Co-evolution of life and atmosphere Moving toward a stable Earth
Bacterial photosynthesis Green plant photosynthesis Autotrophs Sulfate reduction Oxic respiration Heterotrophs 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 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
Fermentation is important to microbes … … and people too !!!
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
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 l Ozone prdn. Ozone destr.
~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 1.9 Ozone screen “established” Redbeds 2.2 2.3 ~0.01 - 0.005 PAL Detrital pyrite and uraninite <10-5 PAL 2.7 Evolution of oxygenic photsynthesis - biological O2 production begins S isotopes 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 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
Note: Te and Ts based on present-day atmospheric composition Solar luminosity curve Ts below freezing! Tg Fig. 12-2 Fig. 12-2 The paradox of the faint young Sun. Assume constant CO2 Assume constant albedo
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 } • Archean • Hadean Present day CO2 concentrations necessary to compensate for changes in solar Luminosity (with only H2O and CO2 greenhouse gases) Fig. 12.3
Temp curves shift up with increasing methane More CO2 not necessary to maintain habitable surface temps if there Was more CH4 Atmos CO2 upper limit from paleosol data 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 “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)
The carbonate/silicate cycle in the early Archaean Atm. CO2 loss in the Archean X CO2 Uptake into organic matter CO2 CO2 Weathering of silicate rocks Ions (and silica) carried by rivers to oceans Ca2+ + 2HCO3- (+ SiO2[aq]) Organisms build calcareous (and siliceous) shells + SiO2 CaCO3 + CO2 + H2O (+ SiO2(s)] CO2 Subduction (increased P and T) CaCO3 + SiO2 CaSiO3 + CO2 CaSiO3 + 2CO2 + H2O Ca2+ + 2HCO3- + SiO2
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 l to particle size • Anti-greenhouse effect as CH4 absorbs red l 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)
Photochemical polymerization to form higher hydrocarbons Sunlight absorbed in upper atmosphere, re-radiated back to space as heat (IR) Most methanogens are hyperthermophiles Runaway warming Fig. 12-5 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.