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1、1Interactions between biosphere and atmosphere on earthlike planetPekka JanhunenFinnish Meteorological Institute, Helsinki(Kumpula Space Centre)Astrobiology course 2009 “Atmospheres of exoplanets” (53849) University of Helsinki Nov 19, 2009, 16:15-17:45 E204, Kumpula Physicum2ScopeEarth history from
2、 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)3OutlineBrief history of life Methods of study Physical processes Consistent(?) model Generalisatio
3、n to exoplanets?4Brief history of lifeArchean: methane-producing bacteria, hot climateOlder Paleoproterozoic: cyanobacteria, high production, ice agePaleoproterozoic-Mesoproterozoic: eucaryotics, modest production, stromatolites, uniform warm climate, Neoproterozoic: increasing production, ice agesC
4、ambrian radiation of Metazoa: oxygen, end of ice agesPhanerozoic: Metazoa, modern climate5Ediacaran biota (635-542 Ma)First multicellular animals6Methods of studyCold periods recognised from glacial debris (e.g. dropstones on sediment bed) E.g., no glacial deposits during 1000-2000 Ma Only possible
5、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 C13Stromatolite fossilsMultic
6、ellulars need oxygen7Stromatolites8Proterozoic 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
7、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.9Proterozoic Mec
8、hanisms 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 todayBecause CO2 greenhouse was strong, also polar regions were warmer than today (no glaciers anywhere) Paradoxically, polar areas w
9、ere warmer although Sun was dimmer Brightening of the Sun reduces CO2 greenhouse which increases tropical-polar thermal contrast10Proterozoic Mechanisms 3 Cold polar seas are highly productive, because no thermal stratification (everything close to 0 C) = easy vertical mixing = nutrients upwell to s
10、urfaceOcean 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)11Proterozoic Mechanisms 4 CO2 production = volcanic activityCO2 loss = carbonate sedimentatio
11、n + organic burialWalker thermostat (once more): carbonate sedimentation increases with temperatureRate of organic carbon burial decreases a lot, if seafloor is oxygenated and has moving/burrowing animals (Metazoa) which of course need oxygen to live12Proterozoic Mechanisms 5 O2 production = burial
12、rate of organic carbon CO2 + H2O CH2O + O2 C + H2O + O2O2 loss = oxidation of minerals (ground, seafloor)13Proterozoic 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 i
13、s 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, instabilityNonuniform E
14、nd of glaciations= Phanerozoic world27Why 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 coldThe excess o
15、xygen 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 production28Astrobiological perspectivesTime when mult
16、icellulars 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 “Ea
