burial

• First glaciers appear during Neoproterozoic

• Well-mixing polar seas new habitat for algae

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Present net primary production

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Meso-Neoproterozoic

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Neoproterozoic development

• Stronger and stronger glaciations

• Higher and higher organic carbon burial

• High-amplitude changes in all variables

• Increased rate of evolution (still unicellular)

• O2 appears as byproduct of algal bloom, first buffered by

rocks

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Neoproterozoic

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Cambrian explosion

• O2 buffering exhausted: Oxygenation of shallow sea

• Multicellulars appear with only 20-30 Myears delay

• Pelagic: Zooplankton grazing on algae, “Arms race”

• Benthos: Burrowing animals promote decomposition

• ==> Reduced organic carbon burial rate

• ==> End of glaciations

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Phanerozoic balance

• New thermostat:

– Increased CO2

– ==> increased thermal stratification

– ==> increased anoxic, multicellular-free seafloor

– ==> increased organic carbon burial rate

– ==> decreased CO2

• Also maintains O2 balance

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Phanerozoic

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“Natural” birth of multicellulars• Brightening sun

• ==> Lesser greenhouse effect (W-H-K thermostat)

• ==> Cooler poles

• ==> Algal paradise (nutrient supply, vertical mixing)

• ==> Organic carbon burial, Further cooling

• ==> O2 as byproduct

• ==> First O2 consumed by rocks, but eventually enters

atmosphere and sea

• ==> Multicellulars appear

• ==> Predation, burrowing; End of glaciations

• ==> Phanerozoic world

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Why coldness promotes multicellular appearance?

• Oxygen is critical for first (primitive) multicellulars to be competitive against their unicellular peers

– Cold water can contain more dissolved gases, including oxygen

– Slower cellular respiration in cold

• The excess oxygen can only come from photosynthesis. Thus one needs an algal paradise, which is readily provided by the well-mixing cold water column

– Penetration of sea-ice edge to mid-latitudes increases area and exposure to sunlight, further increasing O

2 production

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Astrobiological perspectives

• Time when multicellulars appear (out of pre-existing microbial background), may not be universal constant (like 3.8 Ga), but may depend on brightening schedule of the host star (and possibly on slowing-down schedule of planetary mantle convection, i.e. rate of carbon cycling)

• Glaciations seem to be a necessary step before multicellulars can appear

• ==> Should try to observe snowy, icy “Earths”

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Biology, atmosphere, temperature• Role of biology large already during Archean (methane

greenhouse)

• Evolutionary innovations have produced coolings, which have triggered further evolution:

– Cyanobacterial photosynthesis → 2500 Ma ice age → Eucaryotes

– Eucaryotic algae in polar seas → Neoproterozoic glaciations → oxygen → multicellulars

– Land plants (Phanerozoic) → Carboniferous-Permian ice age → mammals (?)

– Grasses+diatoms → modern ice ages → Homo sapiens

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Messages to take home

• Biology has large effect on a planet

• Snow and ice !

• “Naturality” of multicellular evolution (sort of)