Interactions between biosphere and atmosphere on earthlike planet

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Interactions between biosphere and atmosphere on
earthlike planet
Astrobiology course 2009 Atmospheres of
exoplanets (53849) University of Helsinki Nov
19, 2009, 1615-1745 E204, Kumpula Physicum

• Pekka Janhunen
• Finnish Meteorological Institute, Helsinki
• (Kumpula Space Centre)?

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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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Outline

• Brief history of life
• Methods of study
• Physical processes
• Consistent(?) model
• 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 productivitygtcarbonates 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)
  gt easy vertical mixing gt nutrients upwell to
  surface
• Ocean bottom waters have nearly same temperature
  as polar waters (close to 0 C nowadays, warmer if
  no glaciers anywhere) gt 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 gt uniform, steady climate
• Occurred when Sun was weaker (Walker thermostat)?
• Main greenhouse gas in tropics is H2O anyway gt
  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 gt nonuniform, variable climate
• Ice and snow albedo feedback gt variability,
  instability
• Nonuniform variable climate Engine of
  evolution!
• Brightening Sun gt ... gt evolution !

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Proterozoic Consequences

• High production gt high organic carbon burial
  gt CO2 removaloxygen gt coldness oxygen
• Brightening Sun gt lesser CO2 greenhouse gt
  polar cooling gt increased productivity of polar
  seas gt carbon burial gt general cooling,
  oxygen
• Oxygen time integral of (photosynthetic)
  production
• Eventually, mineral oxidation buffer exhausted,
  after which O2 accumulates in atmosphere ...
• O2 gt multicellulars gt less efficient organic
  carbon burial (burrowing) gt 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 CH4)?
• 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
• gt Reduced organic carbon burial rate
• gt End of glaciations

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Phanerozoic balance

• New thermostat
• Increased CO2
• gt increased thermal stratification
• gt increased anoxic, multicellular-free seafloor
• gt increased organic carbon burial rate
• gt decreased CO2
• Also maintains O2 balance

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Phanerozoic
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Natural birth of multicellulars

• Brightening sun
• gt Lesser greenhouse effect (W-H-K thermostat)?
• gt Cooler poles
• gt Algal paradise (nutrient supply, vertical
  mixing)?
• gt Organic carbon burial, Further cooling
• gt O2 as byproduct
• gt First O2 consumed by rocks, but eventually
  enters atmosphere and sea
• gt Multicellulars appear
• gt Predation, burrowing End of glaciations
• gt 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 O2 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
• gt 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 (?)?
• Grassesdiatoms ? 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)?