Atmosphere Evolution PDF

Breathe Deep ¡ Earth’s early atmosphere was dominated by hydrogen (H2), nitrogen (N2), and carbon dioxide (CO2) ¡ There was also methane (CH4) and water (H2O) ¡ There was no oxygen ¡ How did early life survive and even thrive on such an earth? ¡ A source of energy ¡ Raw materials § Car...

Breathe Deep ¡ Earth’s early atmosphere was dominated by hydrogen (H2), nitrogen (N2), and carbon dioxide (CO2) ¡ There was also methane (CH4) and water (H2O) ¡ There was no oxygen ¡ How did early life survive and even thrive on such an earth? ¡ A source of energy ¡ Raw materials § Carbon § Nitrogen § Phosphorous ¡ Means of reproduction Tree of Life based on comparing ribosomal RNA in living organism textbook Figure 11-3 The earliest organisms were heat loving and methane producing. Methanogens produce energy by combining CO2 and hydrogen CO2 + 4H2 ➝ CH4 + 2H2O + energy textbook Figure 11-3 12C has 6 protons and 6 neutrons. It is stable. 13C has 6 protons and 7 neutrons. It is stable. 14C has 6 protons and 8 neutrons. It is radioactive. ¡ Isotopes have slightly different masses, so they behave differently from one another chemically and physically. ¡ As a result, various natural processes can “sort” isotopes: fractionation ¡ The amount of fractionation is determined by the nature and strength of the process responsible for the sorting. ¡ Isotopic fractionation is extraordinarily useful for understanding past environments. Living Thing 13C 12C 12C 12C 12C 12C 13C 13C 12C 13C 12C 12C 13C 13C 13C 13C 12C 12C 13C 13C 12C 12C 13C 12C 13C 12C d13C = (13C/12C)sample (13C/12C)ref Units of δ13C value: per mil Reference material: Vienna Peedee belemnite (VPDB) (13C/12C)ref x 1000 During methanogenesis, archaea preferentially take up 12C by up to 60 per mil The product is "isotopically very light" methane δ13Cmethane≈ - 60 per mil hydrothermal precipitates ~ 3.46 Ga old fluid inclusions δ13C= - 60 per mil http://www.nature.com/nature/journal/v440/n7083/images/nature04584-f1.2.jpg Uneo et al. 2006, Nature 3.46 Ga: First evidence of very light carbon δ13Cmethane = - 60 per mil (Uneo et al. 2006, Nature) 2.7 Ga: Possibly a second fingerprint of methanogenesis textbook Figure 11-5 ¡ Titan’s atmosphere is rich in N2 (98%) and CH4 (2%) ¡ A methane-based, thick aerosol haze gives Titan an orange hue ¡ Curiously, this as an antigreenhouse effect ¡ Surface temperature (Ts) = 94 K http://upload.wikimedia.org/wikipedia/commons/8/84/Titan_in_natural_color_Cassini.jpg ¡ Atmosphere is full of N2 ('di-nitrogen') ¡ N2 has a strong tipple bond § N2 is largely useless ¡ Most organisms need 'fixed' forms of nitrogen which are more bio-available: § nitrate (NO3-) and ammonium (NH4+ ) ¡ Today, certain types of marine phytoplankton (cyanobacteria) are able to break the strong triple bond of N2 ¡ N2 is also fixed on land, primarily by bacteria that live in root systems of plants, and some of this fixed N makes into the ocean ¡ But how was nitrogen fixed before life evolved ? 𝑁2 + 2𝐶𝑂2 → 2𝑁𝑂 + 2𝐶𝑂 𝑁𝑂 + 𝐻2𝑂 → 𝐻𝑁𝑂3 𝑯𝑵𝑶𝟑 ↔ 𝑯 ! # + 𝑵𝑶𝟑 ¡ comes from weathering of rocks on land ¡ A source of energy -> from the Sun ¡ Raw materials § Carbon -> from CO2 § Nitrogen-> from lightning § phosphorous-> from rocks ¡ Means of reproduction The Rise of Oxygen Methanogens were wide spread on the early Earth. There was virtually no oxygen. Methane could accumulate in the early atmosphere to high concentrations. Cyanobacteria were also around and formed stromatolites. These bacteria photosynthesized using an anoxygenic pathway. H2 or H2S is used (instead of water) to reduce CO2 to organic carbon At some point, cyanobacteria evolved the ability to carry out photosynthesis using the to oxygenic pathway. CO2 + H2O -> CH2O + O2 http://universe-review.ca/I11-30-cyanobacteria.jpg Today, most photosynthesis is carried out by eukaryotic algae and higher plants. They acquired the ability to photosynthesize by ingesting a cyanobacteria Textbook Figure 11-3 Multiple lines of evidence: fossils biomarker detrital minerals (uraninite, pyrite) new findings are still being made We will focus on these three: Banded Iron Formations (BIFs) Red Beds Sulfur Isotopes Sulfur Isotopes Sulfur has 4 stable isotopes: 32S, 33S, 34S, and 36S Certain bacteria use sulfate (SO42-) to oxidize organic matter. They reduce sulfate and form the mineral pyrite (FeS2) in the process. If O2 is around, bacteria fractionate against SO42- containing increasingly heavier isotopes in a predictable way: ‘Mass Dependent Fractionation’ Sulfur Isotopes D Mass Dependent Fractionation O2 rose to appreciable levels by 2.4 Ga 2.4 Ga 0 Time before present (Ga) 4 sandy/silty terrestrial sediments deposited by wind/rivers stained red by a thin layer of oxidised-iron-bearing minerals (e.g., hematite, Fe2O3) oldest red beds: 2.2 Ga abundant after 1.9 Ga http://farm3.static.flickr.com/2224/2437850391_91d0ae158e.jpg marine sedimentary rock alternating layers of: Fe-rich minerals (hematite and magnetite) and Fe-poor chert most of them formed before 1.9 Ga http://spot.pcc.edu/~ksutton/GEO207/Images/banded_iron_formation_rock.gif Waters without oxygen (anoxic) Fe2+ is soluble Fe3+ is insoluble Chert (SiO2) Iron-rich sediments Waters with oxygen (oxic) ¡ To explain the large quantities of BIFs, the ocean must have been anoxic ¡ This allows F2+, ferrous iron, which is dissolved, to reach high concentrations • The repeated deposition of solid Fe3+ mineral phases indicates that O2 started to be available episodically in shallow water. ¡ Early life lived in an anoxic world. ¡ Single celled organisms got energy through methanogenesis and anoxygenic photosynthesis. ¡ This is the Archean Eon in the Geologic Time Scale. ¡ A key event was the evolution of cyanobacteria that are able to carry out oxygenic photosynthesis: § These cyanobacteria produce oxygen (O2) § They also fix nitrogen § The presence of O2 killed off most methanogens! ¡ Atmospheric O2 rose to 'appreciable' levels ~2.4 Ga before present.